After a night of boondocking, why doesn’t truck recharge RV’s house battery?

Dear Dave, 
After a night boondocking and furnace running, CPAP run by inverter, my truck will not bring batteries back up sufficiently. Do most motorhomes do a better job of this?

Also, regarding our discussions in January, the Progressive Dynamics converter is working smoothly.—Wayne, 2009 Cedar Creek

Dear Wayne,
There are many variables to this situation such as the type of batteries, how many, what condition of sulfation, and the truck alternator or alternators.

12-volt components used for boondocking

First, let’s look at the 12-volt components you used for the night of boondocking. The furnace running on LP will still draw a large amount of 12-volt power due to the fan. The amount of power used depends on the temperature and how often the furnace is running or “cycling”. According to the specs I could find, your 5th wheel most likely has a 35,000 btu furnace which would draw approximately 8 amps. Without anything else drawing power, it would run a moderate flooded lead acid battery to 50% state of charge (SOC) overnight.

Next is the CPAP machine. The power draw depends on the settings for your usage as well as if it has a humidifier. From what I can learn from some of the manufacturers, it could be as low as 3 amps and as high as 6 amps.

Other components that would draw power would be the refrigerator, which looks to be an absorption model in your rig. Therefore, running off propane would only draw about 2 amps. If you left the water heater on, it would cycle and also draw a small amount of 12-volt power. Then you have to factor in lights, roof vents, water pump and other items that would be used occasionally.

As you can see, there is no “perfect science” when trying to calculate power usage. Then add the fact that the inverter can be a weak link reducing power, as well. Plus, I assume you probably did some boondocking or dry camping during the previous day that would add to the depletion of the battery or batteries.

Batteries in the rig

Now let’s take a look at what battery or batteries are in your rig. Since it is a 2009, I assume you do not have the original batteries in the unit. If they are flooded lead acid (FLA), then you should only draw them down to 50% SOC. You most likely went way past this with all those components running.

Another variable here is how many amp hours (Ah) you have available and what condition the batteries are in. If they have not been properly charged with the correct charge profile of a multi-stage charger, they are most likely sulfated and not storing 100%. But if they are absorbed glass mat (AGM), then they most likely are not sulfated but could still be drained lower than the recommended 50% SOC.

If they are lithium (LiFePO4) batteries, they can be drained to approximately 100% depth of discharge (DOD) without damage. However, they require a 14.6-volt charge coming from the truck charging system, which we will look at next.

First thing to check

The first step is to verify that you do have a charge coming through the 7 pin connection to your trailer. Some trucks have been shipped without the fuse or wiring for the 12V Aux feature, which is an easy fix. You can verify this by connecting your trailer and using a multimeter on the house batteries. Fully charged FLA batteries should read 12.6 volts. When you start the truck, you should see an increase in charge voltage at the battery.

Since you have a 5th wheel, I assume you have a larger truck which might have dual batteries. The engine alternator(s) might be working almost 100% charging those with little to go back to the rig. Once you can verify what voltage is going to the rig, we then look at the charging profile for the different types of batteries.

Typically, the charge profile for FLA batteries is 13.8 volts for 8 hours. Therefore, it would take a very long road trip to properly charge the batteries IF they were only discharged to 50% SOC. Lithium batteries are typically 14.6 volts for two hours. If your alternator is not putting out that much, it will take much longer. And if the alternator is putting out more than 14.6 volts, the battery management system (BMS), which is the internal computer, will shut down any charge to avoid damaging the lithium cells.

 You might also enjoy this from Dave 

How safe is it to store lithium batteries in an RV?

Dear Dave, 
My question is about safe storage of lithium batteries, in what I have started to call the Lithium Craze. Many owners are buying and installing multiple lithium batteries inside the coach—under beds is a big one. I just recently saw them under the dinette seats. I see it as a dangerous installation. Does the industry have a stance on this? Is it as dangerous as I think? —Rudy, 2015 Crusader

Read Dave’s answer.

MORE FROM DAVE ON RV BATTERIES

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• Can different type and age RV batteries be mixed? Can I add a lithium battery?

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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