AVR-Chat

>
>That's another point.
>If backup usage current is below 500mA, then use plain and cheap Alkaline
>ones.
>They are safe, shelf life is much longer than anything else.
>Of course, lithium current limit is a good idea, perhaps a small fuse? :)


Fuses that small are WAY expensive, and I don't think you can go that low. 
I once talked to a Panasonic apps guy about a design we had, that put 600nA into a lithium cell. His comment was that it probably wouldn't explode. 

If the steering diode on the battery output fails, then you need to still have essentially zero current into the battery, by some means.