Can I remove or adjust the BMS in my LiFePo pack?

[grazy+1949]

Hi All,

I am developing my own EV ATV Mobility Buggy. Eventually when I have the funds it will be powered by a LiFePo4 battery pack and have brushless motors.
For the moment I am using a 24v 75 Ah LiFePo battery pack and brushed motors. I will have funds in 2010 to do these upgrades.

Info: After charging the voltmeter shows 27.5v. This drops to 26.5v as soon as I use the vehicle.

Current problem:
My BMS shuts the battery pack down when it still has a voltage of 25v +. This will typically happen after say 5-10 km of driving in the foothills, which are 20 to 30% inclines. Whilst climbing a 30% incline the voltmeter connected to the battery side of the controller, will at times display 23.1v - 23.3v. At some point anywhere between 5 and 10km into the run, the BMS will shut the battery down. If I unplug the battery from the controller and then plug it in again, I get an voltage of 25.5v. However as soon as I try to drive it, the BMS shuts the battery down again. The batter last shut down 24 hrs ago and without being charged is showing 26.5v on a multimeter.

My questions are:
1) Do I have a failed battery?
2) Is the problem that my battery cannot discharge at the rate demanded by these steep gradients?
3) If I buy an intelligent charger can I operate without a BMS to avoid the above issue? (Which intelligent charged do you recommend?)
4) Should I strip the battery pack down and test for failed cells and balance all the good cells in order to end up with a battery that holds a bigger charge?
5) Should I replace the BMS and if so which rating BMS do I need?

Thank you for your help

grazy1949

[martin007]

Can you upload a photo of the Mobility Buggy?

[grazy+1949]

EV ATV Mobility Buggy 2x 600w brushed motors. 2x 25:1 reduction boxes. 24v 75Ah LiFePo battery pack.

[grazy+1949]

EV ATV Mobility Buggy 2x 600w brushed motors. 2x 25:1 reduction boxes. 24v 75Ah LiFePo battery pack.

[martin007]

Where are you from grazy+1949?

[steves1977uk]

BMS=The root of all evil!

What charger are you using? The recommended LiFePO4 charger is the Cellpro PowerLab 8 V2. This will charge and balance your pack far better than ANY BMS!

Steve

[Burgerman]

Current problem:
My BMS shuts the battery pack down when it still has a voltage of 25v +.
This will typically happen after say 5-10 km of driving in the foothills, which are 20 to 30% inclines. Whilst climbing a 30% incline the voltmeter connected to the battery side of the controller, will at times display 23.1v - 23.3v. At some point anywhere between 5 and 10km into the run, the BMS will shut the battery down. If I unplug the battery from the controller and then plug it in again, I get an voltage of 25.5v. However as soon as I try to drive it, the BMS shuts the battery down again. The batter last shut down 24 hrs ago and without being charged is showing 26.5v on a multimeter.

The cells you are using are what? Please dont say its a ready to go lead replacement brick so you dont know.
The reason I ask is because we need to know what C rate this pack is. Because its is a small pack to be using for a powerchair. A high C rate cell (such as a tiny 4Ah 70C 3S pack as big as a pack of cigarettes) can start a truck. And has a better heavy cranking performance than a 100Ah lead brick. But a low C battery like a 1C 4Ah battery can only run a headlamp bulb...

And what is likely happening is complex. It basically stems from any of the following, and maybe a mix of them.
1. Voltage drop under load, caused by low C rate and low Capacity cell. Remember that a 2C cell that is the same size as a lead brick at 75Ah can only do 140A absolute max. While it does this the voltage falls below what the dumb BMS considers is "safe. You MAY have a low, or bad cell. But either way that pack is too small for an off road chair.

