Solar Panel Keeps Car Battery Topped Off Through OBD-II Port

Up until the 1980s or so, a mechanic could check for shorts in a car’s electrical system by looking for sparks while removing the battery terminal with everything turned off in the car. That stopped being possible when cars started getting always-on devices, and as [Kerry Wong] learned, these phantom loads can leave one stranded with a dead battery at the airport after returning from a long trip.

[Kerry]’s solution is simple: a solar trickle charger. Such devices are readily available commercially, of course, and generally consist of a small photovoltaic array that sits on the dashboard and a plug for the lighter socket. But as [Kerry] points out in the video below, most newer model cars no longer have lighter sockets that are wired to work without the ignition being on. So he chose to connect his solar panel directly to the OBD-II port, the spec for which calls for an always-on, fused circuit connected directly to the positive terminal of the vehicle battery. He had to hack together an adapter for the panel’s lighter plug, the insides of which are more than a little scary, and for good measure, he added a Schottky diode to prevent battery discharge through the panel. Even the weak winter sun provides 150 mA or so of trickle charge, and [Kerry] can rest assured his ride will be ready at the end of his trip.

We used to seeing [Kerry] tear down test gear and analyze unusual devices, along with the odd post mortem on nearly catastrophic failures. We’re glad nothing burst into flames with this one.

47 thoughts on “Solar Panel Keeps Car Battery Topped Off Through OBD-II Port

  1. Better than the opposite, learning that I had to unplug my older bluetooth-OBD dongle whenever I left the car to sit for over a week or two or I would have a flat battery.

      1. I still have no idea why that is a permanent connection.
        It should be connected to the convenience timer like all other non-critical loads, because there is no way that an external OBD device is a critical load.

        1. Because it’s a diagnostics connection. You plug in the diagnostics device and it measures the battery voltage through it – which would be ambiguous if there was some logic or relay to turn it on first because that might then be broken.

          1. Right. There’s still not much on the market for an OBD-II port that isn’t a diagnostic device or some sort of engine computer hack. Nothing that would normally be plugged in constantly. Maybe since cars are removing their cigar lighters the OBD will be the new port people plug their mini-fans and other gadgets into.

            New cars often have USB, on the stereo most likely. But that’s very limited compared to raw 12V at a few amps. Maybe USB-C will fix that. It’s amazing how cheap DC-DC convertors are nowadays. Still I wish they’d kept the lighter. Because I fucking smoke, for one reason, but that’s not even the most important one.

            Don’t get me started on health nazis… It’s because the middle-class, doctors and proprietors of society, tend not to smoke, and the working-class do. And they think working-class people are stupid, which is why we’re poor and have to do unpleasant jobs. That MUST be the reason because otherwise it wouldn’t be fair, would it? And the fact working-classes smoke is just, by circular logic, proof of their stupididity.

            That’s why bright colours and now banned on cigarette packets in the UK. Seriously. They’re all plain brown with nothing but the brand name in tiny letters at the bottom. And lurid photos of corpses because they don’t think we can understand what “death” means without examples. Many of which surely are nothing to do with smoking. They had some guy’s leg that had clearly been dead for some time, with mortuary-quality thick stitches, not nice operating-theatre ones.

            Some people gotta meddle. They bitch and tut about a “patriarchy” while acting completely paternalistically to we lesser mortals. Humans are shit at irony up-close.

            Ah shit, I got myself started, sorry.

          2. Some car’s computers will not go to their low power state while there is something plugged into the OBD2 port as well.
            My 2000 Jetta has no issues with a Bluetooth OBD2 adapter plugged into the port, but a friend’s 2012 Mini Cooper would have a flat battery the next morning with the same adapter. Unplug the adapter, and his car was fine for days.

  2. I’ve been meaning to do something like this on mine. I had some issues a while back with the battery mysteriously going dead on me. Even after I replaced what I thought was a faulty battery. But then sometime after I started putting it on the charger once or twice a week the issue went away. I haven’t had to charge it for a while now and it’s working fine. I’m guessing in all of my fiddling I must have fixed an intermittent issue that was draining my battery.

      1. Lol… You can say that again. A real dodgy alternator. Because if your alternator is working 100%… Then you wouldnt need to charge your battery at all. Never mind once or twice a week. Its the alternator’s job to charge the battery while driving.

