dual batteries 75 series Troopy

We are all bloody experts on batteries here....but I will let some of the younger ones reply to your question

Cheers

You asked for experts on this issue and I'm not one. One thing you should decide on is whether or not you want a truly independent battery management system. There are plenty of good reasons to go for this type of system but they are very expensive. By truly independent I mean one that will actually deliver the full electric current from the alternator to the second battery during the recharge phase. The absolute majority of systems are not independent at all despite some very missleading and at times downright dishonest advertising claims. This means that the second battery will only ever receive the "overflow" of current from the main battery once it is fully charged. This overflow will never be as high a current as the one direct from the alternater so the second batery will never be charged to 100% of its capacity. Appart from obviously not having as much available power as you might like, the life of the battery will be shortened because they really don't like being partially discharged all the time. The only independent system I know of (and there may well be more) is made by Rotronics. They cost about $700 just for the controler! I actually bought the Rotronics system because I wanted absolute reliability. It comes with clear instructions for self instillation. A really basic system might be a fair investment however, because you can buy quite a few batteries for $700. Everyone will have a different opinion on this issue and they all have merit. What ever you end up with just understand how it works and what the limitations will be.

I've always only run a simple solenoid and do not have battery problems. I run a fridge, lights and sometimes an inverter.

Our style of travel is to keep on the move so this keeps the battery charged. Would require something else if you went to a location and camped for a week. We tend to change camps every night due to lack of time and so many places to go.

I think the trick is not to short change the cables. Make sure the main cables connecting the batteries are large

Go to www.fridge-and-solar.net
All the info you need is their.

Because the vehicle manufacturer wants a reasonable life from the battery, the alternator is regulated to a maximum voltage output of 13.8 - 14.1v. (sensed at the alternator). This will recharge the battery to about 70% of its capacity. The fact the alternator has a current output of (say) 100a. is to deal with power loads (lights, w/wipers, heater fan etc.)

Having said that, the optimum charging rate for a 12v. battery is 14.7v @C/10 a.
(14.7 derived from 2.45v/cell x 6 =14.7, C/10 being battery ahr. capacity/10, 100ahr. battery/10 = 10amp charge rate)

This 14.7v. has to be at the battery terminals, so voltage drop in the cable/connections has to be taken into account. I also follows that to thoroughly recharge (not simply "surface" charge) a discharged battery it will take around 12hrs.@ 10a. Periodically the charge voltage should be increased to 15.5v. to "equalise" the battery cells.

Because the vehicle alternator initially whacks 30-40a. into the battery then fairly quickly reduces back to 8-10a. does not mean the battery is charged but that the battery voltage has become equal to the control point of the alternator voltage regulator.
As installed in the vehicle the alternator will never fully charge a 12v. battery. A check with a hydrometer will quickly verify this.

A system which addresses the problems here is to use a 12/240v. inverter near the starting battery with a 3stage 240/12v. battery charger located adjacent to the aux. battery. This is the system used for years in motorhomes. Of course, for this to work, the engine has to be running otherwise the starting battery suffers!

Collyn Rivers, who used to post on here, has written some excellent books on this subject, highly recommended reading.
firph

Im all for the KISS projects too. I wanted to run 2 batteries with a marine battery switch.
I had a marine battery switch on a 60 series but alas I sold it before I got to fully explore its possibilities.

Is it ok to run a deep cycle battery for the fridge and a starting battery for the engine and swap over to the deep cycle battery occasionally to recharge it when your on the move?