(disclaimer: this design is only for 1S LiFePO4 or LiFeMnPO4 rechargeable batteries - not for LiPolymer or LiIon! Though it has poly fuses on load and heater, always use own fuses while experimenting to avoid accidents!)
Patrick van Oosterwijck made a nice design for a solar MPPT LiFePO4 1S battery charger, based on the Consonance’ CN3801 ( IC product page and data sheet) and added some neat features around the loader IC:
The schematic (CC-BY-SA) exposes some notable features:
- Low voltage load disconnect at 3V battery voltage to prevent over discharge and maximize battery life.
- Over temperature (58°C to 64°C) and under temperature (-3.2°C to 0.2°C) protection to prevent charging outside of recommended battery temperature range.
- Heater control that turns on battery heating from the solar panel’s power during under temperature conditions.
Undervoltage load cut-off is done by a NCP302 delayable reset controller / voltage detector (+ FET helper). - And for the colder zones: the under- and over temperature detection (LM2903 double comparator) not only manipulates the MPPT input of the CN3801 to avoid charging in either condition, in the under temp case it also uses incoming energy as dump load for warming the battery until it reaches a temperature above 0°C (w/ tolerance, can be tweaked) to prevent loading outside the allowed range (e.g. with resistive heating pads)! As the proper temperature is a prerequisite for charging and discharging, this ‘temperature conditioning’ is a very reasonable usage of energy on around-zero-degree conditions.
Both poly fuses could gain from more solder when really feeding / sinking 3A (or more on shorts) - but this is feature, not a bug: if they need to be exchanged, it gets easier, and reinforcing solder after testing it as needed.
And - hey, it explicitly is sold as a working prototype, so no complaints here! ;)
The kit is equipped with an isolated NTC thermistor which is mandatory to mount as close as it gets onto the battery (if not placed, the system remains in undertemperature state). Some batteries already have an 10K NTC - this is the connection where to connect it to. There is also footprint for overriding thermal protection or placing a SMD NTC there if the PCBA is very close to the batt pack.
At my site, the desired battery has not yet been delivered (and designated solar cell could do better/get bigger), so more news and results on this PCBA to come!
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