Linear solar charger - is MPP relevant? - Page 1
Forum section: electronics / projects

author: Peabody
author_rank: Super Contributor
timestamp: October 29, 2021, 05:26:41 pm
content: I'm trying to learn about solar, and have made some progress, but have come up against something I don't understand, and could use some help.A while back Andreas Spiess posted a Youtube video (#383) proposing that we power the popular and cheap TP4056 lithium charger module with a solar panel for outside projects. He added the usual load sharing parts (aka "power path") so the solar panel could power the load directly when it's in sun.But then at about 5:40 into the video, he starts talking about MPP, and ends up saying that we need to replace the default charge current setting resistor - 1.2K for 1 Amp - with a pot so if the panel is too small to deliver 1A, we can adjust the charging current close to the MPP and maximize power. He says, "Now we can adjust the charging current to maximum current when the panel is in full sun." I think this implies that there is a pot setting that gives maximum current, with lower current being produced on either side of that setting. This makes absolutely no sense to me.The TP4056 is a linear charger. It requires a minimum of 4V to operate at all, and the input voltage must also be 30mV above the battery voltage for any charging to take place. Based on my very limited experience with solar panels, and assuming the charge current setting is left at 1A, I believe a small panel will produce the maximum current it can at the lowest voltage that still produces charging, and setting the charge current to any smaller value will do exactly that - it will just reduce charging current and extend charging time. It will also increase voltage which will only result in increased dissipation in the charger. So what's important in a linear charger input is maximizing current, not maximizing power.If that's correct, there will not be a "maximum current" pot setting. Adjusting the charge current setting down from 1A will have no effect until it gets to whatever the panel is currently producing, and any reductions from that level will simply reduce the charging current and extend charging time. I can see it would be different if the charger were switch-mode because then maximizing power in would permit maximizing power out. But I don't see MPP being relevant for a linear charger like the TP4056.So as I see it, the charge current setting should be whatever is right for the battery, not the panel. And if the panel can't provide that much, don't worry about it - it will provide what it can at the lowest voltage that still works, and you can't do better than that - maximum charge current with the least dissipation. No pot is needed unless the *battery* needs a lower charging current.However, I know that when a hobbyist like me disagrees with an EE, I'm usually the one that's wrong. So if I'm wrong about this, I hope someone can explain what I've missed.By the way, in case anyone is interested, Andreas' circuit doesn't work so well under partial illumination (battery current flows through the p-fet body diode), but I worked out a more complicated version using an opamp that works fine. The output voltage is never less than battery voltage. See below.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 29, 2021, 05:32:32 pm
content: With a linear charging current there is no need for MPP regulation, as the actual voltage the solar panel produces does not matter as long as it is high enough. So the analysis is correct.

author: magic
author_rank: Super Contributor
timestamp: October 29, 2021, 06:18:56 pm
content: Quote from: Peabody on October 29, 2021, 05:26:41 pmThat sounds reasonable. One coulomb in, one coulomb out.One would have to try if the IC isn't doing something silly under dropout conditions, but TP4056 is straightforward enough that it should be alright and basically look like a zener to the panel.

author: Peabody
author_rank: Super Contributor
timestamp: October 30, 2021, 03:54:47 am
content: Quote from: magic on October 29, 2021, 06:18:56 pmYes, I just don't want it to oscillate. Well, it's supposed to be clear tomorrow, so I should be able to test it.

author: Peabody
author_rank: Super Contributor
timestamp: October 31, 2021, 08:55:17 pm
content: I was able to test my solar charging circuit pretty thoroughly over the last two days. It works really well, and as expected, the output voltage feeding into the regulator never drops below the battery voltage, and no load current ever flows through the body diode. And it doesn't oscillate. Yay!!!I also confirmed that there is no charging current "peak" that can be tuned with a pot in place of the charge current setting resistor. I replaced the 1.2K (1A) resistor with a 20-turn 10K pot. When set to 1.2K, I got charging current of about 480mA from the panel I'm using. As I increased the resistance (decreased the current setting), nothing changed until I got to a little above 2K. Per the data sheet, that would be about the right setting for 480mA. From then on, the charging current simply got lower and lower. This confirms what I expected. So with respect to efficiency, since the TP4056 is a linear charger, there's no need to change the setting resistor. If a replacement resistor has any effect on charging current, it will be a negative one.However, the charge current setting also sets the termination current , which is about C/10. So if you have a small panel with output in the same ballpark as the termination current when C = 1A, the constant voltage phase might be really short, and the battery would not be fully charged. In that situation it might make sense to use a lower charge current setting so as to get a lower termination current. That wouldn't reduce the actual charge current, just the setting, so long as you set it no lower than the absolute maximum the panel can deliver in bright sunlight. The panel's short circuit current would probably be a good choice.One interesting fact about the TP4056 is that the input-output voltage differential increases as the charging current increases. The datasheet says the input has to be 30mV above the battery voltage, but at 480mA the input voltage was up to about 4.4V when the battery was at 4V.

author: magic
author_rank: Super Contributor
timestamp: November 01, 2021, 07:42:51 am
content: The datasheet says it needs to be 30mV for the chip to turn on. That may be a safety margin to avoid discharging the battery due to opamp offset errorsThe rest looks like good old RDS(on). Some Chinese datasheets of xx4056 variants (there are a few) have a spec calledRONwhich is given as 650mΩ. The English "NanJing Top Power" TP4056 datasheet is silent...