Disappointing solar panels from Amazon

I’ve posted in other threads that I kept a 10W solar panel on the dashboard to keep the battery topped up.

No more, at least for the time being.

I first bought a 5W panel, thinking that would be adequate to make up for parasitic drain on my infrequently driven Tucson.

However, measurement revealed it was only delivering ~2W on my dashboard in the sun.

I chalked it up to windshield loss and bought a second identical panel, added in parallel.

I would periodically check battery voltage (hood up, accessories off) near the end of a sunny day and find the battery at ~13.5V.

I added a couple of 5W zener diodes to limit the panel output to 14.3V, to prevent over charging.

All was well for 2 years, then a sunny day check was under 13V.

I measured the panels output in direct sunlight and got ~1.5W (10W spec).

Did the heat in the closed car kill them?

I then purchased a 10W panel, which put out a measured 6W in direct sunlight.

Disappointing, but enough to get the job done.

Two months later, I started to notice the voltage display I keep in the 12V port on the dash started reading lower (12.4V vs ~12.8V) when I turned the key to ACC before starting the engine, even on sunny days.

I did the sunny day check and the battery was at 12.55V.

I measured the panel output, sitting on my dash in the sun: less than one watt!

I bypassed the little charge control circuit this panel came with and measured cell output in direct sun: 3 watts.

So that panel was $30 wasted.

I park on a city street, so I’m back to removing the battery periodically for an overnight charge in my basement.

Going forward I’m going to cobble together some 10AH LiFePO cells on hand, and an LDO voltage regulator to apply 14.2V through the under-dash connector I put in for the solar panel.

Overnight every couple months will keep the battery happy.

While I have no experience with them, I’d have spent a bit more for ‘Battery Tender’ brand:
Amazon.com: Battery Tender 5 Watt 12 Volt Solar Panel Charger and Maintainer with Charge Controller, IP67 Weatherproof, Includes Windshield and Handlebar Mounts, Ring Terminal Cable and ODBII Connector : Automotive

Thanks for the link. A trustworthy brand.
I found it also comes without the accessories I don’t need for a little less:
I might give it a try after my bruises heal.
Will I ever learn to avoid cheap crap?

I know, it’s always tempting. And by ‘no experience’, I meant with their solar units. I’ve been happy with my battery tender from them for the last 10 years.

I looked at the reviews for the BT solar panel and found one person saying the older ones were better.

I’m going to build a portable overnight charger with the LiFePo cells and a boost regulator set to ~14.2V and 5A limit.

I have a string of solar powered outdoor christmas lights, solar cells charge up battery, battery powers lights at night. Getting much dimmer. Always have thought it was the battery was on the wane, not holding full charge, but maybe it is the solar cells losing power output. My guess is the protecting cover material is less transparent & blocking some of the light

If the cell cover slides are plastic, UV damage will reduce light throughput. The cover glass really should be glass. UV won’t damage silica. If you are sure the cell covers are plastic, another thing you could try is a headlight restorer kit. If you are going to throw out the solar cells anyway, it might be worth a try.


Here’s a sneak peek at my portable charger.
So far, LiFePO4 cells, booster board, OBD power cord, carry bag.
Waiting for display and cell balancer board.

Thanks for the photos. I have to say , your project seems very well organized. I recently purchased a new filing cabinet, now the drawers are missing, can’t find them … lol …