PSU schematic

[Nick]

When replacing a lithium like this (in Tek 'scopes for instance) I came up with the following method:

• Make sure the unit is OFF - leave it for a while to let any DC rail levels settle to zero.
  
• Measure the voltage on the battery precisely.
  
• Charge a 4,700uF electrolyic to the same value (to be honest, 470uF would probably do) - SRAM only consumes a few micro-Watts in standby
  
• Tack the electrolytic across the lithium battery (not on its leads) - no charge transfer as at same potential - obviously make sure it's + to +, - to -...
  
• Remove the lithium battery making sure you don't disconnect the parallel capacitor in the process!
  
• Replace with new battery - this will obviously charge the electrolytic to the new (hopefully higher) voltage - this is why you don't want too big a uF value...
  
• Remove the electrolytic...
  
• Done!
  

Important thing here is that you are at no time trying to force charge into the lithium battery, which would be a BAD idea. You need a capacitor value which is big enough to keep the SRAM happy while you change the battery, but small enough that you don't waste too much charge when the new battery is placed in circuit and raises the potential on the capacitor to the new (higher) voltage - that charge is essentially wasted as you then remove the capacitor, so you don't want a value that is unnecessarily large.

HTH