How Does Your Car Start?: When you insert the key in your car’s ignition and turn the switch or push the button to ‘ON’ a signal is sent to the car’s battery. Upon receiving this signal, the car battery converts chemical energy into electrical energy. This electrical power is delivered to the starter to crank the engine. The battery also provides power to the car’s lights and other accessories. Show Battery Voltage: Voltage refers to the amount of electrical potential your battery holds. The standard automotive battery in today’s vehicles is a 12-volt battery. Each battery has six cells, each with 2.1 volts at full charge. A car battery is considered fully charged at 12.6 volts or higher. When the battery’s voltage drops, even a small amount, it makes a big difference in its performance. The table on the left shows how much energy remains in a battery as the battery voltage reading changes. Though not fully charged, a car battery is considered charged at 12.4 volts or higher. It is considered discharged at 12.39 volts or less. Note: A fully charged specific gravity of 1.265 corrected to 80°F is assumed. Chemical Reaction: Electrical energy in a battery is generated by a chemical reaction. In the case of a lead-acid battery, a mixture of sulfuric acid and water, known as electrolyte, reacts with active material inside the battery. A battery’s voltage largely depends on the concentration of sulfuric acid. To get a voltage of 12.6 volts or higher, the weight percentage of sulfuric acid should be 35 percent or more. As a battery is discharged, the reaction between sulfuric acid and active material forms a different compound and the concentration of sulfuric acid declines. Over time, this causes the battery’s voltage to drop. Cranking Power: Vehicle engines require cranking power to start. The power needed depends on many factors such as engine type, engine size and temperature. Typically, as temperatures drop, more power is needed to start the engine. Cold cranking amps (CCA) is a rating that measures a battery’s cranking power. It refers to the number of amps a 12-volt battery can deliver at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts. For example, a 12-volt battery with a 600 CCA rating means that at 0°F, the battery will provide 600 amps for 30 seconds without dropping below 7.2 volts.
I just bought a voltmeter that plugs directly into my 12v outlet. The outlet goes into a 12v splitter and one of the outlets on the splitter is connected to the voltmeter. The remaining outlet is attached to the inverter that runs my laptop. When the car is turned off and set to Acc, the voltage reads 12.2-12.3v. I assume this is the actual voltage of the
battery, right? What voltage do I not want to drop below so as to preserve the battery plates and also allow me to continue to start my Honda Fit? When the car is running, the voltage goes immediately up to 13.8-14.0v. I'm assuming this is not the actual voltage of the battery. It is probably the voltage that the charging system is putting out, right? Last edited:
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1) It's not voltage that determines whether you can start the car, it's the starter's ability to deliver amperage. A volt meter will only give you a rough idea of the state of charge assuming that the battery is known to be in good condition. The volt meter will tell you nothing about the condition of the battery. 2) A known-good battery is "dead" at 12.0 volts and fully
charged at 12.6 volts. The voltage you see at the outlet, however, is not the same as the battery's voltage as there will always be some voltage drop between the battery and the outlet. Additionally, since the key must be in the "Acc" position there are other items creating electrical draw than just the outlet. Finally, the splitter itself will create a voltage drop as well. So the volt meter you're plugging into the splitter really doesn't tell you much at all about the battery. 3)
Depending on the battery condition, the voltage that will allow the engine to turn over changes. A battery with 5 good cells and one bad cell may only read ~10 volts but still be able to turn the starter. A battery with all six cells in good shape that reads 11 volts, however, would almost certainly not be able to spin the starter. 4) Yes, when the car is running the alternator puts out ~13.8 volts. 5) Just find a different way to run the inverter. Running it off the car's
battery without the engine running is a bad idea. As people here have told you before. ZV
Wait, so this can't even be used as a ballpark? If I see the voltage drop to 11v there is actually a 50/50 chance that it will still turn? Likewise, If I see the voltage at 12v there is also a 50/50 chance that it won't turn? And with the car on Acc reading 12.3v, the voltage drops due to all the systems could mean the battery is actually at 11v and couldn't turn anything? Let's just assume something that is known - the battery is good. It has never been discharged to a low level. It is a year old. Can this method *still* be entirely useless, even with a known good battery?
