I wish I could do  that, but The battery voltages are fixed at what they 
are, and the maximum voltages of the other parts is fixed too. Looks 
like the next higher voltage supply is 15 volts, that's a little higher 
than I would like to operate, plus the batteries float at 13.8 volts. 
That relay is quite beefy, with 10 amp contacts, and if the move maybe a 
dozen times a year, that would be a lot, plus it could be put on 
maintenance schedule to replace every few years. The whole system is 
based on the Kohler engine electrical which is 12 volts (nominal), then 
all the sensors (pressure, voltages, temperatures) are selected and 
calibrated based on the 12 volts.

What about adding a diode across the contacts with the capacitor ?

Dwayne Reid wrote:
> I think that I'm still gonna beat the same drum.
>
> Eliminate the relay that switches the PLC power line.  Use a diode 
> array of 3 diodes so that whichever voltage is highest feeds the PLC 
> power input.
>
> Ensure that the AC power supply is higher than the float voltage 
> present on the batteries when fully charged.  No relay needed.
>
> Then try it and see how you make out.
>
> dwayne
>
> PS: the only problem with adding that large capacitor after the relay 
> contacts is inrush current eventually damaging the relay 
> contacts.  But I still don't see the need for the relay and think 
> that eliminating it will get rid of your problem.
>
> dwayne
>
> At 01:06 PM 4/10/2007, Carl Denk wrote:
>
>   
>> Powered from the non-generator engine battery, all OK. If it's powered
>>     
> >from the engine's battery, upon starter energizing, voltage is pulled
>   
>> down to 9 volts, and PLC resets, had that issue some time ago, and added
>> a second battery (from a homebuilt airplane we had dismantled) with the
>> diodes to select the higher voltage one as described earlier. No
>> problems when powered off either battery, except when starting. Issue
>> only when switching from the switching power supply to battery, when the
>> relay coil is deenrgized and spring pulls contacts open. No problem when
>> coil is energized and contacts are switching.
>>     
>
>
>   
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