Hi, what is the minimum charge in the battery needed to operate the bike??...mine is RVF400....voltmeter showing my battery charge at 6.8V...

I assume that the bike neither starts or runs? A good charging voltage is 14.5 volts. Where did you measure 6.8 volts, if at the battery, then the battery is stuffed. Get a new battery and then check the battery voltage with the engine at 5000 rpm. Measured voltage should be more than 14 volts if the charging system is working correctly.

Yup...the bike not running for a year...got to work somewhere else...i've taken all steps just like haynes manual stated in leaving the bike for long time...this week i started work back at home town again and take the bike out...but after i turn on the key, no lights, even the Neutral indicator was off ....luckily got the voltmeter...measure it at the battery, without terminal cable attached....reading shows 6.8v.....just charge it 2 days ago at the bike shop........so this is the battery fault is it?

Yup. The battery is most likely toast. If you have a good battery charger that can deep charge batteries, it *might* recover - but I'd just buy a new one.

Did you regularly charge the battery while the bike was in storage, as required by the haynes manual.