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The voltage is only a very general ballpark idea of battery health/strength. It can be misleading. The true test is the specific gravity of the acid in the cells. Last edited: Oct 25,
2013
I've seen batteries struggle at around 12v and crank happily at more like 11.5. The latter is usually with large, high-quality batteries. Technically they're both 12.6v, 6 cell, lead acid batteries, and that shouldn't be the case. But it's what I've observed. Basically, it's totally up in the air. You can throw guesses out based on battery quality and condition...a batt that
tests over its CCA rating with a conductance tester will probably be good to a lower voltage than one testing under. But I would not place any bets on it. I would generally advise not letting a battery get below about 12.2v (roughly half-charged) if you can help it. Not only are there no guarantees at lower voltages, but you're going to shorten the battery's life via constant charge/discharge cycles and generally keeping it at a low SOC. Just driving the car for 20 minutes is not enough
for a severely discharged battery to recover. Voltage isn't even that easy to read...most people will just see surface charge and not the 'real' voltage. If the battery is at rest (no loads) and the car has been started recently, you can usually watch the hundredths of a volt tick away with a meter. I usually get a reading with the headlights on (and engine off, obviously). Even with the draw, voltage should be pretty stable. Oct 12, 1999 6,438 107
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The real answer is a generator instead of continuing to ruin batteries by running AC powered equipment off a car battery with the car turned off.
As stated above, a small portable generator. Or get a second battery and leave the car battery alone. You can use jumper cables to recharge the aux battery WHEN the car is running.
Aw geez... This shit again :facepalm:
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Wait, so this can't even be used as a ballpark? If I see the voltage drop to 11v there is actually a 50/50 chance that it will still turn? Likewise, If I see the voltage at 12v there is also a 50/50 chance that it won't turn? If you have a much larger than stock battery (e.g. stock requirement is 350 cranking amps and you're using a battery with 600 cranking amps) lower voltages are more likely to still be able to spin the engine as well. And with the car on Acc reading 12.3v, the voltage drops due to all the systems could mean the battery is actually at 11v and couldn't turn anything? Let's just assume something that is known - the battery is good. It has never been discharged to a low level. It is a year old. Can this method
*still* be entirely useless, even with a known good battery? Also,
note that this is merely a case of choosing a deliberately very conservative position since the method you described is not even remotely accurate enough to let you get anywhere close to the "edge." ZV
It's not 50/50. At 12.0 volts there's much less than a 50% chance that a known good battery will crank the engine over. Remember, 12.0 volts means 0% charge for a known good battery. At 11.0 volts, the chances of the battery actually cranking over the engine are very, very low. For practical purposes it's 0% since a single bad cell in modern
batteries is pretty rare. If you have a much larger than stock battery (e.g. stock requirement is 350 cranking amps and you're using a battery with 600 cranking amps) lower voltages are more likely to still be able to spin the engine as well. So a battery's ability to start the car has to do with the amount of cranking amps it can put out, not the voltage. Are there any simple devices that can actually display a readout of the cranking amps of a battery? Without actually cranking?
For example, you plug this device into the cigarette outlet and you get a readout on an LCD that says "450 Amps" or whatever. I know there are testers that can display something when you connect it directly to the battery terminals (are they even accurate?): http://www.amazon.com/BA5-100-1200-Cranking-Electronic-Battery/dp/B0017R5EQK But I suppose something
like this is impossible to have if it were simply hooked up to the 12V outlet? No... The voltage drops make the meter read low. It's impossible for the additional draws to make the meter read higher than the battery's voltage when on "Acc." Oh yeah, of course. Duh. Automotive starting batteries are not designed to withstand significant amounts of discharge. Letting a starter battery drop below about 90% with any frequency will significantly shorten its service
life. While this method could be enough to keep you from getting stranded (by un-plugging the inverter if the meter's reading drops to 12.2 volts), it is not a good idea to do this as a usual activity. Just out of curiosity I want to know how long I can run a laptop on just the main battery and still keep the battery healthy. Is it 1% every 5 minutes? 1% every 30 minutes? 1% every hour? The problem is that I don't even have a remote ballpark figure. There are still going to be
times when I need to run something from the battery temporarily with the ignition in Acc. But I have no faint idea how long I can run it - am I ruining the battery after just running it for just 15 minutes? Also, note that this is merely a case of choosing a deliberately very conservative position since the method you described is not even remotely accurate enough to let you get anywhere close to the "edge." ZV Last edited: Oct 26, 2013
So a product like this is next to worthless? http://www.batterybrain.co.uk/faq.html "How does the Battery Brain work? It sends out a pulse every two seconds to the battery that senses the voltage level. When it senses the level falling below what is needed to
start the engine the patented microprocessor activates its single moving part (a solenoid) and isolates the battery from the electrical circuit to preserve the charge you will need to start the engine."
So a product like this is next to worthless? http://www.batterybrain.co.uk/faq.html "How does the Battery Brain work? It sends out a pulse every two seconds to the battery that senses the voltage level. When it senses the level falling below what
is needed to start the engine the patented microprocessor activates its single moving part (a solenoid) and isolates the battery from the electrical circuit to preserve the charge you will need to start the engine."
sounds like a nifty device. Btw if you plan on regularly discharging your battery in this fashion, you should get a deep cycle battery for its replacement. Starter batteries just don't tolerate this very well especially economy cars with smallish batteries.
