Advice on connecting batteries in series

Question

I have a 4 NiMh AA HR6 batteries each with a capacity of about 2000mah. And am working on a LED project which requires 500ma but I need to power it up with a voltage of around 5v. So I was planning to hook these 4 batteries together in series to achieve that level of voltage.

But here are my concerns these 4 batteries are not of same make. Two batteries are GP Recyko and two are Kodak Digital camera batteries. Does it poses a problem when i connect batteries of different make in series.

Moreover I have recharged the battery two days back and when i measured the voltage none of them seems equal even the batteries of same make. The batteries measured 1.24, 1.31, 1.30, 1.27 respectively. If i do connect these batteries together what will be the resultant output voltage?

If this is good to proceed how long can i safely draw 500ma from these batteries?

Answer 1

If you connect those batteries in series, you'll get the sum of the rated voltages: 1.24 + 1.31 + 1.3 + 1.27 = 5.1VDC but this is true on unloaded state.

When the batteries are connected in series, the capacity will be practically the minimum one of the rated capacities, not the sum. Since you claim that each battery has a capacity of 2000mAh, total capacity will be 2000mAh when they are connected in series.

Practically, it can be said that the lifetime will be approximately 2000mAh / 500mA = 4 hours, but please consider the discharge curves. Like the one below:

According to the graph, one battery voltage will be about 1.25VDC for a 500mA load. Then the total voltage will be 1.25 x 4 = 5VDC. If the load current remains constant then the lifetime will be about 4 hours.

Answer 2

Battery voltage is a 'low class' measurement, you can't tell a lot from it, so those four measurements are more or less the same. You do the sums for the present series voltage, it's a bit more than 5v. Of course during discharge the voltage will drop, many people choose an endpoint voltage of 1v per cell, so the final voltage will be around 4v.

More important than the make is the history of the batteries. If two are new and two have had years of use, it would be unwise to run them in a series pack. If they are all the same sort of age, and nominal capacity, then go for it.

NiMH is a fairly forgiving chemistry. They tolerate low current overcharge, and self-balance during that. They tolerate deep discharge, and while it's not recommended, they are not fatally damaged by being taken well below their endpoint voltage, though voltage reversal will damage them. On the first discharge of the series battery, monitor all the cells individually to make sure there isn't one weak cell that will run out early.

How long will a 2Ah battery last at 500mA? The naive calculation is 4 hours. However, the rated 2Ah will be at the 10 hour rate, discharging them faster will give you a lower capacity. The capacity will also vary with endpoint voltage. Check the specs for those cells to see how they're tested. In summary, no more than 4 hours, but probably the majority of that.

Answer 3

The voltage of the pack will be the batteries combined voltage, 1.24 + 1.31 + 1.30 + 1.27 = V_pack.

The current load is 500mA and batteries is specified in mA/H. So the time it can supply current to the load is 2000mA/500mA = X hours

Regarding different brands. Don't mix 'em. Even if they are all NiMH cells the differing characteristics would mean they wouldn't charge properly, and the weaker cells could be damaged during discharge in use.