Have anyone seen this before?

[Anonymous]

(1980 GS750E bike)
Have you guys/gals observed this before?. When the engine is hot (like after riding for 20 minutes in a summer day), if I turn the engine off and then try to turn it back on, the battery doesn't have enough energy to start the engine again. It just sounds like a low battery. However, when the engine cools down just a bit (letting it rest for like 15 minutes), the battery doesn't sound low anymore and it turns the engine easily and immediately!.
The first time this happened I was convinced I had a dead battery. It could be that it just died out of old age; or that the generator (or equivalent device) in my bike wasn't doing its job anymore. But when I came back to the bike (after making some calls trying to arrange a rescue) it turned back on like nothing happened...
Today morning (I didn't use the bike during this past weekend) the engine started without incidents and the battery sounded like a fully charged one. So after I arrived here for work (a 20 minute ride from home), I tried to turn the engine back on right away... and as I suspected: the battery sounded like a dying one, without enough energy to turn the engine on again. By noon however, I when out to the bike and it turned on perfectly...
Has anybody here observed a similar behavior?.
I haven't entirely discarded the low battery theory yet; or the possibility that the re-charging system can be defective. However what confuses me it the relationship this behavior has with the engine temperature... and then I think that it could be something completely different, something beyond my basic knowledge; or maybe some particularity of the GS bikes with a solution known to some of you?.
The bike is a 1980 GS750E, and it has been working flawlessly since I got it in May. The worst of summer is gone now and the days are significantly cooler which throws another variable to my theory about engine temperature...
I would appreciate ANY sort of help, advice or ideas.
Thank you and salud,
Oscarin

[Anonymous]

It's probably a hot starter, not the battery. As the windings heat up, the resistance associated with the windings increases also. Higher winding resistances = lower current = less torque output on the starter = motor turns over slower.

[Anonymous]

I baught a bike once that was doing the exact same as you described. I took the starter to a starter repair place & found no problem with it. I then installed a new battery, same size as the one in the bike. Same problem. By chance I read something that said my bike used a 14LA2, I remembered I had baught a 12LA2. The previous owner had put in the wrong battery, and I followed him by doing the same thing. The leingth & width are the same, only the height is different, by about 1 in. Installed the 14LA2 and never had another problem with it

[Don Lobacz]

Hot engines need more power to turn over than cold ones. A marginal battery will start a cold motor but have trouble with a hot motor. I would bet it's the battery that needs replaced.

[nabrams]

Also, check the brushes and armature in the starter. If they are somewhat dirty they will impede full current to the starter. When it's hot this will be even more of a factor because the resistance is greater (as stated before). After I cleaned mine the starter turned the motor over with much much more force.

[Anonymous]

Thank you!

Thank you guys!!. I am so glad to be part of this group. Now I have plenty to try and test. Thanks to your ideas and experience I now have several paths and options to test instead of being in the dark guessing forever, and finally giving up and taking the bike to the shop ending up paying; when most likely I can fix it myself! (and with the experience, I will be able to help others; just like you helped me)
Again, thank you very much!.
Salud for all of you!
Oscar