I don't think the solar battery chargers have the same level of sophistication is a battery tender IE they don't shut off as a battery get stopped up so they're probably overcharging in a little bit but I don't know that for sure. I know they do keep it topped up right to the top. I would not leave them outside, I throw them on the dash of the truck and park them facing the Sun

 

Simple solar chargers are usually self-regulating, so they don't overcharge. Sophisticated solar panel systems need a regulator to keep from overcharging. I'd be surprise if the HF charger overcharges. Easy to test battery voltage on a sunny late afternoon. Less than 14.4 V or so is not overcharged.

 

I have a 15W Battery Tender brand solar charger (and plug-in chargers), they are regulated and won't overcharge. In my D2 I also run a "dashcam" that keeps going when the ignition is off. There is a voltage sensor inline that will shut down the dashcam when the voltage gets low. For extended periods I like to keep the solar charger going so that dashcam keeps running too.

 

i used to let mine sit for 5-9 days ands its cold here in ontario canada this year and used to fire right up maybe you have a weak battery

 

I feel lucky. I went 3 weeks in December and it started up no problem. I do like the idea of a solar charger.

 

I don’t think 3 weeks should be too long. I can even leave my P38 for 3 weeks without a charger and it will fire right up.