        1. depends on how long your commute is each day – I run into problems every few months as I dont run the car long enough for it fully recharge – yeah i could ride a push bike and some days I do but….

    1. depending on what car/year/model there are all sort of circuits that can get power when a car is left unused, eg door switches can leak power when they get corroded, relays and around relay terminals and then the problem disappears due to a knock or slam or a door or you wobbled the relay breaking away the conductive path. this can also happen at starter motors if you get muddy often or by the sea. I use the 10Amp current setting on cheap multimeters between neg terminal and body neg and connect it inline you will see any leaching current easily then it’s just a case of pulling fuses to see while is responsible then leave that fuse tied to steering to remind you to fit it prior to using the car if it’s standing for long time or at least a idea of what path to trace and inspect.

  3. And now car manufacturers, please put solar panel on the roof of all electric cars. This will reduce other charging energy consumption by at least 10%, since car will be charged 12 hours per day.

    1. Unless you are parked in a covered lot, or indoors, or it is cloudy, or you live in a region that does not get 12 hours of sunlight, or you live in a region where the sun angle is not conducive to ideal charging, or your panel gets dirty, or…

    2. No it won’t. A randomly pointed solar panel with random shade and bird poop on it will on average make less than 5% its nominal power.

      In fact, the amount of energy you collect from leaving your car in the sun for an hour is about as much as you’ll lose when you turn the AC on when you go, because it’s scorching hot in the car. You have 300 Watts of solar panels on your roof collecting 0.3 kWh of energy over an hour, but your AC demands 1500 Watts or more at full blast – so you’ll use up the energy in the first ten minutes. It’s better to park in a garage or under a tree.

      1. Well done completely missing the point there.

        The solar panel is there to offset some of the lost energy used to power things like keyless entry, the alarm system and telematics. It’s not there to run your AC.

        The specific problem this is solving is when he parks his car for 2 weeks while he is away on business. It apparently works fine even with the panel inside the car, and the windscreen does cut some of the light out.

        Nissan actually did this. You could get a spoiler mounted solar panel for the Leaf and it did its job.

        1. Well it is, once you set off driving, because the car will be hotter, so a solution to one problem creates another.

          With a tiny solar panel on a spoiler, that’s different, but you’re not going to reduce the charging power consumption of an electric car by 10% with that.

        2. Yep. I’ve been wanting one of these for my 08 Prius. There’s something dodgy in the electrical, but the battery and charging circuit test out fine. Something is draining the battery if the car sits parked for more than 3-4 days without starting.

          I’ve tried chasing down the phantom drain, but can’t seem to isolate it. Right now, I just remember that I have to plug my car into the batter charger the night before I go back to work after a long weekend.

          I’ve almost purchased a solar panel like one in the article, but haven’t pulled the trigger yet.

    3. A suggestion that only makes sense as an optional – If you don’t have parking at your current frequent trips that is in the sun at all its pointless. For the right people its going to be great but like electric cars in general won’t work for everyone.

      Its also a significant energy investment to make solar panels, to get that back you have to assume a reasonably long lived car, out in the open, with battery capacity to fill or a connection to the grid (useless to capture it if there is nowhere to put the energy). Also have to take care of weight while solar cells are themselves not heavy they tend to be bonded to glass for protection which is.

      1. And with the panel technically only 10-20% efficient, it’s like painting your roof black – it’s going to heat up the car while it’s sitting OR driving around in the sun, so you then need your AC more.

        Sun shines down at 1000 Watts per square meter. The solar panel and converter makes 150 Watts out of that and leaves 850 Watts to heat the roof – because a solar panel is not reflective in the infrared like a sheet of steel or aluminum. You need more power from the AC to keep cool inside the car than the solar panels are providing you.

        1. Or to compare with the usual painted finishes: white paint on steel absorbs 30% and black on steel about 75% of the incoming sunlight at all wavelengths. Just because it’s black to the eye doesn’t mean it’s not reflective at invisible wavelengths – especially if the paint coat is thinner than the wavelength in question because then it’s transparent to it.

          Either way, a solar panel is very effective at absorbing sunlight and turning it to heat, because it’s designed to absorb sunlight in the first place. If you replace a regular silver gray roof with a solar panel, that area is going to heat the car at least twice as much.

          1. Makes sense to me, David. I know a bit about solar panels and physics in general. Solar panels do give off a lot of heat compared to the electricity they generate, like Luke says. Before you slap him, does he at least get the dignity of a fair trial? What particularly is he wrong about?

