# Frustrated with NiMH



## IamIan (Mar 29, 2009)

NiMH ... is no free lunch.

I am a fan of it myself ... but it isn't a fix all by any means.

You can buy NiMH cells and build up sub-packs and modules the way some people have with other battery types ... but this involves a good bit of labor and personal skill and knowledge are required.

Large format NiMH cells are not in demand as much ... so they will be harder to find and not as cost effective.

NiMH will still benefit from a proper BMS ... without a proper BMS any battery system will suffer ... and have a shorter service life.

See the Wiki article on battery Alternative section on NiMH.
http://www.diyelectriccar.com/forums/showthread.php?t=13351

The cheapest $ per kw for NiMH that I know of is:
http://www.batteryjunction.com/tpeh-td12000.html

12Ah 'D' Cells ... if you buy more than 480 at once you get them for $5.50 each... which results in ~$382/kwh ... and they yield ~90Wh/kg but only about ~270W/kg ... but that would be ... a ton of spot wielding you would have to do to make a usable pack out of those.

Power cells for NiMH are things like the ~10 year old version in the Gen-I Insight which has ~43Wh/kg ~666W/kg

Remember you need a certain amount of power from your battery pack for acceleration and such.

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So in short NiMH is still out there ... but it is not a fix all / free lunch ... you will still have issues to deal with ... a BMS is still a good idea ... etc.

Depending on exactly how you build it... some of the Li batteries out there are not significantly more expensive than NiMH.

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Yes I know allot of the above is negative for NiMH... but I am personally still a fan of it... It has proven calendar life ... measured in decades ... other batteries have either failed to measure up to this or are still too new to know... and NiMH is very tolerant of abuse.

NiMH is not without its issues and short comings ... 

So in the end I recommend not focusing so much on battery chemistry ... figure out what you want the battery pack to do... how much usable Kwh do you need .... how much cranking kW do you need ... what is your budget ... etc...etc... then once you know what you want / need ... look at your options and pick the best fit... no matter what the chemistry might end up being.


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## ADAMEV (Jun 3, 2008)

Thanks for the advice.

The small cells seem to be on a par with lithium price wise.
I can't help thinking that large cells would be competetive, and the fact that Nickel metal should be half the price at least. 

Sure lithium is great, but I'd like to see NiMH on the scene as an
alternative.


























Also, The Prius mark 1 used packs of NiMH "D" cells I'm pretty sure, but that was a hybrid, not full electric.


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## IamIan (Mar 29, 2009)

ADAMEV said:


> Thanks for the advice.
> 
> The small cells seem to be on a par with lithium price wise.
> I can't help thinking that large cells would be competetive, and the fact that Nickel metal should be half the price at least.
> ...


The cost benefit for producing large format cells is dwarfed by the cost benefit of large economies of scale.

Li is nice... and it has been making slow but steady progress in increasing its economy of scale while reducing the scale of completing battery types.

For instance Cell Phones ... first they were all powered with PbA when they were in a bag ... then NiMH for the single hand held units ... but now almost every cell phone in the world uses Li batteries.

Same trend is also seen in Laptops.

etc...

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There is a general public concept that Li is the newer better in every way battery than NiMH... it just isn't true... but that is still the impression just the same... reality depends on specifics.

No one battery is the best for every application ... They all have pros and cons.... but technical specs are lost on the general public... yes, there are application where even PbA is still the best battery option.

For instance the media Loves A123 Cells... and they are great for several things... but they are not prefect and are not the best battery for all applications.

for Example:
The A123 #18650 cell has a energy density of ~93Wh/kg
The previously linked to NiMH 'D' cell has ~90Wh/kg ( ~3% less than A123 18650 )
The LG Li-Ion #18650 cell has an energy density of ~207Wh/kg ( ~122% more than A123 )

Of course the A123 LiFePO4 cell has benefits ... but I don't find it is worth the hype most of the media gives it.

The NiMH cell linked to can do 2C continuous discharge rates.
The LG 18650 can only do 2C continuous discharge rates.
The A123 18650 can do 27C continuous discharge rates.

The discharge rate matters of course ... but you still get half as much energy for the same weight of battery pack as the LG ... but the LG gets no press.

If you are building a Kill-A-Cycle that high C discharge rate matters allot... for the rest of us... not as much.

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I also find it interesting how soo many people talk about the poor cycle efficiency of NiMH ... but they always use bad tests.

The standard NiMH charge methods all depend on the abuse tolerance of NiMH cells... they all over charge the cell.

If you want to actually test the cycle efficiency of a battery you start of testing the actual capacity of that battery.... of how much will it give you.

Then from a discharged state you apply less than that amount... if the battery will give you 10.0 Ah ... only put in 9.5Ah... etc....

Then do a discharge to test to see how much of that 9.5Ah you get back out... then repeat this process several times to remove random statistical errors.

Using a C/10 rate for charge and discharge I have found that almost every NiMH battery yields a ~98% cycle efficiency.

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Now the Peukert effect varies dramatically from NiMH cell to cell... I've seen some that fair very poorly with higher discharge rates...and some that fared very very well.

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Take any Li battery pull it into cell reversal and see what happens... it dies a very quick death... I have pulled NiMH cells in cell reversal several times... yes it does damage ... yes it is very bad for the cells... but it is not an instant death sentence like it is for Li... I have several NiMH batteries that I still use after several dozen intentional cell reversals.

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With current LSD-NiMH ... the self discharge rate of NiMH batteries is in the same range as Li.

but I think the trend is clear...

The public is sold on Li = better.

the Public ignores specifics and details.

Li looks to have a bright future ...

NiMH looks to be in for a up hill battle.


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