# How to calculate time for NiMH batteries to discharge?

I want to use 4 battery cells AA NiMH (or NiCd) 1.2 V, 2600 mAh in order to power up circuit that requires 5 V, 300 mA.

I know that fully charged NiMH can provide 1.35 V each, which means 5.4 V in this case. Circuit's electrical specification is Min: 4.8 V, Typ: 5 V, Max: 5.2 V. However, it refers Max voltage without damage: 12 V.

My question is:

Can I calculate time that batteries will be able to power up the circuit, before voltage drops under 4.8 V, if we assume that provided amperage from batteries is 300mA?

• What kind of circuit is this? Keeping something near (and over) its maximum voltage is not a good idea. The 5.2V limit is there for a reason, you probably should respect it. – Vladimir Cravero Apr 24 '14 at 7:57
• It's a single-board computer, BeagleBoard-xM. In case of overvoltage it refers that an overvoltage detector is activated in order to prevent the board. – dempap Apr 24 '14 at 8:04
• That might not work then... I think you are safe not damaging it anyway. – Vladimir Cravero Apr 24 '14 at 8:22
• That kind of specification basically means you should be providing a regulated 5V power supply. You'll be far less likely to damage your device if you use a DC-DC converter and either a higher (with a buck converter) or lower (boost converter) voltage. You'll be able to get much more life out of your batteries as well. – LeoR Apr 24 '14 at 12:19

You have to estimate or use the discharge graph from the battery specification. Different manufacturers may provide different graphs.

The picture below is from a modern Sanyo 2700mAh Eneloop battery datasheet. The manufacturer gives us a capacity vs cell voltage graph:

It looks like 2000mAh should be available above 1.2V if you are discharging with 540mA. There is no 300mA discharge curve, but we can assume that will be similar or better than the curve for 540mA.

We can therefore estimate that 2000mAh/300mA = 6.6h.

For a GP NiMH battery that I randomly found, it gives a voltage vs discharge time graph which gives you the answer directly:

These are the datasheets I used for the pictures and calculations:

Note: Sometimes datasheets provide information about high discharge currents, but I don't think these very high currents are normal. You probably can't drain so much continuously from battery because of heat generation.

• I have some experience with the Eneloop batteries and they can surprisingly supply high currents (for a AA battery). They have a low internal resistance which allows the high currents without too much heat. But even they will heat up if you demand really high currents. I had one of their eneloop "AAA" putting out 8 amps and it did get fairly hot after a few minutes. – Filek Apr 25 '14 at 6:11

As Kamil has alluded to, different batteries will supply different amounts of energy at or above the 1.2 volts per cell that you require.

As a result, the only way to answer your question is to get the datasheet for the brand of battery that you are using and calculate from there. If you haven't chosen a battery yet, then you could choose one of the ones that Kamil has provided info for and calculate from that. IF you need help calculating from one of those datasheets let me know.