New battery and engine, (well was put into service a year ago). Kept 
charged with 6 amp plus float type charger.  Unit has been running 20 
hours in last 6 months that there has been an hour meter on unit. Cables 
are 00 copper with crimped terminals, all new at same time as battery. 
Only time it didn't start was with temperature at -10F, removed started, 
dissasembled, couldn't find any problems, reinstalled, and started. I 
assumed some condensation in the solenoid had frozen the piston in it's 
bore, or maybe the bendix drive jammed. Have been considering replacing 
the starter solenoid. Taking to the local engine mechanics, it's not a 
problem with this engine usually, and they don't recommend replacement, 
but considering the importance of unit starting, I wouldn't mind the 
cost. Have to agree 9 volt is marginal, but can't explain otherwise.

The PLC that manages the generator resets below 10 volts, as backup to 
the 120 VAC power supply from the house power (it would be out if 
generator is needed), there is the generator battery and a smaller 
sealed lead/acid battery, both parallel with diodes. If the 120 VAC 
drops out, a relay switches the PLC to the 2 batteries, with the higher 
voltage winning who's going to provide the power until the generator is 
on line. The PLC holds it's power to the batteries for a minute anfter 
the 120 VAC is stable to prevent chattering.

Just to keep this [PIC} tag, There is a 18F1320 at a natural gas well to 
monitor pressures and enclosure temperature that communicates with the 
PLC via fiber optic > RS-485. The Generator can run on natural gas, 
gasoline, or propane and switch under load between them automatically as 
managed by the PLC.

Dr Skip wrote:
> What is the initial condition of the battery though? For cars, under 11v
> with about 10 sec of cranking and the battery is considered suspect or a
> candidate for replacement/charge. By 'large' I would assume 80-100 Ah
> min. Smaller, less frequently charged or used batteries, with age, may
> perform as yours perhaps. Generators don't get used that often, so
> performance suffers. I would suspect that the lightness of the load is
> what allows a battery under load at 9v to crank the gen. More cylinders
> and 9v wouldn't cut it. ;)
>
> -Skip
>
> Carl Denk wrote:
>   
>> I have a 27 HP. 2 cylinder Kohler engine on a generator with a heavy 12 
>> volt marine battery. On starting this relatively small engine, the 
>> voltage drops to 9 volts at the battery!
>>
>>     
>
>   
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