Hi
Bobii,
you didn't put in a Location so that we can advise you properly...
this question has come up before, several times. Here are my previous posts: (
click here)
Circuit is a
TL431 which serves both as a precision voltage reference and an adjustable comparator that can switch either a relay or big PFET high-side switch.
The load for discharging a SLA battery at 150W @ a nominal 12V requires that it draws a current of I=P/E = 150/12 = 12.5A. That could be accomplished by a combination of
1156 or
1157 automotive lamps. These are cheap, and readily available. The power to the lamps will vary a bit as the battery discharges from 12.5V down to 10.5V
How precise do you have to be on maintaining the load to be exactly 150W as the voltage drops? Building a voltage-independent constant-current or constant-power load can be done, but is much more complicated than just using the lamps as the discharging load.