Ok firstly lets start off with why do you want dual batteries?
The main reason people add a second (or 3rd or 4th...) battery to their vehicle is to run auxillary accesories whilst the vehicle isnt running. Accessories such as fridges, camp lighting & inverters for 240V assessories such as battery chargers (or in Garbages case an expresso machine
). The problem with relying on one battery (your main / starting battery) is that these accessories drain your battery (sometimes quite quickly) to a level that will not allow you to start your vehicle - Uh OH. Big no no - especially on modern cars with sensitive electrics or if your in the middles or no where! So add a second battery and make your battery power last longer! - seems to work on paper but its not quite that simple... Or is it?
Well you can just add a second battery in complete parrallel (ie positive to positive & neg to negitive) to your exisiting battery this will work and will add a combined total aH to your electrical system. But still you run the risk of killing two batteries and still being stuck. So you need to work out a way to connect the two batteries so they charge but seperate them when the car is off so the main battery remains charged and your auxillary accessories drain the auxillary battery.
Basically their are 3 choices
Heavy duty Diodes (or one way "electricity" valves)
Heavy duty Manual switches
Automatic Heavy duty Switches
"Smart" Automatic heavy duty switches
"Smart" charge regulators
Pros and cons
Heavy Duty Diode
Pros - Relatively inexpencive, after they are installed the are pretty fool proof
Cons - Voltage drop between batteries, pain to install, old technology and do have some "log term" reliability issues
Heavy Duty Manual switches
Pros - Extreemly cheap, easy to install
Cons - These are idiot switches cause one day some idiot will forget to switch it - wether thats switch it on to charge or switch it off to isolate and
you'll be stuck with eiher one or two flat batteries
Automatic Heavy duty switches (Solinoid / similar to a relay)
Pros - Good fail safe, joins the batteries when the key is turned on and disconnects them when the key is turned off
Cons - a little expencive, generates heat, some cheaper models suffer longevity issues, voltage drop when solinoid is "on" can play havoc on modern cars
"Smart" Automatic heavy duty switches
Pros - Good fail safe, joins the batteries when the main battery is charged and disconnects them when the voltage in both batteries starts to drop off once stopped.
Cons - Expencive, generates heat, some cheaper models suffer longevity issues, Some cheaper versions can "spike" the electrical system
"Smart" charge regulators
Pros - proven technology, good charge to both batteries, both batteries charge straight away (no "cross pollination"), no risk of "spiking", some models can connect straight into solar systems
Cons - extreemly expencive, reduced rate of charge, cheaper models inefficient
Ok so we have chosen an Isolator... (I choose the RedArc SBi12, which is a "Smart" Automatic heavy duty switches)
Now we need to choose a battery...
Firstly space. How much room do we have L H W. where do we want to put the battery - in the boot? under the bonnet? permenent? removeable?
Next how much reserve current (RC) or Ampere Hours (Amp hours / Ah) do we require.
This one catches many out.
mathmatically it looks like this P = I x V
P = Power, measured in WATTS
V = Voltage, measured in VOLTS
I = Current, Measured in AMPS
Ok so we have a 13watt fluresent light. We know that our batteries voltage will be 12V nominally. SO
P = 13
V = 12
I = ?
To get I we need to Divide P by V
So 13 / 12 = 1.1
Therefore I = 1.1
So our light draws 1.1 Amperes for ever hour it is on (or 1.1Ah)
Now we need to calculate how long we will have this light on lets say 6 hours.
So 6 x 1.1 = 6.6 now lets factor in a bugger factor of 10% so we are up around 7.3Ah of Reserve charge we require.
But now we have to remember that P = V x I so we need to keep the voltage up around our nominal 12V other wise our little 13watt light is going to start
drawing more current the lower our voltage gets!
Now no battery likes to be discharged more than 80% but we are a little harsh on our batteries plus we are using deep cycle batteries so we'll be willing to take it down to 50% so we need 7.3 x 2 = 14.6Ah of RC per night per light! MINIMUM!
Or how about a fridge. My Waeco CF-80 draws 7.5Ah whilst on cycle (I run it on freezer mode). (I havent measured it off cycle yet as my DMM died and I havent replaced it). But if you've ever taken any notice of your fridge at home you know that your fridge isnt "on cycle" with the motor running all the time. So lets work to a fairly broad generalisation that it is on cycle for 50% of the time.
