Connecting NiMh in series and parralel

[Mick FavagerParticipant @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

[Mick FavagerParticipant @mickfavager60486]

[ashley needhamParticipant @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

[Glynn GuestParticipant @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

[Mick FavagerParticipant @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

[ashley needhamParticipant @ashleyneedham69188]

no probs

[Mick FavagerParticipant @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

[60wattParticipant @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 5C 0.2C

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

[ashley needhamParticipant @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

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