The rest is all BMS related.
1. the BMS will cut power every time it sees a cell that drops below a certain level. So if its useless low 100mA balance circuit has allowed a cell to go far out of balance you may have 7 full cells and one at 30% for e.g. And you cant know. So under load, voltage falls after a couple of miles, aggravated by, high impedance cells, or bad cell caused by BMS balance uselessness, or a bad connection to one cell.
2. A BMS may be faulty. Or programmed incorrectly. I have 3 on my bench that failed killing off 3 lawnmower batteries in 3 years... And another from my strimmer pack. They are great at one thing. Problems and unreliability! Which is why I keep telling people not to use them. You may not have that problem but how do you know?
3. BMS may see you draw too much current (amps) due to exceeding the current limit of the lead brick replacement, or the one programmed into the BMS. This happens to almost all BMS equiped chairs, with users that try to climbe, or use high currents. Nandol in Portugal didnt listen, because he got his for free. He was stranded and had to be rescued 3 times befor his lithium "expert" rewired the thing to not use the BMS.

My questions are:
1) Do I have a failed battery?

Maybe. Or a low cell. Or a inadequate BMS, or incorrect BMS settings that you cant change. And once you can, then the BMS does nothing useful.

2) Is the problem that my battery cannot discharge at the rate demanded by these steep gradients?

Almost definitely if its only 75Ah esp off road. It may be the cell C rate that the BMS is protecting.

3) If I buy an intelligent charger can I operate without a BMS to avoid the above issue? (Which intelligent charged do you recommend?)

If your cells C rate is adequate to provide say 220A now and again, then you can use the only half decent charger that also balances and charges and tests batteries available thats much good to us right now. What cells are we charging?

4) Should I strip the battery pack down and test for failed cells and balance all the good cells in order to end up with a battery that holds a bigger charge?

Well you will have to just to know. And the only way to test them is to use the same charger that many of us are using. Where have you been?
Did you read POST 1 here? And follow links? Because 75Ah isnt enough. And BMS are the source of a mass of issues every time someone tries one on a chair! Read, but this is from 2012. Things have moved on!!! viewtopic.php?f=2&t=1813

You need a Cellpro PL8v2 and to read that thread! The red things... http://www.wheelchairdriver.com/BM3-con ... arging.jpg It will charge and balance your cells accurately with 1000mA balance, at 40A and display this on your laptop or PC screen so you can SEE what is happening.

Just to give you an example on capacity, I am replacing my 74Ah Lead brick, on my Salsa chair with 176Ah lithium 8 of these now fit the same space. They are 2C cells, peak. So can cope with 2x 176A so plenty of Amps. And over double the Ah that you have now. Lithium doesn't like a hard life. So fit a big pack. 8 of these for EG https://www.lifepo4-batteries.com/sale- ... f-car.html

5) Should I replace the BMS and if so which rating BMS do I need?

A BMS isnt needed, and more importantly it isnt wanted!!!
Having one is the source of your problems, and the reason most lithium cells are ruined.
READ SLOWLY!!! http://www.wheelchairdriver.com/BMS.htm
And ignore hyperion references, replace with PL8v2 from Revolectrix, and ask me for a profile for it once we find out what cells you have. For discharge testing and for charging properly!

Thank you for your help

I aim to please! I know a lot about batteries. And chargers. And have been using lithium in hobby stuff since 90s, and chairs for nearly a decade. So please ignore all the numpties trying to flog you a BMS!!!

[woodygb]

24v 75 Ah LiFePo battery pack

Please be specific .... Do you have a link to the specs?

[Burgerman]

Just saw the pics. That thing looks like a car. A 75Ah lithium battery is around half whats required to make it work properly, and around 1/3rd what it really needs. Range off road where loads are very high, will still be quite limited.

The entire point of moving to lithium is that its more energy dense. It therefore allows you to fit more Ah into the same space. It actually demands that you do so because most LiFePO4 doesn't like large currents, and so its much better to install 150 to 200% more lithium than you had lead. Which you can. And once you do, a protective BMS becomes unnecessary. Its battery protection BMS system is only there for 2 reasons.

1. To charge cells and balance them. Depending on actual charger and bms setup, they can be spectacularly bad at this, or just bad. So good luck with that!
2. It protects the cells from people who think its OK to fit a pack into a machine that will draw too much current, and damage them. As it is doing now. In a properly implemented setup, where cell capacity and C rate is more than adequate to supply the controller at peak demand (Max Amps) the part of the BMS that pulls the plug is no longer required. So thats both bits redundant.

