Victron energy BMV-700 Anleitung Seite 33

A timely alarm can be generated by monitoring the midpoint of the battery
bank (i. e. by splitting the string voltage in half and comparing the two
string voltage halves).
Please note that the midpoint deviation will be small when the battery
bank is at rest, and will increase:
a) at the end of the bulk phase during charging (the voltage of well
charged cells will increase rapidly while lagging cells still need more
charging),
b) when discharging the battery bank until the voltage of the weakest
cells starts to decrease rapidly, and
c) at high charge and discharge rates.
5.2.1 How the % midpoint deviation is calculated
d (%) = 100*(Vt – Vb) / V
where:
d is the deviation in %
Vt is the top string voltage
Vb is the bottom string voltage
V is the voltage of the battery (V = Vt + Vb)
5.2.2 Setting the alarm level:
In case of VRLA (gel or AGM) batteries, gassing due to overcharging will
dry out the electrolyte, increasing internal resistance and ultimately
resulting in irreversible damage. Flat plate VRLA batteries start to lose
water when the charge voltage approaches 15 V (12 V battery).
Including a safety margin, the midpoint deviation should therefore remain
below 2% during charging.
When, for example, charging a 24 V battery bank at 28,8 V absorption
voltage, a midpoint deviation of 2% would result in:
Vt = V*d/100* + Vb = V*d/100 + V – Vt
Therefore:
Vt = (V*(1+d/100) / 2 = 28,8*1,02 / 2 ≈ 14,7 V
And:
Vb = (V*(1-d/100) / 2 = 28,8*0,98 / 2 ≈ 14,1 V
Obviously, a midpoint deviation of more than 2% will result in overcharging
the top battery and undercharging the bottom battery.
Two good reasons to set the midpoint alarm level at not more than d = 2%.
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