Lipoly Low Battery Indicator
Lipoly batteries are great power source for our AEGs, they offer high current capacities and are available in different shapes and sizes that can fit virtually any AEG. But they have a downside, they are prone to failures when over discharged. Their nominal voltage is 3.7V (4.2V-full charge) but must not go below their critical voltage of 2.7V. New development in AEG electronics protect the battery from going too low by either cutting the power or an audible warning that indicates low battery voltage. While saving up for the "Panther" or "Cheetah" SW-COMP, here is a cheap and easy to construct 11.1V lipoly lo-batt indicator.
"deadbug" circuit construction makes it easy to install (blue resistors=1% tolerance):
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LED directly mounted on the circuit, but later opted to wire them as seen on other pictures.
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Circuit is tapped parallel to the input of an Mosfet switch. Here, its mounted inside the cheek rest of a SCAR:
In case there is no mosfet switch, it can be tapped on the connector of the gun(not the battery).
LED lights up when the voltage is below 9V:
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The project consist of a TL431-adjustable shunt regulator, 3 resistors (must be 1% tolerance for accuracy) and 2 super bright LEDs (to ensure visibility during daytime). All these parts are available at Alexan (P50) or any electronics store.
The LEDs will light up when the voltage goes below 9V, since this is a simple circuit, it will light up even on temporary dip in voltage such as when firing the gun, in my testing, the LEDs are not lighting on a fully charged battery even on full auto. When the LEDs starts lighting up during firing, that should be a warning that your batteries are nearing discharge and when they become steadily lit, you should immediately disconnect the batteries and charge them. Lastly, when the gun is not used, disconnect the battery since the circuit will be on (I think the current drain is very minimal but its still good practice to disconnect the battery when the gun is not used).
The circuit above is for an 11.1V lipoly, if you are using a 7.4V, change R3 to 3.3kohms as indicated on the table. If you wish to get indication above 9V, change R3 accordingly, maybe to a slightly lower value.
