charging a 12v 18Ah sealed lead acid battery?

Unless the 12V adaptor has a no load voltage of 13.5V or higher, it won't fully charge the battery - when charging, the battery voltage will rise and if the supply is not higher voltage than this, no current will flow - no charging.

If the charge rate is limited to 350 mA, you can charge almost indefinately without battery damage - about twice as high will cause the electrolyte to boil out eventually, though.

The thumb rule is charge this at 1.8 amps for 10 to 12 hours. Since your charger can give only 350mA, I guess you need to charge for at least (1.8/0.35)*10 Hrs.= 50 Hrs. After that you can continue to charge, depending on what the load is. If load is 1.8 amps, it will discharge to terminal value in about 10 Hrs.So you need to charge (without load!!) for another 40 Hrs.

12v 18ah Battery

18 Ah battery would draw around 1.8 amps current. Your 12V, 350mA charger can never charge the battery and if you keep it for charging, your adaptor would burn off.

350 mA is a safe 'trickle charge' rate.

Get a sensitive volt-meter.

You can charge it a bit faster up to about 12.6 volts,

then trickle till you get it to 12.8 (after a brief load application),

at which point you can consider it full charged.

Check out : www.windsun.com/Batteries/Battery_FAQ.htm