If you want to keep a car battery charged when your car is not in regular use Solar Panel charging is a good option providing you use this method correctly you will be able to keep your battery in a good state of charge and without risking damage to car electronic components that remain active when parked / ignition turned off .
All modern petrol/diesel cars with 12volt systems when driven charge the battery via a alternator with a rectifier/ regulator to charge and limit the voltage output to about 14.7volts DC
Connect a 12V,10watt Solar Panel direct to your car for charging and on a bright day a 12 Volt panel will output 20 plus volts dc and be constantly changing votage wise as the light alters so its important to limit the panels output unless the panel incorporates a voltage regulator
Some expensive solar panel are sold with a regulator package most dont .Limiting the voltage output can easy be done by adding a Buck convertor (voltage limiting type) to the circuit (see picture ) they are well under £10 to buy, they also come as a current limit type but the one you need is a voltage limiting type a typical one will be 5Amp rating and will cope with input up to 50vDC and provide a stable output set by adjusting a small screw, measure this with a voltmeter connected to the output terminals and set at about 14.5 to 14.7volts and it will be maintained at the selected output voltage when the panel outputs alters above this level
Just as important with Solar charging is to fit a Diode to stop current drain back to the solar panel when its dark and the panel is not providing a charge output, fit the diode in the positive lead NOTE the diode must be connected the correct way around they only allow current flow one way (again some panels are sold with a diode most cheaper ones are not) if in doubt fit one they are not expensive .
The DC-DC converter is an excellent idea. But it’s best to make sure it can produce the correct output to the battery for a range of panel input voltage from both above and below the final battery voltage.
However, a simple single-inductor Buck switcher can only drop the voltage.
SEPIC or Zeta switchers each need two inductors but most importantly, they can cover the possible input range and still produce useful output. Often these still use a Buck switch controller.
The blocking diode is best on the lead from the panel, not on the lead to the battery. This is because the diode’s forward voltage will vary with current.
I’ve done several experiments with solar panels, most recently during lockdown. I used the OBDII socket inside the Mazda3 to charge the car battery because this is not switched off with the ignition.
If I put a panel up inside the windscreen (so it wasn’t nicked) my results suggested I needed one about the size of the windscreen in the winter to have a net gain in Ampere-hours per day over dark current!
The “2.5W” NOCO panel I have is effectively useless and not on sale
I dug it out during again during lockdown in a vain hope it might keep the Mazda3 battery alive. No.
I tried again with a “10W” panel. Also no joy.
This “100W” panel has almost enough ampere-hours per day in the depth of overcast winter, but it’s too big for inside the car, and without a suitable regulator (and blocking diode for the dark) it could boil the battery if well aimed on a really good day! I gave it 3* in my review.
If anyone wants it for further experiments they can have it, but collection only (PM me).
