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).
I mentioned a SEPIC converter. Here is one I built in 2003.
The design is almost standard and could be adjusted for a panel and car battery, but the clever bit here is in the peculiar application and the spreadsheet specifying the components.
The external +24V input is nominally 12V to 24V, but can be anything from 8V to 30V if one includes noise and ripple.
The +22V output is low power, only about 3 Watts maximum, and average current about 80 to 120mA, but with a weird load duty cycle of up to a 3 Amp pulse for up to two milliseconds at a rate of a hundred times a second, so there are also two big low ESR capacitors right in the load area on another board to carry the pulses and limit the current demand on the choke.
As a female of a certain age we were not allowed to study Physics/Chemistry so the detail you both describe is incomprehensible to me
However a ‘very nice man’ from the AA recommended a small Solar Panel for Bullit [2005 NB] years ago and I’m convinced it kept her unwell battery going until I could get her new one
I still put the Solar Panel on her dashboard every time she’s out in the sun
And if its the one with the AA branding and the OBDII plug, it’s a good one for the size while it’s working; but only provided you were lucky to find one that was made properly.
General reviews seem to bear out two of my friends’ experiences after they had several replacements under warranty. They eventually gave up on solar.
During lockdown they asked me to try and fix their last two early AA panels, but each one had an open circuit. After a few weeks a track had peeled off the cell array and cracked where its output lead was attached. At the time I speculated they might have been stressed because of movement from thermal cycling; freezing at night, cooking in the midday sun. The supplier gave us no clues, and I’m still no wiser. Not fixable, alas.
Those faulty panels became part of “device” recycling, along with the host of “solar garden lights” and “fox scarers” we’ve disposed of, all suffering from the same two basic manufacturing problems of solar devices, the other major problem being water ingress.
The AA solar panel I purchased from them in Oct 2017 for £24.00 is approx 14" x 9"
'12V 2.4W with Optimum working current of 0.13-0.2 amps with EOBD two pin plug Connecting Cable [and another you can connect to the actual battery with clamps that I’ve never used]
Needless to say I understand nothing of what I’ve just written !!
But I hear what you’ve said and believe it sound advice
I think the only reason mine is still working is that Bullit got garaged in 2019 so the Solar Panel has had increasingly less use over the years
Because Bullit and I get out less and less [e.g. less than 300 miles 2024]
The Solar Panel just gets bunged on the dash board [e.g. whilst shopping at Tesco] and when there is sun [errr…sun…you remember when we had sun ?]
Probably why my Solar Panel has not gone kaput yet ?
Interesting post, clockjohn, as I have been using the same 10W panel plugged into the OBD port for the past year when my NC is idle for anything more than a few days. It works well other than when there are more than about 4/5 days without direct sun - I am fortunate that the windscreen faces due South, though. I started originally with the AA-branded 5W panel, but quickly found out that the battery was gradually depleting, even with sunny days.
However, having read somewhere else that the voltage is uncontrolled, I fitted a bluetooth monitor on the battery and (admittedly only once recently) found that the battery voltage had climbed to 18V. In the past couple of weeks, I have bought a cheap voltage controller and just now need a box to contain it and fit it to the panel.
i am intrigued about using a diode in the circuit - I had never considered that there may be a backfeed when not charging. My knowledge of electronics stops there, though. Could I ask where a suitable diode could be obtained and what rating/code am I looking for please?
SR5100 Diode 100V 5A is the one I used a pack of 10 will cost about £3.50 post free, you only need to use1 ! 10 x SR5100 Schottky Barrier Rectifier Diode 100V 5A | eBay
you can buy a IP44 box from toolstation in selected sizes hope this helps
kind regards John
Yes, that one is excellent. But it’s only needed if the panel does not have reverse current protection.
For my experiments, at first I put a diode inside the bolt-up OBD connector I added to the chopped end of a spare NOCO battery lead. I kept its fuse holder but now fitted with a 1Amp fuse instead to make sure of no risk to the 10A fuse buried in the Mazda3.
Then I belatedly tested the two panels to see how much reverse leakage current they had. It was only a couple of micro-amps when gently warmed up with a hairdryer and applying 24V from a bench supply. It was effectively nothing at 12V.
Nice article. Whilst I’m an MX owner I actually tried a version of this system on my Hilux Invincible. I was initially confused by the awful measured results from a flexi solar panel that spanned almost the whole of the windscreen. It took an electrician colleague to point out the elephant in the room! He reminded me that almost all modern windscreens are designed to significantly REDUCE transmitted UVA and UVB to protect the driver!!
