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How does a 12 Volt Alternator charge a battery to read 18+ Volts

RAC68

Darth Vader
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Hi All,

The last few days I have had a riddle to solve and haven't. My Son owns a 2011 Kia Soul and had a very nice battery I could use for my Healey. When checking the battery, it red close to 20 volts if the car was running or not. Surprised by this condition, I placed my charger on the battery and for 2 days with the car not running, let it do what ever it could do. It did bring the voltage back to 18+volts but no better.

Changing the alternator with a new Denso unit, I reconnected with the same battery and, after running a bit, found the voltage to still be at 18+ with or without the engine running. Although the replacement Denso Alternator does have a built-in regulator and is brand new (not rebuilt), I was expecting maybe 13.6 or even as high as 15 volts at the battery which would not bring the battery near the 18+ volt readings registering on 2 different separate hand held gauges.

So, what could have caused this battery to go so high and how can it be brought back for use in my Healey (with an alternator).

I appreciate that this is not a direct Healey question and the regulator in the Kia alternator must have failed in some way. However, this is a 1year old battery and would have been a perfect replacement for my 12 year old in place unit.

Thank in advance,
Ray(64BJ8P1)
 

John Turney

Yoda
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My first thoughts:
  • Since you used two different meters and read the same result, I guess we can assume they are correct.
  • Most battery chargers will shut off when the battery is fully charged, so I wouldn't expect the battery charger to have any effect.
  • There may be a circuit breaker or fusible link in the Kia which has tripped/blown when connecting a higher voltage battery. A +/- 6V difference can cause quite a current flow (in the wrong direction) through battery and alternator cables.
  • Although some new cars use higher voltages than nominal 12V, I doubt the Kia is one of them, and those that do, IIRC, are in the 40V range.
  • Is the voltage at the new alternator ~14 volts?
  • The old regulator in the Kia probably failed and overcharged the battery. Is the battery sealed, or can you check electrolyte level? If it's sealed or the electrolyte level is below the top of the plates, forget using it. You'll have to buy two new ones (one for the Kia and one for the Healey).
 
OP
RAC68

RAC68

Darth Vader
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Hi John,

What seems perplexing to me is that each battery, 6V, 12V, and 24V, has a different number of plates with each maintaining an additive 2.4 voltage. Although each standard battery can register higher voltages (i.e 12v - 14v) for a short time, they usually fall back to a range much closer to their base specification. In this case, the previously replaced battery has maintained an 18+voltage reading for over a year.

Also, the battery is a low maintenance battery. Before charging, we checked the fluid level and found it to be a little low on all cells but with all plates covered by electrolyte.

The reason for this replacement on such a cold day was that the car engine would sometimes fail to turn over and start, even though the dash lights seemed fully functioning. However, a short wait and it would start normally and without indication of any problem transmitted from the dash lights. As you have mentioned, I understand the ECU self protects and can shut vulnerable components, or even itself, off if an electrical anomaly is sensed. Although I have not verified that, it stands to reason that it would be true later attempts would start the car and with no visible issues.

Thanks again for your response as I am stumped by this condition and how to rectify it … other then throwing the battery away.

Ray(64BJ8P1) also with an alternator
 
Last edited:
Country flag
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Is your son's battery an AGM? I believe they produce/register a volt or two higher than a regular lead-acid.

But, this makes no sense. The voltage of a battery is determined by the potential difference of the different components--in this case, lead and zinc IIRC--it's a law of physics. This makes no sense. The newer battery maintainers claim to be 'temperature compensated;' i.e. they maintain a different level of charge depending on ambient temperature. Has it been really cold in Jersey?
 

Keoke

Great Pumpkin
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Well:

It may be that the sons battery is an AGM and consequently its memory simply stored the charginging sources out put voltage .???
 
OP
RAC68

RAC68

Darth Vader
Offline
Hi All,

I am sorry to say I didn't look to see what his newer battery was but I understand he purchased it from Sears so expect its a standard Low-Maintenance battery and looks ordinary. I also checked the Soul's original battery with both meters and found this Rocket No-Maintenance battery, left sitting on the floor of the garage for a year, still registered 18+ Volts as well.

Bob, I can attest to the AGM registering a little higher as I have an Optima in my Healey and it does but, as you indicated, not this high. When installing the alternator under my Son's car yesterday morning, my temperature compensation for 19F was to be bundled up with a small ga
s propane heater. However, the same battery reading was also registered at 33F.

