RV Batteries & Alternators: The Easiest Way To Check Your RV Charging System Is To Use A Multimeter

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The worst sound you can hear when traveling is the click, click, click of the starter not turning over.

That sound indicates that your RV battery is dead, and you’re not going anywhere.


The time to think about this is before you leave town — not when you’re stranded along the side of the road.

Fortunately, a little preventative maintenance can help you avoid breakdowns when you least expect them.

Here’s what you need to do…


One Tool Every RVer Should Own

So often I hear people complaining that they purchased a new battery, yet they got stranded a couple days later.

Worse yet, some RVers hastily replace the alternator, not realizing that the problem lies within an aging battery that can no longer accept a decent charge.

With a one-time investment of less than $20, you can carry the most helpful diagnostic tool that every RV owner should have: a multimeter.


A multimeter can quickly tell you the exact status of your electrical system — without any disassembly or extensive mechanical abilities required on your part.

Rest assured, a multimeter is no one trick pony! It will be equally helpful in checking 110-volt household circuits. It can also tell you why the new taillight bulb you installed didn’t work.

For this article I will only be covering how to diagnose and fix your RV’s 12-volt charging system.

Must read: What’s The Difference Between A Voltmeter And A Multimeter?


Using A Multimeter To Check Your RV Battery & Alternator

I like a digital multimeter because it tells you exactly what your voltage is — leaving no room for error from misreading a dial.

Here’s how to use a multimeter to check the RV battery and alternator:

  1. Hook the red lead to the positive battery terminal and the black lead to the negative battery terminal. Turn the meter on, and set it to the scale that reads + 12 volts DC.
  2. With the engine off and nothing turned on (keys out of the ignition), you should get a reading of + or – 12.xx volts. If the battery is fully charged, it will be a little over 12 volts. And if it’s run down, you might see 11 volts or less.
  3. Start the engine, and with the engine running read the meter again. The meter should now read 13.8 to 14.5 volts. The point is now you should see an increase in voltage — indicating the alternator is doing its job and it is indeed charging the battery. If the voltage didn’t go up, then the alternator has failed. It’s as simple as that.


Possible Outcomes After Using The Multimeter

If your battery voltage was low without the engine running, then charge the battery either by running the engine or using a battery charger. Once charged, if the battery voltage drops dramatically overnight, your battery has reached the point it will no longer hold a proper charge and it should be replaced.

Some motorhomes have a solar panel on the roof designed to maintain the battery’s charge during periods of non-use. Most provide what’s called a trickle charge — just enough to keep a healthy battery at its peak.


To verify that the trickle charger is operating properly, all you need to do is:

  1. With the motor not running, cover the panel with something like a piece of plywood to prevent the sun from shining on it.
  2. Check your battery voltage. It should read about 12.30 volts.
  3. Now, expose the solar panel to the sun. Your voltage should go up. It will only go up slightly, because this is just a trickle charge. Mine only jumped up to 12.41 volts — enough to ensure that nothing was draining the battery and that a slight charge was being applied.


Routine Maintenance Tips For Your RV Batteries

Preventative maintenance on RV batteries is the best way to ensure that they last and are fully charged and ready when you need them.

The most common failure is as simple as dirty cable connections at the battery. They may look clean from the outside, but a gray film can build up inside the cable clamps or between the terminal connections. At least once a year, all connections should be removed, cleaned, and re-installed. The battery terminals should be cleaned to bright shiny metal in order to ensure good contact.

Sandpaper is the best (and cheapest) tool for cleaning battery terminals.  If there is a buildup of green acid corrosion, it can be neutralized with a paste of baking soda and water. Rinse thoroughly after the bubbling stops.

Many of today’s RV batteries are maintenance free — meaning you don’t have the option of adding water as needed.

However, if your RV battery has removable caps, then you need to monitor the water level. If the plates become exposed, the life of the battery will be dramatically shortened. If you do need to add water to your bank of batteries, only use distilled water. Tap water may have minerals that could clog and shorten a battery’s life.


One final tip: if one battery fails that is part of a group — such as 4 batteries that make up the house battery group for your RV — you should replace all 4 batteries at the same time. By installing only one new battery next to the 3 remaining so-so batteries, the new one will be overcharged and it too will fail prematurely.

This video provides some good information about troubleshooting the complete electrical system in RVs:


More Great Advice Regarding RV Batteries


I’ve been involved in RVing for over 50 years — including camping, building, repairing, and even selling RVs and motorhomes. I’ve owned, used, and repaired almost every class and style of RV ever made. I do all of my own repair work. My other interests include cooking, living with an aging dog, and dealing with diabetic issues. If you can combine a grease monkey with a computer geek, throw in a touch of information nut and organization freak, combined with a little bit of storyteller... you’ve got a good idea of who I am. To date, I've shared my RV knowledge in over 300 articles here at The Fun Times Guide! Many of them have over 25K shares.

8 thoughts on “RV Batteries & Alternators: The Easiest Way To Check Your RV Charging System Is To Use A Multimeter

  1. how about a more indepth view of the charging system?…like where the delay relay is located?..the charging voltage ranges and reasons they voltage might not be high enough…saying what happens is ok..but showing it with pictures and diagrams would be a lot better…

    1. I have no idea what a delay relay is. If there is such a thing it would be different for every vehicle. Proper charging voltage is 13.8 volts. The only connection you need to diagnose the charging system is a volt meter to the batteries. No diagram needed.

  2. sorry…it was mentioned in the mcbride webinar on this page..I figured you had seen it…he called it a delay relay…or battery isolator…..basically I think it separates the chassis battery from the house battery….unless your charging them or in some cases if you want to bridge them together for more cranking power…your name as the bottom must have confused me..I thought you had contributed to the page and information there in…sorry…..a voltmeter is good..but a schematic is better….in my humble opinion….

  3. Our 2005 Beaver patriot thunder over head lights dim then fail, the only electricity we have when this happens is from the shore power. we are currently hooked up to a 30 amp friends place. during our winter trip started 12/2016 we stopped and had our batteries serviced. we went to MS and now its March ended up staying in Tehachapi Ca again on our way home. My husband checked the batteries again the water was almost empty and corrosion was heavy. My husband cleaned the batteries himself, when he hooked them all up he couldn’t start the coach. after 500 paying a mobile Cat mechanic just to find out he had hooked the cables back up wrong. now we can start the coach, but the power to coach fails after turning on over head lights in coach. there is a loud bang when the power goes off. stuck again in Tehachapi Ca what do you think

    1. I don’t have good news for you. You have a big coach that uses a lot of electricity. It has a 50 amp power cord for a reason, it draws enough current to require that size power cord. Using a reducer to 30 amps for an extended period may cause all kinds of problems the least of which may be insufficient or incorrect charging of the house batteries. Lead acid batteries in the southwest go through an amazing amount of water. I have to top off mine every 30 days at the very least. Once you allow the water level to drop below the top of the plates the life of the battery has been reduced drastically. I suspect your coach batteries are no longer taking or holding a charge sufficient enough to be useful. Hooking them up incorrectly probably finished them off. You need to have your batteries load tested by someone who knows what they are doing. If one comes up bad you should replace all of them because of your history of not maintaining them properly. If you install one new battery in a group of 3 other so-so batteries the new one will degrade to match the already failing remaining three. The loud bang is likely the automatic transfer switch dropping out for lack of sufficient power to it.

  4. Unless you like to fry your meter or blow its fuse. Turn on your meter and set your meter to the proper scale first before attaching your leads.

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