So get a real charger, and throw away the BMS and fit more Ah or/and C rate will likely be your only real solution.


Also you say 600w motors, but that only tells us they can survive 600w continuous. It tells us nothing useful electrically. What we need to know is how many Amps the motors and controller can suck out of the battery at full power? Is its controller, 200A or bigger? Single or double channel?

[grazy+1949]

Hi Guys,

I want to say a huge thank you to both Woody and Burgerman for the great replies you have given.

I will be reading and re-reading all this info and following all the links you have provided.

The EV ATV BUGGY weighs 120 kg with the 24v 75Ah li-on battery pack. I did 10 trips up a steep 6 km climb to the mountain village of Mijas Pueblo, Spain and then the battery started to drop below 24v on the voltmeter. If I stopped it would read 26.5v.

I have temporarily re installed the lead acid bricks, which are a big disappointment of course.

Today I took the lid off the Alu cased golf buggy battery pack. and discovered that it is built from 25 x 68mm. LiFePo cells. The only marking I can find on the cells which are all kind of glued into a pack within an alu box is: Heter Battery.

I do get your message guys and I can see that this golf buggy battery, is never going to get me 6 km up 20 and 30% inclines with reliability. I will put the pack back together and sell it to a single seater golf buggy owner, who is going to drive 200 m,let the battery pack rest, whack a ball, and repeat the process.

You have made it obvious what I need to do. Get hold of some cash and buy enough LiFePo4 3.2v X 180 Ah (or similar) to suit my needs and at the same time buy a PL8V2 from Revolectrics. I have had to recover this buggy 6 times due to BMS "tonterias" (Span. for: stupidities.)

My controller is a PN S-Drive 140 A. I do have a Robotec 150A HDC2459S, but living down in Andalusia, I have never found anyone capable of programming it.
[url]
https://www.learningalias.com/showmod.php?m=75&t=26[/url]

If you guys have links where I can source the LiFePo4 3.2v x 180 Ah, the charger and the power supply , please let me have them.

I have looked at the http://www.parvalux.com website, but they don't mention the amperage that these PM90/150 24v motors draw.

In response to the question : Where have I been? The answer is laying on the floor in 35ºC building the buggy and then , teaching Alias AutoStudio CAD to keep the wolves from the door. My wife gave up her pizzeria to care for me full time.( Here in Spain carers receive zero from the state, but she is still required to pay her soc. sec. of 250€ per month ). We get by, but buying the pack and charger for this project will be a struggle, which has to be undertaken. I plan to buy brushless motors and a new controller some day too. I have a rehab powerchair for sale which has Alber 120w BLDC motors and they are fantastic. I will sell that to raise the funds required.

Thanks for all the valuable info guys. It's very much appreciated-

grazy1949

[foghornleghorn]

I have looked at the http://www.parvalux.com website, but they don't mention the amperage that these PM90/150 24v motors draw.

Is this any use? pm90-data-sheet-10-10-2016.pdf

[woodygb]

.

[grazy+1949]

Hi Woodygb,

Thanks for digging that out, but the PM90 is a 4 pole mine are 2 pole PM90/150's. It's raised another question though.

Q) I suspect that my PN S-Drive 140A single channel controller, is simply too small to handle the loads imposed by twin 400w motors on a 30% gradient?

Q) Which controller would you recommend for these motors and this extreme service?

(I do have a 150A RoboteQ single channel controller which has 2 sets of motor leads, so peaks at 300A. This sounds good. I don't need to program for a joystick, as I have handlebars, but even so I don't have anyone who knows how to program it )

Here are some pics of the inside of my 75Ah LiFePo battery.

Q) Can I remove the BMS and rewire the leads and then charge it with a PL8v2 from Revolectrics , at least until I have the new LiFePo4 battery pack.

I am going back to read all those links you sent me now.



grazt1949

[grazy+1949]

[woodygb]

The Roboteq should not need any real "programming" for your application.

