Victron energy LFP-Smart 12,8/60 Manual página 8

2.5 Why a Battery Management System is essential
Important facts:
1. A LFP cell will fail if the voltage over the cell falls to less than 2,5V
(note: recovery by charging with a low current, less than 0,1C, is
sometimes possible).
2. A LFP cell will fail if the voltage over the cell increases to more than
4,2V.
3. The cells of a LFP battery do not auto-balance at the end of the charge
cycle.
The cells in a battery are not 100% identical. Therefore, when cycled, some
cells will be fully charged or discharged earlier than others. The differences
will increase if the cells are not balanced/equalized from time to time.
In a lead-acid battery a small current will continue to flow even after one or
more cells are fully charged (the main effect of this current is decomposition of
water into hydrogen and oxygen). This current helps to fully charge other cells
that are lagging behind, thus equalizing the charge state of all cells.
The current through a LFP cell however, when fully charged, is nearly zero,
and lagging cells will therefore not be fully charged. Over time the differences
between cells may become so extreme that, even though the overall battery
voltage is within limits, some cells will fail due to over or under voltage.
Active cell balancing is built-in in all our LFP batteries.
The additional functions of a BMS are:
-
Prevent cell under voltage by timely disconnecting the load.
-
Prevent cell over voltage by reducing charge current or stopping the
charge process.
-
Shut down the system in case of over temperature.
A BMS is therefore indispensable to prevent damage to Li-ion batteries.
Important warning
Li-ion batteries are expensive and can be damaged due to over discharge or
over charge.
Damage due to over discharge can occur if small loads (such as: alarm
systems, relays, standby current of certain loads, back current drain of battery
chargers or charge regulators) slowly discharge the battery when the system
is not in use.
In case of any doubt about possible residual current draw, isolate the battery
by opening the battery switch, pulling the battery fuse(s) or disconnecting the
battery plus when the system is not in use.
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