I am still a bit confused about parallel charging.. 
for example: if i have 2 lipos one of them is 3.7V/cell and the other one is 3.5V/cell, can i charge them together?

Not recommended, you should keep the batteries at least inside 0.1V difference of each other, otherwise there will be too much current goes from battery to another when connected to the paraboard, which could burn something up.

From what I understand is that voltage should be close as possible, but if you have 0.2 or 0.3V difference in voltage, plug in only main leads first and let them balance each other out, then the balance lead, leave them to equalize on the parallel board for several minutes before start charging.

the largest voltage difference I do it with is at most .2v and it takes quite a while for that voltage to equalize. I don't know if the type of board makes a difference (i have a fused and non fused parallel board) but I've never had an issue as long as only the main lead is plugged in while cells equalize. I did a lot of research on parallel charging before I started doing it, everything I read stated " the larger the voltage difference between packs, the longer it needs to stabilize with just the main leads plugged in". The .1 difference I read about was to never charge when cells have more than .1v difference.

(18-Mar-2016, 09:46 PM)CurryCat Wrote: the largest voltage difference I do it with is at most .2v and it takes quite a while for that voltage to equalize. I don't know if the type of board makes a difference (i have a fused and non fused parallel board) but I've never had an issue as long as only the main lead is plugged in while cells equalize. I did a lot of research on parallel charging before I started doing it, everything I read stated " the larger the voltage difference between packs, the longer it needs to stabilize with just the main leads plugged in". The .1 difference I read about was to never charge when cells have more than .1v difference.

I've charged at more than .2v and my packs are fine. I just always make sure the voltage has settled before I charge. you can mix packs of different capacity as well, just don't go too high with capacity difference a few hundred mah should be fine.

Yes, I have done parallel with more than .2v for the longest time, and it is fine. The trick is to let the voltage stablise first before charging.

I'm not comfortable with more than a 0.1V/cell difference. I've tested the current flow with my amp meter and even that difference results in an initial draw of
several C. It quickly drops, though.

If I had 3.5 and 3.7 I'd put the 3.5 on the charger alone for just 1 minute and it would bring it up close enough to the 3.7. to put them on the board together.

There is a recent YouTube by Rotor Riot that addresses this. If I remember, they said .2v difference is the safe zone. Seems like there are polarized opinions on this, but I haven't ran even a percentage of packs that some of the pros have, and they seem much less cautious about charging. I think it comes down to the degree of ocd that you have. I am still looking for the evidence of catastrophic failure when charging mismatched voltages, but so far I haven't found it. I guess if I was charging $60 10000mah batteries I would be more careful, but with cheap 1300mah batteries I'm not losing sleep parallel charging them if they are .2v different.

BTW, I'm still fairly new so I would quickly dismiss my opinion if I were you, but if you have a pack that is .1v higher than your others, and you are concerned about the safety of parallel charging it with the others, you could always plug it into your quad and do a 15 second hover to bring it down with the others.

So, we have two batteries with 0.1 V difference

I see that common internal resistance of these batteries is around 50mOhms (or lower)

These are then two resistors each with ...50mOhm resistance in parallel that gives 25mOhm -
I=V/R - I=0.1/0.025 = 4A - not to bad.
but goes to 8A with 0,2 volt difference.

that said, I'm not sure if I use the parallel equiv resistance of the two batteries or just one.

I'm getting my first parallel board in mail now - so I'm going to measure and see if the math fits.. I'm not 100% sure.

edit: One thing to note that can lower the current as the discharging battery has internal resistance the voltage will drop when loaded so 0.1-0.2v difference is probably lost somewhat over the discharging battery

- Benni

I have found that plugging the packs into the parallel charging board (using main plug not balance lead) but not starting the charge and leaving them for 20 mins equalises the voltage in them and then I start the charger.

I did this the other day with a battery that was a fair bit lower than the others and then checked all the packs after half an hour and all the cells were within 0.1v of each other.

Also pretty new to the parallel charging so if this is a really really bad idea then please feel free to correct me!!

(17-Apr-2016, 12:36 AM)DaJudge Wrote: I have found that plugging the packs into the parallel charging board (using main plug not balance lead) but not starting the charge and leaving them for 20 mins equalises the voltage in them and then I start the charger.

I did this the other day with a battery that was a fair bit lower than the others and then checked all the packs after half an hour and all the cells were within 0.1v of each other.

Also pretty new to the parallel charging so if this is a really really bad idea then please feel free to correct me!!

It's a very bad idea if you don't know what the voltage difference is before you plug them in. Some people are happy with 0.2V difference, I won't do it with more than 0.1 difference. Of course they will equalize but that doesn't mean you didn't pump huge amps into the low one and damage it. Worst case scenario: fire.

Not sure what you mean about them being within 0.1 after 30 minutes. Within 10 minutes they should already be exactly the same…

Good video showing what you are wondering.