INRUSH CURRENT LIMITERS
Thermistor Protection for Precharge Circuit on Lithium Ion Batteries
When a battery is connected to a load with capacitive input, there is an Inrush current
surge as the capacitance is being charged to the battery voltage. The Input current
depends on the input capacitance; the larger the batteries and the more powerful the
load, the larger the input capacitance. A large Inrush current (in the precharge circuit,
without protection) can cause the following:
• Damage to input filter capacitors
• Blowing of the main fuse if asked to carry the inrush current without protection
• Contact failure (as well as reduction in current carrying capacity)
due to arcing and pitting that results from high inrush current
• Damage to the battery cell, which is not rated for inrush current
A typical precharge circuitry for battery operation is below with the timing diagram,
showing how the circuit operates. (Courtesy of Lithium -ION BMS)
R1
10
K1
PREC HA RGE
AS Series
For this application note, let us limit our discussion
to the selection of the Thermistor
SE LE CTION OF
THE THE RM ISTO R
The minimum resistance of the thermistor is
determined by the following:
1. Ambient temperature
2. Input capacitance value
(of the precharge circuit)
3. Battery voltage
The precharge surge current reaches 63.2% (1/e)
of its initial value after a time
τ
= RC.
In the selection of the thermistor, we consider a
time value of “five time-constant” when the
capacitances are fully charged and the surge
current reaches the normal operating current.
For the purpose of our design, let us assume the
following quantitative values:
Precharge time: 20 millisecond
+
BATTE RY
CONTROL L ER
K1+
K1+
K1–
K1+
K1+
K1–
K2
B=
H V O UT
+
–
Ambient operating temperature: Varies between
10°C to 50°C.
Battery voltage: 100 volt
Capacitor bank : 50,000 µF
–
K3
B-
5τ = RC
R = 5τ / C = 5 (0.02 sec) / 0.05F = 2.0
.
V
Load
Voltage
Now, look at the at R-T curves for Ametherm
thermistor at ambient of 50°C. The material
“C”exhibits
t
A
Battery
Current
R @ 50°C/ R @ 25°C = 0.412 @ R @ 10°C / R
@ 25°C = 1.70
Therefore, minimum resistance @ 25°C = 2.0 /
0.454 = 4.40 , so our standard part has 5.0
ohm nominal resistance
At 10°C, the standard part will have a resistance
of 5.0 x 1.70 = 8.50 , which will meet our
minimum resistance.
Determine the energy the thermistor needs to
handle with out self-destruction,
E = ½ C V2 = ½ (0.05) (100) 2 = 250 Joules.
The steady state current is not calculated because
in most precharge circuits the steady state current
goes through the contactor.
The part, which would meet your specification, is
AS32 5R020.
Precharge Surge
Normal Operating Current
t
K1
K1
K1
Off
Pre
charge
On
Off
t
In its most basic form, the Precharge circuit operates as follows:
•
OFF:
When the system is OFF all relays / contactors are off.
•
Precharge:
When the system is first turned on, K1 and K3 are turned on to
Precharge the load, until the Inrush current has subsided. R1 shows the location
of Thermistor in the Precharge circuit.
•
ON:
After Precharge, contactorK2 is turned on (relay K1, must be off to save
coil power)
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