# 12v. car batteries ... resting voltage ? When to replace ?



## TooManyGizmos

What should the resting voltage of a 12v. car battery be , 2 hours after driving the car 8 miles ?

I think when bought new , the voltage is 12.70 volts.


What resting voltage would be a hint the batt. is about to fail , and should be replaced ?

Or can this only be determined by a cranking/amp test at a dealership or battery seller ?

I realize age and heat are a factor too , but some seem to last years longer than others. I'm also not sure the Brand name assures longer life either. On some batteries you can't even check the electrolite level or top it off.

Is it best just to replace every 4-5 years , no matter what , so you don't have a sudden no-crank failure ?
.


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## Mr Happy

This reference should give you some info: http://www.batteryuniversity.com/partone-13.htm

Basically you want the battery to read 12.6 V or higher after resting if it is to be in good condition and fully charged. As with most batteries I don't think you can get the whole story about condition without doing more elaborate load tests on it.

(If you have your car serviced regularly by a competent workshop they should automatically test your battery and tell you when it needs replacing. A replacement every 5 years would not be unusual.)


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## terrik_zion

A good battery should be between 12.4 to 12.6v at rest, it will drop some if any accesories are on (lights, radio, phone or gps chargers, flashers, etc) if you have an automotive voltage/amperage tester available, you can do a load test on the battery (hook it up and load it by 1/2 the cca rating of the battery for 15 seconds) if it drops below 10v, it's time for a new battery. Most autoparts stores can do this test for you, but beware, if they don't know what they are doing, they could test it as needing replacement when it does not. Also, the test must be done fully charged. Also, after testing it will be mostly dead and unable to start your car, requiring a recharge.

Source: me, automotive tech student, only need another few shop hours to be certified.

Good luck!


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## J_C

Yes it is best to replace every 4-5 years, your cost/year doesn't go up much trying to squeeze an extra year out of it, though if you drive the car nearly every day and it doesn't drop much below freezing in the winter where you live and the battery is large high-capacity per the vehicle (engine) size, it is possible to go a little longer but the first time it doesn't start you'll wish you just spent the money.

This is especially true when better batteries tend to have a pro-rated warranty longer than 4 years so you aren't losing much money at all replacing it, so long as they take it instead of telling you it's good still.


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## TooManyGizmos

Seems like very good and educated answers ..... thanks .

Could be helpful to others , too .
.


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## KiwiMark

J_C said:


> Yes it is best to replace every 4-5 years, your cost/year doesn't go up much trying to squeeze an extra year out of it, though if you drive the car nearly every day and it doesn't drop much below freezing in the winter where you live and the battery is large high-capacity per the vehicle (engine) size, it is possible to go a little longer but the first time it doesn't start you'll wish you just spent the money.



I have to agree totally with this idea - a new battery may cost you some money but the ability to start and use your car - priceless!


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## Lynx_Arc

I have found batteries last 3.5-7 years depending on how many times you forget to turn your lights off and kill the battery dead etc which can weaken it more. When your battery gets weak (assuming you have good electrical connections) your starter tends to struggle turning the motor over more especially in cold weather. The reason for doing a load test is sometimes you have a failing alternator or voltage regulator that isn't charging things well enough so voltage level isn't always the only factor in battery condition. You could also have a problem with something putting a constant drain on the battery large enough to kill it when not used for less than a week. A bad alternator diode or starter winding or some other electrical device malfunctioning (my dad had a mercury switch trunk light that came on when he parked on a hill killing the battery).


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## TooManyGizmos

snip..snip...
(my dad had a mercury switch trunk light that came on when he parked on a hill killing the battery). 



I luv that last part ....... thanks for the input .


Also some good answers in this other thread ... after mine.


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## fivemega

*Measuring car battery voltage doesn't give you much usefull information unless you do it under certain load which may varry from one to other battery.
It is important to know what kind of current (Amp) battery is capable to deliver within certain amount of time with minimum voltage of 9.6~10 volts which is enough to spin the engine fast enough in cold temperature and still enough energy for ignition system.
Draining the battery is not as harmfull as you leave it drained for long. So if your battery is drained for any reason, immediately recharge it.
Battery life really depends on many factors but 5 years is long enough to consider buying a new one specially in cold weather.
Stocking with dead battery may cost you more than price of battery.*


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## Wattnot

You're getting great info here but just pull into any chain auto parts store. They will test your entire electrical system with their fancy do-hicky tester and tell you if even one cell on your battery is weak. It's worth it and I have never heard about any dishonesty among them.


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## QtrHorse

As FM stated, the battery needs to be tested under a load. There are six 2.1V cells in a 12V vehicle battery. While the vehicle is running, it should be 13.5-14.4V and while off, should be 12-12.5V. 

Cold weather will affect your batteries cranking amps more than hot weather. For instance, my batteries as of today still show to be good when I check the voltage but I know it's time to change them because we have finally started seeing some cold weather and my engine cranks over slower than when it was warmer.


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## dcracing

QtrHorse said:


> As FM stated, the battery needs to be tested under a load. There are six 2.1V cells in a 12V vehicle battery. While the vehicle is running, it should be 13.5-14.4V and while off, should be 12-12.5V. Cold weather will affect your batteries cranking amps more than hot weather. For instance, my batteries as of today still show to be good when I check the voltage but I know it's time to change them because we have finally started seeing some cold weather and my engine cranks over slower than when it was warmer.



Just pondering here:

I have a Walmart Marine/DeepCycle with a consistent resting voltage of 12.4v 
(12.4. resting voltage at purchase date also)

Is anyone aware of Marine/DeepCycle battery manufacturers with higher "tweaked/accidental" resting voltages near at least 13volts? All of my RV appliances would be better off at night time.

