Re: Ubitx batteries / charger #ubitx

Jerry Gaffke

Daniel's three cell lithium ion 18650 holder with protection from looks promising.
Unfortunately, from the "discussion" section there, I doubt it does charge balancing across the cells.
Would like to hear of a 3S or 4S holder like this that does do charge balancing,
and is designed for standard issue 4.2v max lithium ion 18650 cells.

Arv said:  "Most Li-Ion and Li-Po 18650 type cells include built-in FET switching
that prevents discharge
 to less than 2.0 volts." 
Arv must be spending more money on his cells, the ones I get generally don't have protection.

Arv also says:
"If using multiple 18650 Li-Ion or Li-Po cells you have to manage the voltage on each individual  
cell during charge to insure that no cell exceeds the rated voltage.   While I abhor the "it won't 
work" and the "it will explode" scare tactics, this is one place where the nay-sayers may be justified"

The model aircraft guys might discharge a battery in 5 minutes or less of aerial acrobatics.
Then land it, and do a quick charge to almost full capacity in 3 minutes or so from 
a 100 Amp source, send it back up for more fun. 
That's the usage case where most of the explosions come from.
If charging at a fraction of an amp, explosions are much less likely.

Charge balancing is a very good idea, but if starting out with a set of all new
cells from a quality manufacturer, they will all charge and discharge at about the same rate.
Maybe back off from the spec max of 4.2v per cell, 4.1v max per cell would be safer.
Unlike lead-acid batteries, a fully charged lithium ion cell can get damaged if you push
a charge current through it while any slower cells catch up. 

If I were to buy that 3S 18650 holder, I would charge the cells in place
from a 3*4.1v=12.3v voltage source, limited to a max of perhaps half an Amp.
Could use an adjustable 3 terminal regulator set to 12.3v, plus a series resistor to a 20v supply.
And check for balance across the cells with a voltmeter every few charge cycles.

But looks like most of the balance boards are sold separately from the the cell holders.
That's fine, just an extra 4 wires to solder.

Here's a cheap 3S balance board, though from the user's comments, the balance function is rather weak:

Here's another, totally different design (don't see the big FET's),
has a somewhat better description even if it is rather mangled:

Too bad they don't include a schematic of the thing so I can figure out what all it does.

Any suggestions for a good balance board along these lines?
Can it charge the string of batteries from something like a 20v source
without any external voltage regulator?  Without external current regulation?
How large can the balance currents be?
That sort of thing is often missing from the description.

Jerry, KE7ER

On Fri, Apr 19, 2019 at 12:45 PM, Daniel Conklin wrote:
You need a 3 cell protected holder, like this one:

Join to automatically receive all group messages.