Tuesday, 18 June 2019

Solar charging

I'm still working on my mesh network stuff but sometimes it feels like I'm working on so many things at once I never make any progress with any of them.

One of the things I want to make sure I have in place is a number of fixed nodes with decent batteries and passable antennas to do the heavy lifting of infill coverage.

I built ten of these and they worked but battery life with cheap NiMH cells was disappointing so I'm doing a second iteration where I'm throwing more serious batteries, better efficiency and top-up solar charging into the mix. The hope with this is that they will run for 2-3 days at least.

For the batteries I've purchased twenty brand new 2500mAh 18650 LiPo cells, planning to put them together in pairs. A quite naive calculation on battery life, assuming the nodes use about 80mA gives me 60 hours, but I know it'll work out less than this.

For efficiency I'm looking at an efficient PAM2301 switching regulator recommended to me by Mike, who's used them in some of his projects. This will generate the 3.3v needed for the ESP8266.

I'm also going down to ESP8285 modules with no status LEDs or clutter, which should help a bit. I've yet to measure their typical load.

The solar charging is a whim that may not deliver. I've picked up ten 1W 5.5V panels from China and after a bit of reading round the subject settled on using an MCP73871 charger chip and bought an official development board to play with.

With LiPo cells you can't just connect a charger to the batteries at the same time as the load. The charging algorithm for LiPos has constant current/constant voltage modes to ensure safe charging and attaching the load directly during charging messes with that.

The MCP73871 was very specifically chosen because it's not a simple charger but a power management device designed to supply power to the load as a priority, charging the battery if there's excess current available from the supply. It also manages switching the load from the incoming supply to the battery when there's no incoming supply. It's not an MPPT charger but it does reduce charging current when the supply voltage drops and tries to stabilise it. Which should achieve a similar end.

If I've read the datasheet correctly all this means it should fairly transparently 'top up' the 18650s if the solar cells can provide enough current.

There's a LOT of IF coming off this plan. One of the 1W panels delivered 100mA in the morning sun to an LED so I may not be completely out of luck.

I've got some INA219 current monitoring modules coming too so I'm hoping to get some fairly true readings of how this all behaves by putting them into the proof of concept build. I may be able to monitor the panel, charger and battery individually at the same so I can see what's going on and do fairly good efficiency measurements.

Or I may just give up, buy some bigger panels, better NiMH cells and connect the panels straight to them via a diode. :-)

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