BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Batteries, Chargers, and Battery Management Systems.
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user964813698   1 µW

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BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Post by user964813698 » Aug 16 2019 4:18am

I have an old Sparta ION Extension 2009 electric bicycle which has a 20s pack of GP 900 DHC cells. These are NiMH 9000 mAh cells, a data sheet can be found at

https://elektronik-lavpris.dk/files/sup2/900DHC.pdf

The original controller (on the steer), motor controller and BMS are blocked from working when parts other than original are used. I already replaced the steer and motor controllers and the hub motor is running fine at 24 volts, 10 ampere when the wheel is loaded, on the original 20s battery pack. But this is in a test setup.

I want to keep the batteries in good shape and therefore want

top balancing of all cells
bottom balancing of all cells
over charge protection per cell
over discharge protection per cell
over voltage protection per cell
under voltage protection per cell
temperature protection per cell
logging of voltage and ampere per cell in time

I realize that this is quite a list of demands which would perhaps make this effort ridiculously expensive for a hobby project. Which features are realistic and affordable?

And yes, I know I can also replace the NiMH cells with some more modern type of Lithium cells, but I would like to try the original NiMH cells first. The are in good shape and I would find it a shame to simply discard them.

NickF23   100 W

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Re: BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Post by NickF23 » Aug 16 2019 10:06am

NIMH batteries don't need the kind of battery management systems that lithium batteries use. NIMH are designed to be overcharged which allows a series string to be balanced on charge. Typically a charger will connect to a thermistor (or a few thermistors) inside the pack which will help terminate charge when the cells are full. Some other chargers use an algorithm to detect the voltage drop that occurs when batteries are charged and don't need a thermistor but this is rarer.
Home made early version Puma 48V 1500 watts http://endless-sphere.com/forums/viewto ... f=6&t=4933

NickF23   100 W

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Joined: Nov 23 2006 3:41pm

Re: BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Post by NickF23 » Aug 16 2019 10:12am

Your controller should prevent overdischarge of the string by cutting out at 20 volts or around that. This doesn't protect individual cells but if you have a bad cell you can replace it. This isn't such a serious situation as with lithium, if you overdischarge a lithium cell it becomes a fire risk.
Home made early version Puma 48V 1500 watts http://endless-sphere.com/forums/viewto ... f=6&t=4933

Joachim   1 kW

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Re: BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Post by Joachim » Aug 16 2019 11:03am

best way is to use a new Li-ion battery 7S-29.4V. These old Ni-Mh batteries are a nightmare under 15C degrees :lowbatt:

You wil have les weight,more wh/kg/more wh in the same space/les effect with cold weather.

you need a 25,2V-14Ah/350Wh lithium pack(good cells like panasonic samsung lg) incl bms and a good 7S Li-ion charger....€300,- they can make this custom for you.

john61ct   100 GW

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Re: BMS for a 20s NiMH 9000mAh pack, GP 900 DHC cells.

Post by john61ct » Aug 16 2019 1:01pm


user964813698 wrote: top balancing of all cells
bottom balancing of all cells
Assuming balancing is required, you need to pick one or the other.

Doing both makes no sense, in fact working against each other.

Top balancing is the norm for both BMS and chargers, easy to build into every cycle if desired.

Bottom balancing is usually manual and only very occasional.


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