So 7.5Ah x 50% = 3.75Ah
3.75Ah x 24hours = 90Ah x 2 = 180Ah! Minimum! Ouch! (these are theoretical factors I've found in real life that the actual power consumption of my Waeco CF-80 is more like 60 Amperes over a 24H period - better to have too mucht than too little though IMO)
Now this is my "real world" experiances with a Waeco CF-80. Remembering that I run my "fridge" in Freezer mode. There are ways to increase its efficiency. Things like freezing bottles of water (like you would in an esky) to keep the fridge from running as much, Insulation "jackets" or covers and A lot of people run their fridges on "full" whilst driving and turn them down lower when at camp. Also in cooler areas a lot of people turn their fridge off at night.
Personally I feel that if you've just dropped $1300 on a fridge and probably another $300 on a fridge slide to make your camping trip more relaxed and enjoyable then why would you want to stress about making sure you turned off your fridge or turned it lower etc. Just get a good sized battery and be done with it!
Ok how to set it up...
Firstly we have chosen our isolator, in my case a Redarc.
Now park the car (and switch it off) somewhere where you have clear access all around and you can open all the doors and walk around - this makes life a LOT easier.
Next disconnect the negitive terminal from the battery - this is VERY important. Next we need to run a cable, i like to use 8 b&s (or 8ga) or larger cable (smaller then B & S the larger the cross sectional area and the current handelling ability of the cable), from the possitive to a fuse or circuit breaker (I like to use manually resettable circuit breakers) You should aim to make this run as short as possible - prefferably 40cm (15 inches) or shorter. Then we need to run some cable from the fuse / circuit breaker to the isolator - again try to keep this as close to the battery as possible.
Then from the isolator (which you should mount according to the instructions that came with the isolator to the body of the car some how) you need to run cable to the location of your second battery (in the boot - or if you have enough room under the bonnet) again this should cable should be fused within 40cm (15 inches) from the second battery.
Then you need to securely mount the second battery and ground the negitive of the second battery to the cars body.
And thats it your all done.
Now all thats left to do is to wire in some 12V sockets (cigarette lighter sockets) for your accessories to plug into and your done.
Alright Now you have a basic dual battery setup - congratulations. But does it do exactly what you want?
Do you want to be able to remove that second battery for when you arent away and need the extra boot space?
The easiest way of making a seond battery removable is to use anderson plugs
What about solar charging?
Solar charging is a great way of extending the period of time you get out of your second battery. You will need a panel of 50watts or more - anything smaller than this in my oppinion os not worth spending your money on. That said even 50 - 100watts will only extend the amount of time your get out of your batteries - it wont fully charge the batteries as solar isnt super efficient. You will also need a solar regulator. You should connect the panel via an anderson plug to the second battery.
What about keeping batteries in optimal condition?
To keep a battery in optimal condition you need to keep it 100% charged all the time - simply not practical. However you can keep a battery in very good
condition by connecting it to a mains charger when you get back from a trip away - and possibly for a couple of hours every week or so. I preffer to use a
pulsaing charger - but there are many theories regarding chargers and what works and what doesnt. how ever make sure you do get a 3 stage maintenance charger
to avoid over charging.
What type of battery do I use as my second battery?
Deep cycle or marine - again this is an area of great discussion between keyboard experts. I personally choose deep cycle but i do understand the arguement
for marine batteries and I may possibly try marine batteries at a later stage. What ever battery type you choose I suggest going with a sealed type -
prefferably AGM type. these are spill proof and dont let off toxic fumes into your cabin space like a lead acid battery does.
If my main battery goes flat but my second battery still has plenty of charge, can i use my second battery?
This will depend on your isolator of choice. Most solinoid type isolators can be setup with a momentary switch to allow you to "jump start" your car off your
second battery. Refer to the instructions that came with your isolator.
Please use this article as a guide only and read to the instructions that come with your isolator.
Fuses are used to protect your car from fire - make sure you use them! They will not blow unless something is wrong. If you do blow a fuse check for any
faults before replacing the fuse / resetting the circuit breaker.
If you think that any of this type of work is beyond your capability please get a help from an auto electrician or someone who has experiance with 12V electrics.
Any questions PLEASE feel free to ask!
Â© 2007 - Kirk Liam Matthews