Charging NiMH on a Li-only charger?

[BrianG]

My Revo still uses 14 cell NiMH packs, but I only have one NiMH charger and initial charging takes too long IMO. Soooo, I found that certain settings on my Xtrema charger will allow me to charge my 14 cells NiMH packs just fine. 14 cell NiMH setups are still quite common so I thought this would be useful.

Before I post how, I want to state that you MUST know what you are doing! If there are any questions about this, you shouldn't do it. I will not be held responsible for any problems with this setup. For safety, your Li charger should have a temperature probe with a thermal cutoff setting.

14 NiMH cells have a discharged voltage of at least 1.1v/cell (once the pack voltage is disconnected and allowed to "drift" back up), or 15.4v. The max right-off-the-charger voltage is around 1.4v/cell, or 19.6v. Knowing this, a 5s Lithium charge will work. Li cells are 3v/cell, or 15v, and max charge is 4.2v/cell, or 21v. This means the Li charger will "see" the NiMH pack as a discharged 5s pack and allow to charge up to 21v before going into CV mode. Perfect.

I set the charger for 5s pack size and a charge current equal to 1C. I then set the thermal cutoff to 115*F. The thermal cutoff setting replaces the delta-peak algorithm since NiMHs start getting warm at the end of the charge right about the time the voltage starts to drop again. So, instead of the charger relying on delta-peak sensing, it relies on the thermal characteristics of NiMH cells. The charger shouldn't go into CV mode since the NiMH pack will most likely not reach 1.5v/cell.

So, I don't know if this helps anyone, but I've been using this setup for a while with no issues.