Okay, so now the searing heat of the day has dissipated a bit, I’ve dragged the cluster out and got everything running.
No homebrew charge controller, we have:
240v mains → 20A battery charger → battery → volt/current meter → cluster.
Here’s the volt meter, showing the battery voltage mid-charge:
Okay, the 3.3V and 5V rails are lower than I’d expect, but that’s the duty of the PSU/motherboard, not my direct problem.
The nodes also vary a bit… here’s another one. Same set-up, and this time I’ll show the thresholds (which are the same on all nodes):
Going forward… I need to get the cluster solar ready. Running it all from 12V is half the story, but I need to be able to manage switching between mains and solar.
The battery I am using at the moment is a second-hand 100Ah (so more realistically ~70Ah) AGM cell battery. I have made a simple charger for my LiFePO₄ packs that I use on the bicycle, there I just use a LM2576 switchmode regulator to put out a constant voltage at 3A and leave the battery connected to “trickle charge”. Crude, but it works. When at home, I use a former IBM laptop power supply to provide 16V 3A… when camping I use a 40W “12V” solar panel. I’m able to use either to charge my batteries.
The low output means I can just leave it running. 3A is well below the maximum inrush current capacity of the batteries I use (typically 10 or 20Ah) which can often handle more than 1C charging current.
Here, I’m using an off-the-shelf charger made by Xantrex, and it is significantly more sophisticated, using PWM control, multi-stage charging, temperature compensation, etc. It also puts out a good bit more power than my simple charger.
Consequently I see a faster rise in the voltage, and that is something my little controller will have to expect.
In short, I am going to need a more sophisticated state machine… one that leaves the cut-off voltage decision to the charger. One that notices the sudden drop from ~15V to ~14V and shuts off or switches only after it remains at that level for some time (or gets below some critical limit).