Would this work - dual battery question

You say: "It is in a diesel vehicle so I see no major problems with the car running directly off the battery *for a while* I have a voltage sensitive relay that I could use to control the relays to ensure the main battery is kept charged"

So many extra independant switches and parts that will have to do the work of just one battery isolator, but your question is: will it work? - if the sudden switching off of the alternator power did not damage the alternator or REGULATOR etc it may, but I would not attempt to do it.

Maîneÿ . . .

In theory it will work when you pay attention to the dynamics of the switch.
When you switch a field wire for an alternator with a traditional relay you will
create a huge voltage spike unless you buffer it with a largish capacitor.
From the specs of the relay (contact closure time + safety margin) one can calculate the time constant for the buffer cap.
A second possibility is to first add a field wire of the second battery in parallel through a switch the and then open the first field wire through a switch.

Ok this is all theoretic because there are other constraints. If your second
battery is very low and you have a few things running in the car like aircon,
lights, radio it is possible to cook the regulator of that flimsy 55Amp alternator.
The engine may get a problem for a moment with a very low battery voltage (despite being a diesel) through the shutdown circuits.
So at the end of the day it is NOT good practise.

You might put a second alternator in and charge the house batterie(s) with that. So you do not need any isolation circuit.

If you try this to save money you must be really desperate and be warned you will pay a lot to repair all the damage you will create.

There are other cheaper solutions for battery isolators than Redarc etc.
Jaycar has a volstage sensor(switch) kit and you could drive a 100Amp
relais with it to combine the batteries when charging.

good luck
gmd

Michael,

The idea sounds solid to me, apart from two factors. The first is possible spikes during the changeover when the sense line will be momentarily disconnected. If your vehicle is recent enough to be full of sensitive electronics this may be an issue. The other is the battery voltage sensing - You propose to switch over once the voltage on the cranking battery reaches 13.7V. After switching to the auxilliary battery the voltage on the cranking battery will drop down to a point where the VSR will probably return the alternator to the cranking battery (unless you can devise some latching arrangement which would introduce its own issues). Voltage will then rapidly rise to 13.7 again, and the system will return to the aux battery, etc etc - the system will "hunt".

Suggest that you don't need to use such high current relays in any case. I use standard headlight changeover relays (30A ? 55A?) to switch the aux batteries in parallel with the cranking battery. I don't use any voltage sensing - The cranking battery terminal voltage rises almost instantly after starting anyway, so I switch to parallel when I have the combination of both ignition on and oil pressure present. With this combination the batteries are can only be connected together with the engine running. This has worked fine for the last decade or more.

HTH

John

Michael,

if you don't want to go the usually(commercial)-recommended-battery-isolator-way, you don't have to use any isolator - just use the right type of battery.

Spiral wound AGM 50Ah true deep cycle starter battery is what you want, and if you need more capacity, just double up by wiring another one of these in parallel. No solenoids, relays etc.

Now you have close to 100Ah to play with, before you run out of juice for cranking.
You could use a low voltage alarm, and/or a small solar panel which re-charges a flattened spiral wound AGM battery within minutes so that cranking becomes possible again.

A word of caution if using two or more batteries in parallel without an isolation switch:
#6 wire or better between the batteries is a must to prevent part of the high cranking current melting the PVC insulation.
Teflon or other high temp wire coating is definitely an advantage here.

Best regards, Peter

The first statement the original poster has said is:
"I'm hoping to build an independent battery isolator rather than buy a *very expensive* system"

Michael has most of the parts now to do it his way, so $$ cost to him is almost nill.
Maybe, just maybe the $120 for a solenoid (isolator) is classed as expencive?
If so, then some of the offered choices are well off the mark in the respect of price too :-)

Maîneÿ . . .

yes you can parallel those batteries for charging.

Im not so clear on what you mean by 5 pin relay.

However if you chose to use a basic on off relay, or a redarc for a few more $$ which activates at particular voltage readings then.

I would suggest that you put a switch in the dash on the earth line for the solenoid, so you can have manual off or on/ automatic.
That way, if you dont have the 2nd battery connected or its out of the vehicle you can turn that side off.

I have 3 batteries.
cranker n70
backup cranker n55 via a basic 100amp on off solenoid manually wired.
agm 100ah via a redarc which is wired for manual off if I choose which runs fridge etc.
Alternator to main battery, then good 8gauge wire to solenoid to agm, and cranking size wire to the 2nd cranker via the solenoid.

Works well for me.

Basic solenoid is around 30 bucks, redarc if you want to go that way is 125, wire 30 with lugs fitted from any battery place for cranker under bonnet, and whatever jaycar etc charge for 8 or 10 gauge wire to your chosen agm 3rd battery etc.

Trains

Switching the main Alternator output is very dangerous - if the Alternator is turning but not connected to either battery, it can result in instant destruction of the Alternator, depending on its design.

Hi all,

Here's a bit more detail. I have a 91 diesel hilux. I have been using an exide extreme (starter) and 100Ah Apollo AGM (auxiliary) separated/charged by a 100A GSL electronic isolator. Both batteries are almost 3 years old.

I have noticed over time that the auxiliary is not being charged fully, particularly if I have a load on the main battery (lights, radio, heater fan). I realise that this is most likely because I only have a 55amp alternator. I have investigated upgrading the alternator but this is difficult as there is a vacuum pump to deal with as well.

My thought was to build an independent charging system so that the auxiliary battery received more current and had more chance of being charged properly.

On the hilux there is only 1 wire that goes to the battery so my plan was to use a relay to control whether this wire gets connected to the starter (starter charging and car powered by starter) or to the auxiliary (auxiliary charging and car powered by auxiliary).

I have all the bits that I think I need and my main concern was the possibility of damage to the alternator when the relay switches from charging the starter to charging the auxiliary.

Cost is not really the issue as I already have a functioning electronic isolator, I'm just not convinced that my alternator is powerful enough for it to be the best system for my ute.

Thanks again...
Michael

Michael, The accepted standard management system for charging dual battery systems is to have the alternator permanently connected to the cranking battery and the auxiliary battery connected via a controlled solenoid. This has proven to be a reliable method for good reasons and apparently is what you currently have.

As Mike DiD has said, it is not a good idea to switch the alternator output.

Following a "normal" start, the charge is replaced in the cranking battery pretty promptly allowing the voltage-controlled solenoid to then send charge to the auxiliary battery. Reconnecting the alternator to the auxiliary battery will not charge it much quicker and if you change-over too soon you may finish up with an under-charged cranking battery with possible starting difficulties.

Even though your alternator is limited to 55 amps it should still be adequate to charge both batteries within reasonable time even allowing for "lights, radio, heater fan" provided that there is not too much additional load. Bear in mind that the alternator will only provide a specific energy charge to your batteries in a given time no matter how you connect them.

The regulator in your vehicle will be designed to charge a normal Flooded cranking battery whilst your auxiliary, being an Absorbed Glass Mat, requires a slightly higher voltage for complete charge. This is more than likely the reason for your auxiliary not charging completely and is not unusual. Without major system changes you are stuck with that. Connecting the alternator directly to the AGM without modifying its output voltage will gain you nothing I'm afraid.