Solar Car Battery Charger question

[Bear]

I use my truck primarily on weekends driving 72 miles to our trailer. So truck sits three to five days. I purchased this solar charger to hopefully keep the battery from discharging too much. The two power outlets on the dashboard show power when the key is out so I plugged the cord into one of them. After two or three days when I go to start the truck I get a warning that the battery is low ( showed 11.9 v ) but started right up and went to 14,3v . My question: is there some sort of one way blocking diode to prevent reverse flow from the outlet to the battery? running the line from the panel directly to the battery would be a pain.
PS: unloaded voltage at the panel cord is 17.2v.

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[mk378]

There is not going to be a diode, however there is often a "battery saver" relay which turns the outlet off after the truck has been parked for some time with the doors closed. Test by plugging in a device such as a USB charger which lights up that you can see from outside. Park for a few hours then check if the device is still live. Do not have the key fob with you when you walk up to check.

 

[Bear]

There is not going to be a diode, however there is often a "battery saver" relay which turns the outlet off after the truck has been parked for some time with the doors closed. Test by plugging in a device such as a USB charger which lights up that you can see from outside. Park for a few hours then check if the device is still live. Do not have the key fob with you when you walk up to check.

Thanks. Yes, I've checked that. I have plugged in an air ionizer which has a light and that stays on, so I know there's a continuing power flow. And I've swapped the two devices out to make sure both stay hot.
Just saw the other threads on this very issue. So I guess I'm gtg with this.

 

[meep]

Glass blocks 3/4 of the usable light for a PV panel, so they do best on the roof or in the elements. A diode can be added, but there is some loss because they effectively loose 0.7 volts. A cheap solar charge controller on Amazon can do very well in this application. Spend less than $20.

I used this one for a friend and it’s lasted 3-4 years now.

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