Electronics > Beginners
Connecting different-sized lithium batteries in parallel.
KL27x:
You have to derate the max discharge/charge rate in addition to capacity. As long as you are paralleling them to increase capacity rather than max charge/discharge rate, and your device has a relatively low draw compared to the cells' capabilities, then I wouldn't worry about it too much.
If you're stacking them together to increase discharge/charge rate, then your results won't be very reliable.
If a single cell doesn't have the ability to supply enough current all by itself, then you could have longevity issues with your battery.
ogden:
--- Quote from: Rick Law on November 10, 2019, 02:41:05 am ---I might have a partially (or hardly) charged battery while the other is fully charge.
--- End quote ---
Note that 1A current per cell, diodes will dissipate 0.5W per cell. When cells are nearly empty (at 2.5V per cell), 0.5V drop reduces efficiency of your battery pack to 75%!! All this due to your inability/laziness to fully charge cells before assembly.
Siwastaja:
The parallel connection of course, by definition, means that each cell will be at exact same terminal voltage, at all times. Due to different series resistances within cells, they do not share current equally during high-current peaks, but then, after the peak declines, equalize each other, eventually reaching the same SoC, assuming they have the same SoC vs. voltage curve.
Note that if they are of different brand/type, and/or aged very differently, their voltage curves may differ quite a lot at some point of state-of-charge curve. This means that current sharing may differ more than you expect from just internal resistance differences alone. This is meaningless if you derate the current properly. For slow discharge and charge rates (say, around 0.2C?), there likely isn't any problem.
Of course, for the worst case, you could just derate the max current to match what the weakest cell can do alone, but this is extremely conservative.
And, laptop cells tend to have quite similar characteristics.
Just remember to match the voltages carefully (within 50-100mV) when you initially do connect them in parallel, and make that connection properly fixed so they can't accidentally drift apart, then connect again.
If you see someone claiming that li-ion cells can't or shouldn't be connected in parallel, or that diodes are "needed", ignore everything they say. They have never seen a battery pack.
Rick Law:
--- Quote from: ogden on November 10, 2019, 08:12:30 am ---
--- Quote from: Rick Law on November 10, 2019, 02:41:05 am ---I might have a partially (or hardly) charged battery while the other is fully charge.
--- End quote ---
Note that 1A current per cell, diodes will dissipate 0.5W per cell. When cells are nearly empty (at 2.5V per cell), 0.5V drop reduces efficiency of your battery pack to 75%!! All this due to your inability/laziness to fully charge cells before assembly.
--- End quote ---
That my setup uses randomly selected cells from the "charge battery" box is one of the design requirements I started with. I keep a box of charged 18650's each installed with battery protection module - typically for my flashlights. They are normally charged with a commercially available 18650 charger. So, when I need it, my "power bank" always is fully charged (with cells picked from the box) instead of itself needing to be juiced up first.
Had the operational designed been permanently soldered-together fixed cells, I too would agree to use carefully match cells, solder them together with fuse protection instead of my using battery holders and cells with "battery protection modules".
I actually begun with a two matched cell with fuse protected connected in series instead of parallel, and buck down rather than boost up to 5V out. Seeing multiple articles (& youtube videos) how some super-bright flashlights (with >2 in series) blew, I never felt comfortable with it and ended up abandoning that one.
macboy:
Parallel is fine, if they are all the same chemistry (same nominal full/empty voltage).
I would definitely bring them to the same voltage prior to connecting them together. Unlike some other chemistries, which don't take a significant charge current until the terminal voltage is made to be quite a bit higher than the standing voltage, a Lithium cell will sink a significant current from any source higher in voltage than itself.
Parallel is much safer for mixed capacity than series. With any series connected battery, you must worry about the balance of all cells in order to prevent any one from discharging too low or charging too high, either of which is dangerous. When in parallel, all cells are at the same voltage, and therefore at (nominally) the same state of charge at all times - it is self-balancing by nature.
edit: I'll add to stay away from large charge or discharge currents. The differing internal resistances of the cells can contribute to differing cell voltage, when a large current is involved. If the current is kept relatively small (i.e. so that the V=IR drop of the internal resistances is small) then cell balance is naturally maintained.
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