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

[Nick Toop]

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 datasheet 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 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)....?

 

[Dirk Bruere at Neopax]

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 datasheet 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 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?

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org

 

[Roger Hamlett]

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 datasheet 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

Calculate this again...
4C, would be a 16A charge rate on this pack, and would give a charge in
just over 15mins (less the losses etc. - say 20mins in practice).

It occurs to me to use 4 x 12V 1000mAH NiMh packs with individual chargers
(15mins charging). 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)....?

Best Wishes

 

[Nick Toop]

From: "Nick Toop" <[email protected]>
Subject: Re: Can I (safely) run several NiMh battery packs in parallel?
Date: 28 September 2004 14:05

Thank you. I misunderstood what C meant.

Regards,

Nick Toop

 

[James Beck]

From: "Nick Toop" <[email protected]>
Subject: Re: Can I (safely) run several NiMh battery packs in parallel?
Date: 28 September 2004 14:05

Thank you. I misunderstood what C meant.

Regards,

Nick Toop

No I think YOU misunderstand what C means, OR you had a typo in the
original post about the max charge rate of the IC, C is the capacity of
the cell.
If the cell is a 4Ah (4000mAh) then a 4C charge rate would be 16A.
BTW, unless your cells are specified to take a 4C charge rate (not very
likely for NiMH or NiCD) you will fry them in no time. On the other
hand, a charge of 4C will have your pack ready to go in 15min.

Now, if you meant that the charger IC has a max charge rate of 4A (you
said C) then you are looking at an hour to charge. I have found that
most cells will handle 1.5 to 2 C charge rates, as long as dropping
their life expectancy some what is OK.

Jim

 

[Guy Macon]

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.

You could use a 12V 20,000mAH NiMh pack and only discharge it
20%. That would allow faster charging back to 100%.

 

[mike]

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.

Use serially and charge in parallel?

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