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Gazzalene

NiMh Pack voltage?

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Have 2 very new 7.2V NiMh packs different brands and 5000mah and 4600mah

The 5000mah pack will charge upto 8.35v after rest, the 4600mah will only reach 7.68v

Just wondered what the "norm"  should be. Even if dodgy I can not do much because I had to swap the deans out BUT I will stay away from the brand VOLTZ.

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Capacity doesn’t affect the voltage it’s just the number of cells. So nominal 1.2v per cell X 6 = 7.2v. I think mine used to finish charging at 8-something so yes that pack is probably better then the other.

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7.68 sounds far too low as a peak voltage. 8.3 is a bit low too. I'd expect the pack's peak voltage under charge to be closer to 1.5V per cell.

Is this the peak voltage that the charger gives as a readout or is it after the pack has been resting? What charger settings are you using?

 

 

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I agree with @sosidge

My good NiMH packs usually peak around 9V and charge to their full rated capacity. 

Give the packs a few cycles (charge-->discharge-->charge-->discharge-->...) and see if they improve.  I'd recommend charging at 1.5A  (sounds low, I know, but I've noticed my NiMH cells can start venting at higher charge rates even when they are cool - I hear a pssssss noise once in a while and it's very disconcerting) and discharging at 0.5A to 1.0A.   I just cycled some old 5000mAH packs that were not charging well and they got up to 3500mAh capacity after 3 charge/discharge cycles which is better than nothing.  Took over 24 hours to complete, though.

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11 hours ago, sosidge said:

7.68 sounds far too low as a peak voltage. 8.3 is a bit low too. I'd expect the pack's peak voltage under charge to be closer to 1.5V per cell.

Is this the peak voltage that the charger gives as a readout or is it after the pack has been resting? What charger settings are you using?

 

 

that's the voltage after about 30-60 mins.

using a propeak super nova 250s.  set for NiMh and 2a charge rate. normally use 1c but read that lower amp charging can sometimes give false cutoff,so tried 2a this time

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The voltage doesn't really mean anything after the cells have been resting, beyond "it's charged". Best guide is straight after the pack has completed the charge. Is it warm to the touch? If it is still cold - it's not charged and probably "false peaked".

There is a whole lot going on in a NiMH pack that you can diagnose if you are so inclined, and it was what you had to do towards the end of NiMHs in racing. But you would need some pretty high end charging/discharging equipment to do it. Thankfully we have LiPos now.

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Should be charging at 5 Amp. for those size batteries on a fast charge about 45 mins. A slow charger (overnight) would take longer (14-16 hrs.) at a lower Amp. setting but should still get it there.

If you can hook up a voltage meter while charging (fast charge), you can watch voltage meter, volts will increase till it is peaking  , and then voltage will drop when charge should be stopped. That's how to tell when charge has peaked. For the mah of those batteries you have, at 5 Amps. should take more than 45 mins. before voltage drops (peak). Slow charge will work this way also, but it's hard to check meter all the time.

They will get warm-hot when done, that's normal. 

Hope that helps.

 

 

 

Edited by Tamiya Fan 1
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