BATTERY PROTECTION CIRCUIT FOR LITHIUM CABON MONOFLUORIDE BATTERY
A protection circuit for a primary battery pack provides the battery pack with reverse charge protection. The protection circuit features a metal oxide semiconductor field effect transistor (MOSFET) with a steady state source to drain voltage. The source to drain voltage is controlled through a feedback loop that includes an operational amplifier. The MOSFET is configured in series with one or more battery cells allowing current to flow from the cells and preventing current from flowing to the cells. The MOSFET provides reverse charge protection with a small forward voltage drop and a small reverse charge leakage current.
1 . A protection circuit for a battery cell, comprising:
a field effect transistor having a drain and a source connected in series with the battery cell; and
a circuit for maintaining a constant voltage between the source and the drain.
2 . The protection circuit of claim 1 wherein the field effect transistor is an N channel MOSFET.
3 . The protection circuit of claim 1 wherein the field effect transistor is a P channel MOSFET.
4 . The protection circuit of claim 1 wherein the constant voltage is between about 10 and about 200 millivolts.
5 . The protection circuit of claim 1 wherein the battery cell is a lithium carbon monofluoride cell.
6 . The protection circuit of claim 1 wherein the circuit includes an operational amplifier for driving the field effect transistor.
7 . The protection circuit of claim 1 wherein the operational amplifier is powered by the battery cell.
8 . A battery pack comprising:
two or more lithium carbon monofluoride battery cells connected in series; and
a reverse charge protection circuit comprising
a field effect transistor electrically connected in series with said two or more lithium carbon monofluoride battery cells, and
a biasing circuit for biasing the field effect transistor to conduct current away from said two or more lithium carbon monofluoride battery cells and preventing current from flowing toward said two or more lithium carbon monofluoride battery cells.
9 . The battery pack of claim 8 wherein the bias circuit includes an operational amplifier for controlling a source to drain voltage of the field effect transistor.
10 . The battery pack of claim 9 wherein the field effect transistor has a gate and the operational amplifier has an output electrically connected with the gate.
11 . The battery pack of claim 10 wherein the operational amplifier has a first input for receiving an input voltage that determines the source to drain voltage of the field effect transistor.
12 . The battery pack of claim 11 wherein the operational amplifier has a second input for receiving a feedback voltage.
13 . The battery pack of claim 12 wherein the source to drain voltage is less than about 200 millivolts.
14 . The battery pack of claim 12 wherein the field effect transistor is an International Rectifier IRF7470 field effect transistor.
15 . A battery pack comprising:
two lithium carbon monofluoride battery cells connected in series;
a field effect transistor having a gate, a drain and a source connected in series with said two lithium carbon monofluoride battery cells; and
an operational amplifier powered by the two lithium carbon monofluoride battery cells having a first input electrically connected with the drain, a second input electrically connected with the source and an output electrically connected with the gate.
16 . The battery pack of claim 15 wherein the field effect transistor is a MOSFET.
17 . The battery pack of claim 15 further comprising a Zener diode in parallel with the power inputs of the operational amplifier to protect the operational amplifier from an over voltage.
18 . The battery pack of claim 15 further comprising a voltage divider for providing an input voltage to the operational amplifier.
19 . The battery pack of claim 15 wherein a quiescent drain to source current is less than 2 microamps.