            I say we USE that heat. Stick a load of Peltiers under the roof. With gigantic cooling fins. Yeah it would fuck the aerodynamics a little bit, but while the car’s stationary it’ll provide energy to run OBD-II port.

        2. In many places in the world this free heat would often be considered a bonus not a negative. And as Solar cells can tolerate pretty huge heat you can always put reflective and insulating layers underneath (or on top of the cell with the right material selection) to largely mitigate internal heating if it is a problem for you. Even if you don’t the power generated can be used to keep cycling with external air so its not much hotter in the vehicle than outside – that needs very little power so you should still net gain for many users..

          Its not all about added range, but its certainly possible to get 50KM+ of range a day out of solar on vehicles. It passes the mathematically possible test with ease and as some people have actually done it with their camper van things is clearly not impossible in the real world – yes its a bigger area than most cars and there are many other caveats to how far you can actually go but in practice I remember seeing a few cases of people claiming that sort range as some kind of average (think it was usually summer days average and from folks living near the equator which might help meaningfully for efficiency but still its not rubbish).

          1. Yup solar panels perform near spec on summer days, pointed directly at the Sun, in sunny Equatorial countries. Outside of those circumstances though performance drops off really dramatically. On an overcast day in, say, the UK, you might get 5% or less. Car roofs point straight up, which isn’t facing the Sun anywhere outside the Tropics.

            There are solar-powered racing cars for contests they hold. They look like huge bicycle helmets, the ones that make cyclists look like giant dongs. Underneath their lightweight shell there’s basically a recumbent bike, all as lightweight as possible. Every cm2 of the shell is covered in solar cells. They race them in deserts and other similarly suitable places. They can go impressively fast. But that’s nothing like an ordinary car with room for passengers, that can take a bump, and might need to go somewhere in the rain.

            Yeah that’s what batteries are for, but the weight of the batteries would stop it from working.

            Future electric cars might have solar cells on them. But it’s probably more efficient to put the solar panels on the ground, aiming at the Sun, and deliver that power through the grid. Or even put the panels on the roof of your house and store it til your car comes home.

  4. Some models of the Prius have a solar panel installed in/on top of the roof.
    The purpose is to run the cabin ventilation system while parked, and not killing the battery.
    Prevents entering a hot car in summer..

      1. Cracking a window lets thieves in. A pathetic little fan might be the best you could do in those circumstances if you don’t want to spend any battery power. Though yeah, it’s 99% for show, to keep the fart-huffers in San Francisco happy. Most people don’t know how feeble solar panels actually are.

        Maybe Dyson could kit them out with their bladeless fans, they’re apparently good for the environment somehow. That’s if you believe them, and they’re not even bladeless so that’s not a good start. Currently Dyson are expanding into useless gadgets for public toilets, “hygienic” hand dryers that send out a strong “blade” of air that pushes your hand onto the other side of the dryer. Where everyone else’s filthy hands have been. Genius. Then there’s the taps that also shoot out a blade of air to knock your hands into the filthy sink. Dude, just wipe your hands on your pants, it’s a solved problem. Anything recirculating germ-filled toilet air onto you isn’t particularly hygienic.

  5. “Up until the 1980s or so, a mechanic could check for shorts in a car’s electrical system”

    So a family friend took a beach trip on a sunny Sunday, and on the way back his car was overheating. Monday he leaves it at a mechanic, later picking it up the convo goes something like.
    FF:Is it all good, what was it?
    M: Fine, it was shorts in the radiator.
    FF: Shorts? In the radiator?
    M: Yeah, causing the overheating.
    FF: What kind of idiot do you take me for, the radiator isn’t even electrical.
    M: I’m telling you it was shorts.
    FF: The &$^& you say, not even the fan on this is electrical, it’s fanbelt driven. How the hell could shorts in the radiator even happen.
    M: *sighs, goes in the back, comes out with a pair of beach shorts that look worse for wear…* THESE WERE BLOCKING YOUR DAMN RADIATOR.

    They figured that early on the drive back, they must have blown off another vehicle from the beach, either tossed loose in a truck bed, or forgotten spread out to dry on a trunk or roof or dangled on an antenna. Then got blown in through the grille, smothering the radiator.