As to your question is the S-drive under powered.... this really needs YOU to say ...
Do you find that the buggy cannot ( with fully charged batteries ) climb gradients?


http://www.heterbattery.com/products/li ... 00mah.aspx

http://www.heterbattery.com/products/li ... lls-3.aspx

[Burgerman]

The motors have 300A stall.
So 2 of those can potentially pull 600A on a slope or a hill. That will be limited by the controller. And must be. Although you say yours are 2 pole.

The problem with 2 pole is that it will probably half the motor current, and half the torque. So you will need double the pulsewidth most of the time. To give the same torque, performance, speed etc. That actually means you will draw double the BATTERY amps at low speeds compared to a 4 pole motor. So that makes your battery drain issue much, worse.

Lets say you fitted a 150A per motor roboteq. And because the motors are 2 pole, you will be pulling almost full motor amps from the battery at stall/low speeds. That means trying to get 300A out of a 75Ah battery... Thats a continuous 4C, and so no wonder you have BMS/Battery issues. As it is, you have half of this. So will be pulling 2C continuous at least while struggling to get up your slopes with a heavy vehicle.

You need to fit a much bigger battery
Lose the BMS.
Use the roboteq controller.
Fit 4 pole motors as this will reduce the battery current on your steep slopes, and/or increase torque.

Also no battery spec, what exactly is it?
And no BMS spec? It needs to go in the bin anyway. But what is it?

[Burgerman]

That battery if woodys link is correct, is made of many small 3C max cells. That means you should try and not exceed 1 C most of the time, and will be able to do 3C intermittently. So it will murder it to be doing 150A on those slopes. And I expect it will cause a large voltage drop, causing the BMS to cut power... As they do.

The reason you are getting cut outs is the battery isn't big enough and BMS are stupid so dont understand the difference between volt drop under load, and volt drop due discharge. Its trying to protect a too small battery from a too big load. Which will get at least twice as bad with the roboteq. Since it will be 2x 150A on your slopes.

Dumping the BMS will stop it cutting out. You will then ruin the cells. As they are inadequate for the job.

[grazy+1949]

Hi Burgerman,

How does this sound for a plan:

RoboteQ 150A single channel, but 2 sets of motor feed cables, so peak 300A .

*8x3.2v 176 Ah (or 24v Ah if I can raise the cash) LiFePo4 packs (2C)

2 x AMT 4 pole motors, as per your BM3

And please comment on this "* Nº recycles > 6000" . Shouldn't that read >2000 https://www.lifepo4-batteries.com/sale-10635284-factory-direct-sell-3-2v-240ah-lifepo4-battery-cell-wholesale-life-for-ups-telecom-base-station.html

Q) Do I have to have the bus bars made myself?

[grazy+1949]

My Last Q for tonight.

Inside my Golf buggy battery's Alu casing. There are 9 layers, each of 16, 26 mm x 65 mm LiFePo or LiFePo4 cells, I guess they are 3.2v 3000 ma cells, total cells 144.
I can't read any spec on them just Heter Battery. They look very difficult to remove from the alu case, as they are stuck in with strong sheets of double sided adhesive material. Also I suspect that they are spot welded in banks to form a pack. The neutral cables are accessible on the BMS, but there are 2 sets of red battery leads which are not accessible, as they are down the side of the pack between battery and case. In other words, I don't think that the cells can be removed from the alu case. and as I have no way of knowing whether they are LiFePo or LiFePo4 my Q. is as follows:

Whilst I am waiting to collect the money to buy a big LiFePo4 battery:

Q) Could I remove the BMS and charge this battery pack with a PL8v2 Revoelectrics charger.

[It does climb 30% gradients at the moment , even using a cheap 50€ charger. It's the BMS that is shutting the battery down, when the voltmeter is showing 23v]

[shirley_hkg]

There are 9 layers, each of 16, 26 mm x 65 mm LiFePo or LiFePo4 cells, I guess they are 3.2v 3000 ma cells, total cells 144.

Your 26650@3000mAh cells are good , but low C-rate .

Hi Guys,

If you guys have links where I can source the charger and the power supply , please let me have them.

grazy1949

viewtopic.php?f=2&t=8425&p=130551#p130551 While discount last .

