12v battery charging from an 18v portable solar panel?

[Davecg2]

I just bought this portable solar panel for when i go camping and backpacking. I can currently use it to charge my portable powerbank to keep my phone charged. There are times when I'm several miles into a state forest or something and could potentially be stranded if my car battery failed. So I was thinking it would be great if I could find an adapter or something to be able to charge my car battery if I needed to. I realize it would be a very slow charge and potentially take even a day+ or something but even that would be a lot better than nothing.

I searched for an adapter to trickle charge from 18v to 12v for this specific panel, but haven't had much luck. I did find a few things like this though which tells me it's at least a possibility. I have very little knowledge of solar panels and converting current, but it seems like I just need to step the voltage down to 12v and limit the wattage? That sample had 1.5 watts so I'm not sure if more wattage is possible and if it would speed the charge process up or possibly damage the battery.

Any help or advice would be much appreciated. I spent some time researching and I could just buy one of the trickle chargers separately. But I figured since I just bought this really nice 40w panel I can probably charge car a battery with it too, I just need to figure out how.

[inetdog]

I just bought this portable solar panel for when i go camping and backpacking. I can currently use it to charge my portable powerbank to keep my phone charged. There are times when I'm several miles into a state forest or something and could potentially be stranded if my car battery failed. So I was thinking it would be great if I could find an adapter or something to be able to charge my car battery if I needed to. I realize it would be a very slow charge and potentially take even a day+ or something but even that would be a lot better than nothing.

I searched for an adapter to trickle charge from 18v to 12v for this specific panel, but haven't had much luck. I did find a few things like this though which tells me it's at least a possibility. I have very little knowledge of solar panels and converting current, but it seems like I just need to step the voltage down to 12v and limit the wattage? That sample had 1.5 watts so I'm not sure if more wattage is possible and if it would speed the charge process up or possibly damage the battery.

Any help or advice would be much appreciated. I spent some time researching and I could just buy one of the trickle chargers separately. But I figured since I just bought this really nice 40w panel I can probably charge car a battery with it too, I just need to figure out how.

A 40 watt panel with an 18V output will produce only two amps of current, whether at 18V or 12V.
When connecting to a car battery, that 2A current is close to a trickle charge and will not cause battery damage if left on for a few hours (or days) beyond what is needed to fully charge the battery. Assuming it ever gets there.
One nice thing about solar panels is that they are current limited, so no circuitry is required to adapt the 18v output (probably designed for airline DC to DC adapters for laptops) to charge a 12V battery.
If you intended to leave the panel permanently connected (weeks) to the battery while the vehicle is not in use, I would recommend a low cost charge controller. You do not need a DC to DC voltage converter to do that.

[Davecg2]

A 40 watt panel with an 18V output will produce only two amps of current, whether at 18V or 12V.
When connecting to a car battery, that 2A current is close to a trickle charge and will not cause battery damage if left on for a few hours (or days) beyond what is needed to fully charge the battery. Assuming it ever gets there.
One nice thing about solar panels is that they are current limited, so no circuitry is required to adapt the 18v output (probably designed for airline DC to DC adapters for laptops) to charge a 12V battery.
If you intended to leave the panel permanently connected (weeks) to the battery while the vehicle is not in use, I would recommend a low cost charge controller. You do not need a DC to DC voltage converter to do that.

Thanks for the quick response! And that's good news. So it sounds like I just need to find an adapter to go from the DC output from the panel to positive and negative alligator clips. Any idea what kind of adapter that would be? I've done some searching and found this, is it what I would need or is there a detail I might be missing? The panel's output is usb and this 18v dc out. It also comes with these adapters. Can I make it work with that cable or is there something more specific or maybe something cheaper I should search for?

[russ]

Chances of using that little dinky panel to recharge your battery (even enough to start the car) when you have a dead battery out in the woods are close to zero. A panel requires sun light - direct sunlight which is hard to come up with - unless you limit your parking places which no one does.

