Minimum battery voltage to start engine
Discussion
Is there such a thing? I guess it depends on how easily the engine starts. As the original Focus battery comes up to its 15 birthday I'm thinking maybe one day it won't work anymore. I want it to live forever but no doubt in another 5 or 10 years it'll be past its best. Voltage currently after 2 months without being started is 12.0. Car seems quite happy at that though and fired straight up today when I just had to go out somewhere. I'll see what it stays at after a good charge overnight.
Depends on engine health, starter health, losses in cables etc etc.
Terminal voltage only really tells you if all the cells are working to some extent - 6 cells at about 2 V each = about 12v.
It doesn't tell you how well they are working.
What you really need to know is cranking voltage, which depends on battery capacity , which in turn deteriorates with age, temperature, number of charge cycles, sulfation of the plates and more.
Basically, without doing a proper charge / discharge test, the first you will know is when it fails to start.
You *may* notice it takes slightly longer cranking at some point, and failure will be very close by then.
Terminal voltage only really tells you if all the cells are working to some extent - 6 cells at about 2 V each = about 12v.
It doesn't tell you how well they are working.
What you really need to know is cranking voltage, which depends on battery capacity , which in turn deteriorates with age, temperature, number of charge cycles, sulfation of the plates and more.
Basically, without doing a proper charge / discharge test, the first you will know is when it fails to start.
You *may* notice it takes slightly longer cranking at some point, and failure will be very close by then.
LordLoveLength said:
Depends on engine health, starter health, losses in cables etc etc.
Terminal voltage only really tells you if all the cells are working to some extent - 6 cells at about 2 V each = about 12v.
It doesn't tell you how well they are working.
What you really need to know is cranking voltage, which depends on battery capacity , which in turn deteriorates with age, temperature, number of charge cycles, sulfation of the plates and more.
Basically, without doing a proper charge / discharge test, the first you will know is when it fails to start.
You *may* notice it takes slightly longer cranking at some point, and failure will be very close by then.
Not trying to,be smart, and I don't know that much about this, but isn't the thing we are looking for is cranking amps, not voltage, just asking.Terminal voltage only really tells you if all the cells are working to some extent - 6 cells at about 2 V each = about 12v.
It doesn't tell you how well they are working.
What you really need to know is cranking voltage, which depends on battery capacity , which in turn deteriorates with age, temperature, number of charge cycles, sulfation of the plates and more.
Basically, without doing a proper charge / discharge test, the first you will know is when it fails to start.
You *may* notice it takes slightly longer cranking at some point, and failure will be very close by then.
Vipers said:
Not trying to,be smart, and I don't know that much about this, but isn't the thing we are looking for is cranking amps, not voltage, just asking.
Yes There will be a couple of hundred of them and kit to measure that isn't cheap or widely available. What we are concerned with really is battery capacity - the ability of the battery to deliver a lot of amps, at a reasonable voltage, for a reasonable time.
If the battery doesn't have enough capacity, the terminal voltage drops as the current is drawn - so measuring the cranking voltage is a good indicator of the batteries ability to deliver the current.
Broadly speaking, discharge current x time voltage remains above threshold = capacity.
Pumaracing said:
Is there such a thing? I guess it depends on how easily the engine starts. As the original Focus battery comes up to its 15 birthday I'm thinking maybe one day it won't work anymore. I want it to live forever but no doubt in another 5 or 10 years it'll be past its best. Voltage currently after 2 months without being started is 12.0. Car seems quite happy at that though and fired straight up today when I just had to go out somewhere. I'll see what it stays at after a good charge overnight.
Most electronic parts onboard will have a minimum operating voltage. So there definitely is a voltage threshold below which the parts simply will not function.That's ignoring any ability to actually turn the engine over too.
15 years is doing pretty well already though and if it started after sitting for 2 months, again a good sign and better sign the car doesnt have any silly voltage drops whilst not in use that would kill a battery.
The problem is not the float voltage of the battery, but what happens to it's internal resistance and SoC with age! A starter motor takes a lot of current. Typical starter motors are rated at around 2kW, electrical power. Now, 2kW from 12v is 167 Amps. When you pull that sort of current from a lead acid battery, the voltage drops, due to it's internal resistance (and of course all the wiring between the battery and starter has a resistance too) In fact, most starter motors are rated to make there claimed power at just 8V, which for our 2kW starter is 250 Amps.
As a lead acid battery ages, it's State of Charge (SoC) degrades as the plates become contaminated, and it's internal resistance rises as well. As a starting event only lasts a few seconds, the total energy required is pretty small (~6kJ) but as the instantaneous current is large, the battery may struggle to maintain sufficient voltage at those high currents.
