[dcbrown]

There should be enough EE types hanging around here to agree on an alternate backup charger design. Once there is a consensus we can at least offer the design to Ifi - if not for this year, then maybe next.

Attached is a straw design, let the darts fly. I purposely removed the values of most of the resistors to start.

Theory of Operation - abbreviated.

1. output voltage should be set to 8.5V, there will be a .2v(?) drop across the forward biased steering diode D1 presenting 8.3v to battery to be charged.
2. D1 prevents discharge when 12v is disconnected.
3. R1 provides a non-charging current to the backup battery input (100ma)
4. R2 provides charging current to backup battery (45ma)
5. D2 prevents the non-charging current available from R1 from mixing with or contributing to the charging of the backup battery
6. D3 is zener crow-bar to prevent voltage runaway due to the charger failing in some manner
7. D4 shunts any current spikes around LM chip to prevent failure.
8. Change C2 to 1uf
9. R2 and R3 selected per Vout = 1.25v * (1+R3/R2)