Monday, November 28, 2022


RVelectricity – Just Ask Mike (J.A.M.): Winter battery maintenance


Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) with the subject line – JAM. Today I cover winter battery maintenance.


Hi Mike,
I really enjoy your YouTubes and website. Thank you for all your great work on behalf of of everything electrical in our RVs. I live in Florida and this winter, I am planning to keep my RV with 30-amp service connected to 15-amp power to keep the batteries charged and keep the wifi running. I have two 6-volt lead acid golf cart style batteries wired in series. My converter is an old-style three-phase converter/charger (bulk, absorption, float) made by Cheng WFCO.

The run from the RV to the power outlet is about 40 feet. I am using a 25-foot Camco 30-amp extension cord with 10-gauge wiring connected to our rig’s 25-foot shore power cord to make this run. I have put my Southwire Surge Guard Model #34931 on the line and found no faults. Everything is working fine.

I have three questions:

  1. I would like to replace the shore power cord and the 25-foot Camco extension with one 50-foot extension cord. Do I need a 10-gauge cord or can I get by with a smaller gauge extension cord (at lower cost) to make this run?
  2. Do I need to attach a robust surge protector like the Southwire Surge Guard Model #34931? Or can I get by with a lower cost surge protector from Southwire or other reliable provider? The pedestal-based surge protector is the only surge protection I have for the entire rig.
  3. Is there anything else I should consider as I set up this battery charging system? I will, of course, check the fluid levels in the battery every month to make sure that the constant charging doesn’t boil off all the water in the cells.

Thanks. —Stuart

Dear Stuart,
Great questions, all. And I have answers that I think will make you happy and keep your RV batteries healthy.

Answer #1: Extension cord size

Since your charging system only requires 1 or 2 amperes of current at 120 volts, you certainly don’t need a 10-gauge extension cord rated at 30 amps. However, while you could get away with a cheap orange cord from a big box store that is only rated for light duty, I would get a heavy-duty 50-ft. contractor grade cord with 12-gauge conductors rated for 20 amps.

The 20-amp circuit breaker in your house will trip to protect that 12-gauge cord from overheating just in case you do turn on something in your RV that requires a lot more power than a charger. Plus, it’s a lot cheaper and easier to run than a 30-amp extension cord.

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Answer #2: Surge protector types

Since you don’t have anything else on other than a battery charger, I would think that a basic “surge-only” protector from Southwire (or any other quality manufacturer) would be sufficient. Of course, if you’re planning on leaving any of your RV’s electrical system on during storage, I would recommend an EMS/advanced surge protector from a manufacturer like Southwire.

However, you should still check your RV for a hot-skin voltage after plugging in since it’s possible that the house outlet you’re connecting to has lost its ground/bond connection.

Answer #3: Other things to think about

Part A: Since you have FLA (Flooded Lead Acid) batteries, I would consider adding an automatic waterer like the Flow-Rite unit I reviewed a while back.

You only need to pour water in one place monthly, and it’s automatically distributed to all of the cells at just the right level. Makes watering your batteries as easy as watering your begonias.

Part B: I would also recommend that you get a modern float charger such as one made by Battery Tender. I’ve seen a number of older 3-way chargers fail and boil out the batteries in a number of days.

You can likely get by with a very small one that will keep your batteries healthy and perfectly charged. In that case, turn off your charger/converter, turn your battery power switch OFF, then connect the battery float charger directly across the two batteries in series. That’s the safest and best way I know to do this.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there….

Oh, BTWRead this post to learn about some exciting projects I have coming up regarding electric vehicles, presenting seminar(s) at the Quartzsite RV Show in January, and more.

Send your questions to me at my 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 For more info on Mike’s qualifications as an electrical expert, click here.

You don’t want to miss Mike’s webcasts on his YouTube channel.

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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20 days ago

Recently lithium battery fires have been in the news (New York, I believe). I am curious if you have any thoughts on how to safely store a lithium ion battery as used in ebikes and laptop computers. Are the BMS within these batteries reliable?

RV Staff(@rvstaff)
20 days ago
Reply to  Larry

Hi, Larry. Here’s a link to a previous article by Mike on lithium battery winter storage. That may help answer your question. If Mike has more info for you, I’m sure he’ll respond as well. Have a great day. 😀 –Diane

Mike Sokol
20 days ago
Reply to  Larry

Note that the Lithium-Ion batteries in eBikes are a different chemistry than the Lithium-Iron-Phosphate batteries in your RV house batteries. The NYC Fire Marshall reported nearly 200 fires caused by eBike batteries this year. I’m studying this problem and hope to have a storage solution soon.

RV Staff(@rvstaff)
20 days ago
Reply to  Mike Sokol

Thanks for clarifying that, Mike. 😀 –Diane

20 days ago

I use the 1.25 amp Battery Tender when I am not traveling. I have two 12V house batteries in parallel that I connect to. Not sure why I have to turn of the salesman switch and the charge/converter. Anyone have some info on that?

Last edited 20 days ago by Cee
Mike Sokol
20 days ago
Reply to  Cee

When you say Salesman Switch, do you mean the battery disconnect switch? If so, there’s no absolute reason to turn it off except for if the power to your battery tender fails for some reason, your house batteries could be drained to 0% State of Charge in a week or so from parasitic currents, which can damage them if it was undiscovered.

20 days ago
Reply to  Mike Sokol

Yes, the disconnect switch. Thanks for that info Mike, good reason to turn it off. Should I turn off the switch anytime I am connected to shore power or just when I have the Battery Tender connected?

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