To BMS or not...

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Re: To BMS or not...

Postby Burgerman » 02 Dec 2020, 03:56

This BMS balances cells by taking the highest cell to charge the lower cells. After the BMS cuts off charging, all cell volts drop under 3.500v or even 3.450v. I think that's why Shirley set the balance starting voltage to a low value 3.400v.


Thats the problem. First it cuts off charging far too soon. Needs to sit at CV above 3.5V for long enough to properly balance. And to fully charge each cell to the same level. By design it does it wrong. It will always look balanced if you let it do this even when its not.

You must not balance at less than 3.5V. Preferably 3.55 or above. Or what it shows you is that its balanced when it isnt. Thats why I keep asking him to use the non averaged 4 digit data/graphs and only balance above 3.5V for say 30 mins after CV reached a least so we can see what it really does. Preferably without all that bouncing up and down.
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Re: To BMS or not...

Postby Rye » 15 Jan 2021, 20:22

What is a BMS? Just a "smart" charger/monitor that needs a power source?

Are there any programable BMS's?
https://www.youtube.com/watch?v=EZt1oJ3n7jY

Is it unrealistic to build your own with arduino? I'm no arduino/bms guru, but there are quite a few how-tos around suggesting it's possible.
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Re: To BMS or not...

Postby Burgerman » 15 Jan 2021, 20:55

Mostly its a device that protects a lithium battery from over current, over discharge, over voltage, on a individual cell level. And it usually includes a way to charge a multi cell system by connecting to a 2 wire dumb charger.

Basically we DO NOT WANT it to protect the cells from the wheelchair in use other than over discharge. Because your powerchairs controller does a better and PROPORTIONAL job without cutting out. All the other dozen things it does are only needed if the pack is too small or to low C rate for power module. In which case its a band aid to prevent battery failure for the manufacturer. And you should have used a bigger pack to begin with. And it does this protection by suddenly chopping off power. And you stop... Great as you try to climb a ramp or curb.

And the charge part of a BMS is dismal because its in the wrong place. In order to stop a high cell going over the max allowed voltage it applied a tiny brake current. Which is a fraction of the chargers power. And it cannot decrease charger power. So 1 second later the voltage is at above max allowed and then it disconnects the charger. Once the voltage has been pulled down too low to say 3.5V then it re applies full power. Rinse and repeat. Also no way to control termination current or to know it its actually balanced or if theres some cell connection or fault. Meaning it can get more unbalanced over time and you get to stop at 50% discharge or whatever to save the battery a long way from home unexpectedly.

So yes you can. But no you shouldnt! Its the wrong thing, doing stuff we dont need or want, and a thing thats terrible at charging. Because its in the wrong place.
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Re: To BMS or not...

Postby sacharlie » 16 Jan 2021, 19:54

Burgerman wrote:
And the charge part of a BMS is dismal because its in the wrong place. In order to stop a high cell going over the max allowed voltage it applied a tiny brake current. Which is a fraction of the chargers power. And it cannot decrease charger power. So 1 second later the voltage is at above max allowed and then it disconnects the charger. Once the voltage has been pulled down too low to say 3.5V then it re applies full power. Rinse and repeat. Also no way to control termination current or to know it its actually balanced or if theres some cell connection or fault. Meaning it can get more unbalanced over time and you get to stop at 50% discharge or whatever to save the battery a long way from home unexpectedly.

So yes you can. But no you shouldnt! Its the wrong thing, doing stuff we dont need or want, and a thing thats terrible at charging. Because its in the wrong place.


In other words the difference is that a hobby charger both "monitors & charges" each individual cell through the balance wire while the bms only monitors, correct?
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Re: To BMS or not...

Postby Burgerman » 16 Jan 2021, 21:45

No. The BMS can only try to stop the high cell from exceeding its max 3.600V by adding a "drain" of usually a 100mA max. Although some can be more. It varies. This is how it balances the cells. It draws a small current from the high ones. While the charger is charging at say 10A or 30A. So obviously that is futile and the voltage keeps climbing fast. In say 2 seconds it may reach 3.8V or more. At that time the BMS does the only thing it has left. It chops off all power from the charger via a mosfet which is a switch. And it then waits while the feeble drain current pulls the cell back down to around 3.4 to 3.5V befor adding the full power once again. And soon after its back at a too high voltage again. Rinse and repeat for many hours. This bounces the cells between 3.4 and 3.8 or 3.9V repeatedly for as many hours or days as it takes to get it balanced, while the rest catch up, or the charger thinks its done and stops charging. Often while still far from balanced...

Whereas a hobby charger adds say 1A of drain on that cell, and then reduces charger total output to be sure it never exceeds the 3.600V on the high cell. Meanwhile all the others continue to charge until they too reach 3.600V. A charger SEES the highest cell and redices power proportionally to continue charging while not allowing the high cell to exceed its limit. And will do so until a) all cells are within 2mV and b) the termination current is reached so we know it is both properly balanced and full.

