Sudden engine cut out CRF250L

[doggone]

Luckily it was only up a field near the house and managed to free wheel nearly all the way home.
I thought probably main 30A fuse but swapped that and nothing at all completely dead on key.
Put jump leads from another battery to it and then it would start and run normally - but disconnecting attached battery it stopped immediately.
Got to need new battery surely so have ordered one it is 9 years old anyway.
Still struggling to know what can be so bad in the old battery that it makes it cut out completely.
Don't really have anything to test battery but charger says it is halfway to full charge didn't like the look of that so took it off.
It has always started perfectly including when I set off up fields this evening.

[jeffyjeff]

Your charging system should supply plenty of power to keep the bike running. Once started, the battery becomes just another load, as elevated voltage from the stator/RR charges the battery. I suspect something off with the Regulator Rectifier, Stator, or wires in between. Without charging, once voltage drops significantly (say, under 11.0V) there is insufficient power to keep the ECM alive, hence the sudden failure. Swapping out the battery facilitated engine start, but if the charging system is kaput, recurring failure would be imminent.

Once you get the new battery in, measure battery voltage with the engine running. Normal is in the neighborhood of 14 V to keep the battery well charged.
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History shows again and again how nature points out the folly of men - BOC

[doggone]

New battery fitted and all seems well.
My charger gives an indication of the battery condition when connected but not switched on (I don't have a proper meter) and when I start the bike it shows higher volts so it is charging.