Wednesday, September 28, 2022

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RVelectricity – Just Ask Mike (J.A.M.): Can a solar panel overcharge your battery?

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. Today I discuss whether or not a solar panel can overcharge a battery.


 


Dear Mike,
I saw a post on “Will solar panels overcharge my batteries?” I have a slightly different scenario, but I understand now why so many RVs have these small solar panel battery chargers. I read from another RVer that he traced his loss of newer chassis batteries to a continuous overcharge by the OEM solar panel wired directly to his chassis battery bank. When I checked mine, sure enough, there is a red fused wire labeled “solar panel” connected to a positive battery post on my chassis battery bank.

When I checked the voltage from the solar panel side of the fuse (with fuse removed) the multimeter read over 17 volts. (Lots of sunny days here in Florida.) I don’t know if the OEM thought the voltage would be lower due to clouds or just bought the smallest solar panel to avoid installing a controller to limit the charge voltage. I removed the fuse for now, but is there a way to add a controller or other device to get the voltage where it should be? Thanks, and regards. —Dale Justice

Dear Dale,

Oh, yes. A solar panel can absolutely overcharge a lead-acid battery. Even a humble 2-amp trickle charger can overcharge a FLA (Flooded Lead Acid) or AGM (Absorbent Glass Mat) battery if connected for more than a few days. If left connected for a week or more, battery damage is a real possibility. And a burst battery with leaking acid is not something you want inside of your RV.

But, overcharging isn’t the issue with lithium batteries, since they have their own built-in BMS (Battery Management System) that automatically regulates the lithium cell charging. However, if the solar panel voltage rises above 16 volts (which it easily can), even a small solar panel can wreck the BMS with over-voltage, bricking (damaging beyond repair) your expensive lithium battery.


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Get an MPPT Solar Charge Controller

What you need to do is install an MPPT charge controller. MPPT is short for Maximum Power Point Tracker. Yes, I know that’s a lot of acronyms, but I don’t make this stuff up, I only report it…

Think of an MPPT as a smart DC to DC charger that can convert the varying voltage of solar panels as the sun changes, into the correct charging voltage and current for your particular type of battery.

Can it fully charge a lithium battery?

Yes, this is potentially a cheap fix if you have an older converter/charger that doesn’t have a lithium setting. With the AGM mode, these converters will typically only get your lithium battery up to 85% SoC (State of Charge), but a few hundred watts of solar panels with the proper MPPT controller will easily top off the final 15% of charge on your lithium battery. Pretty neat trick, eh? 

Never charge a battery without a charge controller

I have a vintage Sears charger that’s good for 6 amps, which does a great job of charging a dead battery in a vehicle if you left the lights on. But I would never trust it for more than a few hours of charging. That can cause serious damage.

And that’s why modern RV chargers are 3-stage with some sort of float or battery-tender mode. That’s really the only way to keep your lead-acid batteries happy. And, yes, lithium batteries pretty much take care of themselves—as long as you send them the correct voltage.

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….

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.

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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Billh42
1 month ago

I have a serious problem with an RV manufacturer that designs a solar charging system that will intentionally overcharge a battery and cause either early battery failure or damage the RV. Am I missing something here?

Mike Sokol
1 month ago
Reply to  Billh42

Solar installations are a little more complicated than simply slapping on some panels and running wires. To begin with, there needs to be overall engineering design to make sure all the various components play nice together.

Richard Hughes
1 month ago

My Norcold Fridge recently started giving the red flashing light of death. One scenario said, my OEM solar panel had a boosted the voltage and caused an overload to the fridge. Not sure if that is right,, but no repair person listed on the Norcold site would touch it. I had to buy a dorm room fridge and head for home where a local company will do the work. So much for the summer vacation.

Mike Sokol
1 month ago
Reply to  Richard Hughes

Solar panels make a pretty high voltage under full sunlight, so it’s dangerous to connect them directly to your battery without a charge controller. There’s a lot of solar installs that aren’t done properly. Guess I need to write a new series of articles.

Bob Weinfurt
1 month ago

I regularly use my vintage 6 amp charger on 12 volt car, lawnmower, and my RVs lead acid batteries. I frequently check the current and voltage. When the voltage reaches 15 and the amperage has dropped down to 1 or 2, I deem the battery as fully charged and remove it from the charger. I’ve been doing it this way for many years and my batteries have always performed well and lasted longer than the amount of time they were advertised to last. I rarely have to add water to them.
Sure, it takes some effort on my part but the batteries don’t get heated and overcharged.

Mike Sokol
1 month ago
Reply to  Bob Weinfurt

As long as you monitor the battery voltage that works just fine. But I have reports from RV owners who used a two-stage charger on their house batteries over the winter, and in the spring found they had boiled the acid out of the batteries, destroying the floor of the battery compartment.

Bob Weinfurt
1 month ago
Reply to  Mike Sokol

I charge the batteries once a month or so over the winter.
My motorhome is 44 years old. I like the simplicity of the older units.

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