battery test question

My vehicle has a couple of “maintainance free” batteries (i.e. no fluid fill or test ports). Is there an easy way to determine their condition? They read 13V on a DMM when not under load - I assume that doesn’t tell me anything and I need to conduct some sort of load test? Can this be done without removing the batteries and burn testing them on purpose built apparatus? It may be a moot point - starting seems a bit sluggish recently, and they’re old, so it’s probably time to replace them, but for the sake of satisfying my curiosity, any answers?

-FK

First measure the open circuit voltage on the terminals, then take a known resistor value, probably something that can handle say a couple of amps, so you’re looking at maybe 6 ohm wire wound job.

You can then work out the value of the current that should flow through it given the resistor value and the open circuit voltage.

Measure the battery terminal voltage as you connect this resistor value across the terminals.

You will get a slightly differant voltage, this is because the battery has an internal resistance that is effectively in series with the voltage on your terminals.

Using the voltage value across you just measured you can work out the current passing through it.

Having obtained this current value, you can then imagine the battery to be a perfect item, with this internal resistor in series.

You can calculate the value of the internal reistance by dividing the voltage change(between the disconnected battery and the one with the resistor across) by the current you just worked out.

Whar you have then found is the internal resistance of the battery.

If you can get hold of a manufacturers specification, it should mention what the internal resistance is supposed to be, if it is greater that this by more than around 10%, then it is on the way out.

Or you could take it to a garge where they can give it a load test, the device works in the same way, but it just gives you a reading on a meter.

Thanks.

Some auto supply chains do a free battery load test.

One thing to always check, are the terminals clean? Give them a check and while they’re off, go ahead and scrub them anyway.

Fuji, many(most?) maintenance free batteries have removable caps and water can be checked/added. The batteries look to be sealed, but there can be two caps that will remove.