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RVelectricity: Charging batteries in parallel

Dear Mike,
I really like your articles, but now have questions of my own. I want to hook up two 12v lead acid batteries in parallel to double my amp hours. Will my tow vehicle still charge these properly? Will the trailer charger still charge these properly when plugged into shore power? Can I hook up a single remote battery charger to charge these? Thank you. —Robert

Dear Robert,
The answer is yes, yes and yes….

But first, let’s review series and parallel battery connections. There are at least three types of battery connections you can install in an RV (four, if you count just one battery): series, parallel and series/parallel.

First up – Series connections

The only time you’ll put batteries in series for a 12-volt DC system in an RV is when you’re using 6-volt batteries. This is exactly like stacking AA or AAA batteries in your flashlight or other battery operated appliance.

As you can see, in the case of 6-volt batteries placed in series, 6 volts plus 6 volts adds up to 12 volts between the chassis and the top battery terminal. Since your tow vehicle’s 12-volt DC charging system also connects to this top terminal, then both batteries will receive charging current. The same thing goes for the converter/charger in your RV, or even a separate charger with a battery tender function you might use without connecting your RV’s shore power cord to anything.

What about parallel connections?

What you have is a pair of 12-volt batteries connected in parallel. In this case the voltages are not additive, but the amp-hr ratings are. Note that for any of these multiple battery banks, all batteries need to be exactly the same. So there’s no mixing and matching allowed.

And just like the series example, both batteries will be charged by simply connecting the negative of the charger to the negative chassis connection (in this case, either of the 12-volt batteries) and either of the battery’s positive terminals. Be advised that if you want to connect your own charger it will need to be closer to the batteries than any disconnect switch. And, yes, your converter/charger or tow vehicle should charge both batteries at the same time, as long as you don’t have your battery disconnect switch in the OFF position.


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What’s this series/parallel thing?

This is when you have four 6-volt batteries connected together to create a battery bank. First, the 6-volt batteries are connected in series to double the voltage and create a pair of 12-volt battery banks. Then those two 12-volt banks are connected in parallel to double the amp-hr capacity.

As above, you only need to connect a charger between one of the positive top conductors between the batteries, and either of the negative bottom conductors. And your tow vehicle will also be able to charge all four batteries at once (albeit, rather slowly), as well as your converter/charger. And depending on how your disconnect is wired, you’ll probably need to have it turned to ON in order for your tow vehicle or converter to charge them. Just remember that all batteries should be matched by brand, chemistry, size and age for best results.

Let’s play safe out there….

Send your questions to me at my new RVelectricity forum here.

Mike Sokol is an electrical and professional sound expert with 50+ 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.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

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Jerry
1 month ago

The correct answer to the questions is maybe.

There is not enough information to give an accurate answer.

Battery chemistry involved?
Size of the vehicle alternator?
Age of original battery?
Type of vehicle, a Subaru will not have the extra alternator capacity as a pick up?
Capacity of your RV power converter?

It just isn’t as simple as the author makes it sound

Keira B
1 month ago
Reply to  Jerry

Putting two 12 volt batteries in parallel can be a bit of a problem if the batteries are different. One older, and one younger battery will charge differently, and the charger will think that the batteries are charged when actually only one is fully charged. Most battery chargers these days are “smart” and get fooled when you charge two unequal batteries.

Mike Sokol
1 month ago
Reply to  Keira B

That’s my last sentence…
“ Just remember that all batteries should be matched by brand, chemistry, size and age for best results.”

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