RV Electricity – Just Ask Mike (J.A.M.): Charging batteries with a tender


By Mike Sokol

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) noshockzone.org with the subject line – JAM.

Dear Mike.

I’ve stored our 5th wheel indoors this winter and for insurance purposes I had to remove my two 6-volt Trojan batteries. I’d like to put them on a battery tender for the winter season. The Trojans are rated for 225AH and my charger is a NOCO Genius 2.

Before charging my test showed 5.7 volts of charge on each battery. I’ve had the first battery hooked up and charging for 3 days now and the tender shows it took 1 1/2 days to reach 25% and 3 days to reach 50%.

Am I doing something wrong? Should I continue to leave the battery charging or will I damage it? Also, is it better to store the battery in my heated basement during the winter, or elevated off the floor in my garage? I’m in Ontario, Canada, and temps locally will hit -10 C  (+14 degrees F) during January and February.

Thanks so much for guidance on this. —Terry Smith

Dear Terry,
Okay, here are the basics. First of all, batteries are kind of like a debit card with a deposit limit. So, just like a debit card with a $225 max amount, a 225 Amp-hr battery can supply 225 amperes of current for 1 hour, or 10 amperes of current for 22.5 hrs, or 1 ampere of current for 225 hrs. (more or less, depending on internal battery resistance, etc.). Of course, just like any bank account or debit card, when you take money out, at some point you’ve got to put it back in.

Deposits and withdrawals

Just like dollars on a debit card, those amp-hrs in a battery are pretty much the same no matter if you’re charging or discharging it. That is, you can’t get more amp-hrs out of a battery than you previously put into it. So if you take amp-hours of energy out, then you have to put amp-hrs of energy back in for the future.

Now, this is up to the limit of the fast-charging ability of the battery chemistry, so you probably don’t want to exceed 10% of the amp-hr capacity while charging a flooded-cell lead/acid battery or you’ll warp the plates and boil out the electrolyte. So, probably no more than 20 amps of charging would be allowed for a 225 amp-hr battery. But the basic in/out charge/discharge principles are the same, just like any bank account.

A battery tender is not a fast battery charger

Your NOCO Genius 2 charger/maintainer is rated for a maximum charging capacity of 2 amperes of current. Now, let’s take a look at a SOC (State of Charge) chart. You can see that at 5.9 volts your batteries only have 30% charge left. So you have to put 70% of 225 amp-hrs back in.

A little quick math shows at 0.7 x 225 = 157 amp-hrs of recharging. So if you’re putting 2 amps of current back into your battery, you simply divide 157 by 2, which equals 78 hrs. Let’s round that up to 80 hrs. for general losses, and divide by 24 hours in a day. So 80 / 24 = 3.33 days of charging with a 2-amp charger to bring your 225 amp-hr battery from 30% up to 100%.

Why does it take even longer?

Now, this is also assuming that your batteries are in great shape, and the charger is maintaining a full 2 amps of charging current. But the NOCO battery tender may have determined it needs to limit the charging current to 1 amp or so, in which case it could take 6 or 7 days to charge each battery.

Note that you can certainly hook the batteries up in series and charge both of them at once in the same time you can charge a single 6-volt battery. But that assumes both batteries are exactly matched and have the same State of Charge. Individual charging is still the best method. But when you have them both up to full charge, I would simply connect them in series and use your NOCO battery tender on them for the winter in your garage.

Must… Go… Faster…

For faster charging, if you had something like a Battery Tender Rapid Charger it would automatically charge each battery from 30% to 100% in around 16 to 20 hrs., then go into tender/maintenance mode to keep them safe and happy. Here’s one I wrote about a few weeks ago that looks great.

Don’t boil your batteries

Also, remember that a simple battery charger is a pretty terrible maintainer. So if you connect a basic 2-stage 10-amp charger to your battery and walk away for a week, it will likely destroy your battery in the process. You need a 3-stage charger that has maintenance (or float) mode.

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….

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.
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Michael Butts
7 days ago

I see that the lithium- and AGM-based “power stations” are becoming very popular. Could you write an article on why products like the Bluetti, Delta, GoalZero Yeti, etc. can have AC plugs that don’t have grounding?

10 days ago

Mike, This winter I am using a 3 stage 15 watt Battery Tender solar battery charger to keep my 2 12V house batteries topped off. So far it’s working great and I don’t have to run a 100′ heavy extension cord to my MH which has always been a problem due to heavy snowfall.

I have the solar panel connected directly to the batteries (parallel) but I would like to use the Furrion solar port on the side of the MH. I purchased a 10AWG 2 Pin circular connector to go from the Furrion port to the Battery Tender cable. I also used an SAE reversedpolarity adapter. Very disappointed the setup doesn’t work. Any sugggestions?

Last edited 10 days ago by Cee
10 days ago

I absolutely love my battery maintainers. I have Battery Tender brand, a couple bargain ones from Harbor Freight, and two solar panel units.They all “float” the charge, but stay under one amp. I have used all of them for years with no issues. Ironically, I do not use them on our RV when it is stored. We have limited access to electricity in a facility with no windows, so I go there once or twice or month and start the RV and the generator and let them both run for a while. I had to use jumper cables one time to start the RV so I took it home and cleaned the dirty battery cables on the chassis battery. By the way, thanks a lot GM for the painful side post terminals. When that battery gives up I will replace it with a normal battery with top posts like nature intended.

Don Nedrow
10 days ago
Reply to  Dan

When you change to a top post battery, be sure nothing will short between the terminals. GM used the side as some cars did not have much clearance in the battery area.