Scandal !!!  I decided that before I buy a battery, maybe I should check it, and the charging system, and my knowledge of alternators and regulators. Following instructions found on the interweb, we measured the voltage
at the battery, disconnected, connected, lights on, then with starter motor load, then idling, then at 2500rpm, then
added lights, indicators,radio. The results ? Well when the engine is not running, the voltage drops such that I need a new battery. (No scandal there), when the engine is running my multimeter says with load or not ...around 14.8v.
 All the interweb hits say your suppy needs to be regulated between 13.5 - 14.5v and outside that range means NEW regulator (if too high) or possible heat/damage to electrical components, or NEW alternator (if too low). Fine, I'll get a new regulator. I open up the BGB to find out where the regulator is (on the alternator) but SCANDAL !! ...it says Normal is u[p to 15.1v !!!! every generic doc I had read was saying that over 14.7,say, would shorten the battery's life.
 So I searched and there are some hits for SW20, 15.1v and BGB.... but none when I add "mistake".
Has anyone noticed this before ?  My multimeter may be out, but the documentation is generic for all cars!
 and more importantly, batteries.

The failing battery may be the reason for the higher voltage you are seeing at the terminals? Do you have a spare battery you can temporarily hook up to test this?

Hi Skylighter,
 all my readings may be out because the battery needs replacing, or my multimeter is cheap, but how can Toyota suggest 15.1v is normal when (it seems) everyone else is saying 14.5 should be the max into whatever battery (and components around the car)?

Does the bgb suggest 15.1 at the alt or batt?

TwoDogs
Hi Skylighter,
 all my readings may be out because the battery needs replacing, or my multimeter is cheap, but how can Toyota suggest 15.1v is normal when (it seems) everyone else is saying 14.5 should be the max into whatever battery (and components around the car)?

I don't know man. Toyota would have a good idea as they designed the car. I've never seen 15.1v in any of my cars though.
 
Sounds like you need a new battery anyway so maybe go ahead and replace it with the correct one and see what the Alt is putting out afterwards? If it's still too high investigate further.
 
Maybe test your voltmeter on something with a known DC voltage like a newish 9v battery or friends car battery?

Yep, I will get a new battery, but you miss the point about the BGB saying 15.1v. It has nothing to do with me, or my car or my meter. It is your car. Unwanted heat going into every component in the car, including the battery and that shortens the life of those components. I will also raise this with an auto-electrician to see what he says.
It MAY be a BGB error (wouldn't be the first). 

Will be interesting to see what the voltage is once you change to a new battery. I'm thinking it will come down a little to around 14.5v
 
Let us know how you go with your investigation into the 15.1v maximum mentioned in the BGB. Maybe you could check a few other Toyota model manuals from the same era or international SW20 BGBs?

I'm sure you are right. Talking about it with a more experienced car lover, he suggested the high readings that I am seeing (14.8v, 14.9v) is because my battery is flagging, and that with a new battery, the numbers could well come down to low 14s.
 About the BGB saying 15.1v he was a little surprised, but suggested it is likely to be a very short-term condition.
I will still talk to an auto-electrician about this, for better understanding. I'll post here any salient points.

Might be 15.1 alt output but lower once in makes it way through 4m of cable...

Good point - had not considered the voltage drop.
New question: (if I can insert a photo)....hmm asks for a URL, but my photo is on the C:drive...??
 
The braces that hold the battery in place use a longish j-bolt with a nut on top.
Unfortunately that bolt does not reach either of the 2 holes in the bodywork, behind the battery.
I suspect that something attaches to those holes and extends 20 or 30mm vertically, which the j-bolt then
hooks onto. I can't find a picture or part number to order it. 
 I should ask Josh if he has one.. or maybe just make something to fit....