View Full Version : Voltage Question

03-06-2008, 11:51 AM

running a single standard battery (no $$ atm for dual setup) and am thinking of installing a voltage gauge in the car to monitor the battery. Basically when I camp, and am using the car for something, want an idea as to how much I am draining the battery, so I know when to turn the car off keeping enough voltage to start it...

Is this a viable idea? What voltage can the main battery drop to before I will have an issue? Also, is one of the cheapr $10 things on ebay enough to do the job?



03-06-2008, 12:29 PM
Hi njclifford

Assuming you have one, I find a multimeter with aligator clips connected to the terminals is good enough (ie: works and is free). Anything else costs $ and takes time to install.

As for permissable lowest voltage (before battery cannot crank the motor) - really depends but the gurus out there say around 11.9 volts minimum (depends on heaps of things like climate, diesel vs petrol and whether measured open circuit or not etc)

03-06-2008, 12:38 PM

The answer to your question is dependant on many factors but a good guide to a voltage level that should always allow you to start the vehicle is the "kick out" voltage that Redarc use on the their dual battery isolators, which is 12.5V. In fact I would suggest that if you are intending to eventually install a dual battery system, then for now just install a voltage sensing isolator (Redarc or similar) and power your accessories from the auxillary side without installing a second battery. This will automaticlaly protect your cranking battery from discharge, cost less than a full dual bettery set up and allow you to reuse all the items once you get a second battery.


03-06-2008, 12:41 PM
From what I have read the RedArc seems the favoured one on PP.

I didn't think of that ..... sounds like a much better idea.... then whatever I am using would cut out and save the battery for cranking.



03-06-2008, 01:14 PM
The redarc is designed to isolate batteries and will vertually cut out once the engine is stopped, it will not take much power drain to get it to isolate. The drop out voltage is much to high.

What you need is a low voltage cut out relay (similar to the redarc but a lower voltage, I have seen them recently (ebay I think) or you could give this guy http://www.sidewinder.com.au/index.html a call.


03-06-2008, 03:26 PM
Its not only the voltage left in the battery, as its also depending on the cold cranking amp rate of the battery.
If the battery is a 590 cca as opposed to a 700 cca, the battey will not have the ability to turn over the engine fast enough, and allow the battery to go flatter faster. You may think that 12.5 volts is really fully charged but it is very close to being flat enough to not start your car. A fully charged battery is about 12.7 volts and 12 volts is dead flat. The bigger your cca, the better chance you have of turning the engine fast enough with the least amount of volts.
Thats the way I understand it anyhow :wink:

Here is some great reading on 12 volts.

03-06-2008, 04:30 PM
...and take some jumper leads just in case you get it a bit wrong...

03-06-2008, 04:41 PM
Basically when I camp, and am using the car for something, want an idea as to how much I am draining the battery, so I know when to turn the car off keeping enough voltage to start it...

If it's just for this purpose, how about a cig lighter socket voltage guard that cuts out when the voltage gets too low? Supercheap has them for $5 at the moment (75% discount). I think they only handle 10 Amps, so that might be a problem?

I don't have a clue, but thought I'd throw it in here to see what all the sparky-types reckon.


03-06-2008, 06:18 PM
I don't have a clue, but thought I'd throw it in here to see what all the sparky-types reckon.


Shocking suggestion Fishnut...


03-06-2008, 06:36 PM
I think fishnut has the right idea for the right price for you. If you are using something that uses more than 10 amps for long you would have been pushing the car by now.

03-06-2008, 08:25 PM
More good stuff for the purposes of confusion , whether or not a battery has the ability to start an engine is a problem of dynamics ie temp , load ,time this can only be tested accurately by a load test however in most cases a static voltage (no load) of around 12 is about the lowest tolerated for normal flooded cell car batteries :
The open circuit voltage,is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:

Open Circuit Voltage (12V) Approximate charge Relative acid density
12.65 V 100% 1.265 g/cm3
12.45 V 75% 1.225 g/cm3
12.24 V 50% 1.190 g/cm3
12.06 V 25% 1.155 g/cm3
11.89 V 0% 1.120 g/cm3

Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.

The following is common for a six-cell automotive lead-acid battery at room temperature:

Quiescent (open-circuit) voltage at full charge: 12.6 V
Unloading-end: 11.8 V
Charge with 13.2-14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.

03-06-2008, 09:27 PM
Hey NJ. This is my story. When I first bought our Prado at 18 months old we had a 40L engel on about the same age. Took it out for a night as you do for a bit of fun the first weekend we had it and it wouldn't start the next day after one night on the beach. Mine is diesel which are harder to start cold than petrols but even so. Just thought I'd give real world story. Personally I'd turn the fridge off at night and only ever do a one night stop at a time.
Leachys tip is also a good way of controlling when your fridge will work but I do agree that it will cut out a little early. I quite often hear mine click out after a drive home just as I'm walking in the door from the carport so only like 45seconds after we pull up.
Good luck.

jeff s
08-06-2008, 11:46 PM
And without using a hydrometer, a volt meter would be close to useless. I'm happy to be corrected on this.
But, not more than 10 hours ago, our diesel would not start. the main had 12.4 on the voltmeter, would turn over but not fast enough. not flat but damn near useless except for listening to music.
A new battery from auto pro was purchased Century 660cca? $190.
Im also up for a new second battery, (dont know whether to go deep or not)

when the motor is running the volts go up to 14.00 Maybe more but thats where I left it.


09-06-2008, 09:51 PM
Jeff S , its the on load voltage that's the critical issue , you had a classic case of a high resistance cell in the battery any decent battery seller can do a load test for you .
I once installed a 700 AH 48 volt battery (72 batteries x 2v in series and parallel) in a solar install that failed a standard load test (25% load for 15 mins ) .......just one faulty cell in one battery had too pull half the batteries out to find it .
Car batteries are reasonably cheap and most are only good for 3 years or so the above batteries are about 5 times more expensive and are good for about 6-7 years .
Some brands are better than others and if you can find an imported Yuasa you will find that they give exceptional life