Piston Slap: In-car solar charging?

piston slap in car solar charging
Sunway Solar

Rob writes:

I have a question that might be a good timely candidate for the “Piston Slap” forum—at least I’m hoping it is. My vehicles are not getting driven nearly as often as before. A concern is that the onboard computers are still draining energy, and this inevitably runs the batteries down and increases the risk of freezing the battery in the winter.

I’m wondering if you or the Hagerty community have suggestions for a reliable and perhaps hassle-free way to keep the batteries charged without the need for a long extension cord lying in the snow and running out to a traditional battery charger connected under the hood to the battery terminals?

Recent searches reveal a whole range of solar charging panels being advertised as a possible alternative, but would you have any experience using any of these solar charge panels? If so, are smaller consumer-grade ones any good? Are OBD-2 computers pretty much all drawing the same energy over time, and could a certain size solar panel or wattage be determined as a minimum necessary size? And what about the need for a charging controller? Can they hold up to being placed on a dashboard of a vehicle and submitted to the temperature changes of that location? Would they be able to be kept on a dashboard all year around just to keep battery topped up?

Sajeev answers: 

During my research for this question, I was shocked to see the sheer volume of 12-volt solar battery chargers on the market. Then again, the #vanlife trend clearly makes it easier for anyone to keep their stored vehicles fully charged and ready to run when needed. Certainly a good idea, but does it work in the real world?

Sure looks like it! This YouTube channel is one of many that reports positive results, even the folks that tested on a cloudy day admit they’d perform adequately with sunny days added into the mix.

Put another way, it will work over the long term, provided you don’t live in a place like Tromsø, Norway, and your 12-volt battery isn’t connected to a vehicle with an aggressive parasitic drain. That leads me to your other questions, which we can address in a more rapid fire manner:

  1. OBD II computer draw: Yes, but there’s a whole lot more to a modern car than an OBD-II computer. Depending on the application, accessories like stereos and climate control systems can remain powered on. Even older vehicles can have a fair number of components connected to 12-volt power, even with the ignition off. Failing modules inside the vehicle can also cause a tough-to-catch parasitic drain, and that becomes more common as the years accumulate. If the drain is significant enough, the battery needs to be disconnected when not in use, even with a charger. That also makes the charger’s job a lot easier, and that means you can buy a smaller, cheaper solar panel array. Again, all this is discussed in a vacuum, your mileage may vary.
  2. Durability: If you can keep it inside the car (using suction cups to affix it to a window) odds are it will last a very long time as nothing more than a 12-volt battery tender. Just make sure the window stays clean so that as much light as possible gets to the solar panel. (That will be more important as the panel loses efficiency over time.) Again, this is all in theory, and it’s likely moot when were tendering a single 12-volt battery and doing nothing else.
  3. Year-round usage: If the vehicle remains undriven no matter the season, yes, I would most certainly use it year round. Most solar panels are tough enough to withstand outdoor use, so a cheap one on your windshield will likely keep the battery topped up for years.

So go ahead and get one, possibly the cheapest one on the market. Between your constant usage and the solar panel’s “indoor” storage, odds are the least-powerful solar charging system is all you need for your battery. What say you, Hagerty Community?

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com, give us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.

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    I see a possible hiccup here. In his question, Rob mentions the extension cords laying out in the snow. In my part of the world, when it snows enough to cover an extension cord, it generally also covers the windshield of the vehicles. And even after the windshield is cleared, the bare glass often frosts or ices over during the night. Unless it is an extremely sunny day (and the vehicle is properly oriented to catch the best rays), the windshields of vehicles may actually screen much sunlight from hitting the solar charging device. So maybe if you’re willing to go out and scrape or otherwise defrost the windshield each morning – and be sure the car points into the sun for most of the day (more challenging in winter since the sun is low in the south in northern latitudes) it would work. But then if you’re out there, why not just start it up and let the alternator do the charging?

    Good points, but to your last question I would prefer to spent 10-20 seconds scraping the snow/ice off the section of glass in question than I would start up an engine and let cold oil circulate for no reason. Of course this all depends on how often the vehicle is really driven. If it’s every 1-2 weeks for grocery runs, maybe nothing is needed.

    Idaho windshield ice and frost must be a LOT tougher than Houston ice and frost (assuming that even exists) – ain’t no way to do that task in 10-20 seconds up here. It takes 2-3 minutes just to get the frozen-shut vehicle door open to FIND the scraper before one can even tackle the glass. 😁
    I get your point, but I’m not certain that circulating some oil in a sitting-in-freezing-cold engine every once in awhile is “for no reason”. Debatable on either side, IMO.

    Houston ice and frost is a joke, I can do the whole windshield in 10 seconds. I am talking about only de-icing the area of glass directly above the solar panel. Can you de-ice a 4×6″ panel in 20 seconds in ID? 🙂

    Even though I drive it every week or so, my 90 Allante has some parasitic current draws that will drain the battery in 2 to 3 days. The previous owner installed a battery cutoff switch which takes care of that problem. I bypassed the cutoff switch with a single wire to the radio (aftermarket at this point) memory keep-alive wire so I don’t lose my presets

    Standard trickle charger with a cord works for me. Solar stuff made in China doesn’t last, in my experience. Out of eight Chinese=made solar panel lights, only 2 are still working after 18 months. My extension cord with trickle charger, on the other hand, has been working for 10 years, so far, with no signs of dying.

    Yeah the extension cord is a good idea, thanks for bringing that up. It could be a tripping hazard if the car is street parked, but otherwise this is probably a better idea.

    Man, I sure agree with that, Jeff. I’ve got three battery charger/tender rigs (each by a different manufacturer, b-t-w) and all three are going strong after many years (at least one of them I know I’ve had for 14 years, ’cause that’s how long I’ve been maintaining a battery for my trailer winch).
    I installed outside outlets in strategic locations around my place where a vehicle can be parked years ago. I bought heavy-duty extension cords with plastic covers over the ends to protect from any rain or snow getting in. I can not only power a tender, but a heater if necessary. I see the downsides of an extension cord, I really do, but for the effectiveness I’ve witnessed, I’m in the “it works, so why look for another way to do it that might not?” camp.

    The few 12 Volt solar auto tenders I’ve seen have cigarette lighter plugs (awright, “auxiliary power socket” plugs to you youngsters) which would need to be plugged into an “always on” socket which is not all of them anymore.

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