Thursday, March 28, 2013

Determining battery health status

Some days ago I found this 12 volts, 17 Ah battery. It was dead. It's voltage was zero volts between terminals.

The battery should be disposed away, but I took it to play. Deep discharged lead-acid batteries can become damaged, but if the battery has zero volts, it's sure it is irreversibly damaged.

The first step to play with it was to revive it in some way. I used my variable power supply and adjusted it to around 15 volts, and applied that voltage to the battery through a 1W 150 ohms resistor.

This produced a very low charging current (under 100 mA) and I left the battery charging until it reached about 10 volts. It took several hours to get that voltage. Once the battery was at 10 volts, I charged it in the common way using my Imax B6 charger. After fully charged, I made four discharge-charge cycles. In all these cycles, the Imax B6 charged displayed capacities between 7 and 8 Ah: The battery is usable, but it has less than half the original capacity, but the same volume and weight!

At this point I have a damaged battery. What are the signs of a damaged battery? Or in other words, Is it possible to check battery health in a easy way?

Battery impedance

If you look for information in the Internet, you will find battery impedance is a key value in battery's health. The battery's impedance or its internal resistance is directly related to the battery ability to output current without voltage drop. The lower it is, the better.

I have a battery impedance meter. I have used it to check a lot of NiCd, NiMH and Li-ion batteries. I found it also useful to check electrolytic capacitor's ESR. Of course, I used it to check this battery's internal impedance. The reading was 35 milliohms, a real good value.

But when used, the battery has a significant voltage drop. Using some resistor loads and measuring the voltage drop I got internal resistance values from 0.1 to almost 1 ohm, values much higher than the values measured with the battery impedance meter, who uses a 1 kHz sinewave.

So, is it a battery impedance meter a good tool to check battery health?

No, but yes: A high battery impedance is always a sign of defective battery, but the opposite is not always true: A low battery impedance does not mean a good heath battery.


Measuring the real battery capacity seems to be the most accurate method of determine the actual battery health status. But again, this is not always true: I have seen many Ni-Mh batteries with almost complete capacity but high internal impedance, so they were only useful to supply low or very low current devices.

Only your experience, using the capacity and internal impedance values, and the final application will dictate you if a battery is in good shape or not.

But, a half capacity battery with low internal impedance is a battery in good shape? And how about a full capacity battery with high internal resistance? Obviously they are not, but they can suffice in some applications.

Definitely there is not a single test to measure battery's heath status.

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