Solar panel for car battery?

[audi321]

Hi all. I’m a bit of a novice about this kind of thing so might be a numpty question.

I’ve got a 12v car battery which I believe is decent, and it was down to about 10v so I had an old solar panel knocking around (says it’s 4 watt on the back) and I put volt meters on it and it says it’s producing 14v (in the sun!). I connected it to said battery a week ago.

I went to the battery tonight and it’s totally drained and is showing 4v!!

Why is this? Can a solar panel actually drain the battery? Or is it not big enough and I should buy a bigger panel (more watts?).

Thanks for any advice.

[PH User]

Get a battery conditioner, a few hours on charge a week will keep it topped up.

[audi321]

I have no electric where the battery is so that’s not an option

[PH User]

So it's not in a car? Well that makes it even easier to get charged up on the mains.

[audi321]

It’s on a boat

[sgrimshaw]

Does the panel have a charge controller built in?

If not the panel will drain the battery when it's not generating.

[audi321]

I don’t think so. It’s just a panel and 2 wires coming out of it (+ & -).

It was only cheap and it’s about 15 years old but Ive never used it before so it’s new.

Can you buy these controllers separately or are the part of the panel?

Edited by audi321 on Monday 13th September 00:22

[sgrimshaw]

4W isn't going to give much charge BTW.

Kits designed for boats are much more powerful and aren't exactly expensive.

[Griffith4ever]

4w is not enough , and 14v would barely charge a lead acid battery . A cheap 4w panel most likely won't have blocking diodes so it'll drain the battery at night.

A £10 12w panel will do the job of simple maintenance, though you can get a 40w and a regulator for £40

[buggalugs]

Needs a diode or something to stop the panel draining the battery at night

[sgrimshaw]

audi321 said:

Can you buy these controllers separately or are the part of the panel?

You can buy them separately, but they are not cheap as designed for more substantial installations.

Cheaper to buy a proper one, they aren't expensive.

Start by considering how much power you need, that depends on the battery you want to charge/maintain.

[Bacon Is Proof]

We paid about £100 for a 100w panel with built in controller (brand was Dokio, I believe). 18v but that can be adjusted down as necessary. We then have two other panels without controllers plumbed into that, also on a boat but charging 4x115ah lead acid leisure batteries.

[hellorent]

Some info on here: https://forums.ybw.com/index.php?forums/practical-...

[audi321]

Cheers guys I turns out I needed to buy a ‘blocking diode’ as some have pointed out. Picked one up for a whole 25p from the local electronics shop.

Installed it within a few minutes and now the waiting game. I’ll come back in a week and see if we’re back up to 12v.

Thanks again.

[spikeyhead]

4W will be about 0.3A in full sun.

Your car battery? 65A hours for full charge, so you'll need at least 200 hours of sun assuming everything is working perfectly and your battery has negligible discharge. At four hours of good sun a day, you may, just may get there by Christmas.

[Griffith4ever]

4w is not enough. That will produce less current in the winter than your battery will naturally lose.

I see 100w panels produce 0.1 amps on grey winter days. You need more than that to keep on top of natural leakage in a battery. Now imagine what a 4w panel will produce.

Regarding regulators.... (Solar chargers, Pwm chargers, mppt chargers)

Small panels like 12w jobs, on the dash, don't need one as they don't produce enough power to overcharge a lead acid car battery. Anything larger does. Even the 6£ eBay Pwm chargers are good enough.

I run a solar panel business for reference.

[Griffith4ever]

spikeyhead said:

4W will be about 0.3A in full sun.

Your car battery? 65A hours for full charge, so you'll need at least 200 hours of sun assuming everything is working perfectly and your battery has negligible discharge. At four hours of good sun a day, you may, just may get there by Christmas.

It's worse than that. Most "12v" panels are actually 19 - 20v. They have to be higher to allow for charging a battery up to 14v and, allow for voltage drop when hot. They are rated at 20v so it's 4/20 which is 0.2 amps. Now take about 25% off that as they are rated under ideal (unrealistic) conditions and you are down to 0.15 amps, in the perfect sun, at 30 degrees angle, not behind a car screen.

[steve-V8s]

As a slight stray off the topic, if you measured the battery at 10v it is unlikely to recharge to anywhere near its original capacity.