What most folks don't realize is that Li-Ion batteries go out of
balance, (without a BMS,) and subsequently catch on fire, for a lot of
reasons.
Cell-to-cell differences in self-discharge is the primary culprit. In
poor quality cells, variations in manufacture "bake in" this
cell-to-cell difference in self-discharge. You need a BMS to keep a
series pack of these low-quality cells from getting out of balance, (and
catching your house on fire,) in even the most ideal circumstances.
In high-quality, matched cell battery packs, differences in
self-discharge are caused by differences in temperature. Even small
temperature differences make a HUGE difference in self discharge, which
will unbalance the pack without a BMS, (and set your house on fire.) It
typically doubles for every 10 degree Celsius rise in temperature. A 2
degree temperature rise produces a 15% increase in self-discharge. Thus,
a tiny temperature difference works to chew away at your pack, causing a
large imbalance, if not corrected by a BMS, (eventually setting your
house on fire.) Remember, self-discharge works 24/7 chewing away at the
pack imbalance. Self-discharge never sleeps.
https://www.designnews.com/electronics/measuring-lithium-ion-cell-self-discharge
There are lots of factors that easy lead to differences in cell
temperature. The edge cells get heating and cooling from the outside
environment. The end cells, connected to the fat conductors, have
well-documented temperature differences from the bulk of the cells in
the pack. What also causes differences in temperature is the imbalance
itself. The imbalanced cell gets tortured as it is over-discharged or
under-discharged relative to its peers. This causes a cascading effect,
leading to accelerated cell imbalance as the imbalanced cells get hotter
and hotter relative to their peers.
>>>>> Bottom Line <<<
You need a BMS. There is no question about it. However, when you
install a new BMS, you (personally) have to see that you have installed
it correctly and see that it is doing its job. It is a new system on new
batteries, so you have to check it for "infant mortality". During the
first few cycles especially, you need to check every cell voltage
several times and make sure that the BMS is functioning 100%. You should
_never_ ever leave a new pack charging unattended during this
"honeymoon" period.
You need to check on the BMS periodically as well, to make certain that
it is doing its job.
The OEMs have done this exhaustively before they release a design to the
public. Thus, the BMS in your OEM EV is trouble free. They have a mature
BMS design and they have a huge amount of "stick time" to find any bugs.
Going without a BMS on a Li-Ion pack under regular use pretty much
guarantees you will get a fire. You don't want a fire, trust me.
Bill D.
PS
When I saw cell-imbalance compared to not changing your oil, I cringed.
Not changing your oil won't set your car on fire, or burn your house down.
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