LTC3305_15 Datasheet, PDF (19/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Applications Information
Selecting the Auxiliary Cell
The auxiliary cell must be capable of sourcing and sink-
ing current and withstand the maximum voltage of any
individual battery in the stack. The ESR of the auxiliary
cell must be small compared to the PTC thermistor. Any
voltage dropped across the auxiliary cell ESR appears as an
offset voltage at the input of the termination comparator.
The auxiliary cell used may be a lead-acid battery, a stacked
supercapacitor, or a low leakage, high voltage capacitor.
When using a supercapacitor stack, the voltage across
each individual supercapacitor must not exceed its rated
operating voltage.
Figure 6a shows a battery stack made of 4 batteries,
each with a nominal capacity of 50Ah, but with a 10%
capacity mismatch. With no balancing, the stack capacity
is determined by the weakest battery in the stack and is
limited to 45Ah.
In Figure 6b, a small capacity auxiliary cell, such as a
supercapacitor stack, is used to balance the battery stack.
When balanced the stack capacity can be made to approach
the nominal capacity of 50Ah despite the 10% mismatch.
In Figure 6c, the auxiliary cell has the same capacity as
the batteries in the stack. Each of the batteries in Figure 6c
has a nominal capacity of only 40Ah but the stack capac-
ity approaches 50Ah since the auxiliary cell supplements
the capacity of the battery stack. Using a large capacity
auxiliary cell supplements stack capacity. Smaller capac-
ity batteries may be used in the stack which helps reduce
system costs.
Precharging the Auxiliary Cell
When using stacked supercapacitors or a single high volt-
age capacitor as the auxiliary cell, the auxiliary cell may
be initially discharged with a voltage of 0V. At startup, a
large voltage exists across the PTC resistor, which will
cause the PTC resistance to increase. This limits the cur-
rent and hence the charge transfer between the auxiliary
cell and the battery it is connected to. The auxiliary cell
will be charged very slowly with an indeterminate time,
as it sequentially connects to each battery in the stack.
Once the auxiliary cell has been charged to a point where
the PTC device operates as a low resistance device, the
balancing process is sped up.
A more time efficient solution is to precharge the auxiliary
cell to the average voltage of the batteries in the stack. Figure
7a shows a circuit using a high voltage buck regulator to
precharge the auxiliary cell to V4/4 volts. NMOS devices
N2A and N2B eliminate a parasitic charging path from
BATTERY1 to the auxiliary cell when AUXN is connected
to GND through N10. Figures 7b and 7c are scope photos
showing a complete precharging and balancing operation.
55Ah
(+10%)
50Ah
50Ah
45Ah
(–10%)
STACK CAPACITY
=45Ah
(a)
55Ah
(+10%)
44Ah
(+10%)
50Ah
50Ah
45Ah
(–10%)
PTC
4Ah
(AUX)
40Ah
40Ah
36Ah
(–10%)
PTC
40Ah
(AUX)
STACK CAPACITY
≈50Ah
(b)
STACK CAPACITY
≈50Ah
(c)
Figure 6. Increasing Stack Capacity with an Auxiliary Cell
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