Doing My Home Work

A couple of days ago, my GPS started to not charge up properly.
Normally it does this fine when the bike is running, so it was a bit of a worry to see it saying it was low on batteries.
I checked things out a bit that evening when I camped and could see no connection issues, so I measured the bike battery voltage with my little multimeter...It looked like my battery was only at 10 volts instead of the expected 12V!
So I started up the bike and watched the voltage and it only rose to 12V instead of 14-15 volts...So then I disconnected the battery while the bike was running and the bike kept running and the headlights worked fine (though with the expected more dimming as the engine revs dropped toward idle). That means that the bike alternator is generating plenty of power...

So, there is some sort of problem but I dont know what it is...
Thats not good!
Especially since Im about a day or two from heading into Mexico, and if I have bike problems there then Ill have to sort it all out using my bad Spanish.

So, I had a list of symptoms and not much more I could do that evening other than consider the complete list of symptoms and try to figure it out...
1) GPS not charging - from too low a voltage.
2) Battery has plenty of "cranking power" - Bike starts OK but is slower to fire up than usual.
3) All connections at the battery and the GPS are in good condition and tight.
4) Alternator is definitely producing plenty of power.
5) Battery is a good Yuasa sealed unit so cant be out of acid.
6) Battery is five years old and has been fully discharged accidentally only once (note this is very bad for the battery and does cause permanent damage - can only do it once or twice before battery is destroyed)

OK, now a Lead-Acid car battery should sit at 12-14V when not in use.
The bike regulator should "float" the system to 15 volts when charging.
A flat car battery will measure about 10V.
A partially charged battery will measure anywhere between 10V and 14V
A flat battery will also have no strength to turn over the engine for starting.

It looks like I have a strong battery that is only 10V... That makes no sense!

So I go to bed and I think and I think...
It seems I have three possibilities here:
1) Its the battery
2) Its the alternator
3) Its something else!

First possibility is that the bike alternator has somehow changed its output and is only charging the battery up to a certain point. But I dont see how thats possible... Alternators in cars fail by either stopping working all together (and your battery then goes flat) or by producing way to high a voltage (when the regulator portion stops working) and blowing up all the electronics in the car all at once!
Neither of these things has happened.
For the output voltage to drift two volts, the regulator zenner diodes need to change... and thats impossible as far as I know (and I was an electrical engineer, so I think Im right on that point)

OK, then if its not the alternator, is it the battery?
Well, what I said about car batteries above is all true and initially it seems thats not a possible reason either...
But, the battery is actually made up of six 2V cells all connected together and behaving as a single unit. It dawns on me that maybe there is a problem with just one of the cells! Its not common, but individual cells can become damaged... Maybe, just one of the cells has become shorted out internally- could be manufacturing fault, could be from the battery being dropped or something like that, but If one cell is shorted out and the other five cells are normal, then I would have a strong 10V battery.
And when connected to the alternator it would "hold down" the charging voltage as I am seeing...

So, I think I have a strong candidate here and that Ive solved my problem.
Nothing more I can do about it tonight though, so Ill deal with it tomorrow in town.
But, having done my home work, I think I have the answer and will get a "gold star" for my good work tomorrow :))

And in the morning I ride in to town and find a mechanic and take apart my bike and get the battery out and go in and explain my situation. The guy there seems to think my logic makes sense and just to be sure, we try the battery on h charger to see if it will behave differently there (the final test to confirm behavior)...

... And in fact it does NOT behave the same!
It sits at 11.5V not 10V .

ahhh...
Turns out the problem is neither the failed cell Nor the bad alternator on my bike!
Turns out that my multimeter is out of calibration and is reading 1.5V low (I adjust this immediately).
But the battery is still low on voltage (only by a half volt though) and this is apparently getting lower since the GPS has only just started to complain. The battery is five years old though and thats a good long life for a car battery. Im probably just getting into the closing stages of its life and the output voltage has started to collapse...
So, I decide to replace the battery with a new one now rather than waiting for it to fail when Im in the middle of nowhere in Mexico!

But, I was wrong on my diagnosis... No "gold star" for me :(


As always, there is that possibility of "something else" :)))