# NiMh



## Whitey_87 (May 30, 2008)

I was curious what some of the more knowledgeable members of this group might think of this.

Is a 12V 100Ah NiMh pack (built from smaller cells) a good deal at $400? I don't have additional information on the pack at this point but what specs should I be aware of? 

Also, is there any reason a 12v NiMh pack could not be charged by a regular 12v charger or if it was in a 144 configuration, say use a zivian or something similar?

Thanks.


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## madderscience (Jun 28, 2008)

Is it a good deal? Big maybe.

You could build a 120v, 100Ah NiMH pack with 10 of these at $4000 which is about equivalent in overall useful energy capacity to a 220ah 12v lead acid pack (20 6v traction batteries) which would cost probably $2500 at todays prices.

BUT

where does this pack come from? what is its power density (how much power can it dissipate without cooking the cells? What happens when (not if, when) a cell dies? Is each of these units made of 10 1.2v, 100ah cells (best) or 100 1.2v, 10ah cells (more likely, not as good)

Is there any battery management? If no, you will need to source or build it yourself. Can easily double the price or more. NiMH will need charge, discharge, and temperature management ideally per cell. You can't get away with just stuffing a bunch of NiMH batteries in a car like you can with lead.

Who is making these batteries? Any track record at all? Any kind of warrantee?

Perhaps a good expirement would be to buy two of these units and build an E-scooter and see how the batteries perform there before you plunk down enough $$ for a full sized car.

as for charging, the precise charge profile is a bit different but I assume any smart charger like a zivan can probably be programmed for it.


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## Whitey_87 (May 30, 2008)

madderscience said:


> what is its power density (how much power can it dissipate without cooking the cells? What happens when (not if, when) a cell dies? Is each of these units made of 10 1.2v, 100ah cells (best) or 100 1.2v, 10ah cells (more likely, not as good).


I all I have heard is that it has a 2C under a high discharge rate - is that what I should look at? I also remember it has 117W/kg at 100% SOC and can do a 3C discharge in 30 sec. - I don't know if that contradicts what I just said before, I am confused on the "C".
When it dies? do you mean like a catastrophic failure or knowing how to identify a bad cell/pack?
I believe 10 - 1.2V cells for the configuration.



madderscience said:


> Is there any battery management? If no, you will need to source or build it yourself. Can easily double the price or more. NiMH will need charge, discharge, and temperature management ideally per cell. You can't get away with just stuffing a bunch of NiMH batteries in a car like you can with lead.


I thought NiMh did not need BMS?


Thanks for the feedback.


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## madderscience (Jun 28, 2008)

2C means that the batteries are rated to drain current at a rate in Amps equal to twice there capacity "C" in amp hours. So if you have a 2C rated battery that is 100AH, you can safely pull 200A from them. (meaning in an ideal world you could do it for 1/2 hour).

This is fairly low "C" rate (good lithium like A123's can do way over 10C) and large format lithiums like thunder skys are 3C I believe.

what this means in your case assuming you want to build a full sized car is that a single 120-144v string of batteries would not be sufficient as even a fairly low performance car needs to be able to pull 400-500 battery amps at times. You would want either a higher voltage single string (at least 200v or so) or two parallel 120v-ish strings.

And everything I have heard to date is that you MUST have a BMS with anything other than lead acid batteries. The 'exotic' chemistries including NiMH and Lithium all are much more prone to damage due to overcharge and undercharge and temperature extremes. A BMS is recommended but not strictly required with any kind of "maintenance free" lead acid battery, but is not really needed with flooded batteries as long as basic maintenace is done. And even with a BMS and flooded lead, you still have to do that maintenance.

Look at http://www.austinev.org/evalbum for anybody else who has built a conversion with NiMH and see what they had to do. (you can search by battery type) 

It can work I am just trying to make the point that it will be more complex and more expensive than a lead acid conversion to make a car that works well and lasts when using these batteries. The benefit if they are decent batteries is you have the real potential to be able to build a car with 100 miles of range at freeway speed.

Whose batteries are they anyway?


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## Whitey_87 (May 30, 2008)

Thanks again for the info. I'll probably end up doing FLA for my first conversion anyway. It never hurts to keep looking at different options.

the batteries would be from Gold Peak.

they make Li-ion 18650's with BMS attached in certain configurations and certain voltages, but I think they are the Li-cobalt...their other brocure specifically said LFP - I dont recall if there was a BMS with those packs.

If the Li-cobalts turn out to be a good deal, is it worth building a car with them?


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## madderscience (Jun 28, 2008)

you do NOT want lithium cobalt batteries in a car. This is the chemistry that can go into thermal runaway and burn up. (laptop batteries, basically). For lithium traction batteries in a car you want Lithium iron phosphate, or LiFePo4 chemistry. Much more stable at the expense of some energy density. Also lithium cobalt chemistry deteriorates over only a few years, especially in high heat situations (like a car) and even when unused and in storage. This is why you never get more than a few years out of a laptop or cell phone battery.


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## Whitey_87 (May 30, 2008)

I'll stay away from the cobalt. Thanks for the info.


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