Parasitic charge current into lithium coin cell

y

Why not just use a better diode? You'd get 5 years' battery life, and not have to worry about the effect of backcharging the Li cell.

If you need low forward drop, use a FET.

HTH, James Arthur

Suggestion: see "Troubleshooting Analog Circuits" by Bob Pease, page

2. Pease suggests using a transistor's collector-base junction, instead of a discrete diode, if you want low leakage... avoid transistors using a gold-doped architecture as these are leakier. "You can easily find such 'diodes' that have less than 1 pA leakage even at 7V."

Such CE diodes are quite slow (compared to fast switching diodes) but that would be irrelevant in your application.

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r

Yep, that's a good diode, if he can stand its forward drop. The description sounds like he's using schottky diodes to avoid the voltage loss.

James Arthur

In general ANY reverse current into a coin cell is bad 'cos they lack the vents other cell types have. (Example: if you look at say an AA cell, you'll see some gaps under the pip at one end - or on some cells, some grooves carved into one end, these are points of deliberate weakness so the cell ruptures when internal pressure is just say 2-3 atmospheres so any bang is small and controlled).

In the UK the law ued to be that you must have two current limiting components (eg a diode and a resistor) between any Li coin cell and a voltage source to stop them exploding in a spectacular manner. Wish I could find a link to this because the Health & Safety Executive used to have a very explicit pdf on this with graphic descriptions of real life examples of unintended charging which they had investigated. Does violent explosion meet your criterion of damaging the cell? Even 1uA into a CR2032 would really worry me. I *did* find one battery manufacturer's app note saying "don't charge more than 3% of the battery's capacity in its entire life":

I'd suggest as a start, use low leakage diodes (I'd start at

they have low leakage ranges with leakages of e.g. one nanoamp at 100V). These are normal diodes though, I'm not sure what their low Vf Schottkies are like.

There *are* some rechargeable Li coin cell chemistries but, when I looked at these a few years ago, they had very limited cycle life and a memory effect.

How much design freedom do you have? We could go on about "you should use such and such a chemistry cell with a capacity of at least X" but if the circuit's already laid out, and the moulding won't permit a larger cell, there's no point discussing the optimal blue sky solution. Maybe you should just accept that the cell needs occasional changing (is it in a holder, or a tabbed soldered one?)

Nemo

snipped-for-privacy@xxxxx.com enquired

l is bad 'cos they lack

mi.com, they have low leakage ranges with leakages of e.g.

(Nemo, for your info and flame-proofing: SED's preference is to quote the post you're responding to, then respond, i.e. post your reply at the bottom. That keeps things in chronological order. Top-posting is disdained.)

I was going to suggest this:

Q1 PNP

+3.3v >--o----. .----o-----o----> Vbackup | e \\ / c | | | --- [R4] | | |b | | | o------' | [R1] | | + | [R3] --- B1 | | - | o----. | | | | | | |/ | | o----| Q2 '---.|--' | |>. ||

Sure, that's the plan. I have tested OnSemi's MMDL301 as a replacement and the measured leakage current is

Neat - I like it! I'd still be concerned about leakage through Q3 body diode and Q1 E-C. I know low leakage small-signal FETs and bipolars are available - any recommendations? Most manufacturers only quote worst-case specs at

125C, all lower temperatures just have 'typical' figures. Our product never gets above 50C internally.

As I mentioned in another post, I've found a very-low leakage Schottky which looks a suitable replacement, but the circuit above looks like a good alternative to another battery/mains diode-OR we have in the product, which has to pass several amps and wastes a fair amount of power. This battery pack is 2.2 Ah and is rechargeable, so leakage current specs aren't so tight. I'll have a play with it in SPICE...

Thanks

R.

Yes the Panasonic app note I found says more or less the same thing. It doesn't present it as a safety risk though, more that the cell won't meet its capacity specs.

OK, thanks, I haven't looked at this company before. They seem to do a few different parts in the package I need.

The board layout is all done, the cell is in a holder but it's a real PITA for our service guys to get at, so we want as long a life as we can get.

Thanks

R.

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You wouldn't need special transistors--standard junk units would easily suffice.

When the battery is ON Q1 is held fully OFF by R4, so leakage will be nanoamps. Likewise, Q3's body diode's leakage will be tiny. A Fairchild BSS138, for example, is guaranteed to leak less than 100nA at Vds=3D30v.

But, if you can get good enough schottkies then that's obviously easier and smaller.

And yes, I think your CR2032s failed from backcharging. They don't like that.

Cheers, James Arthur

=A0

Around room temperature, I've used regular old 2N4403s down to 100nA or so, and BS170/2N7000 is good for

I don't quite follow your calculation.

225mAhr x 3% =3D~6.8mAhr max. permissible charging.

At 10uA leakage, 6.8mAhr would be exceeded in 6.8mAhr / 10uA =3D 680 hours, or 28 days.

Likewise, 1uA leakage would exceed spec in 280 days, or about 9 months.

Another thing to watch out for is dirt. Dirty boards can easily leak microamps and kill backup cells quickly.

Cheers, James Arthur