[Burgerman]

How does this sound for a plan:
RoboteQ 150A single channel, but 2 sets of motor feed cables, so peak 300A .

Yes, it will double your available torque. And so will then need a much more capable battery.

*8x3.2v 176 Ah (or 24v Ah if I can raise the cash) LiFePo4 packs (2C)

That will barely be adequate. Its OK on a wheelchair. As thats exactly what I am about to fit to my new salsa chair. But you device is much heavier. And you intend using off road, meaning way higher discharge rates. I didnt know how big or heavy it was when I suggested those cells. It would be better to fit something similar but bigger Ah if you have the space.

Those cells are 1C continuous. That means absolute max, for more than a couple of seconds to a full discharge. The rated service life is 1500 cycles to 100% discharged. Or 2000 cycles at 80% discharged. Or 6000 cycles at 70% discharged. But thats at a .1C discharge rate. So an average of 17.6A. If you take out 176A regularly then expect that to be about 500 cycles like lead or much worse if you exceed this. If you try to take out 2C (PULSE rating) then you can only do so for say 1 to 3 secs. They CAN do that, but the battery will suffer and volts fall due to resistance. It doesn't mean its a good idea. So you need a big battery and a high C rate.

They will work, but I would want something 40% bigger for peace of mind and a sensible service life without issues. Dont buy the cells I linked to, buy cells that MAXIMIZE the Ah that you have space to fit! Thats the whole point of lithium. And the best way to make use of its capability. I have to ask, who advised you to fit a 75Ah with BMS battery in that machine?

2 x AMT 4 pole motors, as per your BM3

Not big enough for something that big and heavy off road on slopes etc. Ideally you really want either low gearing (6mph uses half the Amps, at EVERY speed and load than 12mph). They would cook. You really want a couple of 1000 watt scooter type motors. Or you will end up burning out a lot of motors. Or a large scooter transaxle/motor and there are a few available cheap on chinese auction sites intended for this type of application. You can still use your roboteq, and 4 wires, but it will need an adaptation to single channel. Ask roboteq. Its not very complicated.

And please comment on this "* Nº recycles > 6000" . Shouldn't that read >2000 https://www.lifepo4-batteries.com/sale- ... ation.html

Its all in the details.
They quote 6000 cycles and thats what you will get at LOW discharge rates, (typically .1 or .2C, properly charged (throw out the BMS!!!) and to a DOD of 70% and to a residual capacity remaining of 70% of the original rated capacity. I had a full spec sheet for the 176Ah cells by email.
You will get 2000 at 80% discharge level, but not at high discharge rates! Thats why a BIG pack is always needed. The ones you linked to for eg.
You will get 1500 cycles if you discharge to 100% DOD to 80% of original rated Ah capacity. At low discharge rates. If you discharge faster you get a lot less... So bigger is always better.


THINGS lithium hate.
High discharge rates. Rapidly shortens life. So you want high rate cells, and/or high C rate. And so each cell gets an easy life.
High charge voltage. Charge to 3.650 Volts, is already too high, and a BMS allows repeated pulses up to 3.7 or more... We usually set the PL8 chargers to 3.600V initially, and lower to 3.550V after the first few charges. And the PL8 balances 10x faster, so it doesn't hold the highest cells at top (over) voltage for hours as it balances. And it doesn't pulse them and keep turning charger on/off as it attempts to balance, it does so in a controlled proportional manner.
Going below about 2.9V per cell, increases self discharge levels causing balance problems that the BMS cant cope with over time. The PL8 can but dont allow it to ever happen. In fact only use the first 80% of the Ah.

Q) Do I have to have the bus bars made myself?

Bus bars are not needed. Just use some 10sqmm cable, and ring terminals. It allows greater flexibility. You will need to do that anyway to fit the smaller balance wires.

[Burgerman]

Q) Could I remove the BMS and charge this battery pack with a PL8v2 Revoelectrics charger.

Not only could you, its pretty much essential. BMS is always problematic.
Its safety features will always cause problems, reliability issues. But in this case its trying to protect the far to low C rate pack. How do I know its far too low? You say its dropping to 23V! That will damage the cells regardless. So battery isnt capable of making adequate Amps. Because its too low C rate, and way too small for the job. It wont last long doing that to it. But removing the BMS will stop all that cutting out. And allow the battery to die fast as you are overworking it.

