I have a 2018 Ram Promaster city cargo van with OEM 12v 70AH 120RC Sealed Led acid battery and a 160a alternator.

Sitting on top of the battery I have a 1200 Watt pure sine 110v inverter connected with 2 gauge wires and then have a 12 gauge run to the back of the van. I also have a single 12 gauge line running from the batteries hot to the middle of the van that gives in an always on 5v USB source.

To date, my power needs in the van are minimal. when I need 110v, it's for very short periods of time (15 minutes or less) to run 500-800w appliances & tools and when I do I am usually parked with the engine running.

When I've slept in the van overnight I run a 1a 5v fan from 10pm to 6am. My battery has never been below 10v, but I do plan to install a 10v low voltage disconnect to the 12v hot line to be safe. My inverter already has a 10v low voltage disconnect.

However.. In the future, I would like to install a 200 Watt 18v 6a (max) solar flexible solar panel on my roof. It might be overkill, but the 32w x 42l size fits perfectly and will block a lot of sun and help with mid day interior temps. As I ponder how to benefit from this free power, NOTE: I am NOT looking to install or sleep with batteries in the passenger area. I would like to install a ceiling vent rated at 12v 3a when on high (specially a Max Air 14x14 fan 12v 900CFM fan). While I think I can easily run the fan overnight on low (12v 1a) and have no issues starting the van, I would like to run the fan on high (12v 3a) when the sun is strong during the day. Again, I have no plan to drain my 12v battery like a deep cycle. And finally, as I anticipate having more power than I need during the middle of the day even with a 12v 3a fan running, I would like to freeze the content of small 50w 12v freezer (Alpicool C15) during the day (9am-9pm) and then keep it off and closed overnight. I would anticipate interior ambient temps to range from 90F midday to 80F night. The ultimate goal being to keep the van cooler during midday and keep Milk and Cheese from going bad for a week. In a pinch I can always find some ice if need be.

So my questions...

(1) How can I connect the solar panels 20a charge controller (200W 18v 6a panel)to my battery so as not to impact/interfere my cars charging system and visa-versa? do need a switch or isolator? I am presuming my charge controller will have some protections like a reverse diode for night fall leak back.

(2) If later on I want to upgrade my battery to a new dual purpose 12v 100AH 200RC 830CCA battery will my 160a alternator be okay with having to work harder to recover for longer periods of time? Again, I do not anticipate ever draining my battery like a deep cycle battery. I will tell you I have experience with another automotive 160a alternator and a 90AH/140RC battery (running 2 amps overnight) that would get close to 10v many times and never had any issues.

(3) also wondering about Charge controllers that have an output ports. Are their controllers with output ports that can be regulated to 12v? how are they different from connecting directly to the battery? I might not be understanding schematics, but seems like there are controllers that offer Low Voltage cutoff and hours of operations from those ports.. is that right?

  • \$\begingroup\$ How did this work out? What did you end up doing? \$\endgroup\$
    – Russell McMahon
    Dec 28, 2020 at 16:55

2 Answers 2


I would separate the solar panel with charge controller and its own batteries. That way the vehicle can be left as "standard" - handy if you need an ordinary garage to fix it...

There are charge controllers available where you can also charge the leisure batteries when the engine is running, and with some you can control that facility.

And, as for fuses and isolators, YES you need them - controlling is much better than fire...

  • \$\begingroup\$ Thank you for your comment. The question is specifically about not having separate batteries for the solar system. I will have a charge controller and fuses where they make sense, but mostly I am wondering If idea is safe enough and if I would just be running the charge controller to my battery without worrying that at some point both solar and the alternator are sending charge to it. Sorry.. In this simple design (one 200w panel, one 20a solar charger and one primary starter battery) specifically where do I need fuses and isolators? \$\endgroup\$
    – Hell.Bent
    Feb 18, 2019 at 12:48

(1): I would consider a charge controller which is designed to charge a 12 V Lead-Acid battery (like your car's) from the solar panels. Any self respecting charge controller will indeed prevent the battery from discharging when the solar panels do not generate power. Also when the battery is full it will stop charging. When the car's engine is on both the solar panels and the alternator will charge the battery.

(2): Increasing the battery's capacitor does not mean that the alternator will have to work harder. The alternator generates a charging current and that current does not change. Only when the battery is empty will it take longer to fully charge it. There's more energy in a larger battery so that makes perfect sense.

(3): No idea, you will need to study the manual/datasheet of the charge controller. You simply need one that can work with a 12 V Lead-Acid battery and that's it. I would really advise not to look at "schematics" as that means "build yourself". I advise you to buy a ready-made unit. Building one yourself only sounds easy when you do not know yet how to do that and what issues you will run into. And you will run into issues, believe me. I am experienced at building electronics projects yet even I would first try to find a ready-made solution as it will save me a lot of time and frustrations.

  • \$\begingroup\$ Thank you for your comment! yes, was just reviewing some wiring instructions manufacture included. Not building anything.. just wiring. So you don't think I need any additional isolation or switches to keep both sources from trying to charge? I'd hate to have to remember to turn off solar charging when driving. \$\endgroup\$
    – Hell.Bent
    Feb 18, 2019 at 12:55
  • \$\begingroup\$ So you don't think I need any additional isolation or switches to keep both sources from trying to charge? Yes as long as the car battery can handle the combined charging currents. It depends on the car battery if that is OK, it probably is as car batteries can handle large currents. When you upgrade to the larger battery then it should become even more OK as the larger battery should be able to handle larger charging currents. I did not mention fuses but I agree with Solar Mike, you do need fuses to prevent disasters by currents becoming too large. \$\endgroup\$ Feb 18, 2019 at 12:59
  • \$\begingroup\$ Currently I have a 20a resettable fuse on the hot 12v going into the van. My 1200 inverter has a built in fuse/shutoff and Low voltage Disconnect built in. \$\endgroup\$
    – Hell.Bent
    Feb 18, 2019 at 13:03

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