car battery ???

Discussion in 'General Electronics Chat' started by Mathematics!, Feb 23, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok, I was measuring my car battery voltage and got 12.48 DC volts.
    This is when the car is off which is perfectly good value.

    But now I want to measure current ? Is their any indirect way of getting the current for this battery because my multimeter goes up to a max of 10 Amps dial. So I don't think I could use it without blowing a fuse or injure ...etc.

    Also is their anyway to find out how long the battery life is.
    If I get the amp-hours how do I know how many amps it consumes in one hour.

    Or maybe they measure car battery life differently then amp-hours.

    I don't see much on the battery for calculations maybe it is on the bottom?

    Any help would be great.
    I know when I get my electric bill I can calculate the precise amount by the kilwatt-hours. So if I left a 60 watt light bulb on for 1 hour I would have

    0.060 kilwatts * (whatever the price of one kilwatt-hour is)

    I am just trying to find out a way to calculate car battery life.
    It would make since to me that the battery would live for a very long time since it is being recharged every time the car is started. But this is not always the case. What makes the battery eventually waste away.
    Is this not an almost perfect reverseable reaction when recharging it?

    And is their a standard current for car batteries I don't know about or can I just create a 12 volt car battery by putting batteries in series or parrell to get the desired voltage/current?
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    If I understand you, you are way off tracks.
    That car battery capability in ampere-hours has to be available at the manufacturer's web site, You can only measure the consumption amperes. Turn the radio on and nothing else, and your 10A multimeter in series with one battery lead should read the current circulating.

    And its life is not measures in ampere-hours; but years; will vary greatly with use and abuse.
  3. SgtWookie


    Jul 17, 2007
    It depends upon what the temperature of the battery core is. If it's near 77°F/25°C, your battery voltage is low, and the plates are collecting lead sulphate, which will rapidly kill your battery; after a relatively short period of time, it will no longer be able to accept or release a charge.

    For the longest life, keep your battery charged to 12.6v to 13.56v when the core temperature is around 77°F/25°C.

    Lead-acid batteries have a negative temperature coefficient of -3mV per cell per °C; when the core temperature is not 25°C, you have to adjust the charge and float voltages. Since a 12v lead/acid battery has six cells, it's -18mV per °C. The proper float voltage at 25°C is about 13.5V, depending upon the battery chemistry. Consult the battery manufacturer for details.
    Here's a link to some documentation that may help:

    I=E/R, or Current = Voltage / Resistance. Even a straight piece of heavy-gauge wire has resistance. If you can measure the voltage drop across the heavy-gauge wire (say, the ground wire from the battery to the engine block) you can calculate the current flow. If you know the gauge and length of the wire, and the voltage drop across it, you can calculate the current.

    Service life, or for how long it will provide a certain amount of current?

    Batteries don't normally "consume" current. They are sort of like large electrical sponges; they can absorb current and later release it.

    OK. Lead-acid batteries will last a great deal longer if the core temperature is kept relatively low than if it is kept relatively high. This is due to the decrease in chemical activity within the battery when it is at a lower temperature.

    This is one of the reasons why charging the battery at a high current (ie: jump starts) is a bad idea; it raises the core temperature of the battery. Once a battery internal temperature is increased this way, it takes a LONG time to come back down. In the meantime, chemical activity is dramatically increased.

    Keeping your battery charged (but NOT over-charged) and the cells properly filled is your best bet for battery longevity. Keep an eye on the specific gravity of the individual cells; that will tell you quite a bit about your battery's health. If all of the cells have a low specific gravity, then the battery is either heavily sulphated or discharged. If one or a few cells have a low specific gravity, those individual cells may be sulphated, may be shorting out due to sediments, or the plates may be warped (thus shorting) due to overcharging or from severe loads.

    Nope! There is nothing on the bottom.

    My Grandpa could always get a car battery to last for more than 8 years. He would check the batteries in the cars constantly, and would keep them charged, but not overcharged. He also lived in upstate New York, USA, where the temperatures are relatively cool much of the year.

    Here in Florida, I'm doing good if I can get five years out of a battery.

    Our local National Vietnam War Museum has a number of retired military vehicles; they all require lead-acid batteries. Some of the batteries were badly abused (ignition switches/lights left on) which killed them in less than a year.

    Automotive batteries are designed to provide maximum current for a short period of time, and then to be fully recharged. Short trips with lots of accessories turned on without ensuring that the battery is fully charged will quickly kill a car battery.
  4. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    But I don't get this
    Where does the "2" probe's go?
    And is this safe to measure current in this way.
    The multimeter has a max rating of 10Amp dial on it.

    Yes , but I cann't find out what the gage is and I don't know the length?
    Is their a rough estimate?
    I looked online and it said car's batteries use around 30 to 40 Amps?