[Davecg2]

Chances of using that little dinky panel to recharge your battery (even enough to start the car) when you have a dead battery out in the woods are close to zero. A panel requires sun light - direct sunlight which is hard to come up with - unless you limit your parking places which no one does.

There are usually some open spots in the canopy here and there and I figure in the case of an emergency I could just remove the battery from the car and carry it to wherever the most sunlight is at.

[russ]

There are usually some open spots in the canopy here and there and I figure in the case of an emergency I could just remove the battery from the car and carry it to wherever the most sunlight is at.

Think about that statement a bit. You have to park in the right spot, constantly move the panel to stay in the sun and only have a power failure during mid day.

Ain't likely - fool yourself if you want but the concept is about useless.

Better to make sure you turn off the radio, lights etc - take care.

[Davecg2]

Think about that statement a bit. You have to park in the right spot, constantly move the panel to stay in the sun and only have a power failure during mid day.

Ain't likely - fool yourself if you want but the concept is about useless.

Better to make sure you turn off the radio, lights etc - take care.

I did think about the statement when I made it. I said in the case of an emergency(I'd be more than willing to carry the battery to the right spot and move the panel constantly if it's an emergency). It's not like I'm going to go out there and turn a bunch of crap on in my car and then go backpacking its JiC. Your posts seem more condescending than constructive. Which is pretty surprising coming from a "moderator". I'm not sure why you'd talk down to me just because I'm not as well versed in solar technology as you. I'm a software engineer, not an electronic engineer and never claimed to be knowledgeable about solar tech. That's why I came here. I explain quite clearly in my original post why I thought it would be a viable option. I still think that logic seems plausible considering they sell a bunch of trickle chargers with less powerful panels than the one I just bought. Also, most of the places I go have semi-large parking lots that could get clear sun for around 5+ hrs, I generally try to get back to my car by noonish so midday is actually very likely and again I could carry the battery if it was an emergency (I hike several miles with 50+ lbs on my back for fun, a battery for a few miles would be no big deal especially in an emergency). That said, I didn't buy the panel to charge a 12v battery I bought it to charge my phones and tablets and just thought if I could also use it as an emergency car battery charger it would be cool.

I appreciate your reply inetdog.

[russ]

Also, most of the places I go have semi-large parking lots that could get clear sun for around 5+ hrs, I generally try to get back to my car by noonish so midday is actually very likely and again I could carry the battery if it was an emergency (I hike several miles with 50+ lbs on my back for fun, a battery for a few miles would be no big deal especially in an emergency). That said, I didn't buy the panel to charge a 12v battery I bought it to charge my phones and tablets and just thought if I could also use it as an emergency car battery charger it would be cool.

I appreciate your reply inetdog.

Fool yourself all you want. When you said "out in the woods" I think of how we did it when I was a kid - not today's MP3/ipad/cell phone/GPS generation.

[Wy_White_Wolf]

There are usually some open spots in the canopy here and there and I figure in the case of an emergency I could just remove the battery from the car and carry it to wherever the most sunlight is at.

Plan on it taking 10 to 20 days for a 40w panel to charge the battery up enough to start the car.

WWW

[Davecg2]

Plan on it taking 10 to 20 days for a 40w panel to charge the battery up enough to start the car.

WWW

Fair enough, so are products like this one which is only 5w useful at all?

[Davecg2]

Fool yourself all you want. When you said "out in the woods" I think of how we did it when I was a kid - not today's MP3/ipad/cell phone/GPS generation.

