Determine Balancing Current for the LTC3305 Lead-Acid
Battery Balancer
Design Note 539
Jim Drew
Introduction
The LTC В®3305 is a lead-acid battery balancer that uses
an auxiliary battery or an alternative storage cell (AUX)
to transfer charge to and from individual batteries within
a series-connected stack. The balancer controls external
NMOS switches to sequentially connect the auxiliary
battery to each battery in the stack. To prevent damage
to the NMOS switches and their interconnecting PCB
traces, a current limiting device is required. One such
device is a ceramic positive temperature coefficient
(PTC) thermistor.
The PTC thermistor limits the peak current in the
connection between the AUX cell and the battery. For
relatively small differential voltages (VDIFF ) between
the AUX cell and the battery that is connected, the
current through the PTC remains low, as does its
temperature, and the PTC exhibits the characteristics
of a constant value resistor. As VDIFF increases, current
increases, and the temperature of the PTC rises. When …