low power battery charging

[orangeman10987]

Hi everyone,
I've been thinking about ways to charge a battery away from home, like if I took my drone camping, similar to this thread:
3DR Solo portable charging / Solar Charging
I could get a voltage boost converter for like 5 bucks so I could get the right voltage out of a solar cell, so voltage isn't a problem, but my concern is about charge rate, and hurting the battery. Let's say I have a solar cell that is putting out 15 Watts, I raise the voltage to 16.8 V to charge, but using P=IV, that leaves me only 0.9 Amps to work with. The 3DR battery is 5200mAh, so charging at 1C would mean charging at 5.2 Amps, so I was wondering then, would what I would be doing count as trickle charging? I've read that trickle charging is bad for LiPo batteries, I'm not really sure why though, everyone just says not to do it. Is there some lower charging threshold there that you shouldn't go underneath? I looked all over this forum but couldn't find an answer, if anyone knows I'd really appreciate it, thanks

 

[SARDG]

Will you have access to 12VDC from a car or deep cycle battery? How about a simple inverter and plug the Solo charger into its outlet? The cheap ones are square wave or modified sine wave but I'm not sure that matters. I don't know how many times you could charge a Solo battery before the 12V battery itself needs a charge, but at least it wouldn't be a trickle charge and would use the factory charger.

 

[orangeman10987]

That could work, it's a good work around, but I don't want to necessarily be tied down to my car, or have to keep the car running to keep its 12V battery charged. I found this stack exchange article that was more the answer I was looking for:
What is the minimum current required to charge a smartphone battery?
It's about smartphone batteries, but I think those use LiPo as well, so it applies. Sounds like it should be okay, as long as there is circuitry battery side to protect it from being overcharged, which the 3DR smart batteries have. I think when the weather gets a little warmer, I might just try it, and see how it goes. Thank you for the reply though.

 

[dml931]

That could work, it's a good work around, but I don't want to necessarily be tied down to my car, or have to keep the car running to keep its 12V battery charged. I found this stack exchange article that was more the answer I was looking for:
What is the minimum current required to charge a smartphone battery?
It's about smartphone batteries, but I think those use LiPo as well, so it applies. Sounds like it should be okay, as long as there is circuitry battery side to protect it from being overcharged, which the 3DR smart batteries have. I think when the weather gets a little warmer, I might just try it, and see how it goes. Thank you for the reply though.

Good luck, let us know what you discover. I backpack a lot and would much rather carry 2 batts and a charger over 5 batts along with the 30lbs already on my back. With drone and cameras that'd be pushing 60lbs. Ounces count, so.....let me know please.

 

[PdxSteve]

These batteries are a lot larger than cell phone batteries obviously and also require 16.8v to charge (cell phones usually charge using USB tech which is around 5v5). Charging rates aren't linear. The stock charger charges at 3.3A and takes about 2 full hours to go from 10% to full. Almost doubling that charging amperage to 6A only saves 30 minutes (1.5 hours to go from 10% to full). Using even lower amperage than the stock 3.3A will take quite a long time. Yes you can charge at a very low amperage level, but then the question becomes how long you're willing to wait to charge a single battery, let alone multiple batteries.

A side note: the last 20 or so minutes of charging is the actual balancing (stock charger rate), which isn't actually required each and every time. If you're in a pinch, stop charging when the last light is flashing. No point in waiting 20 minutes to get that extra 2-5 percent. Balancing is good for the cells in the long run. It just really doesn't need to be done each and every time. Balancing also wastes power because it charges all 4 cells a bit, then takes a measurement. If one cell is over the target voltage by a milliamp, it discharges the cell that's over limit for a period of time using the built in resistor array and then charges all 4 of them again, takes another reading and repeats the cycle until all 4 cells reach the target voltage. That's why it takes forever for that last light to stop blinking when charging.

It's interesting watching the charge levels per cell going up and down while it's charging and seeing the actual balancing taking place. I did that with my arduino setup as the BMS reports charge levels for all 4 cells individually.