When test eqipment lies?

[KeepItSimpleStupid]

Ok, here's the deal. In January, I did a Hydrometer test of my car battery and noticed that one cell was weaker than the others.

In June, I had some car work performed at the dealer and I asked them to check the battery. Their fancy tester says everything is fine.

The car had been sitting for about 5 weeks and I had it out the night before. The next day, embark on a 40 mile trip to a major city in the US. I get to my destination, and idle the car with the AC on for 5 minutes or so and the battery light comes on just before the car dies and would not restart. I did check the alternator belt when the car was stopped. I just pushed on the belt,

I finish my business and get in the car and drive to another parking lot and I parked under a tiny tree. I idle there for a while and the car dies and would not restart. I do my business and call for towing.

The tow truck arrives fairly quickly and I get towed to a shop about 4 miles away. They said the alternator belt snapped and they could not perform the electrical testing until they replaced that, So, in about 45 minutes thay say everything is fine and I'm on my way home. I stopped to eat a few blocks away. Car started.

4 days later, I tried to start the car and the battery is dead. I had turned the dome light (changed to LED based some time ago) to the off position days earlier.

My troubleshooting the next day revealed all of the cells were dead (Hygrometer) except one which had a tiny indication of charge. Car jumped fine. I did my two quick and dirty tests:
1. Voltage across battery with engine running: 13.8 and doesn;t vary with RPM, so OK
2, From a previous measurement, of the voltage ACROSS the lead connecting the battery to the alternator, I get about 300 mV which indicates charging.
3. Another test I might perform is to take a scope and look at the ripple, but I didn't do that.

So, two shops telll me my battery is fine and a $4.00 piece of plastic told me it wasn't.

I asked the 2nd shop whether their tester measures ESR (Effective Series Resistance) and got "I'm not sure". I do have the ability to perform a "load test", but I didn't do that either.

So, what happend?

Do, the shops not know what a hydrometer is?

 

[nsaspook]

The SG measurement is the primary indicator of SoC so always trust that. You've been under-charging the battery for a while with the loose or broken belt so it's hard sulfated and most likely will never recover to full capacity as most of the current from the ALT is just being conveted to heat instead of moving ions across the plates. It's time for a new battery.

 

[tcmtech]

I went around with Sears some years ago over one of my Diehard batteries going bad.

Their fancy little handheld digital tester somehow measured that it was putting out 700 cranking amps at 11.5 volts through a set of 18 ga wires three feet long after being connected for less than 5 seconds.

I insisted they do a real test with their old style analog load bank machine that put a full and honest 700 amp load on it with big heavy clams wires and a large resistance load.

That machine said the battery was bad after about 2 seconds and outright toast at 5 seconds with a real honest load on it!

 

[Mr RB]

I replace auto batteries when they show any hint of being faulty. It's normal to replace them every year or two especially if you get real cold (or real hot) weather.

Be careful buying new batteries, to only buy the type that has external acid in a container, where you put it in yourself. That's the only way to get a "new" battery. If the acid is already in the battery it will degrade the battery very quickly unless it is in regular use in a vehicle. Be extra aware with a shop that has a row of batteries ready to use, not only have they been degrading there on the shelf, but it is in the shop's best interest to sell you the oldest one in the bunch...

 

[HiTech]

The Specific Gravity test is still the gold standard. A battery is a chemical reaction inside a container. Electronic testers are a safer, convenient method to that of handling the corrosive electrolyte, especially the sealed LA batteries that don't have removable caps. In today's business world where everything centers around efficiency and risk management, service garages and similar services don't want their employees exposed to sulfuric acid fumes, let alone splashes, etc. I still rely on SG when testing a battery.