Friday, February 3, 2023


Keep those expensive RV batteries alive!

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.



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5 years ago

OMG: I’ve been camping/RVing for 50 years & will say batteries have been the most understood & most talked about issue ever! Everyone has different usage & different circumstances, so its really hard to show all the different usages. Actually I’ve had the best results with 2@ 6V off brand batteries for my boon docking. I had a 125W solar panel that keep everything going, except if we ran the heater all night! In todays RV’s with 110 volt everything, it makes it difficult to dry camp, unless you have mega L16 AGM batteries & 800 watts of solar! PS: I still dislike GENERATORS!
I’m thinking maybe there might be something else thats not understood & thats shock absorbers & springs, hmmm!

5 years ago

Actually understood should have been misunderstood! Sorry my finger is the blame. LOL

5 years ago

Testing voltage of a battery in use or recently charged will not tell you the state of charge with the table above. ‘That table is for disconnected static batteries. When it reads 13.1 right after you’ve been charging it…that is a surface charge and you don’t know if you have 100% charge on the battery or 80%. It takes time disconnected (24 hours!) or a dummy load ($20 bucks at harbor freight) to get a true reading.
Of course a battery in use will exhibit lower voltage than when disconnected as so you could easily be seeing voltage corresponding to 80% discharged when the actual state of charge is much higher and the voltage will rise when the load is removed.

IF you boondock you need to know the REAL state of charge of your batteries, how much you’ve used, how much is left at your present rate of use …and when you can turn off the generator cause you’re batteries are full. This requires a true battery monitor like those from Trimetric or Victron. The batteries and gas you save will pay for the unit.

5 years ago

I don’t know if it showed up better on a desktop or notebook computer but the chart columns for flooded and lead acid were so out of line on my iPad they were indecipherable. And say again that discharging down to 30% is better than down to 50%? Might have been answered by Bob.

Michael Absher
5 years ago
Reply to  Ed

This is a common problem with iPad my Android looks clear and aligned

5 years ago

I think Russ stated the scale BACKWARDS. Yes, you discharge less/keep more charge to lengthen the life.

He also unrealistically described rest, because unless you disconnect the battery, there are always some parasitic loads drawing down the voltage.

Rufus Dryer
5 years ago

I need some clarification, please. When you say “Discharge down to 80%” do you mean there is 80% of capacity left in the battery or do you mean there is 20% of capacity left in the battery?

5 years ago
Reply to  Rufus Dryer

I being in the battery business since 1983 and owning a chain of retail battery stores..he means discharged down to 20% remaining..very simply the deep discharges shorten the life of most batteries in the RV world but the older other type of batteries such as camcorder needed deep discharges and then recharged fully as to prevent memory problems but this does not relate to your question..Similar to your auto recharges immediately after starting thus your car batteries last usually 5 years except in super high temp climates and those batteries use a different low heat plate..Keep in mind the #1 killer is Heat ..#2 Vibration…Hope this helps..

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