CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

How bad did I fry my baby when I plugged a 24v solar panel into the cigarette lighter? :(

I wanted to use a solar panel to maintain my battery and thought it was 12v but it’s a 24v panel. I plugged into the cigarette lighter. Is this going to cause damage? After installing a second panel and a 12v transformer, I let the battery charge for 24 hours. Now, when I hook up the battery, the fuel pump clicks on and off constantly and I can’t turn off the alarm (however it’s an old alarm and needs to be replaced anyways, but that doesn’t explain why the fuel pump is tripping out.

That would be called a voltage spike.

Computers and modules don’t like voltage spikes.

Oooops!

Tester

1 Like

Sounds like a problem with the alarm system, are you able to disarm the alarm?

Unless this solar panel was at lest 10 square feet in size it is doubtful that it boosted the vehicles battery to more than 13 volts.

I agree with Nevada, the car’s battery would not allow the voltage to go high enough to cause any damage unless it was huge. BTW, any solar panel designed to charge a 12V battery will have an open circuit voltage of at least 19 V.

For which reason they have voltage regulators, the smallest standard voltage regulator is rated for up to 37 V. Computer circuits are made to work in a small range of voltages. Regulators provide that small range for any input 2V higher up to a maximum. The standard transistors worked on 5V, making them more energy-efficient required dropping that voltage. Modern ICs run on a bit above 3V. Regulators make the input voltage not-matter.

A spike is a transient that can slip through regulation, not too-high a constant voltage. A 24V spike is unlikely damage a 12V circuit. Of course a 24V solar panel probably delivers 30+V into low loads.

Can you elaborate on this? Do you now have two panels or you replaced the first one with this new one? Describe your 12 transformer. Technically speaking, a transformer won’t work for DC voltages. Is it actually a regulator or converter?

How long did you leave the 24V panel connected?
Did you ever measure the output voltage from it when it was connected to the lighter socket?

Do you have a link to the panels you are using and the transformer/regulator? I’d like to see how much power they can produce/handle.

Ok. That’s comforting. I tested the battery and it was only putting out 9 volts. So, the panels weren’t charging as fast as I thought. Possibly the voltage was so low, that was why the alarm was freaking out. I’m charging the batter today on an AC charger. I’ll try again once charged.

But if the entire car is expecting 12v, wouldn’t a higher voltage fry something, wiring or some other component?

Gotcha. I think the open voltage on this one is 23 volts. So then the battery is probably ok, but I plugged it into the cigarette lighter. Do you think theres anything else in the system that could be damaged?

As long as the battery cables are connected, the battery will hold the voltage below any potentially harmful level. If you were to measure the open circuit panel voltage vs the voltage with the panel connected to the battery, you’ll see a big difference. Everything should be OK whether you plugged the panel into the CIG lighter or attached it directly to the battery.

1 Like

Ok,
I recharged the battery and it’s cranking up and runs good. Clearly I don’t know much about how electricity works. haha… Now I see that these solar panels are only putting out about .2 amps on average and at that rate would likely take a long time to charge a dead battery. hopefully with a full charge, the solar panel will maintain the battery for a few weeks at least. Any idea what the power draw is on the average 90’s era car at rest?

If the battery is good why the need for a solar panel ? Are you letting this thing set for long periods of time ? I had a truck that sat for any where from a couple of weeks to more than a month and it always had enough charge to start.

If you want to charge your car’s battery using a solar panel, and you know your panel outputs 12-15 volts DC max, good idea to disconnect the battery’s ground first, then hook up the charger leads directly at the battery posts. This minimizes the possibility of the car’s electronics from being damaged. Charging the car battery from the cig lighter isn’t the way to go imo. As posted above, to properly charge a battery using a solar panel requires using a battery charging electronics package in between the solar panel and the battery.

@George_San_Jose1 - I just ordered a battery tender to do the go between. How does this help? If Volvo_V70 is correct, then my battery will discharge regardless without enough juice. Will the battery tender electronics preven a deep discharge / ruined battery? I don’t have a garage so, can’t keep it hooked up to a legit battery maintainer.

I have thought about using a battery quick disconnect as you suggest for the solar charging, but then every time I drive it, it will have to go through the OBD cycle all over again. Also, I’ve heard that each discconct/reconnect of a battery can also damage the car’s electronics.

@VOLVO_V70 70 - yes it’s a project car. The risk is that I don’t know how deeply it is being discharged on a weekly basis. Deep discharge ruins the battery.

A battery tender has all the monitoring electronics needed to keep your battery charged, but not over charged, as long as it’s getting enough voltage and current. Do the panels match the tender specs’ requirements? Do you get enough sun where you have them?

What exactly is the layout of your system now?

I think it is absolutely safe to “maintain” a flooded-cell lead-acid battery with a moderately-sized solar panel, even if it pumps out voltage higher than 14.5-14.8 volts needed for high-current charge.

The absolutely worst thing which may happen, it will charge the battery to 100% and at that point battery will start gassing out.
Given the meager current solar panel would pump, you would need NASA-grade instruments to detect that gassing out :slight_smile:
Really, that concern versus guaranteed damage of sulfation from long storage of undercharged battery, not even comparable.

My example:

  • I have a car which sometimes has to stand for few weeks between starts
  • Battery is 1.5 years old
  • I have remote start system installed, adding around 12 mA extra draw
  • After 2 weeks, multi-meter test shows battery is something like 30% drained
  • Bought a solar-panel maintainer in HarborFreight, it claims to pump something like 2W of power, my multi-meter test shows more like 1.1W in full sun
  • Now, the battery is always full when I need it

That HarborFreight panel pumps around 19V with no load and connects to always-on cigarette lighter socket, no ill effects, and even theoretically it can not make for any ill effects, as under load it gets to the normal voltage range, simple Ohm law :slight_smile:

Now, I would not suggest to use the same method for the AGM batteries, they are more sensitive to over-charge.

1 Like

The solar panel I have now is putting out about 7 volts at night under the parking lot light. In full sun, it’s probably closer to 20 but not sure… I’ll have to measure full sun output. I think the tender has a pretty wide input range.

How long have you had this setup on this car? No issues with damage to electronics / CPU / etc…?

For a little over a year I’ve been using a 10W solar panel on the top of the dash.
In bright sunlight open circuit voltage is ~20V, connected highest I’ve observed is 13.4V.

1 Like