I am reviewing the handout I received from you at the battery maintenance seminar at the Seattle RV Show and have some questions. I have an Itasca motorhome with two 12-volt batteries wired parallel. I have been dry camping with this rig and have nearly run down the coach batteries after a few days. When that happens I have to start my RV generator (which runs on propane) to charge the coach batteries up some. I thought I remember you saying that this is not very effective. Is running the RV engine more effective or taking it out and driving it a little? I am also thinking of buying a portable 2000 watt generator that I can just connect to the coach batteries and charge them in place. My plan is to buy the little generator and connect to the coach batteries in place without unhooking the battery cables. I have been told I can charge the two coach batteries together by hooking the positive clip to the positive terminal of one battery and the negative clip to the negative post of the other battery. Will buying the 2000 watt generator, which has a built in 8-amp charger, do the job? Jim G., (Seattle, WA)
Jim, I'm glad the seminar prompted some further contemplation on your part! And you remembered correctly; most modern RV generators do not have a dedicated battery charging circuit. They simply produce the 120-volt AC output that further powers a converter/charger or, in some cases, an inverter/charger for on-board battery charging.
As for connecting the load and a charger, both the load and the charger should be connected to the positive terminal of one battery and the negative terminal of the other battery in a two-battery, 12-volt parallel set-up.
But in my opinion, 8-amps would not be sufficient for effectively charging your battery bank. The general rule of thumb is C/5. “C” being the total amp-hour capacity of your battery bank, divided by 5, which equals the output amperage necessary to fully charge the bank in the least amount of time, without the fear of over-charging. This presupposes the battery charger is a sophisticated, 3-step charger. If your batteries are Group 27, you can store about 210 amp-hours, or so, of current since the two are connected in parallel. 210 divided by 5 means you’ll need a battery charger that can perform a bulk charge of about 40-amps. Anything less, simply will take much longer to charge and risk overheating the batteries.
Now the engine alternator will likely put out a lot more current, but it’s not very effective or efficient to actually run the motorhome engine just for battery charging purposes. Your best bet is a dedicated battery charger. You already have an on-board generator, so a portable generator just for charging the batteries is not what I would recommend.
I have recommended the TrueCharge 2 battery charger since it came onto the market. You can read my review here. See what you think. I think it would be a much better option.