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effect of repeated solar charging on 18650 capacity
#1
In some of Wolf's repeated test of 18650's it was evident that the cell capacity reduced with repeated charging/discharging/charging, and that makes sense.
I'm building a 4S9P 18650 pack to run a 4-metre strip of 5050 LED's plus a security camera.
I have a 250 watt 12 volt Mono panel to charge the pack via a 30 amp Epever MTTP Charge Controller.
I'll reduce the voltage to 12 volts with a variable buck converter.

The LED strip will get only occasional use - say 2 nights a week for 3 to 4 hours PN and the camera is motion-controlled so little use there also.

Given the little usage the battery will receive would I be better off just charging the battery from grid supply as required (I'll be monitoring voltage), rather than using Solar?
Solar will (attempt) to charge every day and I'm concerned about increased loss of capacity over time.
cheers
emmsee
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#2
It's typically full cycling, a 100% battery being discharged to 0% then recharged that causes the most wear. The charge controller should not be charging the battery if it's already full. The effects of it putting in a few mAh each day are negligible in my opinion. You also have to consider the effort of monitoring and manually charging from the grid vs just letting the controller manage it.
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#3
And the cost of replacing an 18650 after a couple years of service also is to be considered. Having to monitor voltage and recharge will take a lot of time and ppl are forgetful, or can get too busy to perform menial tasks.
As Mike says, the charge controller should auto charge to proper voltage. And, it's possible the charger won't trickle charge the cells, but wait till several 100mAh have drained. If you want to make your own mppt charger you can even configure it to wait till voltage drops to a specified limit before it re-enables charging.
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#4
Sometimes it's fun & satisfying to do a project like this even though it may be more complex, vs just regular mains powering it.
The camera will drain the pack some over night every 24hrs. Do you know how much it draws (don't forget the night time extra IR current)?
During the day the solar panel would cover the camera's power draw + recharge the pack.
Ball park numbers:
So with 9P pack & say average 2000mAHr cells, you've got approx 18,000 mAHrs available at 100% DoD (you might have better cells!)
If the camera draws say 500mA at 12V for say 14hrs (assuming winter) = 7,000mAHrs so bit under 40% DoD per day.
Haven't considered buck losses, etc.
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#5
My off-grid Solar system's daily charge/discharge cycles is one of my top interests - a lot of $ and work has gone into the 18650 battery bank.    The battery bank is made up of 14s100-120p / 48v@260ah batteries. The design is for low DOD% in hopes of 1,000(s) of cycles. Last year's DOD% average was 32% with the oldest battery at 524 cycles, the next 2 at 346 cycles, and the latest one at 2 cycles.

No obvious loss of power yet...   but  time will tell Smile
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#6
the epever 30 amp mppt maxes out at 17 volts( 9 - 17 volts on user define settings) on a 12 volt system, max charge for a 4s is 16.8 volts. If you want to fully charge the pack everyday you have to make sure there is no voltage drop between controller and battery.

The only bms I would trust on a lithium solar system is the chargery bms8, once setup you never have to mess with it, and since it uses contactors to stop the charge its failsafe if it loses power. If your pack is in perfect balance, you can set the controller to get you near the voltage you desire. The bms will protect from out of balance conditions and also controller failure. The cheaper bms that use mosfets, produce voltage surges when they activate to stop the charge, I would not recommend those on solar systems. 
With the chargery you can set it to disconnect the solar panel (with contactor/relay) at 16.8 volts(or lower), and reconnect the panel if the voltage drops to 16 volts. With the panel disconnected, the controller will be sleeping thinking its nightime. The bms will only allow the controller to charge the battery when it meets the right conditions. 

As far as the battery losing capacity charging everyday, it'll take years before you notice any loss. Its just like using on a laptop, you charge it everyday and occasionally run them down and they still last you 4 or 5 years.
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#7
Many thanks to the 5 of you for your excellent responses.
I now have a clear way forward and will update once I've tested all the options.
jonyjoe505, I do have a Chargery bms8 and will swap the Epever out for it.
offgridinthecity, I too am looking for low DOD (<50%); time will tell if my Samsung 22Q's will give me that result.
Redpacket, I hadn't taken the IR usage into account and will research that now - thanks for the heads up.
Korishan, Now that I'm aware of the Chargery's settable limits, I'll be able to reduce the charge swings (set the cut in lower voltage to 15.5V). Plus I planned to run the packs between 3.1 and 4 Volts per string to extend life further.
Mike, thanks for the clarification on the cycling. That's a top lesson I've learned there!

I'll proceed with Solar Charging the pack.
I appreciate all of your input.
cheers
emmsee
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