battery pack rebuilds

[Luke Herron]

Has anyone had their Li-ion battery packs rebuilt with NiMH? Stupid idea, or no?

[esaj]

11 minutes ago, Luke Herron said:

Has anyone had their Li-ion battery packs rebuilt with NiMH? Stupid idea, or no?

I think that it maybe could work to some extent, but you will certainly get far worse performance with NiMH. First off, because NiMH has much lower nominal voltage (about 1.2-1.4V or thereabouts?) than Li-Ions, you're going to need a lot more of them in series than with Li-Ions. 16S Li-Ion has around 57.6V (16 * 3.6V) to 59.2V (16 * 3.7V) nominal voltage. To get same voltage with NiMHs, you'd need to stack up something like around 45-50 cells in series, even more if you're planning to replace a higher voltage pack, like 20S Li-Ion. This will lead to a very large internal resistance, which might become too high to give enough current for the motor to give enough torque / the pack voltage drops too much under load. The pack would also weigh more and require a lot more space than a Li-Ion pack.

Secondly, even if you manage to build such a pack, it won't take you as far as a Li-Ion pack. I haven't checked the numbers, but the energy density of NiMHs is something like less than 1/3rd of Li-Ion, meaning that your battery capacity would be far less than with Li-Ions for the same weight or size.

You could try it, but even if it somewhat works, I doubt you'd be happy with the end result.

[Luke Herron]

Thanks.

[Keith]

43 minutes ago, Luke Herron said:

Has anyone had their Li-ion battery packs rebuilt with NiMH? Stupid idea, or no?

I’m fully in agreement with @esaj on this!

Having been a very early adopter of electric power for model aircraft, I have a fair bit of experience of using NiMh and NiCad batteries in high power applications. The first VERY telling thing about the power available from NiMh is that, even with the advent of much more torque and power from Brushless motors the battery was half the total weight of the plane I.e. if the battery weighed 1lb then the airframe, radio, motor, etc had to be built at not much more than 1lb as well or the plane would hardly fly. Another significant issue (which was quite handy in the winter) was how hot the packs would get - A high power (for the time) plane could melt the hot glue used to assemble the packs. Flight times of as little as 3 minutes were often the case and more then 6 minutes was a real result.

Lithium Polymer batteries totally transformed electric model flying and any plane built for a Nitro engine can easily be converted to electric and fly as at least as well. Power and or duration are easily tripled compared with NiMh and weight (and size) at least halved.

It really is the power density of Lithium and efficiency and power of BLDC motors that have made electric vehicles of all types viable. Using NiMh would be a bit like going back to steam powered cars (IMHO🤓)