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Is a BMS needed for only 2 cells in parallel
#1
Hi,
I need to make a 2 cell 18650 battery pack in series for 7.2v 3500mah, do i need a bms or some sort of balancing system ? I will charge them with a NP-550 charger.
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#2
Yes, to be both safe and perform optimally, every Li-ion battery requires adequate protection against extreme voltages (OVP/UVP), current (OCP), and temperatures, no matter the pack topology.  If you build a Li-ion pack with multiple batteries in series then balancing is essential for both safety and health (a series pack degrades much more quickly without balancing)

Note: you mean "series" not "parallel" in your title.
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#3
In series, yes, it's good to use.
In parallel (like in title) no.
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#4
(07-21-2020, 09:08 AM)Overmind Wrote: In series, yes, it's good to use.
In parallel (like in title) no.

No, adequate protection i(OVP/UVP/OCP etc) is absolutely essential in both cases (both for safety and pack health).

E.g. if no balancing is done on a series pack then this greatly increases the chance that they will get so unbalanced that one cell will eventually become reverse charged - which usually leads to venting (possibly with flames / explosion). Not to mention that pack capacity will quickly degrade once they become nontrivially unbalanced (it is a positive feedback process).

Generally it is very dangerous to use Li-ion packs without adequate protection (by BMS or protection circuit). Note that in some cases the protection may already be incorporated (somewhat) in the device(s), e.g. flashlight cells are protected by both the light's design and also by the (usually separate) charger - assuming proper design.
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#5
Agree w/@Overmind - for 2 cells in parallel - balance is not needed as the parallel wiring will force the voltages to be the same.
You do want to avoid overcharge - a proper charger can handle that (or a 1s BMS) and of course over-discharge will be harmful (monitor voltage or 1s BMS again)
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#6
^^^ No one claimed balancing was required for a 1S pack (balancing refers to balancing the SOC of multiple packs put in series). The OP is a 2S pack (read the post, not the title) so it does require balancing.

There are good introductions to all aspects of balancing in the papers that I linked here.
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