Hello there,
I would link two 3.7 lipo cells in parallel to double the total mAh.
The two cells are of the same model but one is brand new and the other is 2 year old so that it discharge faster.
I know this could be an issue if you connect the two batterie in series (different voltage/discharge time curve) but what about parallel? Is it okay?
Do the two cells voltage-discharge in the same way?
Many thanks.
squizzy91:
Hello there,I would link two 3.7 lipo cells in parallel to double the total mAh.
The two cells are of the same model but one is brand new and the other is 2 year old so that it discharge faster.I know this could be an issue if you connect the two batterie in series (different voltage/discharge time curve) but what about parallel? Is it okay?
Do the two cells voltage-discharge in the same way?Many thanks.
It's not okay. It should only be done when the batteries are new and are charged at the same voltage level.
It would be better to get 2 new cells and parallel them without using the old cell.
Basically if you play fast and loose with LiPo you should expect cells to burst into flames and
take the relevant precautions (keep outside in fireproof storage). If you can't provide such
provision, you risk starting a fire, endangering life and property.
No, absolutely not. You can only do things like that with batteries of similar age and wear. I have seen boards that take two 18650's and charge them and generate 5 and 3.3v from them, which use in a 2p configuration, but they're not quite connected in parallel (ohm meter shows that) - they are managed individually, somehow - something like that might work. But not just paralleling them and hoping for the best, because that's not what you'll get. I wouldn't even parallel them without protective measures if they were both new, frankly - I'd rather use a battery management board and buck converter and the batteries in 2S configuration. 2S battery management boards are readily available from china for cheap.
You've had the theoretical answers now a practical one.
It's generally fine to parallel lipos even if they're not new or even if they're not the same capacity provided you treat them sensibly. People have been doing it in the RC modelling world for years. Of course if one is old and "discharges faster" that means it now has lower capacity than it used to so you won't get twice the notional capacity. But lipos are remarkably good at load sharing and since they are connected in parallel there's no possible way they can be at different voltages.
I wouldn't do it for a production system but for a hobby setup, particularly with single cells so there are no balancing problems, I would and have (without any flames or other difficulty).
Steve
To give your more information, i'm going to use a LIPO battery management board: it is the lipo pack of my pc and i would couple it with the old battery pack just replaced (500 cycles have passed but still 50% of original mAh remains).
I don't know exactly what the lipo management board do. I think he have a built-in balancer (i can check in some way, i think).
What do you think guys?
Now you have changed the story. There's a world of difference between messing with multicell packs with built-in BMS (which is what all computer packs I've seen are) and what you originally said which was paralleling two single lipo cells.
I wouldn't mess with packs like that (apart from anything else many of them aren't actually lipos).
Steve
Sorry Steve, i've understood until a certain point.
Btw let's go straight to the point...->
The real case is that:
what i have
1 brand new 2S 7.4v 5000mAh PC battery pack with BMS
1 old 2S 7.4v 5000mAh PC battery pack (probably we have just 2500 real mah) with BMS
what i would
make one "big" (more total mAh) 7.4v battery pack
ideas:
way 1: solder the 4 cells to have a 2S2P configuration and attach one single BMS
way2: solder directly the two battery pack maintaining both BMS
What do you think?
Is it feasible or it is a waste of time\ money\ dangerous?
squizzy91:
what i have
1 brand new 2S 7.4v 5000mAh PC battery pack with BMS
1 old 2S 7.4v 5000mAh PC battery pack (probably we have just 2500 real mah) with BMS
When did this go from 3.7 to 7.4v? Your original post asking about paralleling single cells. Now you are talking about paralleling 2 2s packs.
squizzy91:
ideas:
way 1: solder the 4 cells to have a 2S2P configuration and attach one single BMS
way2: solder directly the two battery pack maintaining both BMSWhat do you think?
Is it feasible or it is a waste of time\ money\ dangerous?
Go for your "way2" keeping both BMS systems in operation. The batteries are then effectively 'isolated' from each other by the active components of their respective BMS systems
You can do it, you should not do it. Not to long before your second pack is also waisted because of the imbalance.
I have never seen people do it in my rc hobby world either........
Balance the two BMS output with a power resistor, then connect their output together.
This "could" work but, as other said, because you could it doesnt' mean you should.
You can't connect parallel cells to the same BMS unless you are able to change its termination current set value, which doesn't seem to be the case here.