When you drive your NPN transistor into saturation, a typical value for Vce will be about 0.2V - not very close to 0V when your reference is 1.25V. And Vce is not gauranteed to stay constant.

Due to my aversion to relays, I just have to throw out the suggestion of using a logic-level FET. Using a beefy one that can handle around 10A will give a respectable On Resistance in the neighborhood of 10mOhm. This is comparable or better than most relay contacts. Plus there is no clicking.

An example part number is FDS6670A: n-channel, 30V, 13A, in an 8-SOIC package for $1.39 from Digikey. I bet you could find something better.

- Lee