Mentioned in a thread earlier I found cheap power supplies on Ebay to lift a "to discharged" battery to a level where the bulult in one can kick in.
I have now tested on a Vectrix with a broken built-in-charger.
I did put two of those in series (30-90V 300mA):
I have about +20°C in the garage and the pack held 120V when connecting the chargers.
Now after (about) six days I can read 141V, and the pack shows +27°C.
24h x 6d x 130V x 0,3A = 5,6kWh went in to the pack.
Obviously the pack started to develop some heat, and I have no active cooling. But the top of the battery housing is dismounted.
Future plan is to use two of those chargers to equalize the pack over weekends, or when the temperature allows it without active cooling.