I'm not sure what you mean by fool yourself. I'm asking questions not making statements. And yes I do take some tech with me while i'm backpacking. I like to track my hikes with a hiking app that allows me to track the path I take, the time it took, the milage and avg time per mile. I'm training for longer trips so tracking my progress is important. I also like having a form of communication (if I have a signal and charge of course) in case of an emergency. It sounds like this panel isn't powerful enough to charge a car battery within a reasonable amount of time. If it only took say 8 hours to at least charge it back enough to start my car it'd be a huge bonus. That said, even if it only serves the original purpose I bought it for that's fine, just thought it was worth the time to post on a forum and see if it could be used for a emergency trickle charger also. I don't think there's any harm in asking.

[russ]

Fair enough, so are products like this one which is only 5w useful at all?

That panel is meant only to maintain a fully charged battery while a car is sitting - the ad is shyster in intent. It is also silly expensive.

I understand the tracking usage - I do the same on bicycle rides and when I am running in the hills around home here. It is fun to see where you were and where you need to work harder to improve.

[SunEagle]

Fair enough, so are products like this one which is only 5w useful at all?

Those small wattage trickle chargers for a 12volt car battery are next to useless.

That 40 watt charger you have is a little better but remember that the cells need to be aimed directly at the sun during the peak sunlight hours 10AMm to 2 PM in the Summer. That may be hard to do unless you have a flat board to place it on as flat as possible. If any cell is slightly angled it will not produce as much requiring a much longer time under the sun.

I have found that purchasing a small portable lithium battery is both light weight and will provide enough to recharge a cell phone and costs less than most of those small portable solar chargers.

[PNjunction]

Dave - here's the quick lowdown.

Those 5 watt panels do not have enough current to actually charge a car battery in a *reasonable* amount of time given a normal discharged state. If they do, the charge is a superficial "surface charge" that won't help start the car.

They are mainly designed to keep a fully-charged battery fully-charged - that is to compensate a bit for self-discharge, or small parasitic drains - like alarms, clocks, etc.

There is a BIG catch 22 when using these things without a controller - with a big enough discharge, no they won't harm or charge the battery that well. However, IF your battery is already close to a full charge, than you want to avoid this:

https://www.youtube.com/watch?v=z5RtZe9AW2E

So yes, use a controller if your battery is in a good state of charge already, otherwise you risk destroying them. If your battery is heavily discharged or clapped-out bad, then these panels without a controller won't make much of a difference. That is the main reason why you see the ok given to C/100 or smaller current panels - most are getting away with it because the panels may never actually let the battery reach a full charge in the first place. But if they ARE close, then lookout! Use a controller.

For the average consumer interested in "maintenance" charging, then Deltran (Battery Tender) with onboard controller is a good choice. Me, I'd piece together my own panel and say a Morningstar 4.5a Sunguard myself. If you still want to use your existing panel, the Morningstar SG-4-12v controller would be a good choice as it is entirely potted so I use it when camping myself.

http://www.solartown.com/store/product/morningstar-sunguard-charge-controller-sg-4/

Deltran is not the only game in town. Battery-Minder also has them with controllers, and also ones with specific voltages for aircraft battery use:

http://www.batteryminders.com/categories/applications/solar.html

Yes, you can build your own like I did for maintenance charging at quite a cost-saving. Still, I've recommended and tested both of these for those that aren't interested in DIY, and they work fine for what they are.

[jony101]

you can get a cheap wincong 10 amp solar charge controller (about 20 dollar on ebay) and somehow rig 2 wires from the panel to the plus and minus input of the wincong, and the output to the battery. At 40 watts it will take a while maybe several days to get at least a few amps into the battery. But sometimes just a few amps is all you need to crank over the engine at least one good shot.

But I think its possible, on my van roof I have a 240 watt panel (lying flat) and once I had left the lights on overnight. In the morning it wouldnt start, the battery was reading less than 10 volts. I couldnt get the jumper cables to make a good connection to the house battery. Luckily I was able to rig some wires to the start battery from the charge controller. With 240 watts by 11 am, battery had enough charge to crank over the engine. Worst case scenario and a decent size panel its possible, better than nothing. I know if I was in the wilderness and only had a 40 watt panel I would try it.