Anyone know about solar chargers and stuff like that?

[Wade Lippman]

I have a trolling motor and a large lithium battery on my sailboat. The two are good enough to get me home from about 3 miles out if the wind dies, though it will take a while. I got a small solar panel from HF. https://www.harborfreight.com/15-watt-solar-battery-charger-62449.html.
I hooked it up to my battery and my ammeter showed a half amp going into the battery, but after two days the voltage on the battery hadn't gone up. I know it isn't big enough to fully charge it, but I expected to see something. Or maybe the solar panel is simply too small to have any effect.
Perhaps at night the battery loses power to the solar cell? My EE courses were way before any of this stuff existed, so I have no idea.
But if it is, HF sell a solar panel controller; https://www.harborfreight.com/7-amp-solar-charge-regulator-96728.html. Is that like to help? A more expensive one says it prevents current lose through the panel, but this one doesn't.
Anyone know?

You can't go by their descriptions anyhow... I got a great deal on 100' of 3/8" polyester rope for $8. Got it home and found it was PP.

[Malcolm Schweizer]

Short answer is you cannot directly charge lithium batteries. They need a solar charge controller specifically made for lithium batteries. You will ruin them trying to charge otherwise. The reasons are complicated. Google “Midnight Solar charge controller.” That’s a good one with a lithium battery option.

[David Publicover]

Short answer is you cannot directly charge lithium batteries. They need a solar charge controller specifically made for lithium batteries. You will ruin them trying to charge otherwise. The reasons are complicated. Google “Midnight Solar charge controller.” That’s a good one with a lithium battery option.

Lithium definitely requires a different charging regimen compared to a lead acid battery. Added to your problems is a very small solar panel. I’m not sure how large a battery you have but I would expect you need a much more capable panel depending on boat usage, battery size and charging time requirements.

[Jim Matthews]

The current provided will vary with Sunlight.

https://electronics.stackexchange.com/questions/306379/do-solar-panels-generate-variable-current-or-variable-voltage#306384

Even the best solar generators (with inverters included) have fairly high system losses. With smaller panels, inverter efficiency can make the difference in output current. My guess is that you had enough resistance to show current, but not enough to recharge.

Remember, for a $light additional "charge" anything is possible.

https://www.emarineinc.com/Sizing-Your-Marine-Solar-System

[Bruce King]

If the battery is less than about 3 volts it won’t charge with a controller, you have to put another method of charge on it first.

[Wade Lippman]

If the battery is less than about 3 volts it won’t charge with a controller, you have to put another method of charge on it first.

Battery is nominally 12v. 14v when fully charged.

[Bruce King]

I was referring to the voltage it’s at before you charge it.

[Bill Dufour]

I read that if you clip the wire to the led the output will increase about 50% espically on cloudy days. The panel can barley keep up with the power draw from that one small LED.
Bill D

[Malcolm Schweizer]

I am revisiting this and just seeing that you have a 15 watt panel. My original response holds true that you MUST use a charge controller made for lithium batteries. That being said, you also need a bigger panel. I don’t know what size battery you have (amp hours) but let’s assume 100 amp hours and 12v. To get charging ability, divide the solar panel output watts (15) by the voltage of the battery (12). 15/12= 1.25 amp hours at max output. Rarely do you get max output, even on a sunny day. Most of these low end panels are poorly rated to begin with, i.e. you get less than advertised output. Let’s make the math simple and say you are getting one amp hour output. At this rate, it would take you 100 hours of full sun to charge your battery. Since you don’t get full sun all day, let’s say 200 hours of daylight to charge your battery. Even at 10 hours a day of good light, which is unrealistic (clouds happen, and it depends on you latitude how many hours of sun you get) you will need 20 days to charge your battery from a full discharge.

I would get at least a 50w panel and a charge controller made specifically for your type of battery. Is it lithium-ion or lithium iron? Tricky- one letter difference but they are two different batteries and have different charging requirements. Also a huge warning: you can take a li-ion battery to 1% power, but the moment you take it to zero, i.e. fully discharged, it is toast.