solar car charger puts out 17v, will it damage my battery? - Page 1
Forum section: electronics / beginners

author: tehtehteh
author_rank: Regular Contributor
timestamp: March 09, 2018, 11:17:24 am
content: as my car sits un-used for long periods, I bought a solar charger to help stop the battery from going flat, it's 2.4watts and says it's for 12v batteries, a reverse protection diode is its only other feature, but in daylight it puts out 17v and possibly more on a brighter daywill this damage my battery if I leave it connected indefinitely?I can buy something like this if needs be:https://www.maplin.co.uk/p/solar-charging-regulator-n82auit also says to disconnect before starting the engine, why is this? if it's wired in to the constant live of the battery, it's already hooked up to 12-13v, then when running 14v maybe? what difference should running the car make to it?apologies if this has already been covered, I did a search but could not find the answers I was looking for

author: mikerj
author_rank: Super Contributor
timestamp: March 09, 2018, 11:23:31 am
content: 17v is the open circuit voltage, it will drop considerably with a load on it. If it's the typical 5w or 10w solar charger then it's most unlikely to hurt your battery, but I know from first hand experience that they struggle to keep a car battery fully charged in the winter.

author: Seekonk
author_rank: Super Contributor
timestamp: March 09, 2018, 02:13:57 pm
content: "17v and possibly more on a brighter day"No, it will put out less. It follows the temperature curve of a diode. If the panel heats up any, the voltage drops. On a wintry bright day it will put out more. If it has a diode to prevent reverse flow there is no reason to disconnect it.

author: rstofer
author_rank: Super Contributor
timestamp: March 09, 2018, 06:18:14 pm
content: What happens when you try to crank your engine with a substantially dead battery and the charger connected? Well, the current flow out of the charger increases - a lot! The charger sees the starter as essentially a short circuit. This can't be good...

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2018, 07:00:14 pm
content: That's not going to matter for a small solar panel. I have a couple of these solar chargers and IIRC they only deliver a few tens of milliamps. You can short-circuit the output indefinitely without any issues, all that will result is a slight heating of the panel which I suspect would be difficult to even measure. These things cannot deliver enough current to damage a lead acid car battery. They are barely enough to counter the drain from constant loads like the clock on the dash and/or settings in the radio.

author: Rog520
author_rank: Regular Contributor
timestamp: March 09, 2018, 07:11:37 pm
content: A solar panel is a constant current source for the most part. The voltage will rise and fall with the load, with the current remaining relatively flat until the "knee" or Vmp of the I/V curve. A small solar panel with a Vmp of 17v will very likely not be able to produce enough current to bring a car battery up to that voltage, so probably nothing to worry about. With a larger panel you'd want to monitor the battery voltage and disconnect the panel around 14-15v.