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Motorhome electrical power systems

by Tom Jones

Newcomers to motorhomes are often baffled by the complex electrical power systems on board. This article, compiled from inputs by the author and forum members and staff, attempts to demystify this subject. It is written in a 'question and answer' format.

I'm confused on when/where/how the different systems take over depending on what you are doing (driving, hooked up to shore power, or boondocking).

In many cases, it's almost all done automatically by an energy management system. This will almost certainly be true in the case of newer motorhomes. But it will depend on the make, model and year of manufacture in the case of older motorhomes.

When do we use the generator to charge the batteries?

The batteries (specifically the house batteries) will need recharging when they've discharged to around 50% of their capacity. There will likely be some indicator in the motorhome telling you what the state of charge is. This might be a simple "battery voltage" meter.

Do we have to run the generator when hooked up to shore power?

Not normally. But, if you happen to be hooked up to shore power that doesn't provide enough power to run everything you want to run, you may need to run the generator. However, most campgrounds don't allow you to run the generator.

When should I turn the inverter off?

Some folks turn their inverter off when they're boondocking, but not actually running anything off the inverter (e.g. when they go to bed and don't need TV or any other appliances on). This saves a little drain on the batteries because, even with no appliances on, the inverter itself draws some current from the batteries. However, I leave our inverter on all the time.

When you're plugged into shore power or the generator is running, the inverter is essentially bypassed automatically. In this case it doesn't matter if the inverter is turned on or off.

How far should batteries be discharged before running generator?

Typical deep cycle batteries used as house batteries on motorhomes should not be regularly discharged more than 50%. However, that doesn't mean that you can't recharge them sooner, e.g. when they're discharged only 25%.

How do I know when the house batteries are discharged to 50%?

It depends to some extent on the kind of battery. A fully charged flooded wet cell battery will read 13.6V or more when fully charged and still connected to the charger. Turn the battery charger off and that fully charged battery might read as low as 12.6V. When 50% discharged, the battery voltage will read approximately 11.8V with the charger turned off.

How long does it take to recharge batteries when using the generator?

It really depends on the make and model of inverter, whether it has a built-in battery charger, the quantity and size of your house batteries, and how far discharged they are. If I'm boondocking or dry camping (no hookups), I plan on running the generator several hours a day to recharge the house batteries.

Will the house batteries charge off the engine when running down the road?

Yes on some motorhomes, but not on others. Our Monaco Camelot for example does charge both the house batteries and the chassis batteries when the engine is running.