Peter, you're on the right track here... 1V between the alternator and the battery is a lot! You're gonna lose some more voltage between the battery and the fridge, so you'll definitely be in trouble from the outset. I added an additional cable in parallel between my second battery and the alternator to counter this voltage drop and improve current transfer.The problem with increasing the alternator output to compensate for the voltage loss in the National Luna system's solenoid is that the deep cycle battery ideally wants to charge at 14.5 Volts or so, but a crank battery does not like going this high. Now calculating in the voltage loss means that you have to bump up the alternator's voltage to 15.5V. But the crank battery gets it's charge directly from the alternator, and will thus sustain damage at these high voltages.
So, before you replace alternators, make sure that your cabling is up to scratch. If there's a notable (I'd say anything more than about 0.5V?) between the alternator and the battery terminals, fix that first. Those thick earth & positive cables are crimped to connectors that are screwed onto an exposed bits of bodywork & engine. Crimping is a good way to ensure good current transfer, but needs to be checked every so often on vehicles like our that are partly submerged every so often. A badly crimped or rusted connection will merely convert your alternator's charging current into heat (and even sparks!). The same for for the bolts that fasten the connectors - make sure they are tight. But loosen them first and check that the area underneath is rust-free before you tighten them up
I'd be very careful of tampering with an alternator's voltage regulator to raise it's output voltage. It may be good for your deep-cycle battery, but it is VERY BAD for every single other electrical component in your car that does not have its own voltage regulator. All in car electronics state explicitly that they are meant to be fed between 13.8V -14.2V, no more! If your deep-cycle battery wants a higher charge voltage more, change it to one that doesn't want to make its own rules at the cost of all other electrical components!
You can safely "tamper" with the current output of your alternator (by purchasing a higher output alternator!). To charge a battery faster, you increase the available charge current, not the voltage. Alternators are self-limiting (via their voltage regulator circuitry), meaning that a 145A alternator will not supply its full 145A of current unless its actually required. If the current requirement drops, it will drop its current output accordingly. So... there is no risk of "overcharging" batteries when installing a larger alternator.Christo have you experienced any problems since you did the alternator replacement with "over charging"??
Remember, the initial alternator was selected by the manufacturer to produce enough current to handle the continual current draw (engine management components, ignition coil, lights, interior fan, air-con clutch and fan, radio & amps, glow plugs, 2-way radio, etc.) whilst still having sufficient excess capacity to re-charge your battery. When you have two batteries, you halve the available current for recharging, so it will now require a trip twice as far to recharge them both! Replacing it with a larger alternator creates more excess current, and you're effectively back where you were with a single battery.
Cedricster, the original pulley won't fit due to the fact that the Delco is an American (SAE spec) alternator and the Hitachi uses a DIN spec pulley. Unless you find someone who can machine it for you? Make sure that the ratio between the circumference of the first drive pulley (on the engine block) and the alternator's pulley is more or less 2:1 so that you do not over-rev the alternator. You'll see that the Delco is specced for a continuous 10k RPM with bursts of 12k RPM. On our diesels we may use an even smaller pulley than the one they come with, methinks. Chat to Alan, he easily found me a pulley to go with the alternator....Christo.....If I used my existing pulley, how will that affect the charging ?? Will the revs not need to be a lot higher to get the charge to kick up over the 12 volt earth voltage for the dash light to go off??