Low Battery Cut-off with Indicator Circuit for Cell Phone/Tablet Battery Pack Chargers

The post explains a simple yet effective low battery cut off with indicator circuit which can be integrated with cell phone/tablet charger battery packs in order to monitor and avoid battery draining below a set threshold. The idea was requested by Mr. David.

The Request

Hi Swagatam. I know it was asked before, but still, if you can help me out, I'd really appreciate it. I'd like a circuit/ic for purely low voltage cut off. I have a 8-pack of AA (NiMH LSD) batteries, which should never discharge below 7.2V. I'm using these with a (car) 12V to 5V USB charger, which I'd like to use on the go. The setup of simply using a transistor and a variable resistor in conjunction with a TIP122 transistor for handling larger currents, resulted in draining the battery further than the absolute minimum of 7.2V. I'd like this to use this with no relays (as they use too much power). I'm looking at 0.5-1.5A on the primary side (batteries) and 1-2.5A on the secondary 5V side. For charging both the phone and the tablet. I just hate it, when a circuit like this is used in a 1$ devices. I don't want to buy of the shelf products. Thank you really much! -David

The Design

The circuit functioning of the proposed cell phone/tablet low battery indicator with cut off can be understood as follows:

Assuming the connected battery pack to be fully charged, the potential at pin#2 is allowed to be at a higher level than at pin#3 by setting the 10k preset appropriately.

The above condition ensures a zero or logic low at the output pin#6 of the IC.

The above low output enables the connected transistor TIP127 to conduct and charge the cell phone or the tablet at its collector. 

As the battery pack drains below the mentioned 7.2V mark, pin#2 voltage becomes lower than pin#3 which instantly makes the output of the IC high, switching off the transistor and the load.

The situation is indicated by the red LED which just lights up due to the low battery conditions.

At the above threshold, the output might oscillate for some time due to battery voltage trying to restore at the previous mark as soon as the load cuts off.

Although not necessary, the above outcome can be avoided by adding a resistor network: one across pin#3 and zener cathode, and another across pin#6 and pin#3, the values can be anywhere between 10K and 100K.

How to Set up the Circuit:

It's simple, apply the desired low voltage threshold to the circuit and adjust the preset until the LED just illuminates brightly.

As mentioned earlier, the possible oscillation at the thresholds can be prevented by adding some hysteresis to the above design, it may be done by the following two methods, the second option appears to be more logical and clean.

While setting the preset, make sure the feedback link stays (pin6 to pin3) disconnected, you can connect  it back once the preset adjustment is complete.

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