Solar trickle chargers

So, in this cold weather my car battery is upset lol.
Someone pointed out this deal - LINK
For £11 I thought it wasn’t bad and worth a pop.
Then a guy says this - based on UK winter light.

‘Assuming it’s kept for long time and on average there is enough sun for half of the day, the effective long-time wattage is 2.5W counted 24/7. Your car need to draw less than 20mA current or less when switched off, to have power balance zero or positive.
You need to keep this thing outside to keep this calculation valid, as behind glass it has lower efficiency. And have a warden, to make sure it won’t be stolen.’

Is this correct, have I just bought an £11 sun visor??? lol

surely the 12w 5W rating is per Hour like any other PV cell? that`s about 12v @ 400ma(ish), given half a days light and we`ll halve it again because it`s behind glass, that`s about 100ma per hour equiv, way more realistic than his 20ma figure.
granted this still won`t do much if anything, and 14ish volts would be much better and may put a little charge in the batt than 12v will, but it should keep your dashboard clock going at least :slight_smile:

You may be OK. I got something like this ages ago to put in a Jeep that hardly got used. Mine was about 12" square. I think it was rated at 8w? But I don't remember. It lived in the front dash window for years, and the old Jeep cranked every time.

Without the trickle charger, I needed to jump the thing if it sat for a few months. Went through batteries fairly quickly too. Post charger, never had to replace the battery.

'Course I'm down in Texas where the sun is actually seen in winter. And the old Jeep had very little phantom draw - It had some, but it's been too long for my little gray cells to remember.

Your mileage may vary... A second one in parallel if one isn't cutting the mustard?

[Edit] Just saw KA post. Yeah, it's watts, as power output, not over time. Like voltsxamps at a given moment. My charger put out a bit more than 14v which is ideal for a car battery. I see the one you got has an open circuit V of 17v. No need to worry about overcharging, the little current these things push won't bother a car battery.

All the Best,


Ah well, I have tried to cancel it - but if it arrives I suppose I’ll have a go with it, or send it back and say it isn’t working
Here are the specs of it -
Maximum Power (Pmax): 5W
Open-circuit Voltage (Voc): 22.41V
Short-circuit Current (Isc): 0.3A
Maximum Working Voltage (Vpm): 17.9V
Maximum Working Current (Ipm): 0.28A

that`s actually not too bad (certainly better than I thought given just 12v and 5W as data), depending on the parasitic drain of the car, that would do a fairly good job of maintaining a charge, or at least slowing down the discharge rate.
you could always get a little 5v USB buck convertor and charge power banks when camping! :slight_smile:

None of the cigarette lighter sockets in my newer vehicles are active unless the ignition switch is on.

So depending on your vehicle it may not work, if your plan was to trickle charge the vehicle's battery by using a solar panel located inside of the vehicle and connected to a cigarette lighter socket.

This is exactly what I did with my 8w solar panel. Used a small DC-DC buck converter I set mine down to 15v. While I was in there I disconnected the status LED that ran continuously. I don’t remember the exact numbers but I want to say I doubled the incoming amperage.

Fancy pointing me towards one please Katherine - sorry I’m electronics dumb, and would no doubt buy the wrong item! :person_facepalming:

Check the specifications of your portable solar panel for capacity and maximum voltage. My 100 watt portable solar panel has a nominal working voltage of 18 VDC and a nominal working current of 5.1 Amps. My solar panel's maximum open circuit voltage is 21.6 VDC.

If your solar panel has sufficient capacity for your needs and an open circuit voltage of less than 24 VDC, the easiest way will be to make an adapter by finding a "plug" that will connect to your solar panel's output port or to the plug on your solar panel's cable and wire this "plug" to a "vehicle cigarette lighter socket" (be sure to connect the plus/minus polarity correctly when you assemble the adapter and add an inline fuse if you want):

Then use a USB charger that works in a vehicle's cigarette lighter socket and accepts up to 24 volts DC. I use this USB charger from AmazonUS that accepts 12 to 24 VDC which is expensive but of high quality and is occasionally on sale, here is a link for it on AmazonUK:

Apparently - I don’t have it yet :slight_smile: I think it already has a cigarette lighter attachment
Maximum Power (Pmax): 5W
Open-circuit Voltage (Voc): 22.41V
Short-circuit Current (Isc): 0.3A
Maximum Working Voltage (Vpm): 17.9V
Maximum Working Current (Ipm): 0.28A

sure, I got mine from ebay, there`s dozens there of all different sizes, just put in “USB buck convertor”, here`s one that should work:

I have and use a couple of these in the workshop: 6-24V Input to 5V 2A Output USB Charger Module DC-DC Buck Step-Down Converter | eBay

Thank you Katherine :+1: :slight_smile:

YW :slight_smile:

btw, if you haven`t already found out, anything you now buy on ebay from China for instance will now include VAT ontop :frowning:

OH MY GOD! No I hadn’t noticed! for real?!? :person_facepalming:
Un-brexit us now lol!

