Hello,
I have connected 4 SMD LEDs in series and connected them with a 12v 80 amps lead acid battery.

When I connect other load, like a small fan, the LEDs are getting dim. How do I rectify this? Below is the picture of led beside a centimetre scale. It is not the actual circuit.

 

It is a white led, I got it from a 4 feet led batten.

 

Try 3 LEDs and 100R resistor. 4 LEDs will not work.