Chargers Parallel charging

sneezy007

sneezy007

Active Member
Hey guys,

This thread follows my upgrading charger thread. If I upgrade my charger this brings in the whole parallel charging thing. There seems to be conflicting information on the Internet. Some say the cells have to be the same count but the mah hours doesn't matter. Others say all cells have to be the same mah? Talk about confusing or what.

Tony did you do a video on parallel charging?

Thanks.
 
Smoggie

Smoggie

Well-Known Member
Dino,
Yes, there is a lot of nonsense out there on this subject.

The reality:
  • Cell count has to be same on all batteries.
  • mAh capacity doesnt matter whatsoever.
  • Charge state has to be 'similar' (I use a rule of max 0.2V per cell between batteries).
  • Don't parallel charge a battery that is any way suspect (puffed or bad cell etc) because if one battery cell fails it will pull down all the others connected in parallel with it, which could damage all the batteries or worse.
  • Regularly check integrity of balance leads of each battery as this isnt picked up when parallel charging (quick check with a LiPo cell checker is all that's needed).
  • Maximum charge current = charge C rate (lowest of all batteries connected) x total Ah capacity of all batteries connected.
  • Connect main power leads first to let voltage balance.
  • Take special care when connecting balance leads to not plug in reversed or touch wrong pins.
 
Tony

Tony

Staff member
I did do a video on parallel charging. Let me know if this answers your questions.

 
Smoggie

Smoggie

Well-Known Member
Nice video Tony, good that our advice seems to tie up nicely.

BTW.. The reason the voltage doesnt always add up is that it's measured in different places. The voltage shown on the main screen is the output voltage of the charger, measured basically at the output connections where you plug the banana connectors into. The cell voltages on the other hand are measured through the battery balance leads. The two will be different during charging due to the small voltage drop that happens through the connectors, wires and balance board traces between the charger and the battery. You also get very minor differences simply due to the tolerance in measurement and rounding errors between the different measurements.
 
sneezy007

sneezy007

Active Member
Smoggie said:
Dino,
Yes, there is a lot of nonsense out there on this subject.

The reality:
  • Cell count has to be same on all batteries.
  • mAh capacity doesnt matter whatsoever.
  • Charge state has to be 'similar' (I use a rule of max 0.2V per cell between batteries).
  • Don't parallel charge a battery that is any way suspect (puffed or bad cell etc) because if one battery cell fails it will pull down all the others connected in parallel with it, which could damage all the batteries or worse.
  • Regularly check integrity of balance leads of each battery as this isnt picked up when parallel charging (quick check with a LiPo cell checker is all that's needed).
  • Maximum charge current = charge C rate (lowest of all batteries connected) x total Ah capacity of all batteries connected.
  • Connect main power leads first to let voltage balance.
  • Take special care when connecting balance leads to not plug in reversed or touch wrong pins.
Click to expand...

Thank you again Steve, now this makes sense. Amazing how much junk is out there.
 
Tony

Tony

Staff member
I always follow the 5% rule when connecting the batteries up. If they are outside about 5% difference in voltage, then I don't charge them together. The battery that is higher, more than likely nothing will happen. But the battery that is lower is being charged WAY over what it is rated for which is dangerous. I'm sure you could get away with about 10% difference, I just like to keep it safe with these guys.
 
Smoggie

Smoggie

Well-Known Member
5% actually ties in pretty much spot on with my 0.2V per cell rule of thumb. (4.0v x 5% = 0.2V).. I just find a fixed 0.2V value a bit easier on my brain as i dont have to do any math, but either method will give safe results.

FWIW is did some testing a while back by connecting different charged batteries together with a 100A wattmeter between to see what current was generated. It's actually much less than you might think. Using two 3S 2200mAh 30C I actually connected a fully charged battery to a used battery (3.8V per cell) and i got a short surge of 22A which settled within seconds down to within the batteries 11A max charge rate (5c). I very much do not recommend anyone repeats this test but for what it's worth neither battery suffered any discernible damage or got more than slightly warm. I went on using those batteries for years without issue. What keeps the amps lower than expected is the fact that the resistance of the battery to charging is much higher than it's resistance to discharging and that charge resistance is enough to prevent the current getting too high.
 
sneezy007

sneezy007

Active Member
Smoggie said:
5% actually ties in pretty much spot on with my 0.2V per cell rule of thumb. (4.0v x 5% = 0.2V).. I just find a fixed 0.2V value a bit easier on my brain as i dont have to do any math, but either method will give safe results.

FWIW is did some testing a while back by connecting different charged batteries together with a 100A wattmeter between to see what current was generated. It's actually much less than you might think. Using two 3S 2200mAh 30C I actually connected a fully charged battery to a used battery (3.8V per cell) and i got a short surge of 22A which settled within seconds down to within the batteries 11A max charge rate (5c). I very much do not recommend anyone repeats this test but for what it's worth neither battery suffered any discernible damage or got more than slightly warm. I went on using those batteries for years without issue. What keeps the amps lower than expected is the fact that the resistance of the battery to charging is much higher than it's resistance to discharging and that charge resistance is enough to prevent the current getting too high.
Click to expand...

Just to confirm Smoggie, 0.2V is per cell and not total voltage? 0.2V per cell on a 6S would be 1.2V
 
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