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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?

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    \$\begingroup\$ Tests by the US Army indicate that a NiCd cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formations. During this corrective discharge, the current must be kept low to minimize cell reversal as NiCd can only tolerate a small amount of cell reversal. Check this link: batteryuniversity.com/learn/article/… \$\endgroup\$
    – Gil
    May 27, 2021 at 14:13
  • \$\begingroup\$ @Gil The OP is using NiMH, not NiCd. \$\endgroup\$
    – Elliot Alderson
    May 27, 2021 at 14:16
  • \$\begingroup\$ What is your deviation of Voc,Vfl, or ESR and dI/dt values (or mAh actual) for each cell? \$\endgroup\$
    – Tony Stewart EE75
    May 27, 2021 at 16:39
  • \$\begingroup\$ The above is critical if you want reliability full charge before assembly is meaningless without very tight matching the above \$\endgroup\$
    – Tony Stewart EE75
    May 27, 2021 at 18:02
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    \$\begingroup\$ Rather than speak generically of "all Toyota hybrid batteries" perhaps a specific battery pack should be referenced here. 95% of all visitors to these pages arrive from Google months or years later, and I'd hate to see someone with a different model of pack make a terrible mistake based on advice relevant to your pack alone. \$\endgroup\$
    – Harper - Reinstate Monica
    Jan 26 at 18:39

2 Answers 2

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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.

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  • \$\begingroup\$ I obtained the cells in a used but good state. The guy ran a hybrid battery recon shop but did not give me a great deal of info. All he said is that i should charge the modules at around 1amp and dicharge to 7.2v. Just really need verification before i am confident in beginning the task of charging 28 modules. \$\endgroup\$
    – Lee Atwood
    May 27, 2021 at 14:24
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    \$\begingroup\$ Lee refer to the website that @Gil linked \$\endgroup\$
    – jsotola
    May 27, 2021 at 15:39
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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 :)

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