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PINNED - Lithium battery conversion/info

Postby Burgerman » 04 Feb 2012, 19:45

How many people would be interested if I were to do a group 24 battery to Lithium Phosphate simple 24v conversion that would fit any powerchair that uses standard hi end group 24 batteries?

I get so many questions and answer so many identical emails that it may be worth doing this to a BM1 sat in my kitchen that I never used yet! It hasnt got batteries in it yet and is 3 years old as a spare.

I could also show how to do group 34, and 22nf sizes too. This would be headway, and hyperion or PL8v2 charger.

FIRST OF ALL READ THIS:
WHY A READY TO GO LEAD BRICK REPLACEMENT IS A BAD IDEA

Because of 2 main things.


1. No built in dumb BMS with external charger can ever do a good job charging because its got no control over the 2 wire charger. A BMS is fundamentally the wrong way to try to do this. It isnt that a BMS doesent work corectly, but that its doing the wrong thing! In the wrong place.
As soon as a charged cell goes above max voltage as you charge, it a BMS cant do anything except to disconnect the charger completely, in repeating on/off/on/off pattern while simultaneously trying to pull down the "high" cell with its usual feeble balance circuit. And it must keep disconnecting the charger, over and over, since it cannot control power proportionally. So the quality of the BMS isn't the issue its just fundamentally the wrong way to do it. A BAND AID stuck on top, instead of preventing the problem happening by design. So it repeatedly pumps the high cell(s) over voltage and pulls them down again, over and over.

It can be done well, if a specific computerised charge system is designed into the vehicle. Complete with balance integrated with the charger such as is done in a full sized car. Or a hobby charger. And that's just charging.

Everything else that a BMS does, simply isn't required or wanted at all. Unless the cells are simply too low C rate, or too low capacity for the job in hand, or a current limiting controller isn't used. So a stalled motor may exceed the batteries max Amp limits. Non of which applies to a powerchair unless you fit a too small pack such as a lead brick replacement. All the extra "protection" functions just stop it working correctly or cut power as you climb a ramp etc, if the battery isn't correctly matched to the vehicles requirements. If it IS matched, non of the BMS features are required OR USED AT ALL! Its like trying to improve health and stop people dying by doing heart transplants every time they are going to die. Instead of proper exercise/diet.

2. Lead brick replacements result is a "recognised" plastic battery casing of standard size like grp 22, 24 etc, to make it all look acceptable to a typical buyer. Its "simple"... It makes them feel happy! But its just filled with the same cells (or often cheaper nastier) that we are already using here. And for packaging reasons it always result in either only the same Ah capacity in some cases (or actually less in most!), than the lead battery it is replacing. So pretty much pointless. For packaging reasons there's wasted space inside that case. And because they need to make space too for a (not needed) BMS. Meaning that most of the advantages of fitting lithium, huge range, longer service life, has now gone. Because you now work the cells harder and gain little in range. So its a great expense for little gain.

Read: http://www.wheelchairdriver.com/BMS.htm For all the other BMS garbage we don't want!
Fires... viewtopic.php?f=6&t=4460&start=40#p84092

The whole point of going to lithium (LiFePO4 only, for safety reasons and cycle life) is that its possible (if done correctly) to fit much MORE Ah into the same space. This is the entire point!!!

For e.g. To replace 70Ah lead.
Nandol's lithium chair has = 190Ah.
My rebuilt BM2-Lithium chair has 120Ah.
Expressos DIY lithium chair has 105Ah. + Add-on 36Ah.

All this extra Ah means less frequent charging (you can now do it every 2 or 3 days most of the time) so the limited cycle life now gives 2x or 3x more calendar service life than fitting say a 70Ah battery. And now, each Ah is working much less hard and discharged at a slower rate. Less load per Ah means a very happy lithium.

So with PROPER charging to a carefully restricted voltages and accurate balance, (neither of which a BMS can do), easier daily life (because of a lower average discharge level in Amps), and charging less frequently, you now get a seriously long service life as well as huge max range gain too. These are the reasons WHY you change to lithium.

I lost count of the number of times I said all this... So will post a copy and pin it at the top of the forum on this thread!!!
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Re: Step by step lithium conversion?

Postby Seajays » 04 Feb 2012, 19:47

Me x10, I think this is my next move for my BM2
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 20:07

Well you already know lots, so its easy for you.

Go to http://www.evassemble.com or wherever you choose and for group 24 replacement order 72x 12Ah headway cells. And 72 orange blocks (included), and enough bus bars. (13 x 6 x 2 + 26 on my 78 cell pack). You will need to work out how many...

