Step 4: Smoke Test 1 - pots instead of fixed resistors for R2 and R4
For those of you who used fixed resistors for R2 and R4, skip this step and go on to the next step, Smoke Test 2.
For those of you who used pots instead of fixed resistors for R2 and R4:
First, turn off S2, put a 555 chip in its socket and a 2A fuse in the fuse holder. Set the pots to their mid-range and attach the plus lead clip of your circuit to the plus terminal of a 12V battery. Attach the ground lead clip of your circuit to the minus probe of a multimeter, and set the multimeter to the 10A AC scale. Briefly touch the plus probe of the meter to the minus terminal of the battery. Check for smoke. No smoke? Good! Try it for 5 seconds, then 10 seconds. Still no smoke? Great!
Check the 555. Hang a scope probe (if you have one) on pin three of the chip and check for pulses. Adjust R4 for peak output at around 1000 Hz (the exact level isn't critical).
Now check the output stage. Turn on S2 and briefly touch the plus probe of the meter to the minus battery terminal. You should see a brief spark and hear a faint 1000 Hz tone come from the coils. The LED will turn on in the presence of output pulses. If it doesn't, but you hear the tone, then the LED may be mounted backwards. If you don't hear the tone, or see smoke, then something is wrong and you'll need to check your output stage wiring.
If the fuse blows try adjusting R2 down a bit (the direction of turn depends on how you have it wired). Smile when you get the meter reading below 0.8A -- you're almost there!
If all is good then adjust R2 so the meter shows no more than 0.7A on the AC scale. This should yield a good output into the battery without overheating the output stage. Finger test the coils, C4, FRED, and the FET. If all are no more than slightly warm after 30 minutes then you're in the clear. You can SLIGHTLY increase the pulse width and the current into the meter a little at a time until the circuit reaches about 1.0A but at this level my charger won't flip into trickle charge mode because the combined currents of the circuit and charger are beyond its trickle threshold. I therefore keep it at around 0.7A. Anything beyond 1.0A gets a bit too toasty after a night's use anyway. Also note that the circuit will tend to consume 0.2A to 0.3A more current and get hotter when the charger is on high charge rate. It's therefore best to stay at or below 0.7A to prevent the current from getting too high as the charger adjusts its charge rate from high to low. Be conservative, especially with an undercharged battery because as the lead sulphate crystals dissolve into the electrolyte the battery voltage climbs and this increases the current and the heat dissipated by the output components.