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What diagnostics can be done to to assess battery charge level? I'm interested both in one-off tests like can be done in car service and also continuous monitoring during normal use.

Car panels usually have warning lights for battery/charging related problems. For example, user manual for my car says:

Charge warning light

If this light comes on when the engine is running, it may indicate that the charging system is not working properly.

I'd be interested to diagnose problem further--is the battery failing? Is the alternator failing or too weak to keep up with demand? Are the lights, sound etc. too power-hungry, or maybe I should look for a short-circuit?

I'm asking about battery charge level specifically because, even if the "Charge warning light" is off, perhaps the battery is constantly half-charged and is going to fail in few months.

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4 Answers 4

Measuring the state-of-charge of a lead-acid battery is non-trivial.

The easiest way is to use a voltmeter. You'll need a digital meter with 3 1/2 digits of precision. Let the battery rest, disconnected, for 24 hours. Then measure the voltage across the terminals. According to http://www.phrannie.org/battery.html, anything over 12.60V is at least 85% charged. Maintenance free, gel, AGM, etc. batteries have different voltages.

Batteries vary, so you'll want to get a few good measurements of your battery to establish its behavior. Use a smart charger to get to full charge, rest 24 hours, and measure.

This is why people on boats sometimes have 2 banks of house batteries. One will rest while the other is used. When it's rested for 24 hours, they measure sate-of-charge, and then decide how to recharge - solar, wind, generator, etc. Then they charge it up, put it in to service, and disconnect the other bank to rest for 24 hours.

You can also measure the specific gravity, using a hydrometer. You don't have to rest the battery first, but you do have to correct for temperature.

Because measuring state-of-charge is so hard, people often choose to measure how much energy they use. Basically you install a carefully calibrated, low-resistance shunt on the battery terminal, then measure voltage drop across the shunt over time. If you start measuring when the battery is full, you can get a sense of how low the battery is after a while. This is typically reported in "Ah". There are still issues: cold batteries don't give as much energy; higher currents (over a short time) cause batteries to drain faster (than lower currents over a long time); you have to be fully charged at the start to know anything useful; as the battery ages its total capacity drops.

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I would recommend fitting an analogue voltmeter, and potentially an ammeter as well.

The voltmeter will give you an idea of battery voltage whenthe engine is off, and alternator performance when it is running, and the ammeter will give you an idea of current drain - You can compare this with the rated current of your alternator to know if you're overloading it.

The reason I suggest analogue rather than digital meters is that they are easier to read "at a glance", and so don't distract you while you are driving. Many automotive-specific ones will also have nice convenient red-green bars to highlight the expected range, i.e. 13-14v

Voltmeters should be fitted across the battery terminals, I would generally go with between the switched live feed from the ignition and the main vehicle earth/ground - most workshop manuals will have a wiring diagram to show you which these are.

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Moreover, voltmeters built into stock instrument clusters aren't always accurate. An aftermarket device may be more accurate. +1 –  William Cline Apr 1 '11 at 17:04
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To test the battery, you can check it with a multimeter. It should be around 12v-14v when the car is off, but should be no lower than around 9v.

If your charge warning light comes on while the engine is running, it will most likely be the alternator causing the problem, since even if the battery is dead the alternator will keep the voltage fairly steady.

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BATTERY - As I have just found from a recent incident with a bad battery in my M3, a battery that will hold a high voltage will not always pass a much more rigorous load test. My battery was holding 12.7 V after 24 hours (disconnected from the car.. that's a whole 'nother story), but when I took it down to AutoZone they said it was border-line bad. The unfortunate part was the $150 battery was only 3.5 years old.

The easy way to perform this test is to drive around for a while to get the battery fully charged, let it sit a day, then turn the key to the run position (don't start the car) and leave the headlights on for 60 seconds. The starting voltage should be around 12.7 or so volts. Have your friend turn off the lights (or pull the battery cable). The ending voltage should be 12.0 V (or more). Anything less, and you have a bad battery.

ALTERNATOR- When running, your battery voltage should read 13.5 to 14 V. If it's low, your alternator is not charging properly (lost a phase, probably). If the reading is high, the rectifier is shot. In most cases, this means replacing the alternator as a unit, although I know people who have fixes individual diodes on the rectifier boards.

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