    Peukert's Formula
    T = C / In

    But how do I find C and N for my car once I have the current I?
  5. SgtWookie


    Jul 17, 2007
    Measure the length. Measure the gauge. Then go here:
    Their might be a current draw during starting the vehicle from anywhere between 100A and 400A.
  6. italo

    New Member

    Nov 20, 2005
    A car battery has one purpose only and there is the turn over the engine until it starts. The bigger the engine the more demand on the battery to start the engine.Car battery will completelly drain in one month or even days depends on the health of the battery. A battery floater will keep a car battery alive longer if installed. And your 10 scale on your meter probably means that the shunt iside carries the amps not the meter itself. Analog meters usualy operate on microamperes a shunt will bypass the current so only microamperes flow through the meter.
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    When the car is running the current from the battery is often negative as it it being charged by the engine.

    The reactions are not perfectly reversable, this is the reason for finite rechargable battery life.
  8. Mike33

    AAC Fanatic!

    Feb 4, 2005
    Hi dude,

    I think I might know what you're getting at. You want to know if your battery is still "good", right? There is a machine you can ask the people at AutoZone, VIP, or other auto parts places to put your battery on to test it's ability to deliver current, or 'ampacity'. Ask them if they have one, and then you'd take it out and bring it in the store for them to connect it to. If they're nice, they might do it for you out in the parking lot ;)

    As said above, the job of the battery is to deliver a lot of amps to the starter motor to crank the engine over. All else is just accessories with very low draw in comparison - you can see 12+V on it and still have it going bad on you. So, to know how "good" it is, you have to stress test it by drawing that current and seeing how long it can do it for (maybe 20A!). To make the test circuit at home would require a lot of power resistors to make a load and would be a pain unless you're going in the battery testing biz.

    Google for 'multimeter use' or such to get a better handle on how to measure current - seems you've got voltage down ok. Current is measured in series with the circuit that draws it, and yes, you will fry something if you do this when starting the car! Devices DRAW current from the power source, the source doesn't push it out but must simply have to give it in 'reserve' to allow the device to take it. How much current will flow is a function of the device's physical construction.

    Put a piece of wire across a car battery? Look at Ohm's Law, the key to all electrical understanding. Resistance is about zero, so current=voltage/resistance gives you 12v/zero or extremely small resistance = current near infinity = melted wire, damaged battery! Put a resistance of say 100 ohms across the same battery, current = .012A, 120mA, a manageable number.

    Hope this helps!
  9. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok just got a few last minute questions.

    Is their any way to turn the car engine over with out a battery?
    Like say your battery dies and you are in the middle of nowhere is their anyway to bypass the starter system...etc.

    Or at least find a mechanical means to turn over the engine. Weather it be a magnet .....

    Yes, but when they stress test it by drawing that current and seeing how long it can do it for (maybe 20A!). Won't that take away from the life time of the battery as well? Assuming yes,is this neglectable.

    Question 1
    I have a 2000 alero for a car I am wondering if their is a specific car battery for every model of a car or if you can put any car battery in any car. So I can just go to any place and get a cheep car battery that I can use. Just wondering how universal car batteries are for specific car's.

    Do they all use roughly around the same voltage and current that would probably be the only incombatiblity ? UMH maybe not ?
    Judging trucks would probably need stronger batteries.
    Either way their must be some standard? Because I don't see stores having every made car battey. Say you had a very old car?

    Question 2
    After the car is started and the engine is running would the car still be driveable if you took out the battery. I know the radio , wipers , lighting, air condition ,...etc would go but could you still drive it down the road?

    Question 3

    Is their anyway using a multimeter to check if a battery is total not rechargeable any more?
    And is their anyway to check a battery to see if it will start a car before you install it in the car?
    Like say I read something like 11 volts is the 12 volts thelower bound on what will turn the car over?
    Last edited: Feb 26, 2009
  10. SgtWookie


    Jul 17, 2007
    Vehicles with manual transmissions can be started by rolling down an incline and "popping" the clutch in 2nd or 3rd gear. Doesn't work with automatic transmissions. Very early autos had hand-cranked engines; this worked for low-compression 4-cylinder engines, but wouldn't work for modern engines. Even back then, plenty of people wound up with broken hands and arms if the timing wasn't set correctly.

    Many WWII aircraft used a cartridge start system. A blank cartridge resembling a shotgun shell would be fired, pressurizing a cylinder, causing the engine to rotate several revolutions. If you had the engine primed properly, spark on, timing set, etc. - it would start. Even some USAF F-4 Phantoms had a cartridge start system available.

