Solar lipo charger for meteostation - Page 1
Forum section: electronics / projects

author: error414
author_rank: Newbie
timestamp: June 23, 2021, 11:13:47 am
content: Hi,I would like to create meteostation with solar and 18650. I'm stuck with charging circuit. There are two issues I don't know how to solve.Solar has variable voltage, I use 6V 1.5W solar panel. There can be in sunny day up to 9V (measured). Most of Ic for charging are only up to 8V. So I decided to use CN3791. Latest circuit.https://oshwlab.com/error414/meteo-l0But it seems that CN3791 does not like like "share load" circuit. I set 0.5A max charging current, only 0.2 I get. I measured gate of Q6 and it seems it oscillate between two duty cycles (I will post picture). I have reference board with CN3791 for compare and there is nice stable PWM regulation. It's interesting that CN3791 does not work if ampere meter is connected to battery.Next problem I have with voltage from "Battery Managment" block. 18650 has voltage 3 - 4.1V, so there is buck boost converter to 4.3V, but MT3608 does not like bigger voltage in input than it's set to output. So 9V from solar is not good when MT3608 is set to 4.3. Moreover MT3608 can't reduce voltage to 4.3V. It's a big problem.I found many circuit on internethttps://www.instructables.com/DIY-Solar-Battery-Charger-LiPoLi-Ion/Most of circuit are only copy of GreateScott.So questions:1) how to improve share load.2) stable voltage from "Battery Managment" block.Keep in mind that it runs on battery, so I would like to keep current consumption as small as possible.Thank you very much for help.

author: Peabody
author_rank: Super Contributor
timestamp: June 23, 2021, 03:02:28 pm
content: Does the CN3791 reference board work with the rest of your circuit connected to it? Does your circuit work at 500mA charging current if all of the downstream circuit is disconnected at Q8 so it functions only as a battery charger? Are you sure Q8 is oriented correctly so the body diode blocks solar current from coming back in to the battery? (It's correct on the schematic.)I believe the MT3608 is only a boost converter. So you would need to find a buck/boost converter.Great Scott apparently has never seen the need for a load sharing circuit. I don't think that's good design, and can result in charging not terminating properly. But you should note that the standard load sharing circuit doesn't work all that well for variable solar power. At intermediate illumination levels the mosfet (Q8) may stay off, so any current provided by the battery will flow through the body diode with the resulting 0.7V drop. Basically, anytime any current is flowing through D2, Q8 will be off. That's ok if D2 is providing ALL of the load current, or none of the load current, but in between things are not ideal. I've been working on a way to solve that problem, but no success so far.I'm surprised your "6V" solar panel goes as high as 9V. I have a "5V 2.5W" panel that produces 550mA short circuit, but the highest open circuit voltage I've measured is about 5.85V.

author: error414
author_rank: Newbie
timestamp: June 23, 2021, 10:30:18 pm
content: You helped a lot, thank you very much.I found out what is wrong with charger, but I don't know why. I removed forward diode and replace SB1045L to B340A (my favorite diode), I had to add capacitor front of P-mosfet (Q6).Yellow line is Q6 gate, red line is drain of Q6. On no-ok.png attachment you can see, that there is weird peak. Gate driver is OK, the peak causes diode. I have no idea why. But new diode solved problem. Even share load works.I know about share load issue, the Q8 has gate tht voltage about 2.5V-+ so if there is slightly below under 2.5V then circuit must be powered from battery but Q8 is closed. So current runs through Q8 internal diode, and there is voltage drop. I can live with it, it is reason why there is boost converter. CPU + sensors need +- 800uA to 16mA (for 160uS when radio is active). So there is small power loss.Do you have any experience with good buck/boost Ic? I found only XL6009Hmm, maybe I'm wrong, I will verify it. Tomorrow should be next sunny day, so I will verify to be sure.

author: Peabody
author_rank: Super Contributor
timestamp: June 24, 2021, 02:21:04 am
content: Sorry, I don't know about buck boost converters.With respect to the load sharing problem, one way to reduce the voltage drop, and move the heat dissipation out of the mosfet, is to install a Schottky diode across the mosfet in parallel with the body diode. It would behave the same, but with a lower voltage drop. See first picture below.A second solution would be to drive the mosfet gate with an opamp so the mosfet stays on and there is no voltage drop at all. But when sunlight gets strong enough that the solar panel provides all of the load current, the source voltage rises and the mosfet is turned off. See the second picture. With your 9V panel, you would want to use the TLV2401 opamp.Edit: I breadboarded the opamp circuit this morning, and it doesn't work. It allows current to flow from the source back through the mosfet to the battery, which can't be allowed. I don't know why.

author: skepticelectroneophyte
author_rank: Newbie
timestamp: May 22, 2023, 06:19:48 pm
content: @error414, could you share the updated schematic if this CN3791 project worked for you? Thank you