I have a 2014 Ford Focus, 2.0l flex and automatic transmission. I was testing my battery with a multimeter and noticed that with the engine running the battery was being charged at 15.5V, this was after sitting overnight and it was 30F that morning. I read that it shouldn’t be any higher than 14.7V when charging. My battery was at 12.4V before I started the engine and it dropped to 11V when I started it.
Which is it?
Watch this.
Why can’t it be both?
12.4 before starting / 11 while cranking
15.5 after starting
My question is how long did it stay at 15.5 ? It should drop down to about 14.7 after a couple minutes.
I didn’t know it should that it drop after a few minutes, I’ll retest it thanks.
I just retested it. 12.2V resting overnight, 10.7 while cranking. 15.08 while running, after 5 minutes it dropped to 14.97.
On a cold start the alternator is commanded to work a bit harder to recharge the battery. It should drop down as the battery is brought up to full charge.
That 12.4 volts shows your battery to be at 65% of full charge. If you drove it the day before, that is a hint that something might be draining the battery overnight a bit more than it should OR the battery has reduced capacity due to its age. As long as the cranking speed seems fast enough, you are OK. when is slows down noticeably it might be time to have the battery tested.
Your vehicle has a battery current sensor.
The computer controls the output from the alternator from the reading it gets from this sensor.
The 12.4 reading is about 80% charge.
Tester
So because of my batteries low charge, the computer sends more current to my battery?
Yes it does
How old is your battery?
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Opening the door and turning on the interior lights can bring a full battery down to 12.4 or 12.5. It’s also common for a battery that’s not brand new anymore to have its Voltage drop a little shortly after full charge.
Works for me, but it’s about time to think about replacing the battery.
12.4 day 1, 12.2 the second day
I’d be checking the auto parts stores/ Costco/ Walmart ads in the junk mail looking for a sale on batteries, you should have a few months to shop
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Well the OP didn’t say that. He said:
“When I started it” sort of, kind of, implies it’s running.
11 volt when starting the engine, 15.5 volts after starting the engine.
The voltages look ok to me. But the 12.2 volt before the first start of the day, that’s a little low, probably means battery needs to be replaced pretty soon. How old is the battery? Do you live in a hot climate? Curious what caused you to make these measurements. Just wanted to know? Or are there symptoms?
“So because of my batteries low charge, the computer sends more current to my battery?”
I don’t know the specifics of your car’s charging system design. But generally older cars (like my 30 year old Corolla) use a simple electronic circuit located inside the alternator called a voltage regulator, and newer cars use the drivetrain computer or similar to decide the charging voltage. The voltage regulator method decides the charging voltage on what’s happening at the time.
The computer method is more sophisticated and takes into account what accessories have recently been used, the battery current-draw history, etc. It sort of treats the battery like a gas-tank, measures what’s been going in and what’ been going out, to determine how much is left.
Maybe so, maybe not. Here in Duluth, MN in winter, a bit under 12V does not concern me. It’s about zero degrees F right now. My 2007 Town and Country’s battery read 11.7V after sitting for a few days. The engine started right up as the voltage dropped below 11 while cranking and quickly climbed into the high or mid 14V range.
Even when the temps are more moderate, after sitting overnight that car’s battery is usually close to 12V. At summer temperatures it’s higher. For me the major test is how it starts the car when it’s really cold, say 15 or 20 below zero F. That doesn’t happen very often (Lake Superior moderates the winter lows and I am 1/2 mile uphill from it) but it has been decades since any of my cars has failed to start in extreme cold because of a weak battery. Electronic fuel injection and computer engine management enable engines to start right up even in extreme conditions.
Remember, batteries depend on chemistry, and in lower temperatures they put out less power. And it’s often extreme heat that kills them. For me, that was Montana in summer. Minnesota in winter is easy (as far as car batteries go.)
I prefer batteries that have removeable caps because I can measure the state of charge in each cell. I regret that that design is on the way out.
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I havent seen a battery with removable caps in years.
This tool does the same thing to test battery cells
Tester
Both of my vehicles have removable cap batteries. One from Costco, one from Walmart. The truck’s battery is pretty old, probably 8 years. The Corolla’s, maybe 3-4 years.