Min. Charger output for balancing one module?

You're using the right charger for individual modules, but as I learned last night using just one is painfully slow... The other three were out with my friend who also does this work. Then around midnight he dropped 'em off and things went so fast all of a sudden.

These chargers do weird things sometimes so you should always be close by to monitor them. My experience started out as cautious and getting freaked out about charge voltage soon as it went above 8.2, but over time realized that even when charge goes up to 8.4 it doesn't make much difference as after the battery is done charging in very short period of time the charge is down to 8.1v or 8.0v depending on lots of factors like heat, air temp, etc.

The main challenge is getting battery resistance/capacity being similar among modules, which is a function of battery conditioning. The first module I bought was awful and died after the first 10 miles and after much hassle got a refund and bought module elsewhere. The new module, or at this point 3 new modules, just to avoid future weaker module replacement, were in such good shape that the remaining older modules had way more resistance/less capacity and I went through three rounds of battery conditioning to make them match better: aka: discharging each module with two Prius head lamp bulbs, first down to 6.5v and recharging, then down to 6.0v and recharging, then down to a little more below 6 volts and recharging. Concept behind this is that over time NiMH batteries lose capacity because the electrolytes get stagnant and crystalised into large chunks that don't conduct and hold electricity as well as when all those chunks are broken up, which is what reconditioning does.

This worked pretty well... But after a 100 miles there's still not enough balance to keep the warning lights away so I installed a Prolong system so as to do charging and balancing and conditioning on the entire pack all at once rather than individually.

Hopefully you know about load testing to identify a bad module: Prolong® Battery Module Load Tester User Guide – Hybrid Automotive