nimh pack low voltage cutoff - how?

[andrew.box]

I just rebuilt my wife's honda civic hybrid IMA pack and have several good 6S NiMH sticks left (6.5 AH rated capacity). I am thinking of using them with an Amped kit, does anyone know of a good method to limit discharge to 1V per cell (I'll probably use a 24S 2P configuration to get a 28.8 voltage nominal pack with 13AH capacity). Is this normally the job of a battery management system that's part of an OEM battery pack or can the motor controller be configured to do this? Discharging below 1V/cell is not desired due to the possibility of cell reversal.

Thanks!

 

[amberwolf]

If they're well-balanced cells, then you can just just a pack LVC, either by the one in the controller or by a user-monitored voltage meter.

Meters like the CycleAnalyst can also perform like the controller LVC (but often better from a rideability standpoint) by reducing power usage as it gets closer to the user-set cutoff voltage.

If the cells are old or not matched, and you want to get all the usable power out that you can, and want to use cell-level LVC, you can do that too but you may have to build something for the purpose; all the ones I know of that are cheap are intended for Lithium cell voltages; perhaps there are some adjustable down to 1V/cell, though.


Also, if you use them in parallel, I'd recommend being really careful with that, because NiMH doesn't parallel well due to the way the chemistry works.

Don't parallel them AT ALL for charging, or you risk fire or worse.

 

[andrew.box]

Amped was very helpful, their controller has a 32 v cutoff so if I just run a 30S configuration it should be ok. Thanks for the tip on not running parallel, I wasn't aware that it could be so problematic.

 

[amberwolf]

The problem with paralleling NiMH for charging is that as it reaches full charge, the voltage *drops*, and the cells get hot. So if you have two strings in parallel, and one string has a cell or two that has lower capacity than the other string, then they'll finish first, and that string will then drop in voltage. When it does, the second string will begin dumping power into the first string, which is *already fully charged and hot*, and will then be getting a (possibly severe) overcharge, heating even more.

If you have a thermistor in there and your charger is delta-T as well as delta-V, it will at least stop pouring current into the pack when the first string gets hot, but otherwise the charger will contribute to the problem, probably until the first-finished string either burns or explodes.

That's the main reason that the Honda and Prius packs are (AFAIK) high-voltage series-only NiMH, instead of paralleling a bunch for lower voltage but higher current delivery.

So if you have a high-voltage controller for your bike, you could use as many NiMH cells as needed to provide the total *power* for the motor that is needed, given that the current delivery will be lower but the voltage much higher (like 80-100V fully charged, but maybe only 10-15A at startup and 1-3A cruising). Keeping in mind that the motor will want to go lots faster at this higher voltage (when at full throttle) than at lower voltages, so you'd usually not use full throttle, or else use a different motor that is "wound slower", meant to run on higher voltages and still get the same road speed out of it.

 

[andrew.box]

That makes sense, the fully charged cells with lower voltage draw more current contributing positive feedback to their already fully charged state. The nimh sticks I have do have a built in thermistor strip. I have 2 turnigy accucel 6 chargers that have a plug for a thermistor for use with lithium packs but I don't think it will do any sort of delta t for nimh batteries. Also, if I run more cells in series the 32 v cutoff in the amped controller will be too low. Are you saying the cyclanylist device can also be programmed to limit current, provide lvc etc or is it just a data monitoring device?

 

[amberwolf]

It is designed to be able to control your throttle based on any of a number of readings, if you add the necessary wires/connectors. For some of the things it can do, take a gander at the manual here:
http://ebikes.ca/drainbrain/CycleAnalyst_Manual_Vers223.pdf
and a basic list of the things it can do on it's homepage:
http://ebikes.ca/drainbrain.shtml

 

[andrew.box]

I ended up getting the nine continent 26" fast front wheel kit from ebikes.ca. It comes with a cycle analyst so I should be able to set the lvc wherever I choose. Amped is working on a bluetooth ap to do much of the same thing but I'm ready to start my build now and the cycle analyst compatible infineon controller sold with the kit seems the best bet for what I want.