Hi Martin (and everyone!)
That you’re starting a revision of the 20A charger is very opportune to say the least!
I found the Libre Solar project a few months ago, but I now finally have some time to get involved.
Just some “quick” background to our use case: We’re looking at putting together a open source Solar/Wind/MicroHydro system for developing nations, starting in the Pacific island region first.The main issue there is that because the population is spread out across many islands, there is no viable way of implementing a normal power distribution grid, let alone at affordable prices. The current power generation there is predominantly from diesel atm too, which is a bit ironic given they’re not renewable. Many tropical islands have the terrain for hydro/microhydro, and heaps of rain every month at around 400mm. Wind is also a possibility, as is of course solar PV.
Of course a major constraint is affordability, and that is where a highly optimized system is required to cut costs as well as integrate enough functionality (ie to improve value for money). To overcome this there are three main ideas: 1) Create “generation systems” so that the community, or groups of families and households can share the more expensive parts of the infrastructure (solar panels, chargers, larger battery storage etc) 2) create “consumer devices” that are individually powered for use independently from the generation system in the consumer homes. 3) Allow systems to be gradually improved and introduced into households to fit their cashflow
As with any system design it’s good to start at the customer end to find solutions, accordingly the largest demand for electricity is for light (LED) so that households can get rid of liquid fuel lanterns, and extend the hours in which they can do things with adequate light (schoolwork, handcrafts, cooking, childcare, going out, light whilst walking down the road etc etc).
The second most common requirement, sometimes surprisingly, is the ability to charge mobile devices, in particular phones as there are no “power outlets” to put chargers in. To the point they run generators just to charge phones! Phones are very important, as it’s through facebook market and the like, that they organise goods and services off island or from other communities, as well as make payments to eachother (via phone credit etc) and of course organise a lift if they need to get somewhere (nearly every car driving around is a potential “uber/taxi” rideshare). We would also like to provide fixed wireless internet as a part of the rollout of “Sneakergrid”, as we are calling it, which also requires an off-grid power solution and also provides another low-cost backbone to telecommunications.
For this we’re well along the way of developing a wifi/BT mesh enabled flashlight, that doubles as a interior room light, and includes USB charging for mobile devices using 2-4S 21700 4Ah cells for storage. With this you carry the power in your pocket and can walk your own power around to where you need it…hence the name Sneakergrid! 
These devices can be charged/discharged via USB (including PD up to 20V5A~100W through the integrated bi-directional 6A BQ charger IC) or via a 5-24Vin magnet connector, (that also serves as a mounting system in the home) which means that they can charge in under 2 hours, provided they have a adequate power source. This increases the cycle times, which in turn improves the value for money ratio as they can use it more than once per day and don’t require the device to be connected to charging all day.
We’re also looking at providing low power refrigeration and water pumps for header tanks (typically under 100W) so that they can store their own cold goods and leftovers etc safely and have flowing tap water instead of a bucket to get rain water from their outside rain water tank.
What we still need to develop is a power source…that’s where libre solar would fit really well I think.
If you’re interested I’d be happy to join forces to develop a “new charger” that incorporates some of these local requirements, which btw also cover two other large potential markets quite well, being RV and microhomes.
A few of things on my list:
- Use a USB-C connector instead of the micro USB and maybe add a USB dedicated buck converter so that you can use the integrated USB-PD function of the STM32G431 (or a G441 like ours so you have enough IO) for device charging as well as the USB data/programming.(Provided a cost effective regulated buck converter can be used)
- Add a 24V LAN PoE option to power PoE network devices etc (there’s very little in the market providing a PoE solution from PV)
- Add a optional $4 ESP32 module footprint on the back of the PCB (that can also run the LAN PHY of the PoE port) and of course provide BT/wifi connectivity. The STM32 would be connected via UART/SPI/I2C and use the same LDO. Preferably use a ESP32 module with a RF connector option. This also means that it can serve as a local or internet connected wifi AP for IoT devices, also as a wifi mesh point, including for automation control, and monitoring via phone app, or via webserver running on the EPS32 itself.
