By Greg Illes
There’s a lot of confusion out there about batteries, not the least of which is how to tell if your batteries are properly charged or not. Pretty much the only handy, inexpensive way to do this is with a (good) voltmeter. Let’s make sure we have the basics down pat:
A fully charged battery will measure approximately 12.6V under the proper conditions.
For best battery life, discharge should not exceed 50 percent, at which point the battery will measure about 12.2V under the proper conditions.
You have to check your specific battery manufacturer’s spec sheets for the exact numbers, but they will be near these values.
Now for the confusing parts. You have to provide “proper conditions” to make meaningful measurements. Here’s how:
Charging or loading condition
You cannot get an accurate battery voltage if it is being charged or heavily loaded. To measure the proper voltage, shut off the engine, shore power and/or solar first. Shut off any but the lightest loads (like the fridge or an LED light).
Float charge
After being charged, all batteries will have a “float charge” on their internal plates, making their output voltage look much higher than it will be after the float charge dissipates. You must load the battery to get rid of that charge (this can take many minutes), then unload it and measure the voltage.
Voltmeter accuracy
All the precision setup conditions won’t help if your voltmeter is out of whack. This is a more common problem than you might think, even with brand-new units. Check the voltmeter against a known standard or another voltmeter, to be sure your instrument is reliable. Expect to pay $35 and up for anything that you can count on. You can check out some voltmeters at Amazon.com.
How and where to measure
Measuring directly at the battery terminals is sure-fire, accurate and very inconvenient. If you have 12V sockets for your coach batteries, you can make a connection there. Otherwise, you will need to do some wiring (or have it done). By far, the best overall solution is a built-in unit that you can easily monitor any time you like.
Once you get the hang of checking your battery voltage, you will have the ability to make informed decisions about loading your batteries and charging them. You’ll be able to keep them at 50-percent charge or above, and your batteries will thank you with the longest possible life.
Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his blog at www.divver-city.com/blog.