A BMS cant do this as it is in the wrong place. And cannot control charge rate. Or the termination point. Which is often long befor its balanced. Or never!

A Tesla for e.g. car and all EVs do the same as the hobby charger. You plug the car into the AC. The power supply inverter and the charger are all on board and very complex. They still call that a BMS but it does things the same way that the PL8 and other hobby chargers do. Only also integrated into the entire cars computer system. It is even more complex to get lot more cycles from the lithium ion chemistry than they would normally give. Mostly by only using around 70% of the batt capacity in total. No high or low voltages/states of charge.
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Re: To BMS or not...

Postby sacharlie » 16 Jan 2021, 22:21

OK thanks.
So the PL8 charges the 8s pack at ~24v@variable amps and the individual balance wire to each cell is only to monitor and drain(if needed) but not charge.
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Re: To BMS or not...

Postby Burgerman » 16 Jan 2021, 22:37

Yes. * Under all normal circumstances. And thats also what a BMS does. But it cannot control the charger output power! So it has to repeatedly disconnect it.

* Well actually the PL8 CAN charge via the balance wires too! Without connecting the main wires at all. If you choose FMA wiring in the setup optons)
I do that all the time for small hobby packs, RC transmitter etc. Needs no main wires so slow but easy. But at a max of 3 Amps. With a 200Ah wheelchair pack you need 30 to 40A minimum or you would be there all week. So for our purposes you can ignore that.
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Re: To BMS or not...

Postby shirley_hkg » 07 Mar 2021, 09:04

My first try to put 200A in the stock boxes of an INVACARE Arrow . cheers
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Re: To BMS or not...

Postby Burgerman » 07 Mar 2021, 12:53

You bypassed the BMS so it doesent screw you up?
Use it only for dumb charging at a lower voltage supply?
I hope it works ok. Get yourself a smoke alarm! :D
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Re: To BMS or not...

Postby stevelawiw » 07 Mar 2021, 19:50

With a 200Ah wheelchair pack you need 30 to 40A minimum or you would be there all week.

I have no wish to contradict our Guru, Burgerman :worship , but my real world experience of charging a 200ah pack every 6 days approx is that I charge at 25ah which puts approx 120ah in the pack in 7 hours.
I do sometimes bump the PL8 upto 30ah but then I find the PL8 cooling fans are on constantly and are a bit loud next to my bed all night, they are too frenetic for my liking, it disturbs the telly!
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Re: To BMS or not...

Postby Burgerman » 07 Mar 2021, 21:03

Ah but you are only taking120Ah out of them in a week! Because its winter? Anyway 25A isnt bad. Its already 3x faster than a 8A mobility charger! And almost the 30 to 40A I said above.

With lithium though I typically charge them only before going on a trip into town or something. So dont do it next to my bed when sleeping but sat at my PC doing other stuff and eating etc. So speed is more important. Fans? Dont care when up! At 40A charge you can put back 80Ah in just 2 hours and thats the same range increase as adding FOUR complete weightless MK GRP 24 Batts... :clap
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Re: To BMS or not...

Postby stevelawiw » 08 Mar 2021, 00:22

Ah but you are only taking120Ah out of them in a week! Because its winter? Anyway 25A isnt bad. Its already 3x faster than a 8A mobility charger! And almost the 30 to 40A I said above.

Actually the more I can get outdoors in the summer the less I use generally, don't forget I have the efficiency of GB motors :admirer

I can't charge when I'm in the chair, as soon as I would plug in I would need something just out of reach, or there would be someone at the door.
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Re: To BMS or not...

Postby swalker » 08 Mar 2021, 01:06

I also charge my LiFePO4 pack (176 Ah) at 25 amps with the PL8. I try not to pull more than 140 amp hours out of the chair before a charge. I can charge and balance the pack in less than 8 hours, which is fine for my pattern of use.

That is far better than the 15 hours Permobil says I should charge my group 24 lead acid gel batteries every night:).

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Re: To BMS or not...

Postby shirley_hkg » 08 Mar 2021, 02:44

Burgerman wrote:You bypassed the BMS so it doesent screw you up?
Use it only for dumb charging at a lower voltage supply?
I hope it works ok. Get yourself a smoke alarm! :D



Exactly , except smoke .

It cuts input / output via a contactor, so you can charge at any rate you can offer.

I avoid stressing my gadget , and always recharge through out the night.
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Re: To BMS or not...

Postby terry2 » 15 Mar 2021, 08:45

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Re: To BMS or not...

Postby Burgerman » 15 Mar 2021, 11:31

In what way? Appears to be exactly the same as all the rest.

