Solar Battery Tenders

GLTHFJ60

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Did some searching, haven't found any topics discussing this. Will edit/close this thread if I find a better discussion.

Does anyone have any experience with solar battery tenders on their rigs? I have some pretty expensive batteries in my rig and sometimes it'll sit for a few weeks. This can happen for a myriad of reasons, but if moonshine isn't sitting, my tow rig is, and the question applies to it too.

After a few weeks, the batteries get low, and I'm interested in a solar battery tender to keep them topped up. Are they garbage? Seems like an easy solution, almost too easy, and cheaper than building two dedicated solar systems, one for each of my trucks.

Thinking 2-5w is plenty to keep the batteries maintained.

Amazon examples:

Amazon product
Amazon product
https://smile.amazon.com/Sunway-Solar-Maintainer-Motorcycle-Snowmobile/dp/B06WP95W51

Teach me!
 

george_tlc

 
 
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The main thing with a tender (versus a charger) is that you want it to maintain the battery and for a flooded battery that means maintaining around 13.4V at nominal 25C. This is the 'float' charge voltage that will extend the life (versus boil/damage) of the battery long term. A decent unit should allow you to a) choose the float voltage based on battery chemistry and b) deal with temperature compensation of the float voltage. I doubt most of the cheap little solar units will do any of that.

cheers,
george.
 

e9999

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my experience with maintainers like the one in the link above is that they were dumb little things just outputting at whatever voltage comes out of the cells without regulation. I imagine basically relying on their small power output to not overcharge the batteries. But this can't be too good for expensive batteries that may want specific voltage settings.
Why not use a good solar charger that will give you full control over the float voltage?

added: George beat me to it ^ . As usual... :)
 

e9999

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true, it'll be more expensive. But not that much for a cheapo controller and and little panel. $20 for a 10 W panel and $20 for a PWM controller. That you can use for other things. Or if you have plans to use panels for your vehicles later on anyway, start with a good controller?
 

e9999

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I got curious and went to put out my tender (looks like the amazon link above) on my 80 starter battery to refresh my memory about that thing.

It is rated at 1.8 W. Probably around 10yrs old. Amorphous. About 12"x3" cell. Gave out 19-21V open circuit under bright sunshine today. I don't think there is any serious circuitry in there. Likely just a direct connection to the cell with a diode.
The battery was at 12.46V rested when I connected the tender. With the tender on, it was about 12.53V after about 1/2 hr. Current to battery was about 0.07A or so, so about 0.9W, half of nominal. (Depends on sunshine of course.)
I'll leave it on today to see what the battery voltage will be after 5 hrs or so. I don't expect too much at 1W. It's a 88Ah battery...
But of course, the more interesting question is what will happen voltage wise when the battery is fully charged, assuming it ever gets there? If I have time later, I'll charge the battery fully with a smart charger and then put the lil' tender on to see.
I see that the float current on my BatteryMinderPlus is specced at 5 to 200mA at 13.4V. I think it's usually around 100mA. So the output of the tender might be fairly close to what the charger would do for float.

Btw, GL, the above is for a FLA battery. If yours are a different technology, then I would be even more cautious about the tenders.

added: after 2.5 hrs of good sunshine, the starter battery had gone up to 12.55V under charging, so barely budged. Of course, it's a fairly large battery that is not yet fully charged so not surprising. But that is a reminder that those tenders aren't very good for actually charging a less than fully charged battery. (Of course, that's not what they are intended for.)

So to see things better, I took a smaller 20Ah LA battery that was just off a charger in float mode at around 13.4V. So, fully charged. I put the 1.8W tender on it, and within 1 minute it had brought the voltage to about 14.3 and still going up. I stopped it there. But that is concerning. It suggests that the voltage may well go above 14V and stay there when the sun is good. So you could be overcharging the battery possibly for a few hours a day. Which is of course what a better charger avoids by going to float mode. And I have vague recollections of even seeing 15V on a battery with that tender...

(And then, there may be a loss due to the presence of the tender when it's not charging. In my case, I measured a loss of 15mA when I blocked the tender cell from light. So, presumably, it would lose battery charge during the night at that rate. So I may be getting something like 70mA peak during some of the day and then losing 15mA during the night and more... Not helping, but admittedly, not a critical issue, unlike a possible overvoltage.)

So, I would not use that kind of tender. And I don't. I use a smartish charger off the mains for my parked vehicles. Easy to put an extension cord out, and the charger will do fine by the battery, floating it nicely. Easy to move around too. I think that's a better solution than the dumb solar tender if you have access to the mains anywhere close. YMMV.
 
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89GASHOG

 
 
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I've used a cheap O'Reilly's solar tender for many years on a rig that sits for a few months on end in Hawaii. Seems to work just fine as it always starts up. Recently replaced battery after 6 years of service.
 
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Here is my setup. Similar idea, but I went a more permanent route.

Back story, I have to park my Cruiser on the street (with lift and RTT, doesn't fit in the garage and don't have a driveway). Not my daily driver, so can be a few weeks at a time where I don't drive it. Have a dual battery setup and when I could park in the garage I would connect it to a CTeck.

Parking on the street, I wanted a setup that would keep the batteries maintained. Went the route of a flexible solar panel and a BatteryMINDer (it is a combo solar charge, maintainer, desulfator). Panel is a 65W Lensun brand off Amazon.

This is just to maintain the battery while it is parked.

When we camp, I use a 160W folding solar panel (with its own controller) to keep the batteries charged up.





 

e9999

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^ looks very well done. And you can put on a bumper sticker saying "Solar-powered" or something like that to flummox the ecowarriors...
Although not the most portable solution for the OP I guess.

On a follow-up note, I left my little 1.8W tender on for a couple of days on my 88Ah FLA battery. The rested voltage went from 12.46V to 12.43V FWIW! :hmm:
 

GLTHFJ60

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I've used a cheap O'Reilly's solar tender for many years on a rig that sits for a few months on end in Hawaii. Seems to work just fine as it always starts up. Recently replaced battery after 6 years of service.
Can you give some more details on this tender? @e9999 is convinced that type of charger will hurt your battery, but that doesn't seem to be your experience.
 

e9999

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^ No, I did not say that I am *convinced* that it will hurt the battery. I think there is a possibility some dumb ones could, and I suggest caution and a test before leaving the thing on for a long time, that's all. I would not use one given my situation, because I think there are better approaches for me. YMMV.
For $10 or $20 try it and see. For $40 or 50 you'll have something way better.
 

e9999

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well, if your battery lasted 6 years while mostly sitting, that's great. Does not get much better than that for a FLA. The combination battery / tender must be just right.
 

e9999

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follow up: I have done a test of sorts with my little 1.8W tender . I charged my starter battery fully with a batteryminderplus and left it on float for a while. So presumably it does not get more charged than that. I then put the tender on it and left it connected for a bit in a suitable orientation. I checked 2 or 3 days in a row and the voltage seen did not exceed 12.8V. This is October sun in SoCal. So it looks like, unlike with the smaller 20Ah battery, this 88Ah one does not quickly reach 14+V. So there may be hope for the little tender yet. Will keep an eye on it and see.
 
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