[It does climb 30% gradients at the moment , even using a cheap 50€ charger. It's the BMS that is shutting the battery down, when the voltmeter is showing 23v]

Thats what its meant to do. A BMS is there to stop people damaging the battery by fitting it into a situation that it cant cope with. But if you happen to know what you are doing, you would PLAN things correctly, and would have already figured out that the battery isnt suitable. In a properly thought out system a BMS never actually does anything. Other than help charge very badly. So the best place to put that thing is in the bin.
The problem is that your battery is also too small, to low C rate to do the job you intended.

ABSOLUTE MAX RATINGS THAT YOU SHOULD TRY TO KEEP AWAY FROM!
Remember that a 2C 75Ah battery can do 150A.
A 2C 200Ah battery can do 400A so bigger is always better. If big enough Ah, you can still have higher amps even with a lower C rate battery.

[LROBBINS]

I'll just note three things:

(1) That buggy weighs all of 120kg - lighter than your Salsa, and grazy doesn't have your heft either.

(2) It is tiller steer so doesn't have the load of differential-steer turn in place.

(3) The Roboteq that grazy has is already an "S" model - single channel. The only scripting needed might be motor compensation, but even that is probably not critical for how he''d be using it. Scooterman already has an untested "draft" of a script I've modified for scooter use that also includes some other not-really-needed niceties.

[shirley_hkg]

My Last Q for tonight.

Inside my Golf buggy battery's Alu casing. There are 9 layers, each of 16, 26 mm x 65 mm LiFePo or LiFePo4 cells, I guess they are 3.2v 3000 ma cells, total cells 144.

BM said : Knowledge is power .

144 cells will make a 8S/18P pack . @3000mAh cell, that's 24V 54Ah.

I ues 26650@3000mAh cell too , but 8S/72P , so 216Ah.

[Burgerman]

(1) That buggy weighs all of 120kg - lighter than your Salsa, and grazy doesn't have your heft either.

I was going by the photos and it looks huge? Like a small car. Or one of those large road going scooters.

If its 120kg, with 6mph gearing, then it would likely be OK with the same 1C Cont. 2C peak 176 Ah cells I was planning to use. As long as it wasn't going to spend a lot of time stalled or climbing steep slopes off road. But it is? So if theres the space for more Ah, thats still wise. My salsa for e.g. will not climb the pub ramp at 15 degrees (not percent!), as its too heavy (185kg) it just stalls the motors. With 6mph/4 pole and 2x120A. While I am trying to climb that ramp, and still moving, both motor channels are maxed out at 2x 120A. So thats ok just about. 240A for 2 to 3 secs. The rest of the time its less. unless zero turning. But thats at stall speeds or close so battery amps are lower than motor amps are. If he was running up a steep hill slope off-road, then max Amps AND full pulse-width will coincide at about half max speed. (his motors are 3.5A stall, 5A free running). Meaning that with roboteq, max 150A controller Amps x2 = battery amps.

For off road use the 2x 150A roboteq will still be an advantage. And I would still prefer to see say 200Ah or so if it will fit. Or some 3C cont / 5C pulse cells. As it is, he is dropping an almost fully charged pack down to 23V or just under 2.8x volts per cell under load. The BMS sees the low volts and cuts off power to save the battery. And those are 3C. (75Ah x 3 = 225A supposedly)

(2) It is tiller steer so doesn't have the load of differential-steer turn in place.

True. But that occurs at stall speeds, or very slow speeds. So the 100 or 150 limits is at small pulsewidth. (with 305A motor stall, at around 50% pulsewidth, so 2x 150A motors pull only 150 total from the battery. Under 1C.
But driving up a slope off road at half speed, at 100% pulsewidth, under load of a steep slope, means 300A at battery. And thats his intention? So regular 2C at the (pulse) limit.

(3) The Roboteq that grazy has is already an "S" model - single channel. The only scripting needed might be motor compensation, but even that is probably not critical for how he''d be using it. Scooterman already has an untested "draft" of a script I've modified for scooter use that also includes some other not-really-needed niceties.

