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Let me preface this by saying I'm not that much of an EE expert, this is a "hobby" project, I consider myself as knowing enough to be dangerous. ;) I'd never apply for a full time career as an EE designer, that's for sure.

That said, I saw this: Low Current Battery Monitor Detail Question and this: Low current battery monitoring in regard to battery monitoring, but they don't address the situation I have.

In my case, I have a 6 cell NI-MH battery that is either not attached to the circuit at all, or it's attached and under load. I'm using a battery pack designed for an RC car, and to turn it off, I just disconnect it. However, I want a monitor that will indicate when the voltage drops below about 6V, since that's the point at which I want to consider the battery pack as discharged.

My current design is the following:

enter image description here

Please accept my apologies for the somewhat low quality circuit diagram, there's limits to what I can manage with a generic paint program.

The TI TL7757 is intended to assert /Reset if it's VCC drops below 4.55V, data sheet is here:

http://www.ti.com/lit/ds/symlink/tl7757.pdf

The Bivar 3BC-C-CA-F is a common anode two color LED, data sheet is here:

http://www.farnell.com/datasheets/1756451.pdf

My logic is that the 470 ohm pot will allow me to adjust the voltage on VCC of the TL7757 so that when the battery hits 6V, the TL7757 sees 4.55V.

Above that voltage, /Reset is high, it's an Open Collector output so the Red LED is off, however the leakage through it, the 270 Ohm resistor and the 100K resistor should be enough to turn on the 2N-3904, which in turn illuminates the green LED.

When /Reset drops, current flows though the red LED and the 270 Ohm resistor and the TL7757, illuminating the red LED, the voltage on the base of the 2N-3904 drops, turning it off and with it the green LED.

Have I made any mistakes that might stop this from working as intended?

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    \$\begingroup\$ You know what's really awesome? This site has a schematic editor built into each and every question and answer box. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 12 '14 at 2:14
  • \$\begingroup\$ Your battery's the wrong way though. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 12 '14 at 2:15
  • \$\begingroup\$ @IgnacioVazquez-Abrams You're right - my mistake. The intent is that +ve is at the top \$\endgroup\$ – dgnuff Oct 12 '14 at 2:30
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It looks pretty good, but there're a couple issues I can see.

One downside is you're just about doubling the current by using a 470 ohm pot for the detection part. I don't know how long you expect to have this circuit attached after the battery reaches the threshold for the red to turn on, but this circuit could excessively drain your batteries if you don't unplug it or charge it soon after seeing the red-light.

You should also make sure you're very careful when setting up/tuning your circuit. 1.2V * 6 = 7.2 which is higher than what this chip is rated for. I know you can get slightly higher than that with NiMH's so start at the low side when you're tuning the pot and work your way up. Another option is to add a resistor between the pot and the IC along with a 7V zener diode in parallel with the IC to protect it. Alternately, a higher Vdd IC could be sought.

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  • \$\begingroup\$ Thanks for the feedback. I should have explained the application: I'll be using two of these two monitor a pair of 7.2V batteries in series that are feeding a DC charger for a laptop. So it'll never be used unattended, and the moment either battery shows a red LED, they both get disconnected. Given this use case, I'm not worried about the vampiric load discharging the batteries excessively. \$\endgroup\$ – dgnuff Oct 12 '14 at 2:56
  • \$\begingroup\$ continued ... Given the discharge curve of a NI-MH battery, I'll probably tune this so it goes red somewhere around 6.6V to 6.8V, that's the point at which a 6 cell NI-MH battery is getting pretty low capacity. This has the huge advantage that the IC sees a smaller fraction of the battery voltage, and can therefore tolerate a higher maximum input to the pot. That said, for tuning, I'll discharge a battery to about 6.6V, attach that, and then tune till it just goes red. With the voltage already below 7.0V I should be safe. Discharge curve of a NI-MH cell: imgur.com/vTVZA0w \$\endgroup\$ – dgnuff Oct 12 '14 at 3:02
  • \$\begingroup\$ @dgnuff Sounds good, have fun! Post back if you have any issues once you get into the thick of it. \$\endgroup\$ – horta Oct 12 '14 at 6:19
  • \$\begingroup\$ Looking good so far. I finally got the components from Mouser, and used a "plug in breadboard" to build a prototype. After watching the discharge curve of the batteries, I'm planning to set the trigger voltage to 6.9V. They hold very steady down to about 7.2V, and then the voltage drops quite sharply. One nice thing is that due to the increase in current drawn by the TL7757 when it asserts /Reset, this drops VCC on that device about 0.2V, which gives the circuit some hysteresis. This means that it's highly unlikely to jitter, it'll just turn red and stay red. \$\endgroup\$ – dgnuff Oct 22 '14 at 16:01
  • \$\begingroup\$ @dgnuff Ah very nice. I figured your circuit already had some built-in protection from hysteresis because you have an LED on at all times, so I didn't think there would be a significant difference in current whether it was the red or green LED. It sounds like it's even better than that. Good to hear. \$\endgroup\$ – horta Oct 22 '14 at 19:09

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