- One can use the ESP32 for setting up the device over an app/webserver or a simple OLED/IPS LCD and 2-3 buttons or a rocker switch with click would do that too. In field setup/programming would be much easier that way and can be done by locals without extra gear. ESP32 also handles over the air updates BTW.
5.I like the high voltage option (100V) but I would also be interested in a higher current option up to 40A on the output, so 1kW for community chargers as the cost of PV is quite low in comparison to storage atm for standard PV panels (330W for $100ea) This means systems can be upgraded with more PV as load increases and as they can afford it, even if that power is mostly only available in daylight hours if they’re running small batteries. Extra PV capacity also helps overcome cloud cover/rain losses. Incremental scale-ability is very important in developing nations.
- This might need a 48V battery option, (which could also be 20A again for 1kW) but keeping it useable at 12-24V as well would be good
- Add 3-4 switched load outputs with protection settings - this would be to automate loads at different set points depending on PV availability. Might be able to use one for the wind power dump load resistor - which could actually be a resistive wire water heater to make it useful The other ones could be for DC fridge and small DC pump control. The idea would be that each output, including the flashlight charging (over mag connector) output could be prioritized as required and configured as needed. One thing that would be useful regarding the optimisation of solar output and load switching is the addition of a solar radiation sensor using a small PV cell/LED, so that the available solar power can be calculated, regardless of load conditions. This input would make dynamic load switching easier I think.
- I like CAN in general, but I’d also like to see a cheaper longer distance 1wire interface connector (up to 50m or so) routed out of the MCU. The flashlights have a 1Wire system so they can communicate with other devices on the home power grid cable. (typically a three prong 5-7m cable hung from the ceiling and using magconnectors for simple DIY house installs)
- We’re getting custom aluminium extrusions made for the flashlight part (open ended “0” shape) that can be cut to various lengths as required, for 2S for 4s packs etc. The end plate internal dimensions are 20mm thick and 45mm wide and a 4S enclosure is about 140mm long. The idea is to provision various PCB mounting positions with 1 PCB vertical between the two cells if batteries are included, or two full width horizontal mounting positions (45x140mm) against each side of the aluminium extrusion for cooling purposes when used without batteries. In general I think a extruded aluminium tube would be a better enclosure for use outdoors, and provide some ruggedness and cooling opportunities as well. The tube will cost less than a $1 per 100mm. (inductor height might be an issue)
- Regarding your application list, I agree with all of them, but would also add microhydro charge controller (similar to wind but not as dynamic if you also include an output to control a bypass valve). A DC generator charge controller would also be an option seeing the similarity of wind/hydro outputs, and you could use the same output switch to activate the starting of the generator.
- A point of load converter is essential in that the device should generally function without any battery storage attached, provided it has a power source! There are numerous reasons why a battery might not function so having the ability to “boot” the system and extract something useful from it is essential if alternatives are scarce.
- e-bike and e-scooter charging is also very important I think, but given the budget constraints with our project in particular, having a ebike charge from a large and expensive battery is not really cost effective. I think having a 12-48V output range from the solar charger, and the ability to switch between loads (so for example a stationary battery and a ebike battery depending on charge state and solar availability) would offer more flexibility. In fact we’re considering ebike/escooters as methods of moving larger energy amounts to homes, whilst providing a viable form of individual low cost transportation as well.
- Another application that is mostly just in software is universal programmable power supply with current limiting. I’m not 100% certain but it might end up beneficial going to a bi-directional full bridge converter topology to avoid any user induced connection issues?
- There might also be an option to daisy chain multiple solar chargers together to increase overall output in a system. So operate them a bit like a DC microinverter system and use a comms line to sync the devices according to load.
Sorry for the extensive post, but I thought I’d present most of the items together so that the interdependencies are evident. Let me know what you think might work or not
Regards Sam
Not a simple task and without it it effectively doesn’t exist.
I think that being able to switch loads depending on the solar input and battery state is crucial for demand side management of loads to improve battery reliability, performance and longevity, plus it allows for “excess” solar capacity to be used when it is available automatically without user intervention, for second priority loads. In a RV market setup that would cover most of the large loads.
