Monday, January 30, 2023


Ask Dave: RV is plugged into 30 amp in a heated garage but engine batteries still die. Why?

Dear Dave,
My RV is plugged into a 30 amp that is located in a heated garage. Today and earlier this year the starter batteries were dead. I left the bay lights on for about 24 hours. I boosted the batteries with the house batteries, started the engine, and then shut the engine down. Then I connected a battery minder to the two starter batteries. After a couple of hours, the red light went out on the minder or tender. I was surprised that it went out so soon. —Denny, 2011 Monaco Camelot 43DFT

Dear Denny,
When plugged into shoreline power, your house batteries will be charged by the converter or inverter. Since your rig is a larger diesel pusher, I would assume you have four 6-volt batteries for the house system and a larger 2000-watt inverter/charger that charges the batteries. The “bay lights” that you left on for 24 hours run off the house batteries and would not drain the engine or start batteries and would be charged by the converter when plugged in.

I would also assume you have two larger 12-volt batteries for the engine that are rated in Cold Cranking Amps (CCA).

BIM system

Since your engine batteries are not being charged when you are plugged into shoreline power, I would assume you do not have the newer style battery isolation management (BIM) system. Your solenoid is the older type that isolates the engine and house batteries. It provides a charge from the engine alternator to the house batteries while driving. It can provide a jump start from the house batteries to the engine batteries with a switch that is on the dash. This is the momentary (MOM) switch pictured here.

The newer BIM switches now allow a charge from the inverter/charger to the house batteries and to the engine batteries when connected to shoreline power. Here is an example of one from Precision Circuits Inc.

According to the Precision Circuits Inc data sheet, the BIM performs the following functions:

The BIM monitors the battery voltage of both the chassis and coach batteries over long periods of time. If it senses a charging voltage, it connects the two batteries together. If the charging system is drastically overburdened, the batteries will be isolated, however, if the BIM sees a long-term charging of both batteries it will allow the batteries to remain connected and allow the charging system to do its job. Once the batteries have charged for one hour, the BIM will isolate the batteries to prevent overcharging, and will only reconnect the batteries for charging if one of the batteries drops to approximately 80% charge, and the other is being charged. This long-term monitoring of the batteries prevents the annoying relay clicking that exists in simpler isolation modules today. The BIM does not guarantee 100% battery charge, but prevents harmful battery charge levels.

I would recommend replacing your current battery solenoid with the newer style BIM from Precision Circuits Inc.

What drains a chassis battery?

When you have the unit in a storage mode, there still will be a draw on the engine battery from a variety of functions such as the electronic control module (ECM), which is the computer that monitors several functions from driving characteristics that set shift points for your transmission to oxygen levels in the exhaust. And, since it is a 2011 and is a Monaco, you most likely have the exhaust gas recirculation (EGR) emissions system instead of using DEF.

In 2010, new emission certifications required much more stringent diesel exhaust emissions. Most manufacturers went with the selective catalytic reduction (SCR) method of spraying diesel exhaust fluid (DEF) on the exhaust between the NOx sensors. Monaco elected to recirculate the exhaust back through the system to basically burn it again to meet emission standards. It did not pass testing for the two-year period and was scrapped. However, all the sensors and monitors do require 12-volt power to maintain the codes and settings, which will drain your engine battery. Even the preset stations on your radio need a power source to keep them in your “favorites” list at a touch of a button.

Back to your question

So, back to your question. Leaving the bay lights on for 24 hours would not have drained your engine batteries since you have the older battery isolator and were able to “jump” them with what I believe was a spring-loaded push button switch.

A couple of questions for you: How long did you have the rig in storage before you noticed the engine batteries were dead? Was it the 24 hours you discussed in leaving the bay lights on? If so, the engine batteries should not have gone dead that fast unless they are getting weak. As stated earlier, leaving the bay lights on would not have drained the engine batteries. Next question: How long did you run the engine after jumping the batteries with the house batteries? The engine alternator will provide 14 volts +/- to the engine batteries while running. From your description, it seems it might be just a short time.


I am a big fan of the BatteryMINDer® since researching the product more than ten years ago and having used it on over 25 vehicles personally. I have also had great comments from owners that have been using it after attending my seminars. This is a product that sends high-impact waves into the lead acid battery to break up sulfation and condition the battery. I would first ask that your charger is actually a BatteryMINDer and not Battery Tender®. The Battery Tender is a 2-amp maintenance charger and not the same thing.

If it is a BatteryMINDer and shutting off after two hours, it could be the battery was just below the CCA charge it needed to start your engine, which is fully charged quickly. Or your battery cannot be charged fully due to weak cells and it shuts off. You need to get a multimeter to check for voltage and maybe contact BatteryMINDer to verify what you are seeing at the battery and monitor. I don’t typically rely on just the lights.

 You might also enjoy this from Dave 

A few questions about storing an RV at a campground through the winter

Dear Dave,
My RV will be left in a New Hampshire campground for the winter. How about installing a solar trickle charger to keep the battery charged and using an ultrasonic pest repeller to keep mice and insects away, plus roof edge heat tapes to melt the snow? —James, Keystone Montana 3950BR

Read Dave’s answer

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

Read more from Dave here


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3 months ago

If the coach is outside, a small solar battery charger will keep either battery charged. You choose which one.

Bob p
3 months ago

I used a cheap battery tender bought at Harbor Freight for $5.98 that put out 1.5 amps. Connected from inside the coach under the doghouse to the chassis battery, anytime the house was plugged in or generator was running the chassis battery was receiving a small current to overcome parasitic drains. Before using this my battery would get low during the time it was stored for more than a month and wouldn’t start the engine. I know it wasn’t as good as the mentioned products but it was much less expensive.

3 months ago
Reply to  Bob p

To tag onto Bob’s post- There has been a product on the market for a long time called Trikle Start.- Made by LSL and for years was standard equipment on Winnebago coaches. I’ve had one in my Winnie for 12 years and it’s always kept my starting battery topped off. It works exactly as Bob’s product but costs about 50 bucks now. -Nothing to plug in so it requires no monitoring.

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