By Russ and Tiña De Maris
A critical factor for RVers who boondock – camp away from utility hookups – is that of a viable battery bank. Keeping those batteries properly charged can make the difference between an enjoyable experience and a bust. Even more important, a battery that is discharged too much is a battery that will experience a premature death.
How do you know your battery’s state of charge? Don’t rely on the clever “charge indicators” that most RV manufacturers build into rigs. These “good, weak, charge” meters don’t give you precise enough information. You need an inexpensive digital multimeter. Harbor Freight Tools has them for as little as $4. But do get the DIGITAL multimeter, not the analog or “needle on a scale,” because being able to discriminate voltage down to this precision requires digital.
To properly test an RV “house” battery (the one that powers stuff like inside lights, water pump, furnace motor, etc.), the battery needs to be at rest. This means not charged within the last hour or so, and with nothing drawing power from it. Since you’ve now read your multimeter instructions, hook the probes up to the battery (best to observe polarity), set the meter for the appropriate voltage range, and check the reading.
Here’s a chart you can use to determine the level of your state of charge. We’ve put information here for both the “old fashioned” flooded lead-acid batteries (ones with liquid electrolyte that needs to be refilled from time to time) as well as the newer absorbed glass mat batteries.
State of Charge Flooded Lead Acid AGM
100% 12.70+ 12.80+
75% 12.40 12.60
50% 12.20 12.30
25% 12.00 12.00
0% 11.80 11.80
How does all of this make a difference? The less your battery is discharged before being recharged, the longer it will last – the more “charge cycles” it can sustain before needing to be replaced. As a rule of thumb, only discharging either type of battery down to the 50% state of charge will mean many more charge cycles – effectively longer life. Interestingly, one AGM battery manufacturer puts it this way: Discharge their AGM batteries down to 80%, expect 450 cycles. Down to 50%, about 550 cycles. Or get real clever about it, cycle them down to only 30% of charge and get 1500 cycles. The latter probably isn’t practical for the average RVer, so we suggest shooting for 50% discharge.
Batteries are spendy critters, and getting stuck with dead ones out in the middle of nowhere is enough to ruin your whole day. Invest in a digital multimeter, make it a practice to keep and eye on the charge state, and you’ll have a much longer relationship with your electricity stockpile.