[page 26] | gpelectric.com
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.
6.2.1 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)
6.2.2 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 15V (12V battery).
Including a safety margin, the midpoint deviation should therefore remain below 2% during charging.
When, for example, charging a 24V battery bank at 28,8V absorption voltage, a midpoint deviation of 2% would
result in:
Vt = V*d/l 00* + Vb = V*d/l 00 + V - Vt
Therefore:
Vt = (V*(1 +d/l 00) / 2 = 28,8*1 ,02/2'" 14,7V
And:
Vb = (V*(I-d/l 00) / 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%.
This same percentage can be applied to a 12V battery bank with a 6V midpoint. In case of a 48V battery bank
consisting of 12V series connected batteries, the % inuence of one battery on the midpoint is reduced by half. The
midpoint alarm level can therefore be set at a lower level.
MORE ABOUT PEUKERT’S
FORMULA & MIDPOINT MONITORING
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