Solar charging lithium ion cells

[Rutiger]

Hi,

I need some lighting for my workbench and want to make something myself. I have some spare lithium ion 18650 cells and want to use them to power some chip LEDs, but use a solar panel to charge the batteries.

I've done a lot of research myself and asked questions, but haven't been able to find anyone who's confident enough in their answers. My main question is around whether I need a specific "solar charge regulator" between the solar panel and the battery, or if I can just use an appropriate regulator, diode, and a BMS? What are the technical differences between the two methods?

There seems to be very limited options available for a solar specific regulator for lithium battery voltages that are less than 5A in size. Does anyone see any issues with my idea? What am I missing?

LED example: http://www.banggood.com/5_5W-56led-COB-LED-Chip-450mA-WhiteWarm-White-For-DIY-DC-12V-p-959057.html

Solar panel (17.5v 10w): http://www.ebay.com.au/itm/3-5-4-5-6-12-17-5-18V-DIY-Solar-Panel-Module-System-Solar-Cells-Phone-Charger-BO-/351491824779

BMS example: http://www.ebay.com/itm/3S-packs-11-1V-12V-20A-Lithium-Battery-Protection-BMS-Board-W-Balanced-Charging-/182172387871?hash=item2a6a520a1f:gcsAAOSw3YNXYbbZ

Standard regulator: http://www.ebay.com/itm/DC-24V-to-12V-5A-60W-Buck-Converter-Step-Down-Car-Power-Supply-Voltage-Regulator-/201047000729?hash=item2ecf55c699:g:LFAAAOxyiOxRyoYa

Solar specific regulator: http://www.ebay.com/itm/SL02B-10A-10A-12V-24V-solar-charge-controller-li-ion-lithium-battey-regulators-/252366892935?hash=item3ac23d2f87:gwcAAOSws8ZXHXv5

 

[flippy]

you do no need a specific solar verion. but it would help greatly to have a real MPPT solar charger as that is the most efficient in order to get the maximum from the panel. (do not expect to get anywere near 10w with that panel unless you live in the desert)
real MPPT is hard to find on ebay as most are regular chargers with a MPPT label put on them.
charging it not really something lipo specific, just keep the voltage under 4.2v per cell and have a good cutoff/BMS. that solar setup will never stress the batteries in any way.
you might want to have a DC DC converter as well to keep the 12v constant. as the leds will dim sonsiderably when the battery drains to 9v the leds will do nothing anymore.
personally i would go with a 4S solution with a MPPT charger and a DC DC buck converter to keep the 12v constant and have the most efficient conversion.

 

[Rutiger]

Great advice, thank you.

I was tending more towards cheaper than efficient at this stage, and am really only considering doing it because I already have the cells.

When calculating I often halve the numbers on offer, so the 10w panel becomes 5, 3Ah cells become 1.5, etc.

I like the idea about running a buck/driver to keep the voltage. I had considered the LEDs would dim as the battery voltage lowered, but wasn't sure how to do it. 4S is the answer!

All of the BMS I see have over-charge protection that seems too high, like this: http://www.batteryspace.com/pcmwithequilibriumfunctionandfuelgaugefor111vli-ionbatterypackat10alimit.aspx

4.35±0.025V

Which to me seems stupid to say it's for lithium ion cells then. If I never turn the panel off and only use the lights once a week, the cells would literally spend 80-95% of their life at 4.35v!

Thanks again.

 

[Samd]

Just use one of these. Mount one on your ebike.
You can use a laptop supply, or a floating input like a panel.

http://www.ebay.com.au/itm/281957104828?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

There are digital ones too for a few bucks more.

 

[flippy]

Great advice, thank you.
All of the BMS I see have over-charge protection that seems too high, like this: http://www.batteryspace.com/pcmwithequilibriumfunctionandfuelgaugefor111vli-ionbatterypackat10alimit.aspx
4.35±0.025V
Which to me seems stupid to say it's for lithium ion cells then. If I never turn the panel off and only use the lights once a week, the cells would literally spend 80-95% of their life at 4.35v!
Thanks again.

you can buy plenty of boards that cut off at 4.25.

 

[Ozzzz]

Iv'e been trying to get a simple/lightweight solar charging system worked out before committing to buying my battery.
Without starting another thread (and noticing the Op didn't come back); iv'e considered a few systems, gensun make a charger for lifepo4 with any intake voltage from 5V and they will take a big charge rate.

But the battery at 48v is heavy and bulky.

I was hoping to run my panels at their full current for a fast charge but understand this is not so good for lithium ion,
not quite so bad with bigger a/h packs.

With these buck convertors & setting adjustable voltage, i cant find enough info to understand if they cut off at that rate, can they simply be left to fill at a set voltage (lower than the BMS high voltage cutout)? Pay some price for slightly shorter battery life? Can anyone please advise if they see a problem with this idea or a better suggestion. (lightweight remote touring relying on mobile solar.. I have this vision/plan of being able to recharge a pack quickly given a sunny hour or three each arvo )

Just use one of these. Mount one on your ebike.
You can use a laptop supply, or a floating input like a panel.

http://www.ebay.com.au/itm/281957104828?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

There are digital ones too for a few bucks more.

 

[Samd]

When the voltage is the same on pack and Buck booster they'll stop pushing electrons into the battery as they are at equal potential.

Note that the booster I linked has two adjusters. Voltage and current.


Sent from my iPhone using Tapatalk

 

[Ozzzz]

Thanks Sam.

I'll need to put it in some sort of enclosure, they seem to have a decent heatsink. I'd probably want to try to maximise the current, the charge rate (of whatever battery) will be more of a limit than available panel output.

 

[Samd]

I've mounted one for a mate inside a semi-rigid lunchbox. Side holes drilled under the rim for moisture protection.
Managed to use cable ties in holes for the lid and tie knots inside to stop the cables being pulled out by an innocent luddite.

 

[parajared]

Here's how I would do it
-buy individual solar cells and serialize them to match the voltage of your battery at a full charge. Your solar panel will pull the battery up to it's voltage
-BMS if you want, or just solder a JST-xh connector to your battery and connect a cell balancer like "battery medic" or "Turnigy Dlux lipo balancer" every now and then.
-build your solar panel so that the peak amperage it outputs doesn't exceed the charge rating of your battery
-don't forget to install the diode to prevent backcharge

-I wouldn't fart around with a charge controller for lithium. Charge controllers are built to keep your saggy, wimpy lead acid batteries from getting damaged.
-I wouldn't use a regulator