Lowest voltage I can still start my car?

[fuzzybabybunny]

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?

 

[Zenmervolt]

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

 

[fuzzybabybunny]

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?

 

[LTC8K6]

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.

 

[phucheneh]

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.

 

[Ferzerp]

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.

 

[EagleKeeper]

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.

 

[Richard N]

Aw geez... This shit again :facepalm:

 

[Zenmervolt]

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?

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.

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?

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."

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?

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.

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

 

[fuzzybabybunny]

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

Ah, I see. Uhhh... see bolded.

 

[fuzzybabybunny]

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."

 

[tortillasoup]

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.

 

[fuzzybabybunny]

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.

 

[tortillasoup]

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.

And if 92% or what ever meant 30 seconds of use, would that be enough? how about you tell us what size battery your car uses.

RC (Reserve Capacity) - the number of minutes that the battery can deliver 25 amps while keeping its voltage above 10.5 volts.

I think it's a 151R since you have a Honda Fit, correct? Autozone sells a 151R-DLG battery which has a reserve capacity of 55 minutes. So to use only 10% or leave 90% capacity left of a BRAND NEW battery, you could only use it for 5.5 minutes @ 25 amps. This is obviously a small battery but I think from this point you should be able to calculate your loads and how much "damage" you're willing to do to your battery. Just saying if you used a deep cycle battery of this size, you could get more minutes between having to charge it again. Keep in mind doing too severe of a discharge on a deep cycle or not will cause the cells to overtime get unbalanced as alternators are really REALLY poor battery chargers with their output either too high or just all over the place. A deepcycle battery would make this situation more viable.

 

[CurseTheSky]

I just bought a voltmeter that plugs directly into my 12v outlet.

Get a real DMM. Don't bother with junk like that. You don't have to spend hundreds of dollars on a professional-grade DMM, but get something that is well received by the electronics hobby community.

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.

This is probably ok for short periods, but don't leave your laptop plugged in for long periods of time with the car off. If you need to charge it or run it for more than 10-20 minutes, turn the car on.

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?

No, that is not the actual voltage of your battery. That is the voltage of the battery after passing through numerous wires, connectors, resistors, crud, lousy connections, etc. To measure the actual voltage of the battery, you would disconnect the leads from the battery, then set a DMM to 20V DC and place the negative (com) lead on the negative terminal of the battery, and the positive lead on the positive terminal. Do not attempt this if you do not know what you're doing, as you can cause damage and personal injury.

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?

Correct.

 

[Maverickuk2015]

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
http://www.ebay.com/itm/DC-12V-Batt...-Alarm-Car-Cigarette-Lighter-ZS-/181210624800

 

[DaTT]

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
http://www.ebay.com/itm/DC-12V-Batt...-Alarm-Car-Cigarette-Lighter-ZS-/181210624800

Most modern cars either have automatic headlights, or turn the headlights off after 10 minutes or so of you turning the engine off.

 

[RLGL]

There are better things in life to be concerned about!

 

[LTC8K6]

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.

 

[LTC8K6]

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.

 

[Wyocoyote]

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.
I use many accessories when the car is not running off my 12 volt bank including a 12volt ARB fridge freezer (it turns itself off when batt gets too low to start car) I can tell from my lighter plug adapter when it is time to start the car and recharge, and can tell when it is fully charged and at what rate it is charging. It was a cheap Amazon item which also tells temp and amps being drawn from the USB charge ports. Very handy. Yours will do the same. Yes you can charge your laptop on an inverter without harming your battery if you are careful not to overly discharge your batt. Most say don't dip below 12 volts. Yes you can put a more amp battery in. sure you could benefit from a dual use deep cycle battery or dual batts or an aux batt in your fit, but should not be necessary, maybe a small auxillary motorcycle battery.
A nice solar panel in the windshield or on the roof say 50 watts should keep you charging good enough for a small laptop. Consider a 12 volt laptop then you wouldn't need an inverter.
a device that turns off your accessory when the voltage drops below X volts as aforementioned is not a bad idea at all.
Small lithium jumper packs have become better and better and might be a great option for you. Some have built in inverter s. Your not going to be able to mix margarita s with your Sister but you can Sur charge a typical laptop maybe not some gaming beast.

 

[RLGL]

Something forgotten, the cranking voltage the computer sees prior to turning the ignition OFF

 

[TSpot]

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.