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batery charging Answered

Having had to buy a dedicated charger for some 18650 cells (at my wife's insistence) I took an interest in these cells for the first time and their smaller cousins the 123a. At with maximum capacities I've come across so far of 3700mAh and 2000mAh respectivley that's twice the capacity of the battery in my smart phone and 2.5 times the capacity of the one in my basic bar phone.
The smart phone width would accommodate 3 of the 18650 cells (6 days without charging at my rate of use) and the bar phone possibly 4 of the 123a's (70 hours talk time or 10 months on standby).
Yes this would mean a custom made back, but there is at least one instructable on molding a GRP/ carbon phone back adjusting it to take extra cells and maybe ports just needs modeling clay. I suppose I could just carry a power bank, but why if the phone can be its own power bank? Removing the guts of a power bank and building a new phone back to accommodate them may be the simplest answer to this but, my questions are:-
A) I've got old mobiles laying around could I use their charging circuits to charge these standard Li-ion cells?
Assuming that is a yes that would mean a long charge every 6 days/ 10 months.
I gather there are problems associated with parallel charging/ discharging of Li-ion cells and over discharging so,
B) Any advice on doing this other than don't?
On the assumption that's a don't or even if its not this strikes me as a safer possibility.
C) Could a micro controller be configured to discharge one cell to its limit then switch to the next full cell.
D) Could a micro controller be configured to charge one cell to capacity then look for the next under capacity cell and charge that.
E) if so could it do both at the same time?
If A is possible and the rest not then a multi position switch is  probably the answer.


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4 years ago

A) While you technically could, it would be difficult and risky, I would just buy a charging module off Ebay. Parallel connection should be just fine, but try to use batteries with the same capacity rating (don't use a 1000mAh and a 2500mAh together).

B) A few things;

1. Don't modify your phone, make a battery pack case that is separate from your phone.

2. Don't buy any 18650 battery that's rated over 2600mAh, it's a fake. And make sure to get good brands such as Samsung, Panasonic, of authentic UltraFire. They're 5 or 10 times as much in cost, but well worth it, the cheap ones are awful.

3. You need to protect the batteries, both from over charging and over discharging.

C) It certainly could, but that would be unnecessary and difficult.

D) Again, possible but unnecessary.

E) Also possible but still unnecessary.

You wouldn't need a switch or any kind, just wire the batteries in parallel and make sure they are roughly the same capacity.

Hope this helps!



5 years ago

A) no need to bother about that kind of stuff, there are cheap TP4056 based lithium charging boards available on Ebay (<1$ for a few of them), they go up to 1A charging rate and they are smart, you could use them easily in a parallel or 1 cell configuration

A')Lithium batteries have low self discharge rates so I guess it could be possible

A")I don't have enough knowledge about parrallel/serial discharging/charging but I guess parallel is fine as long as the cells are in the same condition

B)That would make an heavy phone :p

C)Yes but you would need room to fit all that circuitry but any µController with an ADC and Digital outputs is able to do so (with mosfets for instance), sorry I don't have the time to make a schematic tonight (I'll try tomorrow) but I hope you got it the way I tried to explain.

D)Same answer as before, I guess...

E)I guess too...

I'm looking forward to this topic as I, too want to know more about lithium batteries, especially about serial/parallel configurations ;)