Reverse current into a lithium battery

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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
Email address domain is currently out of order.  See above to fix.

Re: Reverse current into a lithium battery


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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
www.abvolt.com




Re: Reverse current into a lithium battery
Rob Gaddi wrote:

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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.


Re: Reverse current into a lithium battery
On Mon, 19 Nov 2012 14:08:04 -0800, Rob Gaddi

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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.  


Re: Reverse current into a lithium battery
On Mon, 19 Nov 2012 17:34:16 -0500, Spehro Pefhany

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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).  


Re: Reverse current into a lithium battery
wrote:
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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.

Re: Reverse current into a lithium battery
On 11/19/2012 5:08 PM, Rob Gaddi wrote:
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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

Re: Reverse current into a lithium battery
On 11/19/2012 2:08 PM, Rob Gaddi wrote:
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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.
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Re: Reverse current into a lithium battery
On Mon, 19 Nov 2012 14:08:04 -0800

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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.

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