NI-MH battery monitor design. Any obvious errors?

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NI-MH battery monitor design. Any obvious errors?

Post by chisha » Tue Nov 14, 2017 4:59 am

I'm a newbie in here, 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 find some messages on internet, like that: Low Current Battery Monitor Detail Question and : Low current battery monitoring in regard to battery monitoring, but they don't address the situation I have. So I post here to take some advises. Thanks all guys.

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:
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: ... 7CDRG4.pdf

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

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?
Thanks all.

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Location: Netherlands, Nuenen

Re: NI-MH battery monitor design. Any obvious errors?

Post by Brightnoise » Sun Jan 21, 2018 8:32 pm

Apart from the battery polarity, and the value of the 100k (should be 33k): this circuit will probably work, but the current consumption of this indicator circuit is more than 10 mA. So this is not really what you want, I assume. I have same problem: many battery packs in use. My workaround: monitor/measure them on a weekly or monthly scheme. (And one large battery is maintained on a safe voltage with a charger. Six cells: keep them on 7.2 volt exact limit, with a 100 mA current limit). Or use the circuit and add a push-button switch.

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