Question about battery capacity?

Okay... here's the deal: I have 3 different batteries.

One is a AA rechargeable Nickel-metal Hydride rated at 2450mAh

Then I have a Ridgid LI-ION pack rated at 1400mAh

Then I have a Milwaukee NiCD rated at 2400mAh

Now imagine the size and weight difference...

Okay so voltages are different. The AA is 1.5v

The LI-ION is 12V

and the NiCD is 18V

So let's break it down:

It would take 12 AA's (NMHI) to make 18V in case weighing ~16 oz or 1LB. Still 2450 mAh

If I take apart the Li-Ion it has 3 cells, so to get 18V you need 4 1/2 cells... weighing about 12 oz and still storing 1400mAh

The NiCD is already 18V and weighs almost 2.5 Lbs! And stores 2400mAh

Are you seeing the problem? Lithium Ion SUCKS!!! They ripped us off! Said it holds more power, weighs less, and is more efficient with longer life span. Well it appears to me that to get equivalent charge capacity, I'd have to nearly double the weight and size of the Li-ion pack. And out of all these batteries, which one do you think stopped working after only a few charges and a years time?

I have 4 NICD packs that have been abused thoroughly for more than 4 years. All still work great.

I have nearly 30 rechargeable Energizer batteries (AA and AAA) and yes they fail sometimes, but as it turns out when one flashes in the charger as "bad" you can usually just drain it down in a flashlight and it'll charge again like nothing was wrong.

I bought a Ridgid kit for 190 bucks about 2 years ago. it came with 2 12v Li-Ion batteries and I tried it out for a while. Kept it with me. Used it now and then. I don't like it at all. It's weak. It dies suddenly. And, you guessed it. I've only had 1 working battery for the last year or so. I'm not paying 60 bucks for a replacement, and it's BS that it failed soon after the 1 year warranty.

I took it apart to see if it's fixable and it really isn't at all. You can't acquire replacement cells without special license (dangerous!) and there's no easy way to test.

I am so freaking poor and broke right now, but I need a good power source for my metal detector. So I'm going with the NiCD pack. I'm splitting the cell group into 12v and 6v to power the unit and a light and I'll just put it in my back pack. I'm just not sure if I can have a wire feeding my detector without upsetting the field. And on top of that it's a two-box detector needing two batteries, meaning a wire has to pass between the boxes. Argh. It's a deep cache finder so the field is huge. I can't wear my steel-toed boots or a belt. But hopefully the battery will be high enough to avoid the field. Is there a cheap way to shield metal wire from upsetting a detector?

Anyone have any advice relevant to my long-winded rant?