Thread: Simplest Dual Battery Setup

Originally Posted by SteveFarmer

Hi

The issue with simply connecting a second battery is that depending upon where the second battery is located, the voltage drop my be too great for the second battery to be charged. This is when you need a DC battery charger.

The setup needs to be different depending upon whether you want a second battery which can start the car, or simply as an auxiliary battery.

As far as electronic components failing, most of our vehicles are full of electronics, so that doesn't concern me in the slightest, provided they are properly specified and installed.

Steve

It doesn't need to be different at all. The system that Bushie and I have used for so long works as both auxilliary and starter. Switch between battery 1 or 2 as and when required. ie either battery does the same thing.

My last 2 batteries lived for 5 + years.

Hi Numpty,

I should have said "Might need to be different"

E.G: If you want to start from the aux battery, the setup is simple. If you want an aux battery isolated from the main battery and it is not used to start, then you might need a slightly more complicated setup depending on the location of the aux battery etc

I am going to use an AGM deep cycle battery installed in my spare tyre area, so I need to add a couple of gizmos to make it all work the way I want it too.

The simplest setup for sure is a second battery of a matching type mounted under the bonnet with a manual isolation switch

Regards
Steve

And you thought you were confused before!!

It's clear that there heaps of different opinions on this. Have a look at Traxide's site, (Drivesafe on this forum), he sells a dual battery isolator which is very simple to install and use and works effectively.
Want one more opinion? here's mine: The whole idea of a dual-battery installation is to protect the starting battery form going flat when running a fridge and lighting etc whilst camping, to avoid being stranded. The size of the second battery will determine how long you can run your fridge, e.g. camping 2-3 days, and the quality of the dual-battery controller will determine how well protected your main battery is and how well both batteries get charged when the engine is running. Ideally a contoller should favour the starting battery, so that it gets charged first when you start the engine, then switch over to recharge the aux battery when the main reaches full charge. A problem with simple solenoid systems is, according to some sources, that the battery with the fullest charge will cause the alternator to 'think' that charging is complete, so the flatter battery will not get charged (this idea may or may not be true, just repeating what some sources say). Another problem is that 'deep-cycle' batteries can take several hours driving to get properly charged after running your fridge overnight, so on a typical touring trip it may never get properly charged.
I think the best way to go is a simple, well-sorted controller like the Traxide SC-80, plus a folding solar panel that can be clipped to the aux battery during camping stays to maintain charge.
Winches should be run from the main battery so that you are not drawing big currents through a dual-battery controller.

I support and re-inforce POD's comment . . . . . "so the flatter battery will not get charged " . . . I won't go into the technical details of why but it is technical fact that if 2 batteries are connected directly pos to pos and neg to neg (typically via a simple solenoid) the lesser charged battery will get less charging current than the charged battery. FYI - a 12v to 12v charger will fully charge the secondary battery using the primary battery as a source. I (and others) use a "Twin Charge" to perform this function - happily. I'm sure there must be other brands, but I know this one works.

Hi folks and before I start, yes I design and sell dual battery controllers and full systems.

But this thinking that something as simple as an ignition operated relay is the cheapest way to go is anything but correct.

What is correct is that it is initially the cheapest to set up, but long term, you can save heaps more by using a system that improves your batteries life span.

If you fit a system that can, in many cases, do as much as double your cranking battery’s life span while helping to extend you auxiliary/house battery life span and add usable capacity without adding more batteries, you well and truly save far more money long term.

This is exactly what my isolators do.

Simply turning a relay on and off when the ignition is turn on and off is cheap but is not very beneficial to your batteries.

Furthermore, charging your cranking battery first then charging your auxiliary battery is a waste of auxiliary battery charging time and no standard isolator does that, even though a number of isolator sellers claim they do ( just deliberately misleading advertising ).

Many years back, when vehicles came with nothing more than a 45 amp alternator, then there MAY have be a reason for such operations but back then we used very little auxiliary power anyway so, even back then there was no real advantage.

Today, with the smallest alternators being around 80 amps, there is no reason why you can not charge your cranking battery and a number of auxiliary/house batteries at the same time.

Next, hi d5disco and you are getting mixed up with how batteries wired in series

Neg ( - ) of 6v battery 1 wired to earth.

Pos ( + ) of 6v battery 1 connected to neg ( - ) of 6v battery 2.

Pos ( + ) of 6v battery 2 is your 12v supply.

If one 6v battery is lower than the other 6v battery, then when the higher charged battery is full, charging of the lower charged battery stops.

With 12v batteries wired in parallel.

Neg ( - ) of 12v battery 1 wired to neg ( - ) of 12v battery 2

Pos ( + ) of 12v battery 1 connected to pos ( + ) of 12v battery 2.

All batteries will be fully charged if you drive long enough.

Furthermore, alternators do not stop charging a low battery if a higher charged battery reaches a fully charged state. An alternator has no idea when a battery is fully charged.

D3s, D4s and RRS have a BMS ( Battery Monitoring System ) that monitors the State of Charge ( SoC ) of the cranking battery just after you start the motor and then sets up a charging algorithm to suit the SoC of the cranking battery.

With the use of any other isolator, you will slow down the of the auxiliary battery, but it will still charge, just at a slower rate.

But with my isolators, because they draw accessories power from both the auxiliary battery and the cranking battery, when the motor is started, the BMS sees the cranking battery needs a charge and sets the charging algorithm accordingly.

This results in both the cranking battery and the auxiliary battery being charged at a higher voltage for a longer time.

There are a number of other features that add to your financial gain when you use one of my system, so while you may save up front with a cheap set up, you loose long term over what my systems can save you.

Couldn't help myself - had to jump in on this one... I run a second battery in the rear of my td5 ( where the dickie seat would be) - its very simple to work out the best set up for you,based on what you want to use the power for.

Winch: always the starter battery
Fridge: Always the second battery
Isolator-I've got a solenoid one with a manual override switch) between the two or the fridge will drain both.

Look at power consumption on the fridge. If its 4 amps an hour - and you've got a 70 amp hour battery - it will only last a couple of days - and 4 hours driving plus to charge it...(at 2000 rpm or more).

So I run an 80watt solar to keep the rear battery charged.

If I'm running the winch I switch on the isolator manual over-ride to try drain the batteries together as much as possible.

If either battery falls to low low low (under 11) voltage levels while the other is sitting up high at full charge - you are going to have problems. ( especially if the batteries are different capacities/cold cranking amps). If possible - run similar .

I do a lot of winching as I have a large boat and front mounted trailer hitch. We use the winch 3-4 times a week, and the fridge hasn't been switched off since 2004.
I'm on my second rear battery and third cranking battery in ten years...they have all been cheap - but corregatted roads destroy all but the best... So if you do a lot of high speed on rough - pay up - they are worth it.
Cheers - mark

Originally Posted by sillsymark

If either battery falls to low low low (under 11) voltage levels while the other is sitting up high at full charge - you are going to have problems. ( especially if the batteries are different capacities/cold cranking amps). If possible - run similar .

Hi Mark and this is actually nothing more than an urban myth.

Any 12v lead acid battery can be wired in parallel with any other 12v lead acid battery and you will not have problems if they are dissimilar.

Great to know - and thanks!!!! I'm really paranoid about huge amps belting though the wiring in between the two batteries. (5m apart in my rig).

Love to pick your brains!!!!

Cheers - mark