Ottawawest said:
Can I charge 12v 8.5ah lead acid with 12v 9A charger?
No.
I charge it now with a 12v 1.2A in 14 hours.
Actually no again. At least not fully.
If not could someone explain why?
Because you cannot fully charge a 12V lead acid battery with 12V. In fact
you'll never get more than an 80% charge.
To charge a lead acid battery requires at minimum 2.3V per 2V cell. A 12V LA
has 6 cells and therefore requires at minimum 13.8V. This won't get you a full
charge, but is safe to charge at indefinitely.
To get a full charge actually requires 2.4V/cell. This is a voltage of 14.4V.
But you cannot charge indefinitely at this voltage.
Heat, gas buildup, water boiling dry are all factors. And the more charge you
drop into the battery, the faster it'll reach a state where damage occurs.
Most battery manufacturers suggest not charging at a rate that is faster than
the AH capacity over 3 (i.e. C/3). For your battery that would be a bit under
3A. A 5 ohm resistor in series would give you 2.88A with a small voltage drop.
You need to do 4 things:
1) Raise your charge voltage to a steady 14.4V
2) Put in the 5 ohm series resistor to limit current.
3) Monitor the current the battery is consuming. when it drops to C/50 (170 mA
in this instance) then the battery is fully charged.
4) DO NOT LEAVE THE BATTERY CHARGING AT 14.4V!!! YOU WILL BURN IT UP!!!
Always trickle charge at 13.8V. This voltage is safe indefinitely.
Hope this helps,
BAJ