And an £8 handling charge.

Yes, that does mean Customs will see your £1 item from China and charge you 20p in VAT plus £8 for processing it.

Used to be that anything under £15 just got waved through, but that exemption got cancelled at midnight on the 31st of December. Everything gets taxed and charged now.

I hear rumours that they’re going to phase it in, choosing a few packages at random to start with and processing a steadily higher percentage of shipments until they’re eventually processing everything. You may get lucky at first, but no guarantees.

OBD port. Always live.

WOW I did not know that……… opens up quite a hole.
For instance……. most Chinese sellers now have a UK address, and claim it comes from the UK, yet it arrives via express from China - how the hell will one argue that point lol!
To be fair, that couldn’t carry on for long surely - it’s pretty underhand., RM taking £8 every time for collecting 20p on a £1 item.
Also anything I’ve ordered from China (so far as i know) was ordered pre brexit so won’t count. Hopefully this won’t come from China - if it does I’ll refuse delivery and go after them for a refund through Paypal for not following the law. It should already include VAT (if I’m reading the gumpf right) I think buying from ebay UK would be ok, but BG, Ali etc well….

’Goods sold before 1 January 2021

If a seller receives payment for an order before 11pm on 31 December 2020 and dispatches the item after that time, these rules will not apply.’


From 11pm on 31 December 2020, consignments of goods with a value of £135 or less that are outside:

the UK and sold directly to customers (not through an online marketplace) in Great Britain (England, Scotland and Wales) will have UK supply VAT charged at the point of sale
the UK and EU and sold directly to customers (not through an online marketplace) in Northern Ireland will have import VAT charged’

Is that saying if you buy something from China on ebay UK you are exempt?

Yes, but that is a "male" cigarette lighter attachment.

My suggestion was to attach a "female cigarette lighter socket" to the solar panel so that you could potentially use one of your existing "male" USB chargers that fits into a female cigarette lighter socket.

You can usually repurpose or find female cigarette lighter sockets for free or very little money and if you already have a suitable USB charger that fits into them, then this would be an economical solution.

I purchased the more expensive Anker dual-port USB chargers from AmazonUS when they were on sale (I included the AmazonUK link in my previous post) and attached two of them to my 100 watt portable solar panel so I can simultaneously connect two USB-C PD and two USB-A devices when the Sun is bright enough. Fast USB-C charging helps while solar charging when full sunlight is only available for a few hours.

Once you can connect to your portable solar panel with USB, it helps to have a USB test meter to monitor the voltage and amperage from your USB port.

I will have to check into that. I currently have a ScanTool OBDLink MX+ plugged into that port.

I do not drive far or often anymore and while the battery charge in my old truck can last for months while parked (since its only parasitic load is running the clock), the battery charge in my new car with keyless entry, car alarm and other electronics runs down quickly if I do not drive it frequently.

Currently (unless I move somewhere else) I am able to run AC power to my vehicles and use a battery tender, but a solar panel connected to the OBD port would be an interesting future option to trickle charge the battery in a vehicle whose cigarette lighter sockets are inactive unless the ignition is on.

It would probably be important to always remember to disconnect the solar panel from the vehicle's electrical system before starting the vehicle.

I got one of those which I use to top off any car that’s been sitting a while. Especially in cold weather, it just kills batteries to sit for too long. Usually just run the clips onto the battery, with the box on top of the battery, and loooooooong extension cord across the sidewalk and into the house. Hate doing that as it’s a lawsuit magnet if some jackass trips on his own feet, but…

With a, say, 2A trickle charger, the OBD port would work fine. They can ostensibly handle 10A or more, but I wouldn’t push it beyond 2A or so, as you don’t want the contacts to overheat.


Solar cells have a deceptively simple IV shape. Current is proportional to incident light as a current source, but voltage is a maximum of ~0.55V/cell. So using an “18V” or even “24V” panel buys you zero compared to a 15V panel. The voltage just quietly drops, no waste heat, nothing. Maxumum power is at the upper-right corner of that shape, so with the right converter circuit you can maximise power to the output, but a straight panel is fine.

Short out the panel, and it’ll happily keep on supplying the same current as when charging the battery. That’s in fact how you measure solar irradiance: short out a solar-cell and measure its dead-short current, taking voltage out of the equation.