    1. Not on any modern car. The least worst thing would be the radio forgetting it’s security codes, and locking up. There’s a few things in cars that rely on constant power. Which is pretty stupid really that they don’t fit them with a backup battery, a long-life primary lithium cell, say, 3V to power low-power RAM.

      It’s actually a problem for mechanics, they have to make sure 12V is provided to the relevant bits, even when they’re removing the battery. Cars should have some sort of secondary power system at, say, 3 or 6V, just for this. Either a lithium cell in a box inside the glove compartment, or even a holder for 4xAA. Or even 4xD cells, they’d basically last til long after the car was rust.

      Even a watch cell inside the stereo would do to keep it’s settings and codes. It could be mounted behind the removable keypad, and replaced any time as long as the main battery has power. When it’s dying the stereo’s screen could tell you. Or it could play back an audio message.

      But nobody really complains about it, car batteries don’t go flat often, and annoying mechanics isn’t something car makers worry about. The “approved” mechanics are their partners in crime! The independent ones can go to Hell anyway.

  6. This is clever, and a good way to think through the issue, but I would feel very much better about putting a dedicated line with a diode and properly sized fuse directly to the battery. Murphy’s law being what it is, a dead battery is much better than CAN bus gremlins caused by something inadvertently dodgy in the charging setup letting the magic smoke out somewhere,

    1. There are commercial devices like this for use by the dealership that also plug in to the OBD2 port. The +12V is fused though, so make sure that your panel can’t blow it or you may have trouble at the next emissions check (some interfaces are powered through this pin, especially isolated ones)

    1. Becoming less of a worry as panels get dramatically cheaper. In the past though it has meant panels needed protecting from thieves by a tough plastic cover, which of course filters out useful light.

  7. Yes, solar panels and cars go very well together. I bolted some harbor freight amorphous panels to my car’s roof rails for camping and hooked them up to a spare 12V in the back. It needs jump cables to charge the actual car battery, but it works great as a charger in the middle of the forest/desert/etc.

  8. Some Volkwagens used to ship (literally) with one of these attached (to the OBD port, in fact) and in the car. Dealers were supposed to remove them before selling the car, but many neglected to do so. For a while, you could find plenty of these at good prices on Ebay.

  9. Thanks,
    Have supercapacitors on my car for last few years just fine go start car, no heavy lead acid, which keeps enough start power a few days whilst slowly self discharging even when all accessories like radio memory feed off. Will be installing a mix of batteries and solar next week or two, seems like really simple stuff. I’ll add the obvious diode protections and charge limiting for batteries to supercapacitor connections, cheers

    1. I can sort of see the point of adding super caps to provide extra starting current. Though aren’t the really high-capacity ones supposed to be limited to quite a low charge / discharge current? The dielectric in there is ludicrously thin. They might not be happy with starting a car. They also of course have much less storage capacity than a battery, that would worry me after a while with the radio etc.

      If it were me I’d just keep the lead-acid, I’ve never heard of anyone having a problem with theirs. Maybe add the caps on top if you like. Yeah they’re heavy but so’s everything else, a car can weigh a ton or two. Twice that if it’s American. Doubt you’ll get any noticable benefit in acceleration or fuel economy.

  10. No, it’s active ventilation instead of passive ventilation AKA cracked window. A car with closed windows in the sun will turn into a child/pet killer inside 15 minutes. If you leave windows open an inch or so, that helps, but not nearly as much as actively pushing outside air into the cabin. A small PV panel to supply current to an inline fan does wonders – I know because I’ve tried it.

    A PV panel on your dashboard can look a bit ugly if it’s not integrated, but that’s a design issue.

  11. I measured the standby current draw for a VW golf 2008 as around 150mA going down to 30mA draw when car locked. You have to wait for the car to go to some low power mode to see this. So the 5mA current will help but i think 20 to 30 mA is required to start really offsetting the parasitic draw when parked. If you have bright sunshine and 200mA then you really do have a good trickle charge. The problem in europe is some airport car parks are undercover. I still carry jump leads still and with one vehicle i had a spare battery !

    1. You can get those emergency car-starter gadgets pretty cheap now. It’s basically just lithium batteries in a box. If I drove I’d get one of those. Often come with built-in handy stuff like a torch or even a compressor.

      They’ve been around for about 30 years. Did the early ones use lead-acid batteries? Lead-acid has always been good at taking abuse without complaining.

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