Understanding NiMH ratings

[castingflame]

Fundamentals question.


If I have a battery Pack rated as 24v, 3000mAh


If 3000mAh  = Capacity of battery =  Amp in 3 Amps for 1 hour


There is no C-rate on the pack but if the rate was say 1C.


does this mean that the battery can provide 12 volts at 3 Amps of current for 1 hour (or close)?

Thank you for any help you can give.

[Buriedcode]

I think you mean 24V @ 3 amps for an hour.  Roughly yes, but the voltage given is the average voltage during discharge.

NiMH are generally 1.2V although can be as high as 1.45v after charging, and drops to ~1.0V when fully discharged.  So if you're pack has 20 cells (20*1.2 = 24v) then its voltage could be as high as 28V or as low as 20V.

As for the current, yes, 3 amps for one hour, 1 amp for 3 hours, 200mA (0.2A) for 15 hours etc..   Often the capacity is given for relatively low discharge currents.  So if you were to pull 3 amps from it, it probably wouldn't last an hour, and I doubt you'll get an amp for 3 hours.

Google "discharge curves NiMH" for pretty graphs of the voltage and capacity vs discharge current.

[castingflame]

Perfect. thanks for the response.

[atmfjstc]

I've found it is easier to understand how battery power works once you realize that capacity, in itself, has nothing to do with voltage. When we say that a battery that has a certain number of "mAh", that means that it stores a certain amount of charge that you can drain faster or slower depending on how much current (i.e. charge per time) you are pulling out of the battery. The current is the only thing that matters as far as the battery is concerned. A 3000mAh battery will be drained in about 3 hours of sustained 1A current regardless of whether the battery or pack is 1.5V, 24V, 120V or whatever.

The battery's voltage becomes important when you take into account that the battery is powering a load that has certain voltage and/or current requirements. If your load needs 5V, for instance, you'll need to place a regulator between the load and the battery, and the current drawn from the battery will then be greater or lower than the actual current your load needs, depending on whether the regulator has to boost or buck the battery's current voltage to get to the 5V. More specifically:

I_battery = I_load * (V_load / V_battery)

and since runtime depends on I_battery:

runtime = capacity / I_battery = capacity / I_load * (V_battery / V_load)

Thus, the battery voltage participates in the runtime calculation only in so far as the battery has to be adapted to a load that has a different voltage requirement. Note that you can have circuits that run directly off the battery's voltage, but then they will have to tolerate significant changes in voltage (up to 50%) over the course of the discharge, and they will likely not use the battery with full efficiency.

[Audioguru]

Usually a battery pack uses standard cell sizes like AA size. A modern Energizer Ni-MH AA cell is 2300mAh, a Duracell is 2500mAh and a Panasonic Eneloop is 2550mAh. They all use the newest technology and hold a charge for one year with the slight loss of capacity since the old ones were 3000mAh.
Maybe your pack is old or is a Chinese exaggeration?

The manufacturers' datasheets show the capacity dropping a little when the load current is higher. The 2300mAh Energizer cell is 2300mAh at a 0.2C rate and drops a little at 1C and 2C.   

[castingflame]

Thanks for the extra info. I am still working on this but was on a different part of the circuit the last few months.

[spec]

The battery capacity that you see is normally at a discharge rate of one tenth of capacity, or in some cases even less than that.

A batteries' capacity is very much dependent on the discharge current.

There is a very good site about batteries called, 'Battery University' just Google it and you can read all about discharge rates etc.

By the way, not only does the discharge rate influence the capacity but so does temperature.

See the attached images below, lead/acid and NMH, which shows the way that discharge rate has an effect on capacity.