My old car battery died because I leave it parked too long. I replaced with new battery and this time hooked a solar trickle charger to it to keep it from discharging. (it's parked in public space)

Solar charger: https://www.amazon.com/Sunway-Solar-Maintainer-Motorcycle-Powersports/dp/B01MYVUSRH

I left the car parked for 6 days, I measured with multimeter before and after the battery. Before 6 days, battery rested at 12:45, when turning ignition on it goes to 10:49, engine nrunning at 14:22. After 6 days battery rested at 11:94, turning ignition on has it rise to 9:96, when engine is running it rises to 13:51 and increasing higher.

The car is small and has a small sized battery.

The solar charger itself in that parking spot which has okay clear sunlight not straight at the sun, when I measure it during day it reads 13. Manual for it says it should be 15 minimum at full sunlight. What I noticed is in places where it's really sunny the solar charger turns on the digital clock even when car is off. So clearly the car receives the power from the charger. But in my usual parking spot that measures 13 with multimeter, doesn't do that.

Edit, what's this red blinking light behind radio? what's this red blinking light?

  • 12
    "the solar charger turns on the digital clock" Your solar charger might not be reaching the battery when the car is off. If you're plugging it into the cigarette lighter/power outlet and the outlet is controlled by the ignition switch, then when the car is switched off there is no path for power to flow to the battery. I have a 95 Dodge Ram that is like this; I can plug a solar charger into the power outlet and the radio comes on and it doesn't charge the battery.
    – Rob K
    Nov 28, 2018 at 15:42
  • 4
    @RobK - why not use this as an answer?
    – Tim
    Nov 28, 2018 at 16:45
  • so the digital clock has nothing to do with the battery you mean? im plugging it into cigarette lighter, but i dont know if its controlled by ignition switch. what i know is the digital clock turns on when i plug solar charger, with the car completely off.
    – Altoban
    Nov 28, 2018 at 16:59
  • @Tim because that is the answer. That and he's using the wrong kind of battery. Nov 28, 2018 at 18:03
  • 1
    @RobK's comment clearly needs to be an answer. The clock behavior means it's certain that the cigarette lighter port is not connected to the battery when the ignition switch is off. There's probably also something drawing too much current when the car is off. Nov 28, 2018 at 23:34

3 Answers 3


12.45 to 11.94 in 6 days seems like a significant discharge. You should be able to leave your car for a month or longer and still have enough power to start. At your current discharge rate, I wouldn't be surprised if you couldn't start after 10 days. I would be looking at parasitic drain. Here is a guide. You can find YouTube videos also.

Some thoughts.

  • How much does your battery discharge in 6 days without the solar charger?
  • If you disconnect your battery and leave it for 6 days, how much does it discharge?
  • You could hook up your multimeter as demonstrated in the parasitic drain test to see if the battery is charging or discharging while the solar charger is supplying energy. The digital clock turning on might be part of the issue. Other electronics may be waking up and causing a larger load on the battery.
  • It's a small battery not a normal big battery, could that be it? I guess the next test is without the charger but it's moot point because it's still discharging. The digital clock turns on only when solar charger is plugged in and full sunlight. My old battery died only after 7 months, I thought it needed a trickle charger.
    – Altoban
    Nov 28, 2018 at 15:17
  • 1
    As long as it is a lead acid battery designed for a car, it shouldn't matter much. Are you using the battery specified by the OEM, or something different? 7 months is extremely short. How did you verify it needed to be replaced? Lead acid batteries can typically be recharged after dying. A load test is the best way to determine the strength of the battery in how many amps it can put out over a couple seconds. It sounds like you have a parasitic drain issue that needs to be addressed. The trickle charging is a band aid for the real issue.
    – rpmerf
    Nov 28, 2018 at 16:03
  • i got it replaced for free so it doesnt matter but the main issue i dont know about, and sounds like really hard to fix.
    – Altoban
    Nov 28, 2018 at 16:57
  • 3
    I believe the main issue is something is drawing more power than normal. This could be as simple as an interior light that doesn't turn off, or a phone charger left in a socket that doesn't turn off when the car does. The best way to test this is a amp meter between the battery and battery cable to read how much power is being pulled from the battery. The draw should be less than 500 mA. If it is more, start pulling fuses one at a time until it drops. Once you see it drop, you know what circuit to follow.
    – rpmerf
    Nov 28, 2018 at 17:20
  • I measured some more with and without solar charger. After driving car the battery was at 12.94. After one day, resting battery with solar charger was 12.54. Second test without solar charger, after driving 13.19, after one day resting its 12.55, showing no difference I guess.
    – Altoban
    Dec 3, 2018 at 16:06

Solar chargers are impacted by many factors:

  • Glass: especially tinted glass filters light, you won't get maximum charge behind glass
  • Direction: solar cells work best when pointed at the sun
  • Sun intensity: you won't get full power on a cloudy day, and in the winter sun is less intense

If you want to test your charger then take it out of the car and see what it produces when when pointed right at the sun on a good day. If it produces the expected power then you know its one of those factors at work.

Keep in mind that a trickle charge is just that, it does not recharge a car battery, just reduces losses. It isn't going to keep up if you have a battery drain, which I agree with @rpmerf that you have.

  • On average it reads 13 by multimeter even when out of car, unless there is strong sun presence. I think this is lower than the min 15 with full sunlight the manual says but what concerns me is, would 13 be enough to keep battery charged?
    – Altoban
    Nov 29, 2018 at 22:08

The cigarette lighter/power port is almost certainly controlled by the ignition switch. When the ignition is on or in the "accessory" position, the circuit is completed to the battery and power is provided to the port. When the ignition is off, there is no path from the power port to the battery and thus the electricity generated by the solar charger cannot reach the battery to charge it.

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