# Battery Min Voltage to Start



## rhaegar77 (Aug 25, 2017)

After being stuck in the middle of nowhere bc of a dead battery due to me leaving the radio on (again)...decided to work on something that can prevent that. It'll cut circuits that are using voltage while the car is off so they're off before it drains the battery 

Does anyone know what the min battery voltage is for the car to still start??


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## PowerslavePA (Dec 9, 2014)

Depends what your battery reserve is.

11.9 will start a car, as long as there are amps behind it.

If you went passed your reserve limit, it may fail.

The battery should say, on top somewhere, how many minutes of
reserve power it has. This means, you can run accessories for that
many minutes before it fails to start the vehicle. 









This means, you have your accessories running for 160 minutes before it
will have a detrimental effect on starting. This is an AGM battery, and they
have a high reserve capacity.









This one tells you 110 minutes at 25 amps per hour reserve capacity (RC).









Good lead acid battery has a decent RC.

I would still not go passed HALF the time the RC says...

Some only have 60 to 90 minutes, and I have seen as low as 37 minutes.


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## cuppie (May 4, 2005)

Also, to add to all of that, it depends on the state of health of the battery (which is affected by total age, time in various charge % ranges, as well as charge/discharge cycles (measured in amp-hours in and out)). 
11.9v with a 85% SoH (a fairly new battery) is a far bit better than 12.0v at 25% SoH (old, aged battery.)

Also depends on that car's specific requirements to be startable - example: a MkVII is going to be a bit pickier than, say, a MkI. 

Instead of trying to build the near-impossible (yes, modern cars an do exactly what you're looking to do - but, there's a lot of engineering and software to do it), why not just get yourself one of those tiny little LiIon jump packs, and keep it in the car?


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## MarkeyseJMundy (Aug 3, 2016)

The real question is how is the radio staying on while youre not in the car?


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## PowerslavePA (Dec 9, 2014)

MarkeyseJMundy said:


> The real question is how is the radio staying on while youre not in the car?


Aftermarket, if that's what he has...

An OEM radio on older cars stay on, not sure when they turn off, or if they do.

All the newer ones turn off with the ignition, then you can turn it back on for a while
without the key in. CAN turns them off.


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## MarkeyseJMundy (Aug 3, 2016)

PowerslavePA said:


> Aftermarket, if that's what he has...
> 
> An OEM radio on older cars stay on, not sure when they turn off, or if they do.
> 
> ...


Well I know that, but if it's staying on, there is an issue. It should always time out after about 10 mins.


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## PowerslavePA (Dec 9, 2014)

#1, we don't know if it's aftermarket, no detail.

#2, no year make and model is specified either...

#3, hasn't said what he may have done beforehand...

#4, never said it USED to, and now doesn't...

#5, he said HE leaves it on, and wants to work on something to prevent that, with no other information; OEM, aftermarket, 
year of car, AND SO ON....

I mean, SO much information provided...

If it is OEM, and it used to turn off with the car, then the radio needs replaced.

IF it's an older one, that does not turn off with the car, well, you gotta' remember to
turn it off. SAME if aftermarket... Since we have NO CLUE, well, guess guess guess...


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