    Some aircraft engines used a clockwork/flywheel start systems; you'd crank a handle many turns to wind the clockwork, once under tension the clockwork would spin the flywheel, and a clutch would engage the engine to turn it. Heavy and complicated.

    re: battery load test

    Not any more than a normal engine start. The battery must be completely charged before the test, and completely re-charged afterwards.

    Get the largest battery that will fit in the space available. Frequently, manufacturers install a smaller battery than will fit to save money. The larger the physical size of the battery, the more room there is for plates.

    The voltage is all nearly the same. It's the amp-hour capacity and cold crankig amps that changes.

    Yes. Truck batteries can be very large.
    Stores generally carry a variety of batteries to fit most makes & models. There are standard dimensions; BCI group size.
    Here's a link to an excerpted BCI group size table:

    I wouldn't recommend it, but it could be done.

    Not without testing it under load that's porportional to the battery AH rating.
    See the answer to Question 3.
    An automotive battery is considered completely discharged when the terminals measure about 11.4v at 25°C. There is very little current available at that point.
  11. thingmaker3

    Retired Moderator

    May 16, 2005
    Most cars and small trucks will use the same physical size and voltage of battery. Some cars and many motorcycles do use smaller (6V) batteries. The battery is used only for running the starter motor, while it cranks the engine. (Prior to starter motors, auto engines had manual cranks with "Z" handles out the front.) The alternator provides all needed electricity once the engine is running.

    Battery analyzers for determining the health of an automobile battery cost about a thousand bucks. I don't know what principles they work on, but I'd wager Sgt.Wookie does.
  12. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    You mean using something like AutoZones battery life tester machine.
    Because I was kind of looking for a practical way of checking a car battery.

    So if I read anything 11.4 v and under. Would it imply the car battery won't start when turning the key. Or is it more important to check the current being delivered?
    And would it be ok to hook my mulitmeter in series with the battery to read the current or will this current blow a fuse...etc?

    Also what would happen if I took 4 9-volt alkane batteries hooked them in series. Then hooked the positve to the positve car battery node , negative to negative car battery node.
    It would then be 36 volts forcing current into the 12 volt car battery. Correct?
    I am just wondering if it would charge the battery enough to turn the car over once?

    Don't really know.

    That means the wire would have to drop 36 volts - 12 volts = 24 volts
    Since the resistance of the copper wire is small we would have a large current i.e 24 / (Resistance of wire) = large current.

    Either the problem is the wire will melt or the current would fry a battery?
    If these aren't the problems then the only thing I could think of is some chemical reaction that screws things up?

    Even so I could add a resistor or two to drop the current?
  13. wr8y

    Active Member

    Sep 16, 2008
    Forget it, there is NOT enough energy in three 9 volt batteries to turn over an engine.
  14. wr8y

    Active Member

    Sep 16, 2008
    Forgot to hit the quote button!

    The AutoZone devise IS a practical way of testing a battery.
    The only way to know it's capacity is to place a great load on it and see how it holds up.

    A battery at such a low open circuit voltage would probaly be too weak to turn over the engine, yes.

    And you can't hook your meter up, unless you have a 300 or 500 amp meter! You would be surprised at the amount of current your car's starter demands from the battery. It takes considerable torque to turn over the car's engine - and that torque is a result of volts times amps. Since you have only 12 volts, you'v got to have a LOT of amps.
  15. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok , get the torque being related to the power and since Volts is small current must be large to give a large power.
    P = VI = V^2/R = I^2R =....etc etc....

    But I don't see where my quote would fail in charging the car battery?
    Their both DC your pushing voltage to the car battery and the current can be made high.
    The only problem is if the batteries die out to quickly to charge the car battery enough?
    Last edited: Feb 26, 2009
  16. wr8y

    Active Member

    Sep 16, 2008
    You know, there is a reason for the difference in size between the batteries.

    Sure, it would charge for a few minutes - but not long and not enough to make any difference.
  17. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ahh so it is just that the 4- 9volt batteries would run out off juice before it could recharge the car battery enough.

    Ok thanks, all set now.
  18. pandian

    Active Member

    Sep 27, 2009
    Great elaboration dude! Thanks
  19. thyristor

    Active Member

    Dec 27, 2009
    So long as the cold cranking ability is similar, any12v lead acid battery will do. The actual ampere.hours used in starting a normal engine is about 1AH so battery capacity is not important for starting.

    If you disconnect your battery whilst the engine is running, you will almost certainly fry the rectifier in your alternator. So don't try it.

    Measure the battery terminal voltage whlist cranking the engine. If the voltage drops below 9v then the battery's internal resistance is too high and the battery is junk.
  20. Blackbull

    Well-Known Member

    Jul 26, 2008
    Measure the battery voltage whilst cranking the engine. If the voltage drops below 9V then the battery is junk - Yep that was what I was taught.