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Full Version: Battery Tender 5-45W Automatic Solar Controller: to keep your starter battery charged
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This is a gizmo that is designed to trickle charge your starter battery, but it's powered by a 12 - 36 volt solar panel rather than plugging into a 120 volt outlet. It is basically a PWM solar charge controller, but designed to trickle charge your starter battery instead of charge a deep-cycle battery as quickly as is safe. It is manufactured by one of the most popular and trusted names in the battery trickle charger industry. It is designed to be permanent installed in your vehicle and connected to the starter battery at all times. Think of it as a one-way valve passing power from your solar system to your starter battery but protecting both from over-current.

Here is a link to the manufacturer's website: They also have other models that come with small solar panels and alligator clips.

The suggested list price is $29.95 but you can get it cheaper. If you are on the move, or have limited opportunities to receive shipments, you can order one at any AutoZone, for $23.99 with no extra charges for shipping or special order, and it will arrive in about a week. I ordered mine through an AutoZone in a town I was planning to pass through. And I didn't even have to pay until I picked it up. I tried other auto parts stores and hardware stores, but they either couldn't get it or it would take too long. 

I contacted the manufacturer, and the maximum current draw is 3.1 amps when it is in maximum charge mode, and it draws about 7 miliamps once the battery is fully charged to 13 volts. That's about .084 amp hours over the course of a night.

So, because this thing can be powered by 12 volts, and it only draws 3.1 amps, it is possible to simply connect it to your "house" battery, which is then charged by your solar panels. (If you charge your "house" battery solely from your alternator as you drive, then this would not help you much.) 

It took me about half a day to install it. Of course most of that time was running the wires where they wouldn't be seen. The gizmo only has four screw terminals. The screws are "captured," so use spade-lugs instead of ring-lug crimp-on terminals. (I never just wrap a stranded wire around a screw.) I used a 5 amp fuse at my house terminal box, and a 5 amp fuse between the charger and my starter battery.

It has a single LED that is yellow when charging and green when charged. So, I installed the thing on the side of the center console in my minivan so I can see it from inside my minivan or by looking in my driver's side window. 

It took about 24 hours to go from about 12 volts to 13 volts  charge on my starter battery. I'm guessing it would go faster if it was connected to a 36 volt solar panel. But then it wouldn't run overnight either.

I have been looking for something like this for quite some time. You can buy similar products from RV suppliers for about $180. But that was too expensive for me. I had even looked on the Battery Tender website before, but didn't find these. So, I think they are relatively new. 

Altogether, it cost me under $50 for the gizmo and the associated parts. I think that is totally worth it to not have to worry about my starter battery draining over the course of two weeks of locking and unlocking doors or putting my windows up and down. That has happened to me twice now. It was easy enough to solve by disconnecting my solar controller from my house battery and connecting it to my starter battery, to charge the starter battery from my solar controller. (Don't try to charge your starter battery directly from your house battery if the house battery is not also lead-acid. I think the chance of damaging your house battery is too great.) But that was kind of a pain in the butt because my electrical system is down under my floor. Now I never have to worry about that again.