I'm not planning on it. I just want to utilize the battery for a little bit and immediately stop before battery damage starts to occur. So maybe stop after 92% or whatever. I just need some kind of a tool to let me know when this level has been reached. RC (Reserve Capacity) - the number of minutes that the battery can deliver 25 amps while keeping its voltage above 10.5 volts.
I just bought a voltmeter that plugs directly into my 12v outlet. The outlet goes into a 12v splitter and one of the outlets on the
splitter is connected to the voltmeter. The remaining outlet is attached to the inverter that runs my laptop. When the car is turned off and set to Acc, the voltage reads 12.2-12.3v. I assume this is the actual voltage of the battery, right? What voltage do I
not want to drop below so as to preserve the battery plates and also allow me to continue to start my Honda Fit? When the car is running, the voltage goes immediately up to 13.8-14.0v. I'm assuming this is not the actual voltage of the battery. It is probably the voltage that the charging system is putting out, right?
Interesting topic guys and good info there. I have a problem with a campervan. Left lights on one evening but got into camper from between front seats. Therefore the left lights on buzzer did not kick in as you would open the door. Anyway, big flat battery in morning so two questions. 1: Why the hell have they not sorted out in this day and age a device that cuts off current
draw when battery gets too low! 2: Is there a reasonably priced unit one could use reliably if the same thing happened again, so that whilst in campervan I could be alerted to having left lights on or something else draining batteries too low? Such as this item on ebay, but given your thoughts here this one would not be anygood as alert at 11.5v is too late - thanks DaTTGarage ModeratorFeb 13, 2003 13,293 117 106
Interesting topic guys and good info there. I have a problem with a campervan. Left lights on one evening but got into camper from between front seats. Therefore the left lights on buzzer did not kick in as you would open the door. Anyway, big flat battery in morning so two questions. 1: Why the hell have they not sorted out in this day and age a device that cuts off
current draw when battery gets too low! 2: Is there a reasonably priced unit one could use reliably if the same thing happened again, so that whilst in campervan I could be alerted to having left lights on or something else draining batteries too low? Such as this item on ebay, but given your thoughts here this one would not be anygood as alert at 11.5v is too late - thanks Jan 8, 2013 1,967 253
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There are better things in life to be concerned about!
Mar 10, 2004 28,520 1,574 126
Battery Saver circuits have been in cars for ages. My 1995 Taurus had it. It did not save you from leaving the headlights or ignition on though, presumably because those are things you might intentionally leave on. Mar 10, 2004 28,520 1,574 126
Why not just keep a jump starter box with you? There are some good ones out these days, including some surprisingly small lithium battery types that hold a charge for a very long time.
I find my lighter plug voltmeter very useful and it it is not much different than readings straight from my battery with my Fluke. I have a much larger diesel vehicle with dual batteries that requires a lot of Juice to turn over.
Jan 8, 2013 1,967 253 126
Something forgotten, the cranking voltage the computer sees prior to turning the ignition OFF
Maybe I overlooked this but I didn't see anywhere in the thread suggest that you get a better laptop or two or three laptop batteries. Laptop batteries can take multiple complete discharges unlike auto batteries which often have a complete discharge listed as the cause of death.
Is 10 volts enough to start a car?When the voltage of the starting power supply is lower than 11.8V, it will be difficult to start the car. You can choose to turn off all other electrical appliances and try several times. When the battery voltage is lower than 10.8V, it is generally impossible to start the car.
What voltage does a car need to start?Measure and Analyze
When the probes touch the terminals while the car is off and the battery is resting, the multimeter display should show a reading of 12.2 to 12.6 volts (full charge). This voltage range means the battery is in good condition for starting the vehicle.
Is 13.5 volts enough to start a car?With the engine running, your battery's voltage should be somewhere between 13.5 and 14.7 volts.
Is 12.2 volts enough to start a car?At 12.4 volts the battery is considered 75% charged and can still ignite your engine. When the battery voltage lowers to 12.2 volts without the engine running, it is considered a bad battery that may start your engine but is no longer capable of holding a charge.
How much voltage do you need to jumpstart a car?For cars with large engines, a jump start cable with a cross section of 25 mm is recommended. The vehicle which has suffered a breakdown must be supplied by a battery with the same voltage. Normally, the voltage of most vehicles is 12 Volts. Only a few vintage cars are operated with a voltage of 6 Volts.
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