Battery wiring help-don't want to die!!!

[knightslugger]

I'd like to increase the capacity of my laptop battery. it is currently a 3-cell 18650 in series. I'd like to wire in a 4th 18650 in parallel to one of the cells. 

 

Is the attached photo considered best practice?

 

[mariushm]

No.

Either add 3 batteries (one in parallel with each existing battery) or you don't add any.

 

If you connect your fourth in parallel with one battery, the pair will charge slower than the other two pink ones and the charger will keep pumping energy in the string of 3, potentially overfilling or overcharging the two pink ones without pairs (and that results in overheating or damaging the batteries).

 

The voltage on each battery has to be pretty much the same, all batteries must charge evenly and at the same "speed", adding a 4th battery slows down that particular battery and screws things up.

 

[Hackentosher]

Yeah that's not a great idea. You would double the capacity of one of your 1s cells which wouldn't really do anything. As one of the single cell cells drains below an acceptable level, while your doubled one would still have a good bit of energy left. However, the bms Will kill connection to the battery. It's not a good idea to mismatch capacities in a pack like this. Either leave it stock or do a 3s2p pack. 

[knightslugger]

well i can't do a 3s2p config. there is only enough room for 4 cells total, and I'm not sure the laptop will like 16.8V battery.

[mariushm]

No, it won't like it.  Check those wires, the white and yellow ones .. they're there to give chips on the circuit board the ability to monitor the voltage on individual batteries in the chain of 3 batteries, and optionally connect a resistor or something across a single battery in order to partially discharge it bringing the voltage down to the level of the other batteries (the charging mechanism aims to keep the voltage of all batteries equal, within 1-2% all through the charging process, but due to manufacturing process some batteries can charge slightly faster or slower)

if you put a battery in parallel with each existing battery, you don't mess around with that, you only change how fast the batteries charge, how fast the voltage on each pair of batteries goes up.

 

Not sure what you could do about it.  There may be a slim possibility to find some flat style batteries that are as long as your 3 batteries. and make a stack of 3 or 6 such flat batteries - may have more capacity this way because you'd probably fill more of the volume with battery...  I don't think it would be worth it ... you'd probably get 5-10% more charge but for huge cost (the 3 batteries)

[ZicMortal]

If you could check capacity of these three batteries, and if they are around 1700mAh (per cell) or less, you could actually upgrade the battery by removing those three, and by putting 3 genuine LG or Samsung batteries with capacity of around 2900mAh (per cell) in same order (pluses and minuses)

 

 

Even if you'd actually manage to add 3 battery in paralell, they would have to be same voltage, and also, BMS could potentially cut off charging (charging current would be higher) 

[mariushm]

@ZicMortal  The pink ones are already rated for 3000 mAh

 

 

datasheet_LG-ICR18650D1.pdf