Winter is coming here in the Rockies and that means I probably won't drive the Nissan Leaf much.  One achilles heal of electric cars is often the 12V battery since there is a constant parasitic drain keeping several of the car's ECUs operating.  In my case I want the 12V system to continue to run because it will allow the main traction battery to heat itself (using its own power) when it gets extremely cold (I just have to charge the car occasionally).

The Leaf doesn't do a great job managing the 12V battery and the forums are full of people coming to dead cars after they have sat idle for a while.  So I made a 12V battery charger out of the following components.

1. An old prototype of my own Solar MPPT charger
2. An old Arduino Uno connected to a Micro-SD breakout board
3. Some ancient Radio Shack project box and a proto board
4. A 45 Watt Solar panel 
5. Misc wire, XT60 connectors and some cheap battery clamps
6. Electrical tape, shrink-tubing and velcro

This use-it-up project was 100% sourced from stuff I had laying around.

A sketch on the Uno re-programs the charger via its I2C interface with appropriate Bulk/Absorption and Float charge levels for the flooded lead acid battery in the car.   It also logs various charge state and voltage/currents to the Micro-SD card every minute.

The charger is mounted next to the battery.  Ideally I would have used some ring connectors for a more reliable connection to the battery but I taped the clamps in place for now.

The charger uses a temperature sensor to automatically adjust the charge voltage levels based on battery temperature.  It is taped to the side of the battery.

Wiring is routed to a hole under the engine bay where it can connect to the solar panel.  I will tuck the wire back up another hole into the engine bay when not in use.  If I make this a permanent installation then I'll dress the wire to a spot on the front of the car near the fog lights and give the connector a 3D printed cover.

Burden current is about 25 mA, 80% of which is the Uno and Micro-SD card.  Supposedly the Leaf's quiescent current draw is about 50 mA average with higher spikes as ECUs do things and the [unused by me] telecommunications subsystem calls home.  So I'm looking at a minimum of about 22-25 Watt power consumption per day.

The charger is capable of about 35W charging power under good sun and less power in lower light levels.  The panel, when clear of snow, should see at least 1-1.5 hours of direct sun each day and the lower light levels for several hours (lots of trees).  So theoretically this charger should have no problem keeping the battery topped off.

It will be interesting to see how this works over the next 8-10 weeks as the temperatures dip below 0°F.  I'll publish the data charts then.