I'm designing a simple circuit to use a lithium-metal coin battery as a backup power supply, and I'm considering something simple like this:
However, during normal operation, \$V+\$ is expected to be at a higher voltage than the coin cell, so I expect some reverse current through D2. According to this source, this absolutely can damage the coin cell if it is too high - coin lithium batteries can't be guaranteed to work after subject to a total reverse charge of 3% total capacity, and a 1μA current will do that in a few months.
My question is, how can I find out how much current will go through D2? Most diode datasheets are completely uninterested at the currents going through the diodes when a small reverse bias is applied, mentioning only maximum reverse current when operating very close to the maximum voltage of the diode. Others have graphs like this, that show reverse current growing roughly exponentially, but don't show any details when close to the origin:
So my question is, is there a good model for how diodes behave when subject to a small reverse voltage bias? Can I have a better estimate of reverse current than simply using the maximum value?
(before you mention, assuming I take Shockley's diode law seriously, the reverse current would be close to the saturation current \$I_S\$ for a large range of negative voltages, but if that were the case I'd expect datasheet graphs as the one provided to look a lot different, and for \$I_S\$ to be mentioned in every single diode datasheet. Since this is not the case, I am assuming the diode law doesn't hold very well for reverse bias)