NiMH fully charged

[Stream]

Would you consider a AA NiMH battery fully charged at 1.43v or 1.46v?

Thanks

 

[SilverFox]

Hello Stream,

That is a good range of voltages for fully charged cells. I believe the ending voltage is dependent upon the charge rate. The higher the charge rate, the higher the ending voltage.

Tom

 

[Stream]

Hello Stream,

That is a good range of voltages for fully charged cells. I believe the ending voltage is dependent upon the charge rate. The higher the charge rate, the higher the ending voltage.

Tom

I see. So based on what you're saying: a charger with a higher charge rate will produce higher voltage numbers when it cuts off. Correct? My current charger cuts out at 1.39v (is this also acceptable?).

Thanks again

 

[SilverFox]

Hello Stream,

Here is an example from a GP data sheet for their high capacity 2400 mAh cell.

Note that they list the maximum charging voltage as 1.5 volts when charging at 240 mA, but when you look at the charging profile graphs for charging at 1200 mA and 2400 mA you can see that the ending voltage is above 1.5 volts.

Your 1.39 volts seems a little low. It should be closer to 1.45 volts, plus or minus a little.

Tom

 

[Curious_character]

Would you consider a AA NiMH battery fully charged at 1.43v or 1.46v?

Thanks

You can't determine by the cell voltage whether a NiMH cell is fully charged. The charger puts a constant current into the cell until the voltage reaches a peak then drops slightly, or until the temperature rises at a particular rate -- it doesn't use the cell voltage to determine when to terminate the charge. The actual voltage the cell has at that point varies with individual cell, cell temperature, the current at which it's been charged, and the charge termination detection method and details. After you remove the cell from the charger, the voltage will pretty rapidly drop to somewhere around 1.35 volts, then over a period of days to something like 1.28 or so. These drops don't indicate significant charge loss.

The only way to determine whether a NiMH cell is fully charged is to discharge it and see if it was.

Li-ion, lead acid, and some other battery types are charged with constant voltage, and cell voltage can be used to determine approximate state of charge. But not NiMH or NiCd.

c_c

 

[lctorana]

IMHO, the no-load voltage doesn't tell you a great deal. Better to test under load.

My rule of thumb is to load the cell with a (say 300mA) load and a fully, freshly charged cell will deliver just over 1.3 volts.

I built a tester that does exactly this. It has a D, a C, an AA and an AAA holder mounted in parallel with a 1.5v meter, and a 1.2v globe is activated by a pushbutton. Then l note the cell voltage unloaded and loaded.

The theory is, for both NiMH and NiCad, a freshly charged cell will, under a C10 load, start at just over 1.3v, falling exponentially to 1.2v, which it will deliver for the majority of the charge delivery.

So -
1.3v = 100%,
1.25v = 90%-ish,
1.2v = anything between 10% and 80%
Anything less than 1.2v - time to recharge.

And I have test leads to connect to other size cells.

This test works for any cell, regardless of chemistry. Your expectations of voltage, as to what constitutes a "good" cell varies - you expect more volts from an Alkaline D cell than you would a ZnC AAA, for example.

 

[FILIPPO]

what about 1,42?:candle:

 

[NiOOH]

I agree that the OCV doesn't tell you much about the state of charge. It should be tested under load. I've built a device similar to what Ictorana uses but with 2 ohms 20 W resistor instead of a bulb. Works great for discharging cells and monitoring their discharge voltages

 

[bob_ninja]

The correct answer is 1.434957823645632586158913656314756183495V

 

[Stream]

Ok, thanks for all the info guys. I think I have a better understanding of how to gauge the charge state on NiMH/NiCD cells now.

Thanks again