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#p84092The 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!!!