Then charge every single cell to 3.600 volts, and leave on charge afterwards for 2 hours minimum to soak. You can do this in parallel in big groups of say 10 at a time. Will take about 2 days. Then put them all in a box, disconnected and loose and forget them for 2 weeks or preferably more. Now charge each one individually (will take just a few mins) and write capacity added in Ma at the charged point in software on each cell in pen or on a sticker. This is a measure of self discharge. How much it lost over time in storage.

When you have done this you need to put them in groups of 9 (each parallel group) that add up to the same Ah lost when averaged. So all the parallel groups are as similar as possible when added up. You will have 8 bags/ boxes with 9 cells in each with similar average self discharge levels.

When you get this far email me or post here! I may have a page up by then.

This is for group 24.

For group 34 it doesent work out well, and is an awkward configuration. With 56x 12 Ah cells. Thats 28 per block but it can still be done.

For 22nf it can also still be done with less 10 Ah cells but ends up with each pack about 12mm wider. This may or may not matter. A tape measure will tell you.
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Re: Step by step lithium conversion?

Postby Step » 04 Feb 2012, 20:09

I am.
And I think anyone in a powerchair should be.
Most people get by with 'normal' 24v systems. Reprogrammed or not.
Some because it really does what they need, some because they don't know they can make their own chair or because they can't.
They all know the limits of their batteries though.
If the industry is not ready to offer the option to go lithium, it'd be great to know there's a place that you can find out how to do it.
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 20:17

Actually going to lithium and getting 2 to 3x the range realistically, and 2000 cycles is dead easy. Once I persuaded them to up the Ah limits on the hyperion amongst other changes...

The hardest part is making a balance cable that works reliably, and connects and disconnects easily. And accepting that the chairs controller will read battery state wrong, and MAY give errors. Although I doubt it.

Its going to more volts, roboteq controllers, etc thats a bit complicated... The lithium battery is simple.
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Re: Step by step lithium conversion?

Postby popschief » 04 Feb 2012, 20:45

When I see all the gimmicky little toys using these batteries it makes me a little sad. :cry:
Here we are in a world full of people in our situation without access to the benefits of
Lithium power and there are kids running around on skateboards and tiny wheel'd scooters
doing 15 mph. Don't they know there's a speed limit? :lol: But seriously my next chair
project will be from scratch and nothing but lithium will be appropriate thanks to the BM
movement. :geek:
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 20:52

I need a set of batteries for my BM1 chair.
Advantages/costs:

2x PC1500 Odyssey 68 Ah (about 45Ah is usable)
112 lb
1 year or 400 cycles @ 80%
Overnight for full charge.
15 Miles range
£450 UK

Headway 72 cell pack 108 Ah (all 108 is usable) Group 24 sized packs.
58 lb
10 year or greater expected and 2000 cycles @ 80 percent.
Full charge at 5C in around 15 mins, but 4 hours with hyperion typical to 100 percent.
38 miles range + due to reduced weight.
£ under 1 k?

About twice the price, but 5 to 10 times service life, 2.5 times the range, much better high load performance, much faster charging. = No brainer...
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 20:54

OK I will do a page... And update it with the pics once I get more stuff ordered!
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 21:26

For those that want to do this today, heres how they connect... They just plug together in blocks, like LEGO.

You want two batteries, laid out as shown: 6 cells x 6 cells like this. It will give you a group 24 shaped plock that is 30mm shorter in length. And half the weight:
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Re: Step by step lithium conversion?

Postby Burgerman » 04 Feb 2012, 22:38

Me x10, I think this is my next move for my BM2


BM2 wasnt designed specifically for lithiums. You may have a problem with height at the front with 6x6 lithium packs. These are 240mm tall. And unlike the later lithium freindly BM3 centre section, there are full flat sides, no bolts protruding or anything that can touch the battery ends or bus bars. So while it may fit, it may need some care or adjustments to be sure that it fits, or that it doesent short out on say motor bolts.

On the BM3 there are full sized flat sides. And the orange "building blocks" have pins protruding to hold the terminals away from the sides. In the unligely event of a terminal bolt comming undone, and contacting the side I have taken another precaution. I threw away 78 x 2 6mm screws. And replaced them with stainless allen screws. These are easy to correctly tighten to a 5lb ft torque setting, and each one has a nylon cap hammered into the end. So even undone, it cannot touch the sides...

You wouldnt want that happening!

Just so you understand the differences.

If the height is a problem then you need to build 2 group 34 replacements instead. This has 8 cells rather than 9 in each parallel group. So 64 in total. But it makes non "block" packs. More wasteful of space.
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Re: Step by step lithium conversion?

Postby Seajays » 05 Feb 2012, 00:45

I would have to go with group 34 for lack of funds to build BM3. My Odysseys are only 6 months old so I have a while.
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Re: Step by step lithium conversion?

