Solar powered LIPO charger with power path - Page 1
Forum section: electronics / projects

author: Peabody
author_rank: Super Contributor
timestamp: May 19, 2021, 01:59:57 pm
content: Andreas Spiess has posted a new video describing a solar-powered single-cell lithium battery charger which includes a "power path" (aka load sharing) circuit. That cicuit lets you directly power the load from the solar cells when they are producing current so you can power the load and charge the battery at the same time and still get proper charging termination.The power path circuit is the standard load sharing circuit described in Microchip's AN1149, and includes a P-channel mosfet, a Schottky diode, and a resistor. (See picture below.) I described all of this in a video I contributed to my local open-source hardware group's Youtube channel, which Andreas links to as "Explanation of Power Path". However, I recommended against using this circuit for solar charging. The circuit works for USB charging because the mosfet gate is always either at 5V or at ground, so the mosfet is always either completely off or completely on. But with solar that's not the case, and I was concerned that at intermediate supply voltages the mosfet would not be turned on when the battery is supposed to be powering the load, and the load would be powered through the mosfet body diode, which may not be a good thing. (The mosfet is oriented "backwards" so the body diode prevents USB, or solar, voltage from coming back into the battery.)I'm posting about this here in the hope that I can find others who would be willing to test this circuit and who actually have 5V or 6V solar cells to test with (I don't). I've done some preliminary tests using an LM317 adjustable linear regulator as a pretend solar source. I used a DMP1045U logic-level mosfet, and an SB220 Schottky. Andreas uses a FQP27P06 mosfet that's not logic level, but for a TO-220 part I think the NDP6020P would be a better choice.With a fully-charged (4.2V) battery installed, I gradually reduced the power supply from 5V down to zero, and noted the resulting output voltage. The load was just a red LED with 470R resistor.Supply(V) Output(V)------------ -----------5.04 4.813.93 3.733.25 4.200 4.20The minimum output voltage (3.73V) occurred at an input voltage of 3.93V, but as the input voltage is further reduced, the output voltage actually rises until it reaches 4.2V, which of course is the battery voltage with the mosfet fully turned on. It seems clear that between 3.93V and 3.25V input voltage, the load is at least partially supplied through the mosfet body diode. As the input drops and the output rises, the Schottky becomes reverse-biased, so no current can flow through it. And at least in the upper portion of that range the input voltage, which is also the gate voltage, is still higher than the output voltage, which is also the source voltage, so the mosfet proper can't be on yet. The only remaining path is from the battery through the body diode.Then I tried one more experiment. I reasoned that it might improve things to install another SB220 Schottky across the mosfet, in parallel with the body diode. That would reduce the voltage drop when the body diode would otherwise carry the load, and move any dissipation out of the mosfet into the SB220. Testing again, I got these results:Supply(V) Output(V)------------ ------------5.04 4.814.04 3.993.61 3.993.25 4.200 4.20So this version produced a higher minimum voltage, and a flatter bottom to the voltage curve. So it seems that adding the extra diode could be a worthwhile improvement. It would only come into play when the mosfet should be on, but isn't, but once the mosfet does turn on, the extra diode and the body diode are both bypassed.But all my tests were done with a low-impedance power supply, and I'm concerned that solar cells might not produce the same behavior. In particular, when the body diode (or extra diode) start carrying the current, the load on the solar cells will drop, and their output voltage may rise a lot, and I wonder if the system might actually oscillate at that point. So this circuit needs to be tested under real-world conditions.It would be great if we could do solar charging using a lowly TP4056 module with a few modifications. I know there are solar charger chips that include the power-path function, but they typically don't come in packages that are convenient for hobbyists, and modules using them tend to be expensive. So it would be good if we can confirm that this circuit works for solar.

author: wizard69
author_rank: Super Contributor
timestamp: May 21, 2021, 04:03:41 am
content: Have you considered using a LiPo charger specifically designed for use with solar? For example there is: BQ25798, which does MPPT which maximizes charge capability that one can get out of the solar array.I don't have much charging experience, but if solar is the goal an IC designed for that makes a lot of sense to me.

author: Peabody
author_rank: Super Contributor
timestamp: May 21, 2021, 04:44:05 am
content: The BQ25798 comes in a 29-pin VQFN package. It would be a bit difficult to use a part like that for a hobby project unless you could find a module that uses it. I don't know of any such modules. The appeal of the TP4056 is that it comes on very cheap modules complete with the DW01 protection I/C. So if modest modifications to such a module would make it usable for solar, that would be a major benefit. However, it seems the MPC73871 charger does appear to come in a module, and I probably need to order one of those to see how well it works.https://www.ebay.com/itm/262879355448I'd still like to be able to use the TP4056 if that's workable.

