Older Generator cranks too slow to start

[Tmargarido]

My 6.5 Kw 1988 Onan NH (Spec j-p) cranks so slow that it no longer starts. The generator has a good ground and the voltage is 12v at the generator under load. This subject has been discussed in previous threads, but those threads reference newer generators that have a starter. The older generators use a winding in the generator to start the engine. I'm hoping that someone out there has seen this problem before and can shed some light on this

[Isaac-1]

This problem is most commonly cause by weak or undesized starting batteries, you need a very BIG starting battery in good condition to start an exciter cranked generator. A battery may start a big V8 engine just fine, yet be too weak to start an exciter cranked generator.

[Arch Hoagland]

It would be interesting to put a battery charger/starter on the existing battery and see if it starts.

If so then what Isaac-1 said will be proven true and you may need to buy a new battery.

[twinboat]

Have you tried holding the boost switch, if it has one, while cranking.

The combination of all the batteries, while starting, will answer the question of needing a bigger battery.

[Tmargarido]

Thanks for the input. I'll give it a try