Best Way to Use a Generator to Charge Your RV Batteries

As more and more RVers begin to consider the fun and benefits of boondocking, as opposed to RV park camping, keeping the batteries charged is a frequent topic of campfire conversations and RV forum entries. Many approach their experiences, not with an on-board generator, but bringing along a portable generator. Lots of them have connections to “charge” 12-volt batteries. Is this the best way to go?

Here’s a hypothetical: Let’s say your portable generator is a 2,000 watt “SoundBlaster” brand. It provides a couple of 120 volt power outlets, and yep, a place to plug in a couple of 12-volt wires for battery charging. Looking at the SoundBlaster specs you note that the 12-volt output is labeled “3 amps.” Yes, your generator will charge your RV batteries, but at a very slow rate. In our hypothetical we’ll say your rig battery is a 110-amp hour battery, discharged to about half it’s full capcity–it needs a minimum of 55 amps to be fully charged. In the perfect world you could plug your wires in, and hey presto! In about 18 hours your battery will be charged. How’s that?

Simple math: Need 55 amps, delivery 3 amps per hour, 55 divide by 3 equals 18.33 hours. But it’s not a perfect world. First, a battery isn’t 100% efficient, so you’ll really need more than 55 amps to do the job. Second, physics teaches that as a battery gets nearer to fully charged, the greater its internal resistance. The greater the resistance, the less the flow of current, and the slower the charge. But even in a perfect world, do you really want 18 hours of SoundBlaster?

Enter better way: Skip the 12-volt circuit on the generator, and plug in a portable 12-volt battery charger. Grab the typical “fast charge” black box charger off the shelf of your neighborhood auto parts store. Plug it into the generator, hook it up to our hypothetical battery, and less than four hours, your 15-amp hour battery charger has brought you back in battery style. OK, maybe a little longer, it’s still not a perfect world. But that’s a topic for a different day, a different venue.

Better still: A smart charger system built into your RVs (most likely) aftermarket battery converter. Hook the generator up to your RV and every time you fire up the generator, your smart charge system goes to work for you.

It’s a simple choice your neighbors will appreciate.