I am currently overhauling and replacing worn battery modules for my Toyota hybrid vehicle. I have purchased a SkyRC q400 smart charger to charge all the six-cell NiMH modules before I assemble them back into the 200 V battery. They are each rated at 6500 mAh with a voltage of 7.2 V (1.2 V per cell).
Could somebody tell me the best charge current setting please and also if it is best to discharge the battery first before I do my first set of charges? What is the best discharge rate?
Given that your info is correct (6S NiMH blocks with 7.2 V rated voltage and 6.5 Ah capacity) here my proposal:
I don't know if discharging them first will bring a big advantage. If you want to do it, I'd not overstress them, maybe around 1 A for discharging is OK, be sure to take care not to let the voltage accidentality below 7.2 V!
For a 6S module I would use of 8.4V (be precise) constant voltage charge with a current limit around 0.4 A (current should be well below 1/10C to be on the safe side (for your 6.5Ah cells 1/10C would be 0.65A). This charging is so slow that even a little overcharging should not damage the cells within a reasonable time.
A lab power supply with CV/CC mode and voltage/current display would be great, but you can go simple with a multimeter and any stable and adjustable supply as well.
To detect end-of-charge there are several options. A timer should be the least - additionally looking at the current (it reduces notably when near full) or the temperature (it rises when near full, but maybe hard to detect at this little current) do work.
Charging with the mentioned 8.4 V and 1/10C is said to take 12-15 h. If you choose the lower current, prolong the timer accordingly (~ 20 h).
Of course a multi-cell NiMH-charger with delta-V detection would be better and could charge quicker, but it seems you don't have one. Also the -dV comes after the +dT so looking at temperature seems more clever to me.
There are 28 modules - will take you a lot of time :)
As always, let the datasheet be your guide. The manufacturer of your cells should have specified the appropriate charging methods. If not, then you really should contact the manufacturer and ask for this information.
I'm assuming of course that you did not buy cells without the necessary documentation.
Googling generic NiMH charging procedures, seems some say "0.5C to 1C" and others say "never more than 0.5C". Some say < 0.5C (or 0.33C) and your charger may not detect "termination" (negative delta V) correctly. 0.5C for 6 cell modules Toyota uses is 6.5Ah * 0.5 = 3.25A. At that rate it will generate plenty of heat as it approaches full. Some say 1C if you want a "fast" charge...(6.5A in this case). Even more heat.
That being said, hybrid cells in the pack and with "a fan" on them are under compression and have extra fins for cooling (compared to AA NiMH batteries that are most common), so it's conceivable you can get away with from 0.5C to 1C.
https://batteryuniversity.com/article/bu-408-charging-nickel-metal-hydride for instance mentions charging at 1C is an option, though discourages overcharging for long at that rate (i.e. your charger must be smart enough to be able to detect when to stop when it's full, to go this rate). It's harder for the charger to detect NDV with lower charge rates so it's a bit of a tradeoff.
0.1C (0.65A, I use 0.7A) is said to not generate heat/bulging while charging, even without any compression or fanning required, however "harmful overcharge" even with it cold like that (i.e. any overcharge, even at lower amperage) is said to be bad for the cells, and it's hard to autodetect termination at lower amperage, meaning it's easier to accidentally overcharge, since all you have to rely on is a timer.
