This is a circuit primarily designed to prevent Lithium Polymer (LIPO) batteries from being over discharged.
I have a few LIPO batteries, they are used in a lot of RC models these days. A 3 cell LIPO battery puts out a voltage of 12.6V fully charged and are quite handy for powering battery powered gear requiring 12V.
LIPO batteries, however, really do not like being discharged below 3V per cell, and for long life you really ought not to discharge below about 3.3V per cell. A LIPO battery left connected to a piece of gear left switched on can discharge the LIPO to such an extent that not only is the battery ruined, it can be dangerous to attempt re-charge.
Of course you could perform the same function as this PICAXE circuit with a pure analogue design, but using a PICAXE does make it rather easy to configure for the battery in use, and the particular cut off voltage you want.
This circuit uses a 08M2 to monitor the battery voltage and switch off the power via a MOSFET if the battery falls below a set limit. There are two switches, one for on and one to manually switch off. There is a LED which flashes briefly when the battery is checked, about once every 20 seconds or so. The LED flashes several times when the battery is measured below the limit. The battery has to be low for a period (about 5 x 20 seconds) before its shuts down. You can fit a piezo sounder which will give you sound effects too.
The poly fuse acts as both a current limiter, and via D1 will also protect against reverse battery connection. It can be omitted and D2 fitted to protect against battery reversal. Take care if using LIPO batteries without a fuse, a typical LIPO designed for RC model use can easily put out 100A or more when shorted, cause a fire, or burn you if you are holding the wires at the time.
There is no over voltage protection, the LP2950 has a maximum input voltage of 30V, so dont exceed that. Also make sure that the capacitor C2 has a voltage rating that exceeds the voltage of the incoming battery supply.
The resistor divider puts approx 1/10th the supply voltage at the PICAXE AD input, and with the 2.048V reference in use the maximum input voltage that can be read is 20V.
At a current consumption of 500mA, I measured 1.2mV across the MOSFET, thats 600uW of dissipation, so at 5A, the MOSFET would still only be dissipating 6mW, which would raise the junction temperature a mere 0.21C above ambient.
There are 3 pads for the PICAXE programming connection on the edge of the PCB, either make a adapter of a 3.5mm stereo line socket and 3 test clips and hook into the pads holes, or fit a DIL socket for the 08M2 and program it elsewhere.
I have a few LIPO batteries, they are used in a lot of RC models these days. A 3 cell LIPO battery puts out a voltage of 12.6V fully charged and are quite handy for powering battery powered gear requiring 12V.
LIPO batteries, however, really do not like being discharged below 3V per cell, and for long life you really ought not to discharge below about 3.3V per cell. A LIPO battery left connected to a piece of gear left switched on can discharge the LIPO to such an extent that not only is the battery ruined, it can be dangerous to attempt re-charge.
Of course you could perform the same function as this PICAXE circuit with a pure analogue design, but using a PICAXE does make it rather easy to configure for the battery in use, and the particular cut off voltage you want.
This circuit uses a 08M2 to monitor the battery voltage and switch off the power via a MOSFET if the battery falls below a set limit. There are two switches, one for on and one to manually switch off. There is a LED which flashes briefly when the battery is checked, about once every 20 seconds or so. The LED flashes several times when the battery is measured below the limit. The battery has to be low for a period (about 5 x 20 seconds) before its shuts down. You can fit a piezo sounder which will give you sound effects too.
The poly fuse acts as both a current limiter, and via D1 will also protect against reverse battery connection. It can be omitted and D2 fitted to protect against battery reversal. Take care if using LIPO batteries without a fuse, a typical LIPO designed for RC model use can easily put out 100A or more when shorted, cause a fire, or burn you if you are holding the wires at the time.
There is no over voltage protection, the LP2950 has a maximum input voltage of 30V, so dont exceed that. Also make sure that the capacitor C2 has a voltage rating that exceeds the voltage of the incoming battery supply.
The resistor divider puts approx 1/10th the supply voltage at the PICAXE AD input, and with the 2.048V reference in use the maximum input voltage that can be read is 20V.
At a current consumption of 500mA, I measured 1.2mV across the MOSFET, thats 600uW of dissipation, so at 5A, the MOSFET would still only be dissipating 6mW, which would raise the junction temperature a mere 0.21C above ambient.
There are 3 pads for the PICAXE programming connection on the edge of the PCB, either make a adapter of a 3.5mm stereo line socket and 3 test clips and hook into the pads holes, or fit a DIL socket for the 08M2 and program it elsewhere.
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