Most critical automotive electronics is these days rated to run down to at least 8v and more typically for engine ecus/ fuel pump controllers etc down to 6v.
When you measure the voltage of a lead acid battery that is not under any significant load (it's Float voltage) you will find that even for a pretty knackered battery, that voltage will still be quite normal (say 12.3v to 12.7V at 25degC).
As a lead acid battery ages, it's State of Charge (SoC) degrades as the plates become contaminated, and it's internal resistance rises as well. As a starting event only lasts a few seconds, the total energy required is pretty small (~6kJ) but as the instantaneous current is large, the battery may struggle to maintain sufficient voltage at those high currents.
Most critical automotive electronics is these days rated to run down to at least 8v and more typically for engine ecus/ fuel pump controllers etc down to 6v.
When you measure the voltage of a lead acid battery that is not under any significant load (it's Float voltage) you will find that even for a pretty knackered battery, that voltage will still be quite normal (say 12.3v to 12.7V at 25degC).
usually mid-low 11 and I'll call for another or charge, tdi's and new stuff can suffer weird problems from a lazy battery, and puts more demand on other systems, its that hart of the electrical system so I like to make sure its at its best, not just good enough, most have a 3-5year life cycle
Some moderns will go into LOS if the battery voltage drops too low on cranking, Mercs mainly IME, never had that issue on a Ford , if it starts the Ford ECU is happy 
although TBH most batterys that read 12V at rest i would class as flat and charge but a lot depends on how many amps it can provide for cranking?
I tend to use old merc batterys that have thrown a low voltage MIL as jump start batterys and they tend to last years in this role where as in a merc they would initiate a LOS, one i changed has been doing stirling service in my 106 diesel for the last 4 years after failing in a Merc, mind you it has a wee bit less load on it
On a similar note when i was given a fiesta Mk6 3 years back the battery was totaly dead, not even a spark if you shorted it, a 48Hr charge and 3 years later it's still wanging over the fiesta easy as pie (is pie every REALLY easy ??) that's the origional silver bosch that came with the car new, a decent battery will do that, cheap ones can refuse to crank a 1.2 petrol whilst still showing 12.4V at rest, drop (load) test them and they sink like a stone.
although TBH most batterys that read 12V at rest i would class as flat and charge but a lot depends on how many amps it can provide for cranking?I tend to use old merc batterys that have thrown a low voltage MIL as jump start batterys and they tend to last years in this role where as in a merc they would initiate a LOS, one i changed has been doing stirling service in my 106 diesel for the last 4 years after failing in a Merc, mind you it has a wee bit less load on it
On a similar note when i was given a fiesta Mk6 3 years back the battery was totaly dead, not even a spark if you shorted it, a 48Hr charge and 3 years later it's still wanging over the fiesta easy as pie (is pie every REALLY easy ??) that's the origional silver bosch that came with the car new, a decent battery will do that, cheap ones can refuse to crank a 1.2 petrol whilst still showing 12.4V at rest, drop (load) test them and they sink like a stone.
Effing battery. I put it on charge for 8 hours, switched the headlights on for a bit to kill any surface charge and then left it for 6 hours to stabilise further and it's showing a perfectly normal 12.62 volts that's about typical for a new battery not a 15 year old one.
I'm wondering if 20 years is now not unreasonable to expect and in fact what the record is for car battery life.
I'm wondering if 20 years is now not unreasonable to expect and in fact what the record is for car battery life.
Pumaracing said:
Effing battery. I put it on charge for 8 hours, switched the headlights on for a bit to kill any surface charge and then left it for 6 hours to stabilise further and it's showing a perfectly normal 12.62 volts that's about typical for a new battery not a 15 year old one.
I'm wondering if 20 years is now not unreasonable to expect and in fact what the record is for car battery life.
With engine off, turn on:I'm wondering if 20 years is now not unreasonable to expect and in fact what the record is for car battery life.
all lights (full beam headlamps)
Heated rear screen
Cabin heater fan
Heated seats if you have em
+anything else you can find
Then re check voltage, immediately and after 1min.
Turn off everything, check voltage immediately then after 1 min
1st reading allows you to estimate battery IR and second battery SoC
With ignition on and everything that would enable me to switch on, headlights, heated screen, radio, fan blower, voltage was 11.2 immediately then dropping to 11.1 after 1 minute. With everything switched off again it was 12.1 and then rising steadily. 12.2 after 1 minute, 12.25 after 5 minutes, 12.27 after ten minutes and now about steady.
guzzler4 said:
My friend
It is a waste of time posting this .35volts stuff unless you are using a meter that has been calibrated at the required intervals
Hardly! It is a waste of time posting this .35volts stuff unless you are using a meter that has been calibrated at the required intervals
Firstly: A typical 40,000 count hand held multimeter will be accurate to around 20mV, even when abused and well out of cal. 0.35v is 350mV, ie massive!