Postby Mr T » 05 Feb 2012, 01:27

Count me in please!
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Re: Step by step lithium conversion?

Postby ex-Gooserider » 05 Feb 2012, 07:12

Agreed, I think it's a good idea... As a minor addition, and I think you did this one a few days ago, is the U-1 size batteries - I think this is also a fairly common size for some of the smaller chairs like the Pride Select 6 series...

It sounds like the procedure is pretty much the same regardless of the battery size, just the number / size of the cells varies... Presumably you could do the page as a table for the different size batteries, maybe a series of pictures of the layouts for each one, and then a write-up of the common procedure...

Another thought that might help on the BM2, though you'd need to measure it to see if the idea makes sense, is to build a sort of tapered pack with more cells high at the back than the front? I don't know if the height difference would be enough to matter or not... Also would it help any to get rid of those angle iron tracks that you currently set the batteries on?

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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 10:50

Re BM2 batteries, there IS room just on mine for 2 of 6x6 cell 240 high packs with 72 cells. Because my seat dump is about 8 degrees.

If you had an exact clone of my chair they will fit. If not then the only configuration that will fit due to height at the front is the group 34 pack. It already has more cells at the back. It is the only way its possible to arrange 60 cells and have 24v.

But since you get about double the range from lithium Ah per Ah and theres no volt sag on hills etc then its not going to be a problem... Your range will still be double. The extra cell in each row (9 instead of 8) would have made that 2.5 times better.

You need to see if you have a clear 240 h x 480 l x 174 w for your BM2 with nothing protruding like bolts. for the 72 cell battery. 2.5 times range.

For the 60 cell one you need 200 x 480 x 174 which is 40mm lower. and 2x the range.
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 10:52

Sorry the 22NF sizes above were actually UI batteries I was thinking of the wrong dimensions. I never worked out 22nf... :oops:
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Re: Step by step lithium conversion?

Postby Step » 05 Feb 2012, 11:10

would it be possible to change the battery box design to go wider above the motors?
There's room for more cells there...
It's all about funding too of course but since weight is not an issue... why not fill every gap with energy?
stick with 24v but squeeze in as many ah in as possible?

Of course that would be a totally different approach to fitting lithium into the existing battery box of any stock chair...

Never mind :-)
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 12:27

You could. But that gets complicated. The idea was to keep it as a swap. Its also better to go with more volts, for "free" extra speed as per BM3 with no other torque/range loss. Or to use lower gearing, smaller motors, higher voltage, for efficiency gains at same speed... Etc.

But most people, and dummy manufacturers need a "simple" swap over or it gets too complicated for them.
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Re: Step by step lithium conversion?

Postby Step » 05 Feb 2012, 12:48

just for better understanding...
You're making 4-group (group being 9 battery cells) packs because 4x3.2v is close enough to 12v, right? 12.8 to be precise...
the 9 or other number of cells linked is the ah you'll get?
So making any 24v pack would mean making 2 sets of any number of battery cells you can divide by 4, right?
With your battery box size being the limiting factor, ah-wise.
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 13:15

Yep.

The thing with lithiums is that they are not really 3.2v or 3.3v Thats a nominal figure. They are generally 3.4 or above for at least half the discharge cycle.

When you charge FOUR cells you do so at 3.600v x 4 = 14.400. Exactly the same as lead batteries. The difference is that they will STAY at 14.4v for a very long time and only drop a fraction, where the lead batt drops instantly to say 13.2 and eventually 12.8v ish.

So a fully charged 4 cell lithium phosphate stays at the 14.40V you chargeD it to.
Some are 3.6v some 3.65v, 3.7v... But the last "bit" doesent do anything. 1/1000 of an amp hour will send the voltage from 3.55 to 4v plus! This is why balancing and care is needed. So almost every charger is 3.60v CC/CV.

As you use the chair, the volts will fall from 3.6 (14.4v) to 3.35 to 3.40 in the first few meters. It will then stay exactly the same and only drop a tenth of a volt - even under load on a hill - for say the next 15 miles. And lose another 1/10th only over the next 15 miles... Then it will drop off a cliff. And fall very fast at about 95 percent depleted. At this point its done. The next half a mile will see the volts drop not by 10ths of a volt but by a volt, and after that its not good for the battery.... But its easy to see/feel this happen as it will fall from 3.2 volts to 30 percent less. And will slow accordingly. So just stop and do not try to drive.

The other thing is that when you come to a hill (do a wheelie, climb a ramp etc) the volts of a lead battery drop like a rock due to the surface charge (peukert) affect. Lithiums do not change. They stay exactly the same.
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Re: Step by step lithium conversion?

Postby woodygb » 05 Feb 2012, 15:28

B.M.
I've taken the liberty of converting your sketch to a pic and adding in the balance connector.

