Will parallel cells in a power bank eventually imbalance each other? - Page 1
Forum section: electronics / beginners

author: skillz21
author_rank: Frequent Contributor
timestamp: June 02, 2018, 11:34:00 am
content: I saw a video by GreatScott! where he made a regular bike into an e-bike. When he made the battery back, he said that, if you just connect two calls in parallel and put around 12 of those in series (not 100% sure of the number), over time, the cells' voltage will drift apart and once the difference is great enough, one of the cells will burn or explode. He used a balanced charger where a circuit took care of the voltage of the individual batteries. I built a power bank, which is just the circuit connected to four cells in parallel. Will my power bank cell voltage grow apart over time? Also, I'm pretty sure, that in commercial power banks, then use this battery configuration as well... does this mean that it's fine?Thanks

author: Rerouter
author_rank: Super Contributor
timestamp: June 02, 2018, 12:09:29 pm
content: If all cells are in parrellel, then they will remain balanced in voltage, The individual capacity, and current provided by each cell may differ, but the voltage will remain the sameIn his case he had those parrellel chunks in series, and it was those series packs that drifted from one another,E.g. lets say you have a 100 salvaged laptop batteries, you carefully measure each cells capacity and balance all in your series string to be the same capacity, can you now get away without balancing? not long term, In this case the ESR of each cell, and to a lesser extent the exact chemistries discharge curve come into play. If a cell in the string has a higher ESR, it dissipates extra energy when charging or discharging, resulting in a lower apparent capacity, and an imbalance of charge to the other cells.

author: skillz21
author_rank: Frequent Contributor
timestamp: June 02, 2018, 12:18:23 pm
content: Ah ok thanks!

author: station240
author_rank: Unknown
timestamp: June 02, 2018, 02:11:17 pm
content: Lets say you have 2 cells in parallel, both 2000mAh, they both behave as one cell of 4Ah.If you put multiple sets of these cells in series you get a 4Ah battery with higher voltage.All goes well assuming all the cells remain the same capacity, but lets say one set of parallel cells loses 10% capacity.Now it's 3.6Ah or 1800mAh per cell. When you charge the battery, the 1800mAh cells will charge 10% faster, reach 4.2V sooner and will take damage if overvoltage happens.That 10% is removed from the other sets of cells in the battery, so they take longer to charge, (eg 6s battery, 10%/5s = 2% longer per series cell pair.From the point of view of the charger, the battery reaches a certain combined voltage so it stops, but 2 cells are overcharged by 10%, the rest are 10% undercharged in total.Another major issue, discharging the battery, those lower capacity cells are very likely to hit a lower voltage when the battery is discharged heavily. In extreme cases that can be 0V (or even negative!), when other cells remain at a usable voltage. A 0V cell need to be very slowly/carefully charged at low current/voltage, or just discarded , a conventional charger will either give up, or kill the battery.Yes ESR also comes into play, often cells with age reduced capacity have higher ESR too.It's pretty hard to get modern Li-Ion cells to catch fire, eg 18650's. LiPo pouch cells on the other hand, yes those can burst into flames, especially the high current ones.