Its still got no purpose on a powerchair that I can see that you would actually want, other than as a cell log. And those are cheap anyway.

Can you think of any reason to want one? Maybe temporarily for charging till PL8s are available or we evaluate a different cell balancing charger or three.

Now if you connected the cells directly to the power module so that non of its unwanted protection stuff interferred with use, and just used it as a cell log, great! But if the pack is big enough, you dont really need that anyway as you are out of day before you are out of battery. And the balancing/charging stuff is way inferior to the PL8. Other than right now they are in short supply!
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Re: To BMS or not...

Postby shirley_hkg » 15 Mar 2021, 14:02

Burgerman wrote: And the balancing/charging stuff is way inferior to the PL8. Other than right now they are in short supply!



But how to ? I've been waiting for 18 months.

I made 3 packs with the BMS .
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Re: To BMS or not...

Postby Burgerman » 15 Mar 2021, 18:49

But you know what to do to make it work. And the right way to charge. And yes theres not much of a choice at the moment anyway.
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Re: To BMS or not...

Postby expresso » 15 Mar 2021, 20:14

when they start shipping - everyone needs to buy - 2 or 3 - to be safe for later on - it it happens again or if they even stop making it
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Re: To BMS or not...

Postby Burgerman » 15 Mar 2021, 21:03

I realised that 5 or 6 years ago.

Theres 3 on my bench, and a BT one in my kitchen in use right now. And one on the shelf in an unopened box...
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Re: To BMS or not...

Postby terry2 » 18 Mar 2021, 11:05

expresso wrote:when they start shipping - everyone needs to buy - 2 or 3 - to be safe for later on - it it happens again or if they even stop making it



Now everything over £35 will have 20% VAT +UK duty and you have to pay the shipper for doing the paperwork and paying the 20% VAT.

It's not worth importing anymore.
Also in the UK. If it breaks we have to pay for it to go back to Singapore and back.
So more costs and some more duty.
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Re: To BMS or not...

Postby terry2 » 18 Mar 2021, 11:15

Burgerman wrote:In what way? Appears to be exactly the same as all the rest.

Its still got no purpose on a powerchair that I can see that you would actually want, other than as a cell log. And those are cheap anyway.

Can you think of any reason to want one? Maybe temporarily for charging till PL8s are available or we evaluate a different cell balancing charger or three.

Now if you connected the cells directly to the power module so that non of its unwanted protection stuff interferred with use, and just used it as a cell log, great! But if the pack is big enough, you dont really need that anyway as you are out of day before you are out of battery. And the balancing/charging stuff is way inferior to the PL8. Other than right now they are in short supply!



Have you tried any of the new programable BMS units?
Github https://github.com/FurTrader/OverkillSolarBMS
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Re: To BMS or not...

Postby Burgerman » 18 Mar 2021, 11:27

I dont need to try them! They are in the WRONG place for charging properly, and they all work in exactly the same way. Do they work? Ish... Using a flawed method. They cannot control the charger output power and so can never do it right. Right now theres not much choice because PL8 are impossible to get. As long as you choose the RIGHT settings, and have a charger that is charging at a lower 3.55V per cell (depending on exactly how/when the BMS balances as that wont work with some) and keep an eye on what its doing.

As for the rest of the stuff they do, it simply isnt wanted. So its best to connect the chair directly to the cells and not through the BMS so as to avoid all that other nonsense they do that cause issues. Or set all of the parameters well beyond what the chair will cause so that they literally never cut in. At which poing what are they for???

If they offer a cell log, so we can see the individual cell voltages then thats useful. But only if you didnt fit a big enough pack to see you through any day and so be suffering the same old range anxiety problem.
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Re: To BMS or not...

Postby expresso » 18 Mar 2021, 23:39

if they stop making the PL8 today - i should still be good for some time - but if we have no choice - i would consider adding the BMS external - when i get home to charge - i connect it on my charge cable from the chair and charge it - if thats the only way - we have no choice till PL 8 comes back or find another charger

it would be good to find a BMS that can only balance - are they not any that can be made custom ? worse case - i would just use the same one the bounder uses - i have no experience with any others -
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Re: To BMS or not...

Postby Burgerman » 19 Mar 2021, 00:55

They all do that. If you just connect the chair direct to the battery it prevents the BMS safety nazi interference from being able to do anything. Or fail. All thats left then is its cell balancing - albeit not very good...
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Re: To BMS or not...

Postby expresso » 19 Mar 2021, 01:39

thats the only solution i can think of if it comes down to that - i am not adding it to my pack in the chair - i just make it external -

come home - connect it to my charge cable on the chair - and charger to the BMS - charge and when done - remove - chair pack never gets touched inside - besides i have no room either way

i would have to figure out to do it - i dont remember how i connected a BMS first time at the very beginning - i may still have that one laying around here - have to look
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