Cheap solar top up - will it work!??

Lambeth Cali · Oct 19, 2025

Hi
In order to keep my batteries topped up I have plugged a 20w dash solar panel into my bench cigarette socket on my 2008 T5. I’m not sure if it’s working.

Should I have the isolation switch in the boot on Generator or other/plugged in?

I’ve had it on Generator as that’s how I usually leave it parked up. The panel has an isolation box so not worried about it tripping anything.

yanathem · Oct 19, 2025

The isolation switch in the rear boot has nothing to do with the battery 12v dc system only 230v ac mains.
Are you sure a 20w solar panel will be anywhere near enough powerful to trickle charge your batteries over winter placed behind the windscreen. A 100w panel on a van roof connected direct to one of the batteries would maybe just about cope keeping healthy fully charged batteries charged from now through to late spring.

Lambeth Cali · Oct 19, 2025

Possibly not but thanks for clarifying.

Last year I had a choice of getting roof solar to top up my 16 year old batteries which had been amazing or get new batteries. I didn’t want roof solar, so I did the latter, but perhaps I needed both.

My original OEM batteries used to sit all winter long without ever being plugged in. They lasted 16 years. I’ve owned these Tayna Varta replacements 6 months and they’re already looking far far inferior.

I may connect the panel directly to the underseat battery as the wires came with the panel and resign myself to having to move it to my drive and plug in every 8 weeks or so.

Tayna customer service seem to think 6 weeks without plugging in was a long time. I thought these things were used on boats etc??

A shame. Trying to be glass half full, I suppose I should think I was lucky for 16 years.

dspuk · Oct 20, 2025

Couple of points I would identify here based on my limited experience and knowledge.

1 - As mentioned, 20W is not a lot and also the sun at this time of year is pants, evidenced by the rapid drop in our solar export numbers for the house. You would need to be somewhere very sunny and constantly turning the panel to maximise power.

2 - I think in the T5 the socket is connected to your starter, but for others reading this with later 6.1’s, it’s not, so you would be trying to charge the leisures. So it should work, but see point 1.

3 - The VW manual does also state that it does not reccomend that the lighter be used for power input. Given the thickness of the wires to the lighter and fuses this makes sense, but for a 20W panel not likely to cause any overheating issues, again, see point 1.

We have the usual 100W on the roof and keeps our leisures topped nicely. Starter, not so much as it is a trickle situation so we just drive it every week when not using in anger, like now.

Lambeth Cali · Oct 20, 2025

I think I was going off the very positive Amazon reviews of people trickle charging car batteries. I thought these were leisure batteries that were designed to just sit there without loosing charge.

I think the bench socket is connected to leisure batteries. Did seem to have a positive effect on control panel when I first plugged in in summer.

I now suspect the tiny led on the panel regulator is probably discharging them rather than helping.

I don’t believe these are superior to the OEM 16 year old ones that would happily keep the fridge running 4 days.

I wasted £380 on what I thought were “leisure batteries”.

Lambeth Cali · Apr 16, 2026

With the bright sun and my slightly crappy new state of the art leisure batteries improving a little, I decided to give this another go. See if it would get me through to the summer with minimal moves to the drive to top up.
The panel seemed to be doing ok the last week with the batteries showing 100% 12.8V.
I’ve never had much faith in using the cigarette in point at the end of the bench (2008 T5) so today I have connected the panel properly to the battery under the seat as the panel came with some good choice of wires allowing you to plug in and unplug when not needed. Quite a good setup, potentially!