RV Electricity – Inverter generator battery charging

10

By Mike Sokol

Dear Mike,
I have a Predator inverter generator, and have done any battery charging to date via the 110V outputs and a good wall charger (yeah, a little inefficient but easy). The genny also has a 9A “battery charging” plug, which I plugged into for the first time last week just as a DC source – and found it puts out 19V?!? Despite the included cable’s battery clips tempting direct connection, that’s not a good voltage for charging batteries. I found the manual itself says as much, urging the use of a solar charge controller or similar if you actually want to charge batteries from that port. Trivial to remedy with a $3 controller,  but you may want to point out to readers this “fry” waiting to happen, as it’s a popular genny… —Wolfe

Dear Wolfe,
Thanks for bringing that to our attention. Yes, many of these inverter generators (including products from Honda and Yamaha) have a 12-volt DC output connector right on the control panel. Looks like a quick and easy way to charge a battery, doesn’t it?  But as you noted, that output is not voltage or current regulated, so it won’t properly charge batteries directly.

And also be aware that these generator DC outputs are generally limited to a maximum of 8 or 10 amperes of current. And while that’s not enough for a fast charge on a depleted battery, it’s certainly enough to boil out a fully charged battery pretty quickly since there’s no current regulator circuit. That’s why if you read the manual it states that the 12-volt DC output is not regulated and will require a charge controller for battery charging.

So while it seems a bit crazy, the correct way to use a generator to charge a battery is by using a proper 120-volt AC charger. Yes, just like the one you already have in your RV’s converter/charger or inverter/charger. It’s properly voltage and current regulated (or it should be), so your built-in charger will match the voltage and current rate to your battery’s condition and chemistry. The DC output on the generator won’t do that, and will simply keep pumping out the 8 or 10 amps of current, and at a higher voltage than the battery really wants.

And because your RV’s built-in battery charger can utilize up to the full 2,000 watts available from your generator (actually, 1,800 watts in most cases, but who’s counting), on the 12-volt DC side of the converter/charger you could easily be charging your batteries at up to 160 amps of current (2,000 watts / 12 volts = 166 amps). But that’s a little extreme for most RV battery chargers, so the normal 40- or 80-amp battery charger you already have should work very well powered by a 2,000 watt genny connected to your RV’s shore power cord, no matter what brand generator you have.

Let’s play safe out there….

 

 

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

 

##RVT916

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Kevin
10 months ago

Thanks for the info. I have a mid 80s travel trailer, how do I find out how many amps the inverter is putting out? No info in manual.

Mike
1 year ago

Having just installed a 2500 watt inverter on our Travato 59K and I find that the alternator isn’t really designed to charge the 2 AGM house batteries. I have read of the DC to DC chargers as a way of improving the charging of the house batteries. What is your experience with these chargers and can you recommend them?

Mike
1 year ago
Reply to  Mike Sokol

Thanks Mike, Fortunately the existing wiring from the chassis battery to the isolator relay and on to the house batteries is #1AWG. The run is less than 15′ so even if I go with the 40 amp charger it should be adequate ( I think? ). I will of course have to add the negative cable. It looks like a 20 amp model will be lots of capacity since it is capable of charging a 300 AH battery. It’s an interesting technology. I saw where a fellow was having trouble with his ‘under the hood generator’ (alternator) and had replaced it with the DC to DC charger.

Gordon Davis
1 year ago

Hi Mike, do you have any recommendation for a 12V regulator, so the 12V output from a generator could be used properly/safely? Could an old automotive one be used?

Kevin
1 year ago

Thanks Mike. Spouse and I just had the battery charger discussion. So the battery charger can stay! Now of course the question is where to store it so I can find it if needed….

Wayne
1 year ago

I mounted a manual transfer switch in the basement of our Cedar Creek fifth so I can switch from shore to gen. The gen sits in front storage. But I also fished a good cord from the front storage forward to hitch box where I can pull it out and plug it into gen after setting the gen in the truck box.
This allows me to plug in a smart charger in the front (also where the batteries are) or plug in the whole trailer via wiring to the transfer switch.
This allows us to get a better charge to the batteries as we drive down the road. Useful if we have boon docked for the night and have run the furnace all night.

chris p hemstead
1 year ago

Is the 19v measured with a battery hooked up?

Wolfe
1 year ago

I know what you’re thinking, and no, it’s not just floating high without a load… I believe this DC port is just a tap into the variable-DC output between the DC generator and DC-to-AC inverter module. It is unregulated because the inverter stage will tolerate a significant range of input voltage as long as the wattage is OK.

What IS the 19V DC good for? A strong DC source for “12V expected” motors and heaters that do NOT have electronic controls. So, I might run a light 12V winch (10A at 12V is OK on 19V@9A) or seat heater pad, for example. No not all THAT useful which is why I never hooked up to it before this week. 🙂

If you WANT to use the DC port to charge batteries, you can get a 10A solar controller for about $3 from China and splice it into the provided cable. If you already have solar, treat the generator’s DC as a 170W@19V-nominal solar panel and let your existing controller handle it.

As for the RV’s CONverter being the best charger, it IS the easiest/safest — but if you’re trying to charge an ATV or MoPed battery, you might be tempted to use that port directly, which brings you back to the $3 controller above — cheaper than most 120V wall chargers.