Thats perfect.

44 cells will make a 8S/18P pack . @3000mAh cell, that's 24V 54Ah.

Oops...

[grazy+1949]

Q) Where can I source this battery spec:

26650@3000mAh 8S/72P , 216 Ah

This sounds like the size of pack I need to climb 6 km @20-30% up a mountain to my next village on a regular basis.

After 10 trips up the mountain, my LiFePo4 24v 74 Ah battery pack with BMS has become weaker, to the extent that the voltmeter shows 23v when climbing 30% measuring from the battery leads.

Q). If I remove the BMS and charge it with a Pl8v2, will it recover at all. I am planning to redeploy it for use in another EV on much flatter roads

Thanks guys

[Burgerman]

Q) Where can I source this battery spec:

26650@3000mAh 8S/72P , 216 Ah

Shirley bought 26650 cells and built a pack from it. For a wheelchair.
There are MANY 26650 cells available, some are what it says on the tin. Some are much lower capacity than claimed, some are much higher resistance (impedance) than claimed meaning lower C rate. Greater voltage drop.
It will be much easier to build a pack using the large prismatic cells such as you were already looking at.

26650 just means, 26mm diameter. And 65mm long. They can be lithium ion and 7S (bad idea unless you like fire) lithium polymer (same problem) or LiFePO4 which is safe and requIres 8S but is a lower energy density.

After 10 trips up the mountain, my LiFePo4 24v 74 Ah battery pack with BMS has become weaker, to the extent that the voltmeter shows 23v when climbing 30% measuring from the battery leads.

Someone did the maths and it actually added up to 54Ah? Rather than 74. But either way inadequate on a mountain!
Reasons it could become "weaker" are:
Too many amps removed from the cells, (too much current) so damaging the cells. Thats very likely.
BMS has let the pack become unbalanced so when the BMS/Charger tells you its charged only some cell groups actually are. One cell group goes low a short time into your trip since its already lower than the rest. This will actually damage it too making it get worse as you use it. Another reason to throw away a BMS.
BMS is cutting power earlier as it is going high resistance as its mosfets begin to fail.

Without testing each cell, and throwing away the BMS, and properly charging the pack, its not possible to know.

Q). If I remove the BMS and charge it with a Pl8v2, will it recover at all. I am planning to redeploy it for use in another EV on much flatter roads

If its a balance problem, it will properly balance the pack.
If cells are damaged, it cant help.
If the pack is simply too low C rate, or too low capacity, it cant help.

It can show you if the pack is charged, balanced and ready to go.
It can then test the pack, measure the Ah, show you if any group out of the 8 is lower capacity than the rest.
It can test each cell individually for Ah.
And can graph and monitor all of this in future.
And it allows you to chose the charge rate up to 40A, and charge voltage, and many other details that keep your expensive pack correctly charged so it has a long service life. And it allows you to SEE this is happening.

[grazy+1949]

Hi Burgerman

OK, so drop the idea of: 26650@3000mAh 8S/72P , 216 Ah

Get back on course with the 178 Ah pack. I have a quote for $1,500 shipped. These are 2C. They are talking about sending them by sea @200€, but I really need them quickly, so I'll get a quote for fast shipping.

In the current 74Ah lead acid golf buggy replacement LiFePo4 pack , they have soldered or spot welded the cells together.

Q). Can I test cells in this type of pack or do I need to separate each cell?

I have a spare buggy chassis, which is 30cm shorter and I plan to recycle the drive train from my existing prototype on this shorter lighter chassis and then use it in town, where it's flat. There is just a 1 km climb to get back home.

I plan to use the AMT 4 pole motors with the RoboteQ 2450S controller. I will have to find someone to machine new wheel flanges. I can make the CAD model.

I would dearly love to go for BLDC angle drive motors, but as yet I have't found any with 25:1 reduction and I am frightened that if I fit 19:1 , they won't be able to deal with the very steep climbs.

