Hi Jon
If you are having problems with chargers not working correctly after a few uses - it wood be a good idea to run a multimeter over the batteries & the pack to see if there is problem with wiring or a battery that may be damaging the chargers.
I charge my 48v SLA battery pack - pretty much twice a day with a 48v e-scooter charger (cheap chinese electric motorcycles type).
Cost about $48 off ebay - they do pop from time to time @ realistic prices.
The eibike sellers were asking as much as $100.00 for similar chargers!!
example:
http://cgi.ebay.com.au/48V-Lead-Acid-SLA-Charger-4-Electric-Bicycle-Trike_W0QQitemZ260296627342QQcmdZViewItem?_trksid=p3286.m20.l1116It pays to think outside the box - as you can get 48v chargers for other such as mobility chairs, e-scooters etc which will work fine.
Just need to get a 48v model that is suitable for your AH's.
You could always pick up a cheap 36v charger or use your own if you already have one to charge 3 batteries & charge one battery with a 12v car charger - but this could be a real pain - but is a good band-aid until you a 48v pops up for sale.
I use 4 12v car battery chargers to deep charge my SLA's individually about every dozen "cycles" - once a week or so.
(these chargers are the cheapie - $14.00 items -brought 1 each pay day at the local car parts shop)
Charging in a cool area - like a concrete floor of the garage - seems to get the best results. Before & after charging - I check the pack & individual batteries with the yet another cheap item - a $12 multi meter. This allows any lower discharged batteries to be identified & given more time to charge up to similar volts as the others. I float charge with the 48v charger for about 1hr before long rides just to top up the pack. This setup has worked well for over 18 mths - even though the batteries are deep discharged often. The batteries are storing less than when new but still cheaper for me to use them with this set up than paying crazy prices LIFEPO4 packs are demanding.
Hope this helps