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Trickle charger on a deep cycle trailer battery question

I’ve removed my deep cycle trailer battery for winter storage in the garage. I’m using a Nextool Battery Charger Maintainer 261-9678 with an output of 12V DC 0.8A to maintain it. The charger has a flashing green light to indicate a greater than 80% charge and a solid green light to indicate full charge. I’ve had the battery charging for several days and the charger never goes to a solid green light.
The instructions say…
Green light flashing: "…Whenever possible, leave the battery on charger until the green light is solid."
Green light solid: "…can also stay connected to maintain the battery for an indefinite period of time."
Does anyone know if leaving the charger connected for several months while it’s flashing green could cause a problem?

Why not have the battery checked to see if it might be close to replacement time. If it is just don’t charge it and use for exchange when time to reinstall.

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If it was my battery, I wouldn’t treat a deep cycle battery like I would a car or motorcycle battery. I’d treat it like a deep cycle golf cart battery by connecting it to a golf cart battery charger connected to a timer that changes it every week.

Have you considered asking a knowledgeable and trustworthy golf cart mechanic for his opinion?

Based on my own experiences with batteries like these, your battery will no longer take a full charge. It won’t get the fully charged voltage up high enough to trip the trickle charger into “fully charged mode”. Take off the charger and check the voltage about an hour later. If its down around the 12.2 volts or so, the battery has sulfated to the point it can’t be fully charged so the maintainer will never show its finished. The battery still may work but it won’t provide power for very long, not nearly as long as it once did.

If the battery was considerably less then full it might take several days to charge it at 0.8A.
How big is this battery, that is, how many amp-hours?
A basic digital volt meter can be had for ~$20, a very useful tool for working with batteries.

This might be an occasion to do Google searching about deep cycle batteries.

Thanks circuitsmith,

The battery sticker shows:
Marine crank @80 F: 795
Cold Crank Amps @ 0 F: 530
Reserve capacity: 140

I did have the battery checked last month by my trailer service center and they said it was OK.

This battery is used on a travel trailer to provide 12 v current for interior lights. It’s the battery that came from the manufacturer of the trailer and is two years old. It is constantly charged when the trailer is towed by my Highlander, or in a campground when the trailer is hooked up to 120 V.

My chargers have a slide switch on them to select deep cycle vs std starting battery. If I select the wrong type, it doesn’t do well in assessing the % of charge.

I would do the test Mustangman suggested and if the battery is OK, then just remind yourself to plug it in once a month for a day and forget about whether or not the charger can figure it out.


I used to maintain my mother’s camper trailer and motor home for her, and I’ve never found or bought a deep cycle battery for use as a house battery on an RV. Did you buy this RV new or used?

The only way I’d consider spending extra to buy a deep cycle battery for this application is if I planned to frequently or primarily boondock off the grid with it. If you primarily use your RV at campgrounds where it is plugged in or with a generator, any old car battery of similar size will do, and you’ll save money on house battery replacement.

Maybe, rather than consulting with a golf cart mechanic, you should consult with an RV specialist.

When I took over maintenance of an RV-size bus at work, I found someone had put deep cycle batteries in it because of how often it sits unused. When the batteries wore out, I replaced them with regular car batteries and installed a solar trickle charger. It’s worked out to be the better solution.

That device can work wonders. A little one can keep an RV battery always fully charged but not overcharged while in storage.

This is a deep cycle battery. My understanding is that that type of battery can handle being totally discharged with no damage, unlike ordinary auto batteries.

So let it discharge, and charge it when you need it.

edit: thanks for the corrections…

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I have a battery maintainer for the deep cycle battery on my boat. I have a battery maintainer. There is no electricity over the winter. The battery is at least 8 years old. It usually takes about 12 hours in the spring to bring the battery up to full charge. If your maintainer is a lower watt output I would expect it could take longer.

We go up for a couple weeks at a time, and I connect the maintainer after pulling the boat, though it performs flawlessly it still takes 12 hours to go from yellw, charge to green full charge. The biggest issue is to be sure it is a maintainer, not a constant trickle charge as that can eventually kill the battery by overcharging it.

I cannot find a manual for the item as listed, If after a week it still shows as charge I would be doubtful it is actually working as a maintainer. In that case I would either buy a dedicated maintainer or check voltage and charge as needed.

Both types can be fully discharged (within their acceptable range) and then recharged without any adverse effects. The difference is in the amount of discharge they can tolerate.

Deep cycle lead-acid batteries only differ in terms of their construction using solid plates instead of a sponge like lead plate in the standard starting battery. Deep cycle batteries can be safely discharged to around 80% repeatedly without any ill effects versus a starting battery that ideally should not be discharged to less than 95% or it will shorten it’s potential service life.

No lead acid battery should ever be fully discharged and intentionally left in that state. It will lead to sulfation and very early failure.

Lead acid batteries are best kept fully charged (float charge) and then periodically (once a month) brought 10% higher to equalize the electrolyte (equalizing charge) to maximize lifespan.

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Not totally discharged… that kills deep cycle batteries. Nothing below 20% state of charge or about 11.7 volts.

***posted before I got to @TwinTurbo 's post ******

Here’s a graph of cycle life vs depth of discharge for a typical deep cycle battery:

Some trickle chargers don’t have what it takes to put enough charge into a battery for it to indicate the state of charge. I wouldn’t let that bother me.

You seem to know a lot about deep cycle batteries. Maybe you can explain why I can’t use a battery tender on my golf cart, and can only use the charger that came with the cart?

You’d have to explain why you can’t use a tender. Is it because they state in the instructions not to do so or because you had some bad results??

Tenders vary greatly across the pricing spectrum. Cheaper units may not be as effective at meeting all of the requirements for maximizing life. Probably work reasonably well however.

There are 4 stages of charging for lead acid batteries; bulk, absorption, float and equalizing. The requirements of each stage can be very specific to the battery type/construction. Of course, a general purpose charger will suffice for average lifespan. To maximize life, it’s important for a tender to be able to perform the last two stages properly for the battery in question. The manufacturer will always default to “use of anything but the OEM charger may void the warranty”. The supplied/recommended charger should be tailored to their battery and therefore perform within their specifications for best service life.

It’s something I was told by a golf cart mechanic, that each electric golf cart comes with its own charger, and only that charger can be used with those batteries. He didn’t explain why this is necessary, only that golf cart batteries are different from other batteries, which is why I advise most people who have this question to speak to a golf cart mechanic. I was hoping to have a more technical explanation than “this is what I heard from a golf cart mechanic” for future use.

OK, that helps. Actually, the built in chargers are usually tenders as well and have that mode designed to be used with battery packs. Most golf carts have multiple batteries to get the higher voltage they need to run motors efficiently and have enough capacity. That being said, there are a number of manufacturers that make charger/tenders. Maybe yours is one with an onboard computer and it simply will not play nice with anything but the OEM charger?