I have a few solar panels I wanted to play around with and charge some 18650s till I get a little more comfortable that I won’t blow something up. My main goal is to understand how to charge batteries, use solar panels, diodes, and learn more about electronics on this side.

The plan is to take come 5v solar panels in parallel, then run them to my little voltage regulator, and from there a diode is run off the + side to a 18650 and the - is bare. I will set the regulator to have a continuous output of ~4.1v so I don't blow anything up or charge too fast.

This brings me to asking things I don't understand, can’t really google, and think I know but I just am not brave enough assume.

1) Battery a 18650 generally has a low end of 3.6V and a high end of 4.3V. Outside of that will damage it/catastrophe.

Question - If I set a controller to feed 4.1V from a solar panel continuously, could I damage a battery or cause it to go over 4.2V? When I think of voltage, I think of it more like the width of a river. If you keep feeding something 4.2V 100ma, won’t the item go over 4.2V eventually? It would just take longer to “charge” given the lower voltage? (like a 2mm straw to fill a glass vs a 4mm one?)

2) Diode I get that a Diode controls the flow making it go only 1 way, so if I use one on the + side then the power cant back feed, but do I need one for the – side?

I can see this being a waste of a diode and you would need to invert the direction to make it useful at all, but I feel like leaving it open like that is bad mojo? Someone please tell me it would 100% be a waste to use one on both + and – to prevent back feed drain?

3) Theoretical Question If you had a 12v battery and a 12v solar panel in parallel leading to a motor or device of some sort, and used a diode on the + side of the solar panel (preventing back feed and forcing a charge only situation, would this cause any catastrophes? In my head, it would work so that when the motor ran, it would draw from the battery and the solar panel. If the motor were cut, the panel would charge the battery. Is this how it would work? I feel like I am 100% wrong and this would cause a short or something having a panel in parallel with a battery.

note None of my batteries have a Protection Circuit, which is why I feel so scared of trial and error with this kind of stuff.

If anyone has any related info to topics I asked about, Please post it, I find it difficult to google anything related to charging/batteries/solar panels on google unless it is raw statistics. Real life applications, experience, and lessons learned are really hard to find.

  • \$\begingroup\$ Also if anyone knows where to find cheaper but properly labeled LiPos/Nicad/NIma/batteries. Please tell me... ppl from Japan and China love to say a 500mah battery is 1000mah and etc. \$\endgroup\$
    – Tab Carmel
    Commented Jul 27, 2015 at 10:16

2 Answers 2


1) This should be fine with a low-power solar panel, but it will charge very slowly.

First a bit of theory. Charging LiPo batteries needs to follow two rules:

  • the end voltage (called float voltage) must be lower than or equal to 4.2 V
  • the charge current must never be higher than a certain value (typically 1C, meaning 100 mA for a 100 mAh battery, 1 A for a 1000 mAh battery, etc.)

If you have a 4.2 V regulator then the battery voltage can't go above 4.2 V. The charge current will decrease to zero as the battery voltage approaches the regulator target voltage.

When using a wall adapter, you need something to limit the charge current, even if the adapter has a low power rating. Otherwise you might damage both the battery and the power adapter. This is because a LiPo battery has a very low resistance and there will be a rush of current when you connect the power.

But with a low-power solar panel you could rely on the fact that your particular panel is incapable of supplying more than 1C of current to the load. Then your solution might work but it will be very, very slow to charge. That's because whenever you try to draw too much power from a solar panel, its voltage collapses abruptly and you get no power at all from it.

For a 5 V, 1 W solar panel in good light, this is what happens to the voltage and power when you increase the current draw by decreasing the resistance of the load:

  • 20 mA: 6.0 V (0.12 W)
  • 100 mA: 5.5 V (0.55 W)
  • 200 mA: 5.0 V (1 W)
  • 250 mA: 0.1 V (0.025 W !!)

So you would have to get an oversized panel and use a resistor in series with the battery after the voltage regulator to limit the charging current. This will work but it is wasteful.