author: Peabody
author_rank: Super Contributor
timestamp: June 04, 2021, 09:13:34 pm
content: I ended up learning LTspice and doing a simulation on the Spiess circuit. It's all posted on my Github:https://github.com/gbhug5a/Solar-Power-Load-SharingI clearly did not understand how solar panels work, but the simulation shows that even under a little solar illumination, the solar panel ouput voltage has the effect of turning off the mosfet completely. So all the battery current flows through the body diode. The mosfet turns on only at night when it isn't really needed. In the end, a two-diode circuit (both Schottky) would work as well, with a Vf of about 0.25V.

author: wizard69
author_rank: Super Contributor
timestamp: June 05, 2021, 12:46:52 am
content: Thanks for sharing! Hopefully I will get some time to digest. I've become far more interested in solar battery charging of late, so any bit of info helps.Quote from: Peabody on June 04, 2021, 09:13:34 pm

author: Peabody
author_rank: Super Contributor
timestamp: June 05, 2021, 03:27:58 am
content: I'm close to finishing another version which has an opamp driving the mosfet gate. The outout voltage is never less than the battery voltage, which is exactly what you want. It works in simulation with the LT1494 single opamp, but that costs $5, so I need to find something cheaper that will work. Needs to be rail to rail, with inputs at or slightly above the upper rail. Film at 11.

author: Peabody
author_rank: Super Contributor
timestamp: June 07, 2021, 03:21:35 am
content: I finished the alternate load sharing circuit that uses an opamp to drive the mosfet gate, and put everything up on my Github repo:https://github.com/gbhug5a/Solar-Power-Load-SharingIt behaves perfectly - in simulation. :-) The opamp runs on about 1uA, and there are no resistor dividers to eat up current. So it's a chicken dinner. Anyway, here's the schematic:

author: Peabody
author_rank: Super Contributor
timestamp: June 27, 2021, 10:57:53 pm
content: Having done as much as I could with simulations, I ordered the parts needed to physically test this stuff in the flesh - a 5V solar panel, TP4056 modules, a P-channel mosfet, Schottky diodes, and some MCP4061 opamps. I already had the 18650 batteries.I confirmed what the simulations predicted:1. While the standard load sharing circuit works fine with a fixed 5V supply, it doesn't work well with variable solar power. The mosfet turns on ok at night, and turns off ok in full sun, but in between it is also off, which means that any current contributed to the load from the battery is going through the mosfet body diode, with a resulting 0.7V Vf drop, which is huge for a 3.7V cell. In effect, except at night, the load sharing circuit is nothing more than a two-diode OR circuit.2. Adding a Schottky diode across the mosfet effectively converts the body diode to a Schottky, with a 0.3V drop instead of 0.7V. It may be better to do this and forget the mosfet altogether.3. It is possible to completely eliminate the voltage drop by driving the mosfet gate from the output of an opamp so that it remains on during periods of intermediate illumination. This results in the output voltage never being lower than the battery voltage - i.e. - no voltage drop. The opamp is the MCP6041, a 6V part which draws less than 1uA, and which has a common mode range extending a bit above the upper rail, so no resistor dividers are needed. (The 10V TLV2401 would be better for 6V panels.) The circuit is shown below.As much as I can, I've confirmed that the circuit has no tendency to oscillate. I believe that's because it has a built-in bias towards turning the mosfet on even when its gate is already grounded. That's because the mosfet's RDSon always produces some voltage drop. In addition, when the mosfet switches state, that only occurs at a time when the load current contributed by the battery is already zero, so the solar panel sees no change in load current, and produces no change in voltage, when the switch occurs. It's basically zero crossing point switching.I've written this up and posted it to my Github:https://github.com/gbhug5a/Solar-Power-Load-SharingI should say that the opamp circuit would also work fine with a fixed 5V supply, so it would be a "universal" version of a power sharing circuit that works with both USB and solar.If anyone has any issues with what I've done, I'd like to know about them. I'm not formally trained in this stuff, so I get things wrong sometimes.

author: FBMinis
author_rank: Regular Contributor
timestamp: February 23, 2023, 04:52:27 pm
content: Could a TL431 replace the MCP6041?

author: Peabody
author_rank: Super Contributor
timestamp: February 23, 2023, 09:43:43 pm
content: Quote from: FBMinis on February 23, 2023, 04:52:27 pmI don't see how that would work, but would be happy to consider any alternative to the opamp.By the way, the opamp circuit is now more complicated. I had to add feedback to deal with input offset voltage, and I added a schottky diode at the input of the charger to make sure any load current provided by the panel doesn't go through the charger.