Using a panel with 5v output to charge a car battery -

[paulg22]

Hi, very very new to solar panels, having recently bought a 15w Portapow to charge my phone etc whilst outdoors. This portable panel has a couple of USB ports and does a reasonable job. I also wanted to charge a walkie talkie which requires a 10v input, so I bought a step up convertor from one of the Chinese sellers and this seems to work OK too.

I was also wondering about using it to charge a car battery, mainly when we go camping (to top up from charging phones and stuff) and maybe as a trickle charger when the car isn't being used much.

Can anyone tell me if I would need a charge controller to do this properly? I can get 12v output with the step up convertor, but guess there are potential losses in running a charge through all of these boxes and 15w isn't a whole lot to charge a car battery with?

The solar panel has a built in voltage regulator, and the circuit/diode to prevent it discharging devices when there's no light, and there's a little meter that comes with it that tells you how many amps/watts it's using.

thanks in advance and apologies if it's posted in the wrong place (or any other etiquette!)

Paul

[Sunking]

Don't waste your time or money. A 5 volt 115 watt panels is just about worthless except for charging a cell phone or very small flashlight with AAA rechargeable batteries. Trying to step a 15 watt panel up to even Trickle charge a car battery is like trying to smell a fart a mile away in a hurricane up wind. Even if it were a 18 volt 15 watt panel made for 12 volt batteries is pretty close to useless. 15 watt is for cel phones and USB chargers which can take two days to just recharge a cell phone.

[paulg22]

Thanks for that - I did think that I might be being a little optimistic

[inetdog]

Thanks for that - I did think that I might be being a little optimistic

There are small, easily portable panels with an output greater than 14V Voc which can be used directly attached to a car battery. But for the most part they can only counterbalance the effect of self-discharge and tiny constant loads; they cannot really charge the battery in a useful way. For that reason they are most often called battery maintainers rather than battery chargers.

115 watts is larger than most of those panels, and falls into a gray area of its own. But the 5V output makes it useless without a voltage converter to get >14V out of it.

PS: I see that the OP has edited the post to make the panel 15W rather than 115W. That is a battery maintainer size.

[paulg22]

I've got a couple of these step up convertors made up in to usb cables to charge some Chinese walkie talkies that need a 10v input - and these let me charge them up from the car through a usb port. The convertors will output up to 36v, so setting them at somewhere 12-14v is easy.

I've got a little usb battery pack that I normally charge off the panel and then charge phone etc, but if I connect the walkie talkie direct to the panel via the step up converter it seems to pulse power on and off (according to the meter)

With the panel being 15w (a bit more than the advertised maintenance panels) I wouldn't want to leave it connected and potentially damage the car battery over time.

IMG_0443.jpgIMG_0444.jpg

[Sunking]

With the panel being 15w (a bit more than the advertised maintenance panels) I wouldn't want to leave it connected and potentially damage the car battery over time.

Stop worrying, it cannot do any damage because it is not enough power to charge the battery.

He is an analogy. A 15 watt panel is enough to charge up your cell phone or small flashlight using 2 AA batteries. So think of that as a 8 ounce glass glass of water. A car battery is like a lake compared to a cell phone battery.

Now think of your 15 watt solar panel like taking a piss. You can over fill an 8 ounce glass of water by peeing in it, but not a lake.