I figured I had a load tester for my batteries already, my truck's lights i.e. 200w of high beams, + 200w of driving lights + 110w of fog lights, + HVAC blower on full, + hazard lights on.
Essentially, turned everything on and measured the battery volts. With all of that load on the batteries held 11.6 volts.

After turning everything off I removed the neg lead and hooked up a Amp gauge between the lead on the neg post. There was hardly any drain - had to use the milli-amps scale to find it, and even then the reading was near minimum. So I just cleaned up the post & clamp and refitted the lead.

I checked the battery rack at the local Supercheap. You can get the H/D Century with 3 yr warranty for $175 each.

I've got a Ctek conditioner/charger hooked up to the batteries now, if it doesn't lift the batteries to 100% within a week or 2 I think I will pull the pin and replace the batteries with a couple of the top spec Century H/Ds from Supercheap.

I did figure something out though, and I know I'm not the first.

When I switch on the truck the WTS and glow plugs come on taking the voltage down to 11.6. The WTS goes out, but the glow plugs stay on for maybe a minute. As soon as the glow plugs go off the volts jump to 12.2. If I wait & then start right at the point when the glow plugs go off then I get the easiest start.

 

Seems these motors could use bigger batteries than what is fitted
but I can't see me fitting any larger battery trays, there is no spare
room under the bonnet.
It looks like I'll just have to keep the truck constantly on the trickle
charger.

 

get a 200amp alternator. remember my moto. "bigger is better"

 

Mate, plan to fit a higher output alternator but
a 200,000giga-amp alternator aint gonna help
start the truck if the batteries aren't up to it.