Diode Question - Backup Battery

[glennpm]

Hi,

I'm building a kick start only'79 and am going to run an Acewell Speedometer-Tachometer. The unit has a clock and trip meter which I'd like to take advantage of, trouble is I'm going to use a capacitor and no battery.

I'm wondering if I could put in a small 12V battery like an A27 used for garage door openers. The ground of the battery and my ignition would be the same but I don't want to charge the battery, blow it up, when the PMA is running. Would a diode that allows current from the + battery work but prevent back flow from the PMA? If so what size would I need?

Thanks

 

[glennpm]

From what I've found, a 1N4001 diode would work. It will allow 1A from the A27 12v battery and protect it from the PMA voltage. It will protect against 50V from the bike side charging.

http://www.bcae1.com/diodes.htm

 

[TwoManyXS1Bs]

Hi, Glenn. A 1N4xxx diode usually has a high forward voltage drop rating, and that 12v battery may end up only supplying less than 11v, which may be an issue for the backup circuitry. If that is indeed a properly designed back-up battery feed line, it may not supply the 14+v you're concerned about. You could power-up the instrument, minus any backup battery, and measure to see if any voltage is present on that 'backup' line. If no voltage shows, then you could wire-in your backup battery without the diode. Otherwise, since backup currents are quite small, you could use a 1n9xx type signal diode, which has a lower Vf forward voltage drop, about 0.6v, yielding at least 11.4v to the backup system...

 

[glennpm]

Thanks Steve!

I'll check out the voltage in the backup line first and hope there is none. Appreciate the tip on the less voltage drop signal diode, 1n9xx type.

 

[glennpm]

Thanks for the great advice! There is no voltage on the "back-up" connection, actually a short pulse of either .1 or .2 volts but steady state of 0. I'll be wiring an A27 or A23 battery to this connection.

 

[TwoManyXS1Bs]

AllRight, good to know!

 

[RFLIVES]

Had the same issue and I ended up diching the cap idea and tucking a battery in where the starter was.
http://www.xs650.com/forum/showpost.php?p=347217&postcount=19

 

[glennpm]

Nice bracket and idea! I'm saving a copy of this just in case my little battery for the Acewell and cap don't work out.

Thanks