An off-grid solar car charger

The plans have come to life.

It all came together as designed - placement of components first, then wiring, PV isolation switch, PV mounting rails and wiring, then panels last of all.

Total spend was approximately AU$5,000

• $1270 for the panels and mounting hardware from Bunnings
• $1300 for electronics - MPPT charge controller, inverter from EnergyEco
• $1019 for the battery from Amazon
• Approximately $1000 for wiring, connectors and tools (Amazon, SparkyDirect, Jaycar)
• Probably a few hundred dollars for things from Bunnings I've forgotten to count
• Not including my labour but a lot of that was learning
• Also no Government subsidy!

General arrangement

Battery is at the bottom, there's a corflute box that fits snugly over the top to keep the weather off and acts as a bit of a blanket in winter. Above that is a home car charger enclosure from Bunnings that was about the right size to serve as the wiring box.

The blue box at the top is the inverter.

The panels are 21.6kg each and needed a bit of thought as to how to lift them onto the roof of the car port. I ended up making a plywood sled on rails with blue teflon slides and a small boat winch which worked brilliantly.

The lifting sled

Inside the wiring box. The portable charger hangs inside and is powered from the electrical socket. Blue box at the upper right hand side is the solar charge controller.

The panels fully installed

On the fine sunny day in  October below, the battery starts charging from about 50V at dawn, and reaches about 65% full at 54V by10:30am. The inverter restarts by coming out of the low battery alarm state at that voltage and starts charging the car. By this time there's enough solar (~2400W) to slightly exceed the draw from the charger (~1750W) after losses, and the battery keeps charging slowly throughout the day. After about 3:30 the panel output can no longer keep pace with the car charger and the battery starts emptying after reaching a peak of about 85% full. By around 7pm the inverter switches off on low voltage at 48V.

Note the slightly truncated top of the yield curve, due to running up against the MPPT's capacity limit.

Total output from the MPPT over the day is 21.3kWh.

The car went from 35% full to 51% full, which amounts to 14kWh.

So the overall system is only about 67% electrically efficient. The rest is lost by converting voltage so many times; 130V DC at the panels down to 52V DC at the battery, up to 240V AC at the inverter, and then up to 400V DC in the car.

Another thing that was a bit unexpected is that the inverter trips off on high temperature trying to run flat out all day at 2300W (10A going into the car). I had to derate the charger to 8A to stop the inverter from overheating. Not a big deal as this doesn't limit the system in any way - the MPPT still spends all day running flat out in "bulk" charging mode. The battery accommodates the difference, and also carries the system through intermittent cloud cover.

I could have squeezed a bit more capacity out by going with the 70A MPPT charge controller but it would only make small difference for a very short time each day, at nearly double the unit price (~$700 vs $350).