Reverse current into a lithium battery

Hey, maybe one of y'all might know this.

I'm working on a super low power personal project, MSP430 based, that will hook up to a garage door opener's pushbutton circuit. My guesstimated average load current on it is under a microamp, and I'm thinking of powering it from a lithium coin cell.

I'm also looking at needing to sense whether the opener is hooked up to my board correctly. If so, it's +HV; if not it'll be -HV, where HV is in the 12-24V range. I figure megaohm resistor into a GPIO pin, let the ESD diodes clamp it, and there you go.

This works fine, except that it means that when things are hooked up correctly I'll be dumping some few microamps back into the battery; as I won't be burning it up as a load.

Yes, I've thought of pitching the battery entirely and just parasiting the power, and I'm still thinking about it, but there are issues there too. Likewise, I could add a zener diode clamp, but at these sorts of trivial currents repeatability could be an issue.

So what I'm wondering is, does know anything about what would happen over the long term (5-10 yr) with a lithium battery receiving a trickle of reverse current?

--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com 
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Reply to
Rob Gaddi
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FWIW: in old times, we used to charge lithium watch batteries. Batteries worked like rechargeables just fine; the trick was keeping charge current no higher then 1mA or so.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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Reply to
Vladimir Vassilevsky

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When I did that, I chickened out and used the raw input to drive a MOSFET, so I don't know the answer.

Mel.

Reply to
Mel Wilson

Hi, Rob:-

They get a bit of reverse current when isolation diodes leak a bit.

For a personal project, I personally wouldn't worry about a couple uA, since if you work out the internal shunt leakage, it's probably in that range, even for something small like a CR2032.

If you want to be really safe, Energizer has a datasheet limit of 1 or

2uA (depending on battery size) for what they call "Max Rev Charge", which is probably what you are talking about.

If you want to be really safe and don't like the idea of 10M-20M resistors, then you could always use a single transistor to avoid the problem entirely.

Reply to
Spehro Pefhany

Consider a 2N4403 base to battery + emitter to two 1M resistors R1 and R2 collector to ground

R1 other end to HV input R2 other end to port pin

This 3-component < 3 cent circuit will allow less than 500nA back into the battery and uses no resistors higher than 1M.

If the pin is not connected though, it floats around so it might drain your battery faster than you'd like, so maybe another 1M from the

2N4403 emitter to ground (For 2 components total, use a 4-resistor network + transistor, if you like to live on the edge).
Reply to
Spehro Pefhany

I looked at some coin cells once, both rechargable and primary cells and the primary cells were very sensitive to reverse current over the long term. This was to the point that you would need to pick your reverse current blocking diode carefully. I suggest you check the data sheet of the particular cells you are considering.

Rick

Reply to
rickman

Do you REALLY need this? How often are you gonna reconnect it? What's the consequence of connecting it backwards? Stick a diode in series with the output and be done with it. Or stick it in the center of a diode bridge so it won't matter which way you connect it.

Reply to
mike

There are tons of transistors in the MSP430 itself. The easiest way is to drive a LED with another pin, when you detected the incoming reverse current. Just drive enough power somewhere else to balance the power load.

Reply to
linnix

Thanks everyone for the advice. On balance, it sounds like reverse current into a lithium battery is bad, and winds up not being necessary to worry about. After some quick measurements this weekend with a DVM and a bag of resistors, my garage door circuit is looks like (to a rough approximation) 18V with a series 1.5K, and reads a contact closure somewhere around 0.7V. That means there's actually a huge amount of power available for the taking; I could draw an average load of a full milliamp without even the slightest risk of an accidental trip; and I don't expect to wind up using even a tenth of that.

--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com 
Email address domain is currently out of order.  See above to fix.
Reply to
Rob Gaddi

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