Connecting NiMh in series and parralel

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19 March 2009 at 18:22 #19537

Mick Favager

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@mickfavager60486

I have two 7.2v 4200 mah NiMh connected in series to drive a brushless motor through 30Amp ESC. This comfortably powers a water jet drive at an excellent RPM.

I now wnat to power two water jet drives using two brushless motors and two 30 amp esc. No problem.

However, will there be a problem if I connect two 7.2v 3800 mah NiMh in series and then parralel them with the 4200mah?

I know that this is not possible with LiPo’s.

regards

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19 March 2009 at 18:22 #5031

Mick Favager

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@mickfavager60486

19 March 2009 at 18:46 #19538

ashley needham

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@ashleyneedham69188

Mick. The theory is ok,, but I am wondering what the ramifications are when using cells of different capacity like this, as obviously one set will drain before the other??? I have a feeling that further advice may be that it is best to run each battery set in turn, changing as one goes down. 60Watt will no doubt provide more info. Ashley

19 March 2009 at 19:15 #19542

Glynn Guest

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@glynnguest59245

Mick,
It is generally considered bad practice to connect batteries in parallel. Even with nominally the same EMF’s and capacities you could end up with the currents not going in the directions you might expect.

Why not use the pair of 4200 mAh on one motor and the other pair on the other? Yes, the 3800 mAh pack will “dump” first but it will tell you switch off that motor and recover the model before the second motor dies.

GlynnGuest

19 March 2009 at 22:14 #19546

Mick Favager

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@mickfavager60486

Glynn

Thanks, I did wonder if my plan may cause problems somewhere, I guess the 4200 may charge the 3800 at an accelerated rate and overheat or worse.

I will stick to operating each motor as you suggest

Thanks

Mick

20 March 2009 at 06:46 #19547

ashley needham

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@ashleyneedham69188

no probs

20 March 2009 at 16:16 #19565

Mick Favager

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@mickfavager60486

Another thought, apart from the weight to power ratio can Pb batteries deliver suffieicnt continuous power to two brushless motors rated at 1650 kv?

regards

Mick

20 March 2009 at 21:36 #19571

60watt

Participant

@60watt

You just need to arrive at a current figure. C= amps and is numerically = amp hour capacity.

Load Current*
– peak
– best result

NiCad
20C
1C

NIMH
5C
0.5C or less

Lead Acid
5C0.2C

Li-Ion
>20C
5C or less

Li-Polymer
>20C
5C or less

Rechargeable
Alkaline
0.5C
0.2C or less

There is no mention of what type of lead acid battery as even gel cell types come optimised from standby to deep-cyclic and car use.

source http://www.rfcafe.com/references/electrical/batteries.htm

Edited By 60watt on 20/03/2009 21:37:27

Edited By 60watt on 20/03/2009 21:43:38

Edited By 60watt on 20/03/2009 21:44:15

20 March 2009 at 21:46 #19572

ashley needham

Participant

@ashleyneedham69188

Mick. In general, no, Gel batteries do not like large amounts of current being taken out of them, NiMh is the way to go….depending on your projected amperage, as above, Ashley