Groove replied:

How each battery is charged depends on what type of dual battery management system is installed.

A simple cheap solonoid will charge both batteries at the same time regardless of the level in each battery.

A so called smart solenoid (and other battery managers pirhana, some rotronics etc will only allow the aux battery to charge only when the starting battery reaches a predetermined level , at which time both batteries are charged at the same rate.

Some battery management systems will actually charge both batteries at different rates (more ex bleep ve rotronics etc).

Wayne (NSW) replied:

Johno,

It will all depend on the isolating system that you have.

The way I look at it is that the alternator was designed to charge one battery at one time. So the isolating system should charge one battery at one time.

The system that i have got will charge the main battery first, then charge the second battery. I have two heavy duty batteries and it is designed to charge the second battery as a heavy duty battery. Other systems will charge the second battery at a deep cycle rate.

The life of both batteries will not be affected with either of these methods and changing them around will not be needed. Sometimes the batteries can't be swapped because of the pole position.

Now you will get all the post with all the tech advice with all sorts of numbers and fact read out of a book. I can only go by what I have and how it works and has done for over 13 years. I don't need to know all the charging rates at what temp and if it is a Sunday or not. All I need is a system that when I start the vehicle the batteries are charged.

The system I use is IBS available from TJM and the other is the Piranha.

Wayne

richopesto replied:

Hi guys,

Wayne has a very good point. Im sure theres many a auto-elec who can elucidate better than I....
Ive been reading a bit about RAPS for shacks and caravans....
"Other systems will charge the second battery at a deep cycle rate"

This point is (apparently) crucial when it comes to the float level (the point at which the battery stops charging) Despite all I've read I dont quite yet understand it sufficiently to explain it fully.....
Suffice to say, certain regulators/isolators will go into float level at slightly different voltage levels and you may never fully charge your battery, thus shortening its lifespan. Its important to match your battery to your isolator type. That's all Im prepared to say on my very thin level of knowledge. :-)

cheerio
richo

Derek from Affordable Batteries & Radiators replied:

This is an open question and can't be answered without more information. The thread could go on for days and members will be abusing each other by the end of it. (Again)

There are many isolators and battery management systems available. The voltages and the charge rates vary and like Groove mentioned battery type also plays a role.

If you want a straight answer without getting totally confused let us all know what batteries are fitted and in what position and if possible which isolator you have fitted.

Regards Derek.

peter d replied:

hi everyone, i just have a cheap solonoid with a toogle switch that i turn off and on when i need second battery charged and i haven't had a problem in over two years. but then how long should the batterys last.
peter d

Muzzgit (WA) replied:

Johno, if your solenoid is the older style which "clicks in" as soon as the ignition is on, then both batteries are open to charge from the alternator.

If it is the newer style it will wait for the starting battery to charge back to around 13.8v before bringing the second battery into line, and will "click out" as soon as the starting battery gets below 12.5v, car running or not.

I have the newer style solenoid in my patrol with a calcium starting and wet cell deep cycle aux, and I'm not convinced this is the best setup. The deep cycle takes too long to charge and I don't think it reaches full charge properly even when it hasn't been used for 3 or 4 weeks, and I drive to work every day (15 - 25 minute trip)

I would not swap a battery that has been used as an aux to become the cranking battery unless it was an emergency.

phil replied:

A solenoid system, smart or otherwise, just connects the 2 batteries in parallel so they both have the same charging voltage.
However... having the same voltage does not mean that they have to have the same state of charge, even with identical batteries.
For example if the starting battery is fully charged and the auxillary battery is half flat the alternator will still keep the voltage on both at the regulated voltage but most of the current will flow into the auxillary because it will have the lowest internal voltage. Most alternators these days can deliver enough current to lift the terminal voltage of even a quite flat battery to 14.2 volts, or whatever it is set to regulate at, so a smart solenoid will still connect the auxillary even when the main battery is near flat. However the alternator can probably deliver enough current to charge both at whatever current they will take.

Hope this helps,

Phil I