We still have the issue of load sharing unresolved. Attached is my latest thinking on adding load sharing to a TP4056 charger for a 5V Arduino. It's just a P-channel mosfet, a Schottky diode, and a resistor.
However, the circuit ends with a 5V boost converter, so I don't think it is appropriate for use with something like a 6V solar panel. The TP 4056 could handle 6V, barely, but when the sun is shining you would have 5.7V at the converter input, and it would just pass that right through its coil and diode, sending 5.4V to the Arduino, which may be too high.
But if the regulator was a 3.3V, then I think the circuit would work. And I think it would also work if the regulator was a 5V boost/buck regulator. And it might even work with the 5V boost regulator if you change the load sharing diode to a standard rectifier such as a 1N4004.
The other major issue is whether the mosfet would properly switch the battery in and out as the solar input voltage varies. The circuit is basically designed for stable 5V USB power input, which is either on or off. But solar is going to fluctuate all over the place, and I don't know whether the mosfet will turn on fully when the solar voltage sags. Are there any spice jocks here who can test that? Basically, as the solar voltage goes up and down, the input to the regulator should never drop below the battery voltage, and no current should ever flow through the body diode.
I've confirmed that the load sharing circuit I offered in the previous post doesn't work with solar. The mosfet doesn't switch properly when the input voltage is variable. The MCP73871 charger chip apparently deals properly with solar, and includes load sharing. There are modules with that chip, and if the charging and discharging current limits aren't a problem, that may be the way to go.
However, note that the module has no USB connectors.
Have you looked at the schematic of a tp4056 module?
Thanks for the heads up about the MCP73871, I'll be checking it out.
MCP73871 on order. Meanwhile, I upped the solar cell from a 10W to a 20W. The time of day the solar cell is putting out 5V is earlier in the AM; 0600 instead of 0700 and charging was done in 1.5 hours instead of 4 hours. I am on day 3 of testing the the 20W solar cell. The solar cell remains in partial shade, to create a less than ideal operating condition.
Idahowalker:
Have you looked at the schematic of a tp4056 module?
Yes I have, and it illustrates the problem. When solar power is both charging the battery and providing power to the load, the load current can distort the charging algorithm of the TP4056, which can be dangerous. So we're looking for a way to provide solar power directly to the load without going through the charger, while at the same time isolating the battery from the load when that's happening so it can charge to termination without interference. I think my circuit handles that, but it depends on the 5V USB supply being either on or off. But I don't think solar is like that, and my circuit won't work at intermediate input voltages.
In your circuit, so long as the "charged" LED comes on when charging has completed, then you aren't drawing enough load current to mess up charging. If you ever get to the point where the "charging" LED just stays on when the sun is shining, then you could have a problem.
But I have a question about what happens to typical solar cells when sunlight is reduced. Assuming there is some material load, do the cells just output lower and lower voltage, or do they do a sharp cutoff below a certain point? And the same question about your DROK regulator - if it isn't able to maintain 5V output for whatever reason, what does it do? Does it just pass through whatever is coming in, or does it shut down?
Reduced sunlight, lowered voltage out, yea, I know the solar cell is a current device, the closer to the trip point for the DROK shutting off. Once the threashold is reached the DROK shuts off.
The TP4056 has a minimal operating point of 4.7volt, if memory serves me. Once that trip point is reached charging is stopped and the battery becomes the voltage source.
Idahowalker:
Reduced sunlight, lowered voltage out, yea, I know the solar cell is a current device, the closer to the trip point for the DROK shutting off. Once the threashold is reached the DROK shuts off.
Do you recall what that voltage threshhold is for the DROK shutting off? Is it adjustable?
The TP4056 has a minimal operating point of 4.7volt, if memory serves me. Once that trip point is reached charging is stopped and the battery becomes the voltage source.
From the datasheet it's not clear. The definition of the Vcc pin says it will shut down if the voltage there comes within 30mV of the voltage on the Bat pin. That makes sense since this is a linear part. But then in the table below it says the minimum input supply voltage is 4V. So I don't know.
