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Charging voltage and starting problem

brandonsmash

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I've been on this forum for a while but have been inactive for a bit. My apologies.

TL;dr: '15 GT with an odd no-start/no-charge/no-jump.

Recently I've noticed that my '15 GT (18,600 miles) had been getting slightly slower to start. I installed a battery tender pigtail because I was going to be gone for the month of July.

I came back and the car started and drove normally through August, but I wondered about the charging system; the voltage display on the dash read ~13.9V. I have not had any CELs or warnings on the dash.

On Wednesday I had a job call on the other side of town and I drove the Mustang (which started maybe a touch slowly, but without real problem). I stopped to check in at security for the location and was cleared to proceed, but the car wouldn't start at all. It would crank once (slowly) and then go dark.

Attempt 1 was a jumper pack. No luck.

Attempt 2 was a guy with a truck. The truck was equipped with a 180A alternator and dual batteries. Still no luck. I pulled the positive battery cable and cleaned it, and connected the jumper cables (4AWG welding cable) directly from the good battery to the positive battery cable (without using my battery at all) and the frame. No luck -- same thing.

Attempt 3 was the guy that Ford Roadside Assistance sent out. He hooked up two jump packs AND his running car. Still no luck.

At this point I called for a tow and had the Mustang towed to the dealer where I bought the car.

The next day the dealer called and told me that they'd replaced the battery and that everything else was fine. The service advisor told me the car "started right up" for her before even replacing the battery, and that the battery had a shorted cell. I asked them to test the alternator; she said it was at 14.2 volts. The replacement battery cost me $197. (The vehicle is under an extended service plan but the battery is not covered.)

(Okay, yes, a bad battery can sometimes revive itself after sitting for a while, yes, but a battery with a shorted cell can never do this: If a cell is shorted, all the current will always take that path to ground.)

The tech blamed my battery tender pigtail for causing the fault, disregarding that I used factory ring terminals on the pigtail and it was installed directly to the OEM accessory posts and was installed after I had my first slow start.

Driving home yesterday I checked the voltage using the dash display; it read 13.6V, not the 14.2 they'd claimed.

This morning the car started fine and I drove to work. The dash display read 12.7 to 13.3V irrespective of RPM, load, and driving condition. I popped the hood once at work and checked the (running) pole-to-pole voltage with two multimeters, both of which read 13.3V.

I called the dealer again and asked what the alternator was putting out. The service advisor confirmed 14.2V. I mentioned that I had photos of the voltage as between 12.7 and 13.3 under running conditions. She came back after a minute and then told me that her tech said that "anything above 13.1 was okay" because the alternator is controlled by the PCM. She did not address the fact that she specifically told me that the alternator was measured at 14.2V before I tested it myself.

I phoned another dealership. Their SA told me that they like to see between 13.5-14.5V.

Here's where I'm confused. For one, I thought that an automotive battery was comprised of 6 cells of 2.3V each and so required 13.8V to charge at all.

For another, I don't understand why the vehicle wouldn't start even with the positive terminal disconnected from the battery and attached to another (good) battery and the negative lead of the other battery clamped directly to the frame of the Mustang. After the 3 jump attempts it sat in that spot for 6 hours and wouldn't take a jump or even get enough power to unlock the doors.

So am I being paranoid? Has my understanding of batteries and charging systems become outdated? Am I overlooking something with regards to the no-jump condition?

Apologies for the long post; I thought details may be merited here.

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GT Pony

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What you describe after getting a new battery is normal. The chatging system on the S550 is a "smart system" - not like old systems that continually put out 14.5 volts across the battery.

The battery voltage read-out on the dash will start out high, around 14.2~14.3 after starting the car if it's sat for a few days. The charging system will slowly cut back the charging voltage as the car is driven and the battery gets charged up. I've watched this happen every time I drive my GT. After a long drive the charge voltage will be down around 13.5~13.7 volts on the dash. These observations were after installing a very good new battery.

My OEM factory battery went bad, and one thing I noticed with that bad battery is that the charging system voltage never went down after a long drive - it always stayed over 14 volts, which means the charging system was over working and trying to push cutrent into the battery, which could have had high resistance as it aged.

So anyone reading this, if the charging voltage doesn't get down in the 13s after a long drive, the battery is probably not totally healthy.

Update: I just looked at notes I took while observing the charging voltage reading on the dash after replacing my bad battery. After a long drive the dash voltage was showing as low as 12.7 volts while cruising at 2000 RPM. At idle (600 RPM) I've even seen the voltage as low as 12.1~12.2 volts after a long drive. Just means the system cuts back the charging voltage because the battery is basically 100% charged. Keep in mind that the dash voltage reads about 0.5 volts lower than the actual voltage across the battery terminals. I've verified that on my car, and it's probably true on all S550s.
 
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brandonsmash

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Awesome, thanks! I'm glad that's normal. I still wish that the SA hadn't lied to me (she distinctly said 14.2).

Also, good to corroborate that the dash reads less than actual value: I found this when my dash said 12.7 and two separate multimeters both read 13.3 across the terminals while the car was running.

This doesn't explain the no-start while my battery was disconnected. I'm just going to hope that was a fluke or maybe I did something stupid.
 

M151A2

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Everything GT Pony said is correct. The smart charging system is trying not to overcharge or heat up the battery thereby the battery lasts longer. At least in theory.

Attempting to jump start a vehicle with a shorted cell is futile. It will not work and creates havoc with your electrical system. Trying to daisy-chain another vehicles battery to yours in an attempt to trick your vehicle, well, I don't know what kind of troubles that might give you. If it did start (lucky for you it didn't) and you drove away on a shorted cell, your alternator would soon follow the path of your battery and you would be replacing both. It's anyone's guess as to how the 'puter would react.

Either way, it sounds like your problem is fixed after the new battery was installed. And agreed, do not think that the dash gauge is accurate. Mine reads about the same as GT Pony's with a Fluke meter.
 

VinnAY

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Good time to remind guys that Ford did not put maint free batteries in this car and given that the battery lives under a cover you wont' plainly see the caps on top and otherwise know that the cells needs checked and filled with distilled water when low. I had the same problems, discovered the battery was dry, filled it but it never came back and a battery replacement solved the slow/no start problem.
 

GT Pony

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Awesome, thanks! I'm glad that's normal. I still wish that the SA hadn't lied to me (she distinctly said 14.2).
Doubt she lied ... probably just doesn't exactly know how the smart system works. But like I said before, if the system senses the battery isn't fully charged (like the car sat for a few days), it will definitely put 14.2 volts or more across the battery until the battery starts charging up, then the system will slowly cut back the charging voltage. Same way that a smart battery tender type charger works.
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