Here's a neat little gizmo for $15 that will keep those 6v lead acid battery's in tip top shape.
http://www.batteryspace.com/index.asp?P ... rodID=2723
In the good old days you have to mess with hydrometers, temp charts, manually tend to chargers etc. These new battery tenders have the ability to bulk charge, then automatically go into a float state that will keep the charge at the correct voltage.
If you shell out for a repro 6v battery (several hundred dollars) a $15 charger is very cheap insurance to keep it running for years. Esp with antique MVs that may sit more than they run.
As for the $15 chargers vs the $50 ones.. other than having a fancy sticker on them I see no technical reason for the added expense.. UNLESS it is a 4 stage charger vs a 3 stage charger.
They also have a 3Amp 6/12 charger.. This covers both battery voltages and has a quicker recharge ability.
http://www.batteryspace.com/index.asp?P ... rodID=2909