Motorized chair manufacturers VS lithium

[Burgerman]

Untill you try it when the battery is low or in winter.

[emilevirus]

It does its job and protects the cells from dropping below 2.5v. It's your job to keep them charged when needed.

[Burgerman]

The point being that under load, a cell thats not really big enough (like 100Ah), is too heavily loaded. So that its internal resistance allows the voltage to drop below that which the cell manufacturer deemed safe. This obviously gets lower and lower as the battery discharges quite normally during use. Depending on variables it may do this at 50% charge. Again the smaller the pack the faster this happens, the higher the voltage drop for a given load, and the sooner the thing chops off power under load. It depends on use case, chair mass, user mass, cell c rateing, BMS choosen but its supposed to protect the cells anyway, and on programming/usage etc. So its MEANT to do this!

So I know that these things can be problematic. As theres been a good few examples on this forum, that have seen this happening. So as long as everything lines up, and you are one of the lucky ones then it can work reliably. But certainly dont count on it.

A bigger Amp BMS stops it tripping, and hurts the cells instead. In the cases where this was leaving people stranded, which can be dangerous depending on where it happens, the users fixed the issue by charging via the BMS which they dont do very well, but by connecting directly to the cells for output power inside the casing. Now the BMS cant inadvertently chop off power as you try to get onto a train or up some ramp or whatever. But instead you are allowing the voltage to drop too low for cell longevity now and again under load. So again not a great idea.

100Ah on these lithium drop in replacements in a grp 24 case seems to be the point where this can work mostly reliably for many users. In the past when these were 50 or 70Ah they were very problematic. And there are now some around that hare sold as starter batteries. So supposedly high current capable. Theres 2 ways they do this.

1. They dont actually run the starter loading via the BMS at all and the cells are directly connected to the terminals. So this is the cheap chinesium way to make that "work"... But it kills off the cells. It allows a large capacity battery. As a smaller BMS can be used that just balances.

2. They use high C rate cells but these nesassarily are of lower capacity LiFePO4 (Theres always a trade off), and sometimes the chinese use hobby lipo pouch cells (see the ready to explode ones I photographed when I took one apart recently!) This results in a lifepo4 battery that can do several hundred amps, for starter service. But it now needs a huge volume robbing BMS as well as high C rate low capacity cells so Ah is compromised.

Theres always a compromise! This is why the manufacturrs recommend NOT doing this, and why some offer their own solutions using higher C rate cells, and a larger BMS. Which limits AH.

Of course as your own 230Ah DIY pack shows, the real way to do this is to junk the space wasting BMS that doesent fit with 230Ah cells in any normal powerchair, and the plastic "box" they fit in, allowing a huge gain in capacity to 230/244Ah. That negates any need for a BMS to keep chopping off power and allows a hobby type charger to better charge and balance the cells too. But this means a little work that some wont or cant do.