Secondly: The absolute value accuracy is irelevant!. We don't care if Pumas meter says 12.35V when it's a "True" value of say 12.25V, we just care about how the measurements change. Even on a totally knackered multimeter, the linearity will be close to that when manufactured, even if the accuracy has been degraded!
Pumaracing said:
With ignition on and everything that would enable me to switch on, headlights, heated screen, radio, fan blower, voltage was 11.2 immediately then dropping to 11.1 after 1 minute. With everything switched off again it was 12.1 and then rising steadily. 12.2 after 1 minute, 12.25 after 5 minutes, 12.27 after ten minutes and now about steady.
Seems ok to me! A ~1 volt drop with probably a 50Amp load is reasonable (~20mOhm IR) and as it only dropped a tiny amount after 1min, then the basic SoC of the battery is ok too!The recovery profile of the voltage when the load is removed suggests that the battery dielectric ions are still mobile and that the plates are likely to be fairly mildly sulphated.
Me and a mate bought an old Audi Coupe 2.3 from Scotland once, 300 quid off eBay with no alternator.
Obviously what you do next is head up there with a few spare batteries and a 10mm spanner and hit the M6 like heroes!
We got into a routine where every hour or so the Audi would die from what felt like weak spark / low fuel pressure around 6-7 volts and we'd swap that battery into my Primera, and my little roller rocker SR20 would actually crank and start off that. I'm guessing it recovered a few volts during the breather, and the engine was hot which helps.
6 hours it took us to get home, that Nisan's alternator must have been white hot!!!
Obviously what you do next is head up there with a few spare batteries and a 10mm spanner and hit the M6 like heroes!
We got into a routine where every hour or so the Audi would die from what felt like weak spark / low fuel pressure around 6-7 volts and we'd swap that battery into my Primera, and my little roller rocker SR20 would actually crank and start off that. I'm guessing it recovered a few volts during the breather, and the engine was hot which helps.
6 hours it took us to get home, that Nisan's alternator must have been white hot!!!
I had a similar experience as a teenager when I was heading for France with a mate in 1978 to tour the continent, both on identical Honda CB550 motorbikes. Half way to Portsmouth to catch the ferry my bike started coughing and spluttering and finally died. No lights or other electrical items working. The voltage regulator had died and the battery was dead flat. We swapped the batteries over and carried on while his bike charged my battery back up. Made it to Portsmouth with half an hour to go before the ferry left. Found the address of a bike breakers in a yellow pages in a phone box, hacked over there, luckily he had the right voltage regulator off another similar Honda on the shelf. Bolted it in and made the ferry with seconds to spare. That was the one and only thing that ever broke on my bike in 30,000 miles and two trips round the continent.
Max_Torque said:
Pumaracing said:
With ignition on and everything that would enable me to switch on, headlights, heated screen, radio, fan blower, voltage was 11.2 immediately then dropping to 11.1 after 1 minute. With everything switched off again it was 12.1 and then rising steadily. 12.2 after 1 minute, 12.25 after 5 minutes, 12.27 after ten minutes and now about steady.
Seems ok to me! A ~1 volt drop with probably a 50Amp load is reasonable (~20mOhm IR) and as it only dropped a tiny amount after 1min, then the basic SoC of the battery is ok too!The recovery profile of the voltage when the load is removed suggests that the battery dielectric ions are still mobile and that the plates are likely to be fairly mildly sulphated.
I've ordered a new Yuasa YBX5075 silver calcium off Ebay for £56 delivered. 5 year warranty, much higher 620 amps capacity than the original Ford 390 amp battery so hopefully that's me set for a long time to come.
Pumaracing said:
Max_Torque said:
Pumaracing said:
With ignition on and everything that would enable me to switch on, headlights, heated screen, radio, fan blower, voltage was 11.2 immediately then dropping to 11.1 after 1 minute. With everything switched off again it was 12.1 and then rising steadily. 12.2 after 1 minute, 12.25 after 5 minutes, 12.27 after ten minutes and now about steady.
Seems ok to me! A ~1 volt drop with probably a 50Amp load is reasonable (~20mOhm IR) and as it only dropped a tiny amount after 1min, then the basic SoC of the battery is ok too!The recovery profile of the voltage when the load is removed suggests that the battery dielectric ions are still mobile and that the plates are likely to be fairly mildly sulphated.
I've ordered a new Yuasa YBX5075 silver calcium off Ebay for £56 delivered. 5 year warranty, much higher 620 amps capacity than the original Ford 390 amp battery so hopefully that's me set for a long time to come.
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