Lithium Battery 3.7V / 4.2V / Charger

Thread Starter

GregJ7

Joined Jun 7, 2014
42
I bought a companion power supply PCB with a 3 x 18650 battery holder soldered on (2/3 down the web page for the main board). I need to supply the main board with at least 12V. I bought 4 batteries labeled 4.2V on the idea that 3 x 4.2 = 12.6V is enough (as the web page states). (The full label is "GTL GtIenrgy® Li-ion, 18650 4.2V 9900mAh.") noticed that most of the 18650 Li-Ion batteries on Amazon are 3.7V. The batteries came charged to 3.7V and I popped 2 of them into my $5/no useful markings/very bad English 18650 charger and it looks like they aren't going to charge over 3.8V. (I've tried to go cheap because I do not anticipate working with 18650 Li-Ion batteries in the foreseeable future.)

There are a variety of different chemistries for Li-Ion batteries. Do some produce 3.7V and others 4.2V? Do I need to buy a 4-battery holder or get a charger for 4.2V Li-Ion batteries?

In trying to figure this all out, I am lead to ask, how do I interpret the voltage readings my multimeter is giving for Li-Ion? I wouldn't wonder, except—How come my MM/Battery Tester has three different dial selections for measuring battery voltage (other than the general "V" setting)?
1. AAA/C/D
2. AA
3. 9V
 
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Ya’akov

Joined Jan 27, 2019
9,071
Lithium Ion cells are 3.8V but charge to a 4.2V open terminal voltage. You will need more of them to get 12V, and a buck converter to reduce the higher voltage to the 12V you want.

Your meter has battery test ranges which include a load to realistically test the battery. When not under load, "open terminal", the battery voltage can appear higher than it can produce in use.
 

Thread Starter

GregJ7

Joined Jun 7, 2014
42
I presume that "open voltage" means with no load. Does that mean I would do well to use the test method of using a small Ohm resistor to calculate power consumption & amps and then calculate volts from that? Thanks for the help!
 

oz93666

Joined Sep 7, 2010
739
I presume that "open voltage" means with no load. ...
Normally they are charged up to 4.2V (you can go to 4.35V with greatly reduced life) they have such a low internal resistance you will see 12V from 3 in series .... As they are discharged the voltage drops .They deliver power over 4.2V to 3.5V during normal discharge ... so that is the problem ..

If you need at least 12V supply , and going as high as 16.8V is no problem then use 4 cells in series ....

You do not need to charge each cell to 4.2 ... if you charge only to 3.93 the cells will last 10 times longer (but hold 65% normal power)

So @3.93V 4 in series will go no higher than 15.7V and give the performance you require.

The cells you have are cheap junk .... no cells have been manufactured over 3500mAHrs they are probably salvaged from scrap laptop batteries ( a big industry in China) . If they won't charge over 3.8V they are defective ... 18650 cells is one item you have to buy respected brand ...panasonic , sony .... any 18650 cell advertised as over 4000mAHrs is fake junk...

There is confusion over voltage because some manufactures advertise the highest voltage (4.2) others the averaged working voltage (3.6 or 3.7) ... the cells are the same Li-ion.....
 
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Thread Starter

GregJ7

Joined Jun 7, 2014
42
Great info, thanks! I guess I may be using 18650's in the future since I am about to have 4 extra (cheap junk) ones.
 
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