the last Batt. i put in my 640 was a SLA= sealed lead acid , Dia Mec, has Dm12 v- 7.2 Ah. 20hp
written on it, i don’t know what the 20hp means. but it’s 12 volt 7.2 amp hour.
$35 from the local Jaycar stockist, i just measured up the old Batt. and got the closest one in size, only ended up a few mm smaller over all.
has not missed a beat in last 6 months or so. has not needed to be charged via an external charger and the bike has sat for up to 3-4 weeks at a time without being started.