'90Cherokee

The dash voltmeters almost always read lower than battery voltage and when more accessories are turned on they get even farther from battery reading. A fully charged battery at rest should read about 12.7-12.8, not 13.5-13.7.

dave1123

Well if that's the case, why does my digital voltmeter show 13.35 on my battery with the negative cable disconnected?

Bugout4x4

Here's the deal Gentlemen...I am completely up to speed on how a voltmeter works But a voltmeter does not show how much current in amps are being consumed or putout by the system.


A voltmeter in a car is pretty much just a fancy idiot light designed to tell a layman driver whether a system is charging or not. It doesn't really accurately show how much current the system is asking for and how much current the system is working to try and satisfy this demand. There is a reason why most modern aircraft still use ammeters, even the Space Shuttle had amp gauges that could be monitored remotely from earth


OK...so you have the alternator maintaining a flat 13.7 v, but how much demand in current are the accessories drawing? How many amps are required to run that powered subwoofer system, headlights, engine management system, the added trailer light/electric brakes, or even when you key up the CB radio you have cranked up to 250 watts? How many amps are required to use them all at the same time? A voltmeter doesn't show us this.


While we have been using voltmeters there have been many technical advances in ammeter designs and hook ups. Ammeters are now coming back into demand as a necessary requirement especially in the electric and hybrid car systems. In no way could a voltmeter alone be used to regulate the engine/motor load management systems. The loads have to be monitored using an ammeter.


Ammeter circuits are not what they used to be. There are remote shunt designs that send only minimal current through the gauge and only require small gauge wires. There are remote "Hall effect" or induction pickup systems that are simply installed around the outside of the alternator output cable and some can be installed as just a secondary parallel circuit with integrated shunt. All of these are now extremely accurate and many are even digital


But back to the original point...An alternator can indeed be reading correctly at 13.7 and still be producing far more amps than is required for the system demand and cook a battery if the regulator has issues. Only an ammeter can tell you this.

Bugout4x4

A very good or brand new battery can sometimes read over 13v. The new one I bought just the other day read 13.2 before I put it in

dave1123

Well there you go. My battery is only 2 months old. It's a group 65 Willard Classic 650 CCA with electrolyte monitoring caps. 800 CA @ 32*

I just parked it about 3 hours ago after a long run and it reads 12.62. The dash gauge reads about the same. When we installed it, it read 13.35 but it was just off the charger, which it didn't need. BTW, the temp outside right now is 37*

I knew about the shunt sensor, but I had forgotten about that part of my history. You tend it forget about stuff you don't use much anymore. I seem to remember a snap-on ammeter for starter draw amps. I also remember being told that if a 12V starter draws more than 600 amps, it should be replaced. Is that right?

GETTING BACK TO THE ORIGINAL QUESTION, What would cause the alternator to overcharge the battery enough to boil it?

Bugout4x4

Yep...Ambient temp plays a huge role in voltage and cold cranking amp output of the capacitor called a battery.

'90Cherokee

Most likely because it had not settled back to it's resting voltage. Depending on how a battery is charged they'll sometimes sit above their normal voltage for a long time. A small load should quickly remove the surface charge and settle it back to a 'normal' voltage.

'90Cherokee

600 I think would be for a big diesel engine. Our starters probably pull under 200. I haven't tested one in so long I don't recall what normal numbers are.

If it's overcharging it would most likely be a bad voltage regulator. Tho it can be replaced, if the alternator is old it's rarely worth working on and replacing it is usually the way to go. If your charging voltage isn't above 14.5 it isn't overcharging. If your charging voltage is never over 13.7 at the battery (I think that was your number?) it's undercharging and likely the cause of the batteries dying. I believe you had it tested at a parts store, try another one and make sure they're using an analyzer not just a voltmeter. A good analyzer will have you go through steps of applying loads and changing RPM. If you were local I'd run a free test on it as I have this equipment. If you have any doubts about the amp/volt disagreement between bugout & I you can check with a battery manufacturer. You'll probably find a lot of info one their websites or you could possibly call their tech department.

dave1123

I agree with your assessment that it's the voltage regulator, HOWEVER on my 97 ZJ AND my 00 WJ, the voltage regulator IS THE PCM! It somehow controls the voltage and charge rate with the use of the battery temp sensor. I think the temp sensor is used as a monitoring devise, not a control.

As far as the high initial voltage was concerned, the tech at Battery World used his voltmeter after taking it off a rack of new ones maintained with a trickle charger. He chose the replacement to match the group size I already had in the jeep. It had a sealed Interstate battery in it but I told him I'd like one where I could monitor the electrolyte level. He was amused because he told me nobody ever checks that. I said "That's why they come back it buy new ones, right?"

The Interstate gave me no warning, it just all of a sudden started cranking slow. I checked the date. 4 years. Okay, time for a new one.

dave1123

Okay Jonesea74, are you still with us? What's going on with your jeep?

Bugout4x4

We ran him off...


Sorry about that Jonesea, I hope it was an educational ride...

dave1123

Damn, I hate when that happens.