[current set up: six 12volt batteries in parallel with 1000watt solar roof panels/charger controller 3000w pure sine inverter]

Thanks in advance.

dc


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## J_C

dcracing said:


> Just pondering here:
> 
> I have a Walmart Marine/DeepCycle with a consistent resting voltage of 12.4v
> (12.4. resting voltage at purchase date also)
> 
> Is anyone aware of Marine/DeepCycle battery manufacturers with higher "tweaked/accidental" resting voltages near at least 13volts? All of my RV appliances would be better off at night time.
> 
> [current set up: six 12volt batteries in parallel with 1000watt solar roof panels/charger controller 3000w pure sine inverter]
> 
> Thanks in advance.
> 
> dc



Whether they are designed to run directly from a 12V system or you're powering them via the inverter, gaining 0.6V isn't enough to make a fuss about and it could be a problem because charge controllers aren't set up for the higher voltage, so they'd likely switch to trickle too soon or shut off.

If you find your batteries draining too quickly the solution is more batteries in parallel, or another solar panel if the present one isn't topping off the batteries.


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## inetdog

Putting batteries in parallel should be avoided as much as possible, since it is difficult to get the charge and discharge currents to split equally among them.
When possible you should use lower voltage higher capacity batteries instead.
In your case, three strings of two six volt batteries each or even better one string of six two volt batteries would be far more reliable.
If you must go with parallel, you would need a bus bar system and carefully cut and assembled connecting wires to keep the resistances in each battery circuit matched.


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## smokinbasser

Quite a few people think that only cold temps can kill a lead acid battery. Both of our cars batteries died smack dab in the middle of a Texas summer the same day. What are the odds? The tech they sent out said it's not unheard of.


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## J_C

inetdog said:


> Putting batteries in parallel should be avoided as much as possible, since it is difficult to get the charge and discharge currents to split equally among them.
> When possible you should use lower voltage higher capacity batteries instead.
> In your case, three strings of two six volt batteries each



There's no advantage to three strings of two six volt batteries over three strings of double the capacity 12V batteries. Generally it's the opposite, that high capacity 6V batteries are rarer and more costly as a result.



> or even better one string of six two volt batteries would be far more reliable.



No. There is no reliability increase by merely splitting up individual cells into more plastic containers. It just decreases power density (per volume) and increases cost and wiring complexity.



> If you must go with parallel, you would need a bus bar system and carefully cut and assembled connecting wires to keep the resistances in each battery circuit matched.



For matching wire resistance, positive should be taken from one end of the parallel array and negative from the other end. Otherwise, beefy enough wires should be used for their own benefit of reducing losses, which would also mitigate differences in drain rate.

You do not necessarily need to make the charge and discharge currents equal in a parallel array of cells. For example if you had a 10Ah battery in parallel with a 100Ah battery, the 100Ah would be supplying more current but this is fine. As its voltage drops equal to the 10Ah then the 10Ah contributes its share, with them both maintaining an equal voltage and both fully discharging, and equally, both charging at different currents because they should do so, until full charge voltage is reached.

A larger problem is faced when you instead have a series of batteries and one becomes weak, at which point current flowing through it when drained too far begins to destroy it when it would have kept working if it were in parallel instead, only discharging as low as the other batteries in parallel and only charging as high since being parallel makes them stay at close enough to exactly the same voltage.

The main thing to watch out for when connecting batteries (of same chemistry) together in parallel, for the first time, is that no battery has substantially higher charge voltage than the rest, especially higher than another of significantly lower capacity so it doesn't dump a lot of current into the weaker cell immediately upon connection.


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## Biggoggs

I'd replace it when it begins to struggle to start the car on a cold morning. Discharging it too often (ie leaving the headlights on overnight, or not driving the vehicle for weeks) kills lead-acid batteries earlier, if you keep them mostly-charged they usually last me around 10 years.

Resting voltage isn't a particularly good health indicator either because old batteries can hold a good voltage but sag under any sort of load. I'd get it checked at a mechanic or an auto electrician, they have expensive testers that prints out a pretty graph with colours.

http://batteryuniversity.com/learn/article/testing_deep_cycle_lead_acid_batteries/2


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## mattheww50

You can design lead acid cells to either be very tolerant of overcharing (car batteries) or very tolerant of deep discharge (deep cycle batteries), but not both in the same product. Consequently deepd discharging of a car batteries will substantially reduce life. Life is a function of where you live to a certain extent. While heat does eventually kill batteries, the problem is usually more about cold weather. As the battery ages both its capacity falls and internal resistance rises (which also happens with decreasing temperature). At the same time reducing temperature increases the power required to turn over the engine. Cold weather=more viscous libricants, and simultaneously reduces the peak output available from the battery. Having lived in both the upper midwest and the desert Southwest, I can assure you that very cold weather is much more likely to cause trouble than very hot weather. In cold climates, I have always simply replaced batteries after 5 years. That greatly reduces the risk of starting issues during the winter. I have kept batteries as long as 7 years in the southwest. The warmer weather reduces the energy required to turn over the engine. The higher temperature also tend to reduce the internal resistance of the battery. In warm climates just make sure you keep the cells topped up with distilled water for maximum life.

Technically a lead acid cell should be 2.2 volts, however very few automotive batteries today are truly lead acid cells . There have been a number of 'tweaks' to the chemistry over the past several decades, to improve the life and cold weather performance, and this has slightly reduced the cell voltage.


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