For good efficiency you need a circuit that reduces the charge current when the solar panel gets less light. See for instance this one (tutorial). What this board does is reduce the charge current whenever the panel voltage goes below 4.75 V, and increase it when it goes above. It also handles the charge current limit for the battery. For further reading google "Maximum Power Point Tracking".

2) You only need to interrupt one wire to open a circuit. One diode is enough. You don't need two switches for your lights.

3) This would work with the reserves outlined above. But there is another danger: LiPos should NOT be discharged too deep, otherwise they will be damaged. So you really should use a protection circuit to cut power to the load if the battery voltage is too low, and also to make sure that the load current isn't too high. Solar charger boards like the one I mentioned should do all that.

I recommend reading the tutorial above, and especially the "design notes" section if you want to understand more about solar panels.

  • \$\begingroup\$ #1 Awesome that answers just about everything... but if i have a solar panel attached to a controller that is set to output lets say 4.3V and my battery hits 4.2V or even 4.3V... where is all the power the solar panel makes going then? #2 opps I meant rectifier diode #3 No worries I poorly wrote this. The motor has an ESC so it will cut out automatically. I wanted to know so I could add a solar panel and thermoelectric generator to my RC boat to prolong it's run time. Someone was telling me I would fry something doing this. I would think if motor draws too much lipo will take over. \$\endgroup\$
    – Tab Carmel
    Commented Jul 27, 2015 at 14:14
  • \$\begingroup\$ What happens is that the regulator will stop drawing current, so the panel will produce no power. \$\endgroup\$ Commented Jul 27, 2015 at 14:21
  • \$\begingroup\$ Beware that motors for RC boats and cars typically draw a LOT of power. They can empty a 10 Wh battery in minutes! So you would need a really large solar panel and a really beefy LiPo charger. By large I mean something at least 50 cm across. \$\endgroup\$ Commented Jul 27, 2015 at 14:21
  • \$\begingroup\$ You'll find plenty of MPPT modules of various power on ebay and other sites, typically for off-the-grid home power. I don't know of another small DIY board, but there may be one. \$\endgroup\$ Commented Jul 27, 2015 at 14:23
  • \$\begingroup\$ any chance you know where to find the module with Maximum Power Point Tracking more afford-ably? It does not look like something i could replicate. Do you know if the make one with a more forgiving input voltage range (or could I use a dc-dc down stepper? if I do, will it still be able to pull the max avail current from the panel?) Do they make an adjustable output voltage also? of would i need to use 3 modules to make it 100% universal? \$\endgroup\$
    – Tab Carmel
    Commented Jul 27, 2015 at 14:25

Not that I have tired. but if I am in your shoes.. I would get a DC to DC stepdown and calibrate it to 5V then use a 5V to LiPo charge circuit to do the job.. All these you can get easily off the shelf in plug and play boards.

  1. The DC to DC board has the ability to deal with fluctuating DC input. Using LM2956 chips..

  2. The 5V to LiPo charge board will depend on how big a current you can get off your solar panel.. I know of some TP4056 chips that can take 1A load..

This was is my intended setup I intend to use for a 2Phase (stepper motor) generator for my bicycle..

Boards are from Aliexpress and very cheap.. so blowing them up will not be to big an issue. Hope it work for you..

  • \$\begingroup\$ This is not a very good answer.. #1 is totally useless #2 This is kind of helpful… I could add a heat sink and it would be able to put out 1A and it would cut out when it needs to. I wonder if it will turn back on when it gets low enough. I was really hoping for something I could manage myself… like run multiple LiPos in series and parallel and switch up the configs and ultamitly use on my RC 3s Lipos. Just a side note, No one should ever be ok with blowing up a board when there is a LiPo on one end of it, too high a risk of fire and injury. I hope you don’t use LiPos on your bike O.O \$\endgroup\$
    – Tab Carmel
    Commented Jul 27, 2015 at 14:01

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