battery charging - Materials required to convert NiMH rechargeable 24V/6Ah circuit (20 C-cell) to Li-Ion rechargeable circuit

Below is a picture of a battery shell for a NiMH circuit for a power assist wheelchair battery. I'm taking a guess that each of the 20 batteries is 1.2V because the entire circuit is 24V. These are rechargeable.

I'd like to convert this circuit to the same voltage (24V) and same capacity (3Ah).

First, is this possible given the space you see in this wheelchair battery shell, and the circuit board in the pictures?

Second, what are the material requirements to be able to convert this circuit from NiMH to Li-Ion and the type of charger as well? What materials will I need to solder/weld the materials together, etc.. for the actual labor? I'm an electrical novice, so please ask me more questions (as comments) and I'll do my best to answer.

Note: right click image and open in a new window to make it larger!

Answer

Li-ion batteries put out 3.7V per cell, while NiMH generate 1.2V. So, 20 cells at 1.2 V generate 24V. With Li-ion you will need 24/3.7 = 6.5(!) cells. This is somewhat difficult to solve... Maybe the chair can accept 7 cells? That would be the equivalent of 25.9V - a bit much, though 8% might be acceptable.

If you have the space to fit more cells - maybe the same quantity as before (20 cells), you could include a switching converter to lower the voltage to the original 24V, and make better use of the space, and have more capacity.

The pack would then have 20*3.7 = 74V, which should be convertible to 24V. But you would need to look for a charger which works at 74V which probably would not be so easy to find commercially. There are plenty of circuits for chargers around. Don't skimp - find a good circuit to avoid overheating problems!

You could also think about two sets of 7 in parallel - or 3 if you can fit the extra cell. You will have 21 cells total. More info on paralleling packs is available here.

The BQ2013H IC on the board is a controller which is for high-discharge battery packs, but is only for NiCd, NiMH or lead acid packs. It doesn't seem to support Li-Ion packs. So there is some work to be done there.

I looked up equivalents for Li-Ion, and Texas Instruments has a new one (bq34x100) which shows promise. You'll still have to redesign the board though.