Completing the first stage of my battery was a great feeling, but it has been sitting underutilised for a few days. My iSDT Q6 smart charger takes a 7-32 V DC input (nicely covering my 24 V system) so I’ve been able to power my cell testing from the battery. This bootstrapping is nice, but it has been slowly draining my battery and I’ve been reluctant to charge from the grid – the main point is to store sunlight! This weekend I’ve “plugged-in” to the sun.

A few months ago I bought six QCELLS Q.PRO-G4.1 265 solar panels on ebay for a fantastic bargain. At 265 W each these panels give me just over 1.5 kW of electricity production – way too much for my 0.5 kWh battery. For now, I’ve wired up a single panel to give a nominal 0.5C charge (2-hours full charge if the sun is bright).

Along with the solar panels I bought a second-hand MPPSolar PCM 60x charge controller. Again, this is oversized for my battery at this stage – it can put out a 60 A charging current – but I’ve dialled it down to 10 A. This is 1 A per cell in my current battery, which matches my testing regime and represents my “upper limit” in the design parameters.

For now, the solar panel is leaning against the back of the house. With only 9.23 A of short-circuit current from the single PV panel, I have taken a temporary shortcut and used 2.5mm twin-core-plus-earth (2.5 square mm cross section area for each of active, neutral, earth – normal internal wiring for household power sockets) to connect the panel to the PCM 60x charge controller. This wire is rated for 20A, and is currently routed under my house so does not have any UV or thermal load to deal with.

It took a lot of fiddly time to measure the cable distance, attach MC-4 connectors, triple-check polarity, connect into the PCM 60x terminals – and by the time I’d finished the sun was low enough to leave my PV panel perfectly in the shade!

Despite this, I was charging at 0.1 A! Everything seems to be working perfectly, and now I need to wait for some sun.