Well, I've equalised them, glued 'em down, done a long float charge of about 36 hours (with the scooter charger) and put them back in the bike. I didn't draw off the excess after all - it either wouldnt' have helped or it would have hurt the batteries.
Good news: They're in GREAT condition. Almost as good as new - I took them to Stanton Drew and they got me there and (just) back (23km), just like when they were new. OK I did drive very slowly when I could and was on sparks by the end, but it never got so low that it started cutting out. The problem of sagging under load is much, much reduced, although not eliminated entirely - I could probably have been more agressive with my equalisation charge. I have isolated the hot wire from the others to as large a degree as I can, used a high temp terminal block and put some heat shrink on it so I'll know if it gets hot. It doesn't, but I haven't put the second string back in yet.
Bad news: It still gasses noticably and stinkily on float charge. I'm giving it a long float charge - over 12 hours - and then think about putting the second string back on. (which I've also equalised, having learned the lessons of the first string)
The second string is the scary bit - the scoot's weedy 800w motor isn't really designed to take the weight and neither is the luggage compartment, so the hot wire probably will heat up if and when I put it back in. So maybe I should try a few more runs and float charges before I do that... also the extra batteries will block some of the ventilation, on the other hand it might not gas so much if its' kept under the gassing voltage by the other batts.
Long term, a Lithium battery - almost certainly another Ping battery - looks like the only way to get proper range on this bike without shortening its' life, not to mention mine.
There's a real risk of accident from having four 20ah batteries in the luggage compartment so I can't reccomend it except as a stopgap. Maybe if I reinforced it... I digress.
Acceleration performance is just fine just as long as it isn't overloaded with extra batteries, but sadly to get any kind of usable range that's exactly what I'm going to have to do - for now. I'm going to go all out to save up and after I MOT it... maybe I'll go for it. I really like this machine despite its faults, and don't want to waste money buying another one when I could be enjoying the benefits of Lithium.
Has anyone else tried putting a big Ping battery on a proper moped? I know someone on the Electric Motoring
forum did but he seems to have vanished... hopefully not because it didn't work out! He seemed a little bit down about it, although he didn't say anything to indicate that it was an outright failure.