Archive for March, 2012

What’s the battery doing? Start with a DVM

Ever wonder about what’s left in the battery when off-grid? Worry about running the battery flat on a chilly night?

You’ve got two different measures to consider. One is the state of charge which is like a gas gauge in telling you how much of the battery you’ve used and how much you’ve got left. Another is the battery state of health which, when compared to the original battery specifications, tells you what you can expect from the battery. The state of health is good to know because a battery is like a fuel tank that shrinks as it ages.

Assessing state of charge used to be a matter of sampling battery electrolyte to determine specific gravity. That means lifting and moving a heavy battery to get at cell caps and then pulling a caustic acid sample from each cell for testing. This is serious hazmat territory and OSHA says several thousand people are injured in the U.S. every year from this sort of activity.

A more modern means to determine state of charge is to use a digital voltmeter. Here’s one way to keep an eye on your battery voltage that only costs about $15:

Equus 3721 Battery and Charging System Monitor

One feature that makes this item attractive is that it is a three and a half digit voltmeter. That means it reads voltages out to two decimal places. That’s one digit more precision than you really need which means you can do your own rounding and can easier spot small changes in voltage response to loads and such things.

The Equus 3721 also includes a level graph and indicator lights to qualify the voltage reading. It has 2 modes for the indicators so that they indicate both battery and charging status. Switching to charging mode is not a simple voltage decision where higher voltages are considered charging. It appears that the Equus looks at the voltage it is measuring and switches to charging mode if it sees fluctuations that indicate a charging device is in the circuit. That sort of smarts adds value to the meter but it’s not perfect because a very smooth charge, such as from a good RV converter, will not trigger the mode change.

This DVM is intended for the automobile cigarette lighter socket. That is convenient but not always the best place for assessing RV battery status.

A bit more sophisticated is the BatteryMole Car Battery Warning System (12 Volt Automobile Battery Monitor with Battery Failure Prediction Logic). Made in USA for about twice the price. This one analyzes what engine starting does to battery voltage to infer a battery state of health. The description notes that battery voltage is best measured after 4 hours of no significant charging or discharging. Since it uses engine starting load for its measuring, it won’t help much in an RV situation.

One the high end are items such as Clipper BM-1CW Battery Monitor Compact White
or New Xantrex LinkPRO Battery Monitor and Xantrex LinkLITE Battery Monitor. These are integrating ammeters or ‘electron counters’. They not only watch voltage but also current. They use the data collected to try to figure out the actual state of charge.

“Defining the amount of energy available in a battery is a complex task since battery age, discharge current and temperature all influence the actual battery capacity. High performance measuring circuits, along with complex software algorithms, are used to exactly determine the remaining battery capacity.”

Numbers can be fun but tend to get old after a while. If you use that $15 DVM you can keep an eye on what your battery is doing so you can judge when it needs a recharge or you need to turn off the TV or when the battery just isn’t up to snuff anymore. The $250 class turns a lot of the judgment over to a robot whose conclusions should be properly qualified.

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