Weird battery problem

Hey guys, 

My son's Pontiac Vibe battery is acting weird. He doesn't have his license yet, so it has been sitting around for a while. I had been trickle charging the battery all last winter and moving the car around from time to time. Now a week and a half ago we needed to move it because we were having a graduation party for him and we needed the parking space. 

So I broke out my jump box and we fired it up. Moved it out back, and it has been sitting there since. Today I'm off from work and decided I was going to try to vacuum down the A/C system that we were working on a month or so ago. I jumped it again and pulled it in front of the garage. I decided I would hook my battery tender to it and let it charge while messing with the A/C. Hook up the leads and get a flashing red light. Battery tender says that means battery hooked up incorrectly. It's not hooked up incorrectly. However I notice the positive connection is corroded and decided to remove the cable and clean up the connection. In the meantime I hooked the positive lead from the charger directly to the battery post. Now I have a solid red light meaning charging and a flashing green meaning battery is 80 percent charged. With the cable attached I get the incorrect battery flashing red. What gives? 

Is the key on? I have seen key on cause battery tenders to be weird. 

Are you sure both battery terminals are clean and making good contact?

Professor_Brap (Forum Supporter) said:

Is the key on? I have seen key on cause battery tenders to be weird. 

No key in the ignition even. 

That's an oxidization agent on the posts. 

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Charging according to the battery tender. 

Not charging, because battery hooked up incorrectly according to the battery tender. 

What does the battery tender do on another battery? Or can you put a different battery in the vibe and see if it charges correctly? My guess is the vibe battery is doo-doo. 

sergio said:

What does the battery tender do on another battery? Or can you put a different battery in the vibe and see if it charges correctly? My guess is the vibe battery is doo-doo. 

Just checked the battery with my midtronics battery tester. 57 CCA. Yeah it's bad. 

Okay, so here's a question. Why did the battery tender say the battery was 80 percent charged if it is essentially dead? 57 CCA is not very much, but the battery voltage was reading over 12 volts. Is that why the battery tender was acting funky? 

At a guess, it's shorted internally. That really pisses off automatic chargers. 

A battery can have over 12 volts but if it has a bad cell it won't start a car. I've seen batteries have 12.5 volts and drop as low as 3 volts when the key is turned to start. A charger can leave a "surface charge" , but the battery doesn't actually take or hold a charge.

Gotta break this down a little.  Car batteries have six 2.1v cells in them hooked up in series which adds up to ~12.6v when fully charged. Unless a battery charger was fairly expensive it's probably only looking at voltage to determine 'state of charge'/SOC. A 12.6v battery IS fully charged, but that doesn't really tell you anything about how it will act under use because you could build a 12.6V battery the size of a nickel and it clearly wouldn't start a car. Capacity and the rate at which you can get that capacity out are different things that a typical charger wouldn't be able to figure out by itself. The CCA measurement you took is an attempt to figure out the rate at which you can get the charge out.  'Starting' batteries are designed to favor quick release of energy over a higher total amount of energy. This is also referred to as a C rating although it's usually not used when talking about car batteries.  Anyway, the CCA measurement is usually an extrapolation and conversion because any handheld tester with ~12ga or smaller jumper leads is not going to ACTUALLY flow 800 amps through it when you try to test an 800 CCA battery (or 600, or 500, or anything over 30 amps really). They apply a small load (which is an internal resistance of a precisely known value) and see how much voltage drops during that. From knowing how much the voltage drops and how much resistance is in the tester itself, it can extrapolate how much resistance is in the battery itself, and then convert that into a CCA equivalent. So basically, it's doing a very light load test and adding the Scientific to Wild A$$ Guess (swag) to guesstimate what the battery would actually do if you hooked it to a real car and cranked it. Usually when you do a real load test (hint: if the cables you hook to the battery aren't massive, it's not a real load test) it will generally do WORSE than what those tools say. So that's all about internal resistance. When a battery has high internal resistance, it drops a bunch of its voltage internally and what you see coming out is very low voltage. So when you take a 12v battery and crank a car and it drops to 3v, that's telling you that due to high internal resistance the battery is burning up the majority of its voltage as heat inside the battery before it ever comes out the terminals. You might still be able to get a lot of energy out of that battery, but you'd have to do it more slowly.  Then there's actual capacity which we never really measure on car batteries but we have some clues about. For example, if you charge a battery with low voltage and the voltage rises very quickly, that's a clue that the actual capacity is low. Likewise, when you do a load test or crank an engine, if the voltages jumps back up close to where it started afterwards then you still have some reasonable amount of capacity. But if every time you apply that brief load the battery is only bouncing back up to .5 or 1 volt lower than where it started, you are using a huge amount of its capacity each time which means the battery has very low capacity now. 

So basically with the whole saying the battery is charged but then testing 57cca, the battery probably is charged but has high internal resistance which means you cant get the charge out fast enough to crank an engine. You may still be able to use it to power a small work lamp for hours or something like that, but it can't release its charge fast enough to be a good 'starting' battery any more. 

again, i raise my glass in the general direction of Vigo. he smarter than a motherberkeleyer, and good at splainin' things too.