Well, almost all of the battery reloc. kits on the market overcharege for the compnoentry.
Honestly, I agree with him. I am going to assume you used about 15' of cable for the hot side. Now-- did you run a like piece of cable up to the engine block for the chassis ground, or did you simply bolt it to the chassis at the back of the car (if you did the latter, this is half your problem).
Go to a welding shop. Get whatever length of cable you used to reloc your battery, in at least a 0-gauge cable, preferably 00. Get enough to carry the + side of the circuit from the new batt location to the starter solenoid. Then get a piece of at least 4AWG long to go from the Alt + output to the new battery. Then get a piece of whatever gauge you ran for the new + side (0 or 00) long enough to go from the engine ground point the factory used, all the way to the battery new location. I STRONGLY suggest you solder these connections- a much superior connection, and it will keep out contaminants as well.
#@$%!, if you get me the measurements, I will build them for you. You pay materials and ship, and $20 for my time. I have built these sort of kits up for Jeep buds quite often, with no complaints. If you build them yourself, make sure to use electronics-grade solder, not plumbing solder.
Also, assuming you ran the pump motor supply from the battery-- another 15'? You did run through a relay, of course...
I suggest you spend a little time playing with the table and calculator here:
http://www.powerstream.com/Wire_Size.htm
With it, a 2AWG cable at 12VDC, with a 150A load (easy to do in a vehicle in the summer, with thin oil, when the starter initializes-- and surely will be worse now), we see
a .72VDC volt drop. And we're being conservative here.
Go to 0AWG and you see a .45VDC drop.
Go to 00AWG and you see a .34VDC drop. We seeing a trend here?
In short-- I have found your problem. Now go fix it.
I suspect your pump has a similar issue. Assuming a 15' run of 10AWG and leaving thick oil, cold temps, etc, out of the picture, you see a .3VDC drop. Run a piece of 4AWG to that puppy and you see a .077VDC drop. You read that right.
Overkill is ALWAYS the best way to go. My guesses are that you have undersized your mains cables somewhat, but undersized your ALT and pump cables even worse. Let's say you ran a piece of 10AWG to the battery, and it's obviously low. The Alt is trying to push 100A through it to recharge the battery, run the lighting, heater, etc. It's losing 3.08VDC. Yes, you read that right.
Here's what you need to do:
1) + AND - leads for battery reloc need to be replaced with 00AWG welding cable.
2) Piece of 8AWG from the ALT to the Starter Solenoid battery terminal. If you can run bigger than a 8AWG on your ALT terminal (ie: the lug will fit), do it.
3: Piece of 4AWG or better to the preluber, and make sure you're fusing it and running it through a relay so you don't fry whatever you're triggering it off of.
4) Of course, make sure you fuse all + leads at the appropriate amperage rating.
or, you could just move the darn battery back up front where it belongs. I forget, why did you move it to the back, again?