# When is a NiMH battery fully charged?



## EugeneJohn (Sep 10, 2008)

Is there a voltage when a NiMH battery, in this case size AA/AAA, is fully charged?

I had an Eneloop battery charger that would charge both my Eneloop and UniRoss cells to about 1.45 volts, hot off the charger. I gave that charger to a friend and with the one I'm using now, a DX cheapie, the cells come off the charger at 1.39ish volts.

Is my current charger giving the batteries a full charge? 

I will eventually get a better charger for AA/AAA cells but its pretty far down my wish list.

Can anyone chime in on this or is there a faq or other resource out there to answer these questions?

Thanks in advance.


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## metlarules (Sep 10, 2008)

My eneloops(both aa&aaa) come off thecharger at 1.45 to 1.47 so that sounds about right for your original charger. The second one sounds a little low but I don't think it will matter much in use.


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## Marduke (Sep 10, 2008)

No, those cheapie DX and Kai chargers use a termination method of max voltage of about 1.38v, which does NOT fully charge your cells. A typical charger will get a NiMH cell to at least 1.42v for a full charge.


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## crofty (Sep 10, 2008)

My eneloops charge to 1.50v - 1.51v on this charger.


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## Turak (Sep 10, 2008)

The voltage that a battery is at when it comes off the charger can be influenced by quite a few things;

1. The method the charger uses to terminate the charge. Many chargers use -delta V. Which basically means that it looks for a negative dip in the voltage which usually occurs when a Ni-Cd or Ni-Mh battery is fully charged (actually slightly after full charge). Some chargers use Vmax. Which is another may of saying when the voltage reaches a certain point for a certain period of time, it is fully charged so it terminates.

2. Does the charger have a maximum voltage cutoff point. Almost all smart chargers have a maximum voltage cutoff. This is, technically, not meant to be used as a termination method but more as a safety mechanism.

Note - The Maha MH-C9000 claims to use -deltaV as its primary method, but the fact is that more often than not, it reaches Vmax (1.47V) well before it reaches -deltaV. Which is one reason that an Eneloop DOES NOT get charged to its true maximum capacity when used with this charger and many others.

3. Does the charger apply a topping off and/or trickle charge after the main charge is terminated?

4. How fast does the battery self discharge. I have seen a battery (Energizer 2500's in particular) lose .25 to .5 volts within an hour of charge termination.

5. The batteries chemistry and construction. Different batteries from different manufacturers can reach different maximum voltages, assuming a maximum voltage cutoff is not present.

I am sure there are a few other reasons. This is all I could come up with off the top of my head....


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## waltereo (Oct 20, 2008)

Turak said:


> The voltage that a battery is at when it comes off the charger can be influenced by quite a few things;
> 
> Note - The Maha MH-C9000 claims to use -deltaV as its primary method, but the fact is that more often than not, it reaches Vmax (1.47V) well before it reaches -deltaV. Which is one reason that an Eneloop DOES NOT get charged to its true maximum capacity when used with this charger and many others.
> 
> .



Do you mean Maha C9000 is not worth to buy ?
So which charger is recommend for Eneloop battery to charge at its maximum capacity ?


Thanks


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## Bones (Oct 20, 2008)

waltereo said:


> Do you mean Maha C9000 is not worth to buy ?
> 
> So which charger is recommend for Eneloop battery to charge at its maximum capacity ?
> 
> Thanks



You can't go wrong with the MH-C9000 waltereo:

http://www.candlepowerforums.com ... post2650477

It's gentle charging methodology and refresh and analysis features will allow you to maximize both the performance and the lifespan of your Eneloops.
-


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## qwertyydude (Oct 21, 2008)

You can't tell the state of charge in batteries by the voltage they come off a charger, in fact let the batteries rest and it'll drop to about 1.35-1.40 or so. Too many influencing factors like internal resistance of cells, charge current, trickling current, and a host of other factors. The only way to tell if they are truly full is the delta v method of charge. That is when charging and the battery is full, voltage stops increasing and starts to decrease. Along with this is a sharp rise in the temperature of the cell during a fast charge. There is no set voltage, but another kinda safe way is to just trickle charge them overnight at less than C/10 current, for a 2000 mah cell that's less than 200 ma.


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