I hope it's correct and that you don't mind.

lith bats.JPG
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 17:59

Not as good as mine... :lol: :oops:

I am going to steal your drawing! And use it on site.
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Re: Step by step lithium conversion?

Postby woodygb » 05 Feb 2012, 17:59

Feel free... :)
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 18:03

You could do a group 34 one? (so 6 long and 5 high. So cant be divided by four. Top row not complete...)
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Re: Step by step lithium conversion?

Postby woodygb » 05 Feb 2012, 18:10

4 off 7 cell packs combined = 28 cells
A pair missing in the top row?
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 18:39

Excellent. So nothing now stops anyone from ordering some 12Ah Headway cells/blocks/connectors.

Batt 1 is 240mm high, and 174 wide, and 480 long. And 12v 108 Ah. Equal to 180 Ah of lead...
Batt 2 is 200mm high, and 174 wide, and 480 long. And 12 v 84 Ah. Equal to 140 Ah of lead...

Batt 1 is group 24 approx 75Ah lead sized replacement and will be a straight fit.
Batt 2 is group 34 approx 62Ah lead sized replacement and ditto...

Now we need a group 22nf sized one, and I already did a smaller 10Ah cell U-1 sized one somewhere...
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Re: Step by step lithium conversion?

Postby woodygb » 05 Feb 2012, 18:48

Just a note ...and pic ... to say that balance wires can go anywhere on the on the single packs + or - rails ..likewise the main battery leads would be better like this on a 84 Ah pack ..thus the Anderson etc could be folded into the gap left by the "missing" cells.
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Re: Step by step lithium conversion?

Postby Burgerman » 05 Feb 2012, 19:04

And another note to say that balance wires need to be kept as short as practical. And fairly thick. If not charge time, or the balance at the end of chargeing will take a very long time...
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Re: Step by step lithium conversion?

Postby ex-Gooserider » 06 Feb 2012, 13:12

Can you clarify what you mean by "fairly thick?" - Obviously AWG 30 wire wrap is out, and judging by your BM3 pack pictures, you aren't down in the AWG 10 range either... I notice that your pack appears to be using a DB-? style computer plug, and those usually use AWG 24-26 wire in data cable applications, and the cups on the connectors get difficult to solder if putting more than about AWG 22 into them... The balance connectors I got with my Hyperion are very short, but look like they are only about AWG 26...

So what would you consider adequate / appropriate? I'm assuming that the charge and balance cables are likely to end up being 3-6' in length just so that the place where I'm parking the chair and where I'll be putting the charger setup can be easily tied together...

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Re: Step by step lithium conversion?

Postby Burgerman » 06 Feb 2012, 14:23

There is no correct thickness. But the longer they are, and the thinner they are, the slower the balance works, due to resistance. I use the size that the hyperion balance board come with for the first few inches, and step up to "the next size up" on the way to a 16 pin connector. Mine are a bit small, because I needed to get 14 wires in a loom and it adds up! This is 18 inches long. On the battery side I use the same wire, found on my bench, and all kept as short as poss, maybe 14 inches?

If balance leads are longer than about 3 feet then they need to be bigger. Double the length needs double the x section. Same with charge leads. Its best to have the charger sat on or sat right next to the chair.
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Re: Step by step lithium conversion?

Postby Step » 06 Feb 2012, 18:31

This is excellent information.

I know this thread is about directly swapping 'normal' 2x12v battery setup to lithium but it could be so much more...
I've been playing around with Woodygb's sketches and these cells allow for so many layouts, provided you keep the 8 (2 x 4) cell-groups to get 24v.
Using 6, 7, 8 or 9-cell groups gives you 72, 84, 96 or 108 (lithium) ah packs.

Imagine 'someone', say 'a manufacturer' just for a laugh, came up with a modular design that lets you add the motors & wheels, casters, seat and armrests to the battery box that suits your needs best, with that battery box being the structural base frame.
You'd transplant the parts to a new battery box with more cells as you grow (kids could even start with less ah I assume) or as your budget or mobility needs change...
It's basically the BM design without the tubular frame but with the casters and seat to be attached to the battery box as well.

You can choose a layout based on your budget, your size, your activity... anything really.
Place them in 1 18cm row with fat tires for grip or small tires for kids' size chairs... more rows in a wider, more 'classic' layout, ... anything goes really.

Parents would be able to get their kids the exact fitting chair without having to worry about coughing up another 15.000£ in 2 years, a bigger base battery box & some screwing/unscrewing will do.
Anyone else can change to bigger motors, a different seat, whatever, whenever you want without having to buy a new chair...

The batteries will probably outlive the parts too, right?

Manufacturers, call people here! fire the committee. :)
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