Today's breakdown

[Man_Named_Dave]

Put it on the Optimate last night, and it's reading 12.37V before I even connect it to the bike.

So it looks like it is indeed kaput! Time to order a Yuasa, methinks.

[magg]

As a load test you could put it in the bike and hit the starter button. If the voltage drops at under 12 volts I would say kaput.

[Man_Named_Dave]

As a load test you could put it in the bike and hit the starter button. If the voltage drops at under 12 volts I would say kaput.

Voltage before fitting this morning was 12.35.
I hit the killswitch and tried the ignition and the voltage dropped to 10.40.
Flipped the killswitch and the bike started but again dropped into the 10.40-ish region. At 5000 rpm it was ok, so it would seem that the charging system is fine but battery has had it?

[amaechizzle]

Not really... Holding charge is not the best way to determine whether your battery is kaput.

Technically, if it's a 12v Battery, as long as it's above 12v, it's holding charge.
Your battery will have a certain CCA, or rather Cold cranking amps.

Here's a quote
"What are CA's (cranking amps) & CCA's(Cold Cranking Amps)?

Cranking amps are the numbers of amperes a lead-acid battery at 32 degrees F (0 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).

In other words, CA/cranking amps determine how much power you have to start your car in most climates. The basic job of a battery is to start an engine; it must crank, or rotate the crankshaft while at the same time maintain sufficient voltage to activate the ignition system until the engine fires and maintains rotation. This requirement involves a high discharge rate in amperes for a short period of time.

Since it is more difficult for a battery to deliver power when it is cold, and since the engine requires more power to turn over when it is cold, the Cold Cranking rating is defined as: The number of amperes a lead-acid battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).

In other words, CCA/cold cranking amps determine how much power you have to start your car on cold winter mornings."

A Battery load test is the only way to tell whether a battery is screwed or not.
This is when they test the CCA of the battery...
Take the battery out, or even ride to any mechanic, and get them to test it for you.

I'm only trying to save you money

[Man_Named_Dave]

^^^
No offence, but what?

[magg]

Not withstanding the battery's CCA rating if the battery drops much below 12 volts when cranking the engine it is either flat or stuffed. The CCA rating is a method of rating batteries for comparison purposes. I very much doubt a battery that has a terminal voltage of only 7.2 volts under any load is going to be very successful at starting anything.

As I have commented in another post on this site, my text book source says 10.2 volts is considered a flat battery and any further discharge is likely to damage the battery structure.

If the open circuit voltage is greater than 12.8 volts it would be considered fully charged and should not drop below 12 volts for the 30 seconds it should take to start the engine.

[amaechizzle]

When a battery is load tested, they load up the battery with as many amps as it takes to drop it down as close as possible to 8v

When your car starts, it drops to around 9-10 volts.
Something tells me that is alot harder to start then your small motorcycle engine.

[Man_Named_Dave]

Ok, Magg says battery, Amaechizzle says it could be something else.
I can't pay a mechanic just to test a battery, but I've got plenty of time to check, so what else could I test at home? The reg/rec and alternator readings are fine, as are all fuses.

[amaechizzle]

You can't pay a mechanic to just test a battery :S
I'm not saying that your battery will not be at fault, I was just suggesting that whacking out for a new battery may not be required.

Nearly all mechanics will have a load tester, surely you local bike mechanics will do it, for an extremly minor fee, or even free?!
You can ride there, take the seat off and hey presto!

[Man_Named_Dave]

Won't be riding anywhere for a while! The tank's off and emptied, today the air box and carbs probably.

Not sure where my nearest mechanic is. Might be within cycling distance. I'm just looking for any checks I can do in the meantime.