Since these high voltage readings don't seem consistent with battery science to me as well as many of you, I am now turning my focus to the 2 trusted meters and wondering the probability of having both malfunctioning. To address this potential, today I intend contacting one of my friends and use his meter to check out my 2 with the original high-reading battery. In a way, I hope my gauges are bad. I will post the results.

Presently, my Son is driving his Kia Soul with the new Denso Alternator and is having no issues.

Thanks all for your thoughts and ideas as mine ran dry,
Ray(64BJ8P1)
 
Last edited:
OP
RAC68

RAC68

Darth Vader
Offline
Hi All,

To close this thread, I drained the battery of power (to a 4 volt reading) and then connected my 12V charger at the 2 amp level for a long slow build. After 4 hours of charging, although not yet fully recharged, the gauge was at a 13.9-14.0 volt level. This was far from the 18+ volt reading prior to the drain and, once fully charged, will be sustain a normal 12.5 volts after removing the charger. I will let you know if this doesn't prove true.

I have no idea how the faulty alternator could have caused 2 batteries (original "Rocket" no-maintenance and newer replacement "Sears" low-maintenance) to have a sustained 18+ volts reading or what 12 volt battery chemistry could have caused this high volt level. I can only state that I had the same readings on 2 multi-meters over these last 2 days and will share if I ever discover the cause.

Thanks again and all the best,
Ray(64BJ8P1)
 
Last edited:

John Turney

Yoda
Silver
Country flag
Online
Is your son's battery an AGM? I believe they produce/register a volt or two higher than a regular lead-acid.

But, this makes no sense. The voltage of a battery is determined by the potential difference of the different components--in this case, lead and zinc IIRC--it's a law of physics. This makes no sense. The newer battery maintainers claim to be 'temperature compensated;' i.e. they maintain a different level of charge depending on ambient temperature. Has it been really cold in Jersey?

A couple of thoughts:
AGM batteries are, like regular automotive wet cell batteries, lead-acid. One plate (positive) is lead, the other (negative) lead oxide. No zinc, unless it's a minor element. AGM stands for absorbed glass mat, which means to space between plates is filled with a fiberglass mat soaked with electrolyte, a sulfuric acid solution. The chemistry is the same in both types of batteries. With an AGM battery, the cells are not open vented, but have relief valves. This allows the oxygen and hydrogen that might form from charging to recombine. If AGM batteries are overcharged, the relief valves will open and the hydrogen and oxygen will be released, drying out the mats, and ruining the battery. If a wet-cell battery is overcharged, the hydrogen and oxygen will be released, but if the plates aren't uncovered, water can be manually added with little harm done. The cell voltage is somewhat dependent on the electrolyte concentration, but 2.1 VDC is typical of a fully charged cell.

When I was trying to figure out which, if any, of my four voltmeters was correct, since they all read different voltages on the same 12-volt battery, I saw a reference that a new Duracell AA battery is 1.65 VDC. Eight in series (13.2 VDC) makes a good standard for the average person.
 
OP
RAC68

RAC68

Darth Vader
Offline
Hi All,

Although I thought an 18+ voltage battery reading to be somewhat of an anomaly, It turned out that this issue was not uncommon but more a condition of being unrecognized. This situation is primarily as a result of automotive dash-installed electrical fault indicators being sensitive to insufficient or 0 voltage generation and ignore excessive voltage levels. In shout, the dash warning light will not go on when too much voltage is being created by the alternator or generator and, over time, the battery's chemistry can become more excepting and reflective of this higher voltage level.

Although most automotive electronics can accept up to 15 volts, 18+ volts seems to be on the cusp of acceptance and, as I understand, can cause the ECU to temporarily turn off (causing the car not to start). These higher voltages often too high for oter critical components and transient electronics failures begin to randomly appear. When addressing these random failures, battery load testing often shows the deterioration of the battery and many attribute all the problems to this as a cause and just have them replaced as the correction. However, not recognizing the underlying issue as a failing alternator only propagates the condition and a new battery helps for a short time before continuing the cycle.

During the last couple of days, I have found that a battery with a reading of 18 volts (as I encountered) can be brought back to a fully charged 12.6 volts. Since battery chargers are designed to bring up discharged batteries, the 18 volt battery must be discharged down to a reading of 2 or 3 volt and slowly brought back at a slow 2 amp or less charger setting. I have followed this approach with my Son's original no-maintenance "Rocket" battery and have achieved a fully-charged 12.6 volt reading. Also, by continuing to operate the car with the new alternator, its normal now-regulated charging cycle has brought the installed battery back to a normal voltage reading of 12.6V with the engine turned off.

Hope this will help another confused person having this type of issue,
Ray(64BJ8P1)
 
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