Re: Can I (safely) run several NiMh battery packs in parallel?

From: mike (spamme0_at_juno.com)
Date: 09/28/04 

Date: Tue, 28 Sep 2004 12:07:50 -0700

Dirk Bruere at Neopax wrote:
> Nick Toop wrote:
>
>> Hi,
>>
>> I have a portable application for which I require 12V at 1A for 3 or 4
>> hours.
>> 12V 4000mAH NiMh packs are readily available.
>> However, I want to be able to recharge the system quickly i.e. in about
>> 15mins.
>> I do not want to change the battery pack.
>>
>> The data*** for the Maxim MAX712 or MAX713 appears to imply that the
>> fastest acceptable charge rate is 4C which for the above battery pack
>> would
>> mean 1 hour
>>
>> It occurs to me to use 4 x 12V 1000mAH NiMh packs with individual
>> chargers
>> (15mins charging).

There are a bunch of issues here.
As already stated, C is 4A for your 4AH pack.
You probably can't get 1A for 4hrs out of a 4AH pack. The capacity
is often stated at a MUCH slower discharge rate.
You can't charge NiMH batteries in parallel. You can split the parallel
chains and charge the chains simultaneously, but not in parallel.
One chain will hog the current and the whole thing will blow up.
Charging batteries in 15 minutes is a recipe for cell failure...no
matter how you split up the cells. Cells are designed differently for
different applications. Typically, fast charge cells have much lower
capacity. Yes, we're starting to see 15 minute charge AA cells in the
stores. We'll see how that goes...I wouldn't bet anything important
on it.
mike

  There then appear to be 4 ways I might use them.
>>
>> 1) Run the 4 packs in parallel...this makes me feel uneasy..there
>> might be
>> large current flows between the packs after (partial) charging
>> 2) Put the 4 packs in series (48V) and use a smpsu to generate 12V.
>> 3) Switch between the packs on a regular basis i.e. every 5 minutes
>> 4) Use one pack but monitor its activity and switch to next best pack
>> when
>> it starts failing etc: (a bit complicated at the end)
>>
>> Obviously 1) is the simplest but is it likely to give problems?
>>
>> I am inclined to 2).
>>
>> Is there a 5), 6)....?
>
>
> Use serially and charge in parallel?
> 

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