Another posibility is to get a Chines company to modify their existing BLDC 1.5 Kw to 2 Kw transaxle to the track width I need, as they are either 70 cm (Mobility) or much wider for larger dia wheels.

I have an Alu Netti 4U CED rehab powerchair with an Alber E.Fix 25 hub motor 120w BLDC. These motors are really fantastic for a 60 kg powerchair and have climbed one particular short 30 gradient many times. Unbeleivably they never get hot. The problem with this powerchair is that it has large 540 rims and the widest tyres available for them at 1 3/4 inch. Due to the small tyre foot print and the crappy joystick, it has skidded out of control on several ocassions. This chair is powered by a 24v 12 Ah LA battery . I will have to break it up to use the reahab seat and leg supports on my second buggy.

[Burgerman]

Get back on course with the 178 Ah pack. I have a quote for $1,500 shipped. These are 2C. They are talking about sending them by sea @200€, but I really need them quickly, so I'll get a quote for fast shipping.

There are US suppliers of lithium cells if you want them quickly, and can likely find 3C cells. I already said that those 2C cells are 2C PEAK not 2C cont. They are OK in a powerchair, but I wouldnt like to use them climbing a mountain/off road. They might be OK, and they will be better than what you have, but they are really 1C cells that are 2C pulse or peak for short periods. How much actual battery space, or battery volume do you have available? If ther is only enough for 2x grp24 lead, then these will be the pest option. If theres space for more then bet to use that!

In the current 74Ah lead acid golf buggy replacement LiFePo4 pack , they have soldered or spot welded the cells together.
Q). Can I test cells in this type of pack or do I need to separate each cell?

Didnt someone say that this was only 54Ah?
You can take out and unwrap the pack so you can access the cells in the parallel groups. remove the Battery Murdering System, (BMS).
There is 8 groups of cells, each connected in parallel to give either 54 or 75Ah depending on the actual correct figure, at 3.2 volts. Give or take a little dep on state of charge.
So you can then use the PL8 to fully charge each group, (8 separate charges). And then set it to DISCHARGE and monitor and graph the result, and to stop at a safe 2.900V. And then read off the Ah removed. Then you will know if each GROUP of cells is equal. Then recharge each group. So you will then have an Ah number for group 1 to 8. And a full battery. If all are the same to with 1 to 2 percent, then you are good to go, and will have a REAL Ah capacity figure for the pack. You could also discharge the pack as ONE all 8 groups together, and have a full discharge curve for the full pack graphed so you can see everything together. On your PC. All this will need a little learning and correct cables making. Theres no plug and play easy solution. People think there is. Thats how you ended up with the problem you have now. You listened to a lithium "expert" that hasn't a clue. Please read the thread pinned about lithium batteries at the top, and or the addon thread by expresso. And the dedicated PL8 thread on the forum.

I have a spare buggy chassis, which is 30cm shorter and I plan to recycle the drive train from my existing prototype on this shorter lighter chassis and then use it in town, where it's flat. There is just a 1 km climb to get back home.

How steep is this climb? Because 1km is way too far if the scooter takes more Amps than the small pack likes. It will fail or die very fast. The closer you get to the C rate, the faster they fail. Its like an every increasing curve.
You may get 10k cycles at 0.1 of C rating.
Or 1000 cycles at rated cont C
Or 100 to 250 cycles at peak C
Or non to 50 at above that. Since nobody has an accurate figure. C rate isnt a fixed point. It just means that they picked some number along an ever steepening line.

Some cells can do 130C Some do 1. Or less. C rate is a guide. Best to be WELL inside their max rating. Or load pulls voltage down to the 23V danger area.

What does load mean?
Hills. Off road.
Speed. (as in gear ratios, top speed)

[shirley_hkg]

Someone did the maths and it actually added up to 54Ah? Rather than 74. But either way inadequate on a mountain!
.

He still didn't aware of his so-called 74Ah pack is actually 54Ah (claimed ) only , and 1C.
He is lucky to get 40Ah out of it in actual use .
We all can imagine the voltage drop for such a small battery , stressed under enormous load.

So moving from a 54Ah 1C battery , to a 176Ah even @2C is a huge , huge jump already.