Panel with built-in electronics for charging car battery?

[hyperknot]

Hi, for a very minimalistic setup, I've ordered the following panel off Amazon:

https://www.amazon.com/TP-solar-Portable-Foldable-Charger-Generator/dp/B07SNP8XZ1

I'd like to use it for camping and for maintaining my car's AGM starter battery when not using the car for weeks. My car has some parasitic loads and if I don't use it for 4 weeks it goes under 12.0V.

The reason I choose this panel was that it has a built-in regulated 14.4V output, allowing me to directly charge the car's battery, without any electronics involved. At least this is what the description says.

My question is: do you know if it's safe to keep this one plugged in for weeks inside the car? Or should I buy an external solar charge controller for this purpose? My car's battery goes down about 0.02 V per day, so I don't care about efficiency, I'm just looking for something which can safely maintain it.

 

[HRTKD]

Deploying the panels inside the car will reduce the output of the panels.

I had a couple of AGM batteries in my side-by-side. I used a BatteryTender solar panel/controller to keep the battery charged while it sat in storage for months. Turns out that the charge profile wasn't quite what an AGM battery wants and the batteries were not happy. I had to recondition the batteries to get to the point where I could use them. They're OK now but I don't use that solar panel anymore.

Otherwise, it will work. I switched to a programmable solar charge controller (that understands AGM batteries) and larger solar panels. The larger solar panels probably won't work in your case.

 

[Bob B]

I bought this cheap little thing and it actually works pretty good as a maintainer.

https://www.harborfreight.com/home/electrical/solar/15-watt-solar-battery-maintainer-64251.html

 

[hyperknot]

I'm not worried about the performance of the panel at all, I mean it's a 60W panel and I'd need about 1W of power 4 hours per day to maintain the battery level. I'm more worried about what happens to the battery if I leave it connected for a few weeks.

@HRTKD, what do you mean by the battery not being happy and reconditioning an AGM battery? How do I do that?
Also, how do those super small trickle charge panels work? Are they just over-charging the battery but with such a small power that the battery doesn't care? I guess it all gets turned into heat, right?

 

[hyperknot]

For example the reviews for this trickle charger say the output is 18V @ 0.5 amps:

https://www.amazon.de/-/en/ECO-WORTHY-Portable-12-solar-panel/dp/B08K3BQTNK

So what happens when this is connected long term to an already full battery? It's just converted to heat without damaging the battery?

 

[hyperknot]

My TopSolar SolarFairy panel arrived. The first thing from the user manual I noticed is that even the 19V output is PWM controlled, there is no output for just getting the panels's open circuit voltage directly. They state something like 21V for open circuit voltage.

The good news is that it does have some mechanism for disconnecting at night, it just starts charging whenever I guess the voltage gets over 14.4 V, right? I mean these simple circuits don't boost voltage probably right?
I imagine the following:
Cloudy day - only USB port works
Bit better day - DC 14.4 V port works but DC 19V doesn't yet
Sunny day - DC 19V works as well.

I'll need to measure this with a multimeter to make sure though.

 

[HRTKD]

@HRTKD, what do you mean by the battery not being happy and reconditioning an AGM battery? How do I do that?
Also, how do those super small trickle charge panels work? Are they just over-charging the battery but with such a small power that the battery doesn't care? I guess it all gets turned into heat, right?

The solar panel charger was undercharging the AGM. My flavor of AGM needed a higher voltage. Other AGM brands may have different demands. I reconditioned it by drawing the battery down to 10 volts I think, then applying the right charge profile to bring it back to 100% state of charge. The reconditioning instructions were specified by the battery manufacturer, in this case. Odyssey.