Look for the neutral safety switch / range sensor on the transmission, by the shift lever.
A good, fully-charged battery will measure 12.6 volts. If it is higher right after it was on a charger, that is due to the "surface charge". That is the abundance of free electrons that haven't been absorbed into the plates yet. Turn on a load, such as the head lights or heater fan, for about ten seconds to remove that surface charge, then the voltage readings will be more accurate.
A good battery that is not fully-charged will measure around 12.4 volts, and a good, fully-discharged battery will read close to 12.2 volts. An additional clue that a battery is discharged is the voltage will drop a lot when any load is applied, like head lights, and especially when trying to run the starter system.
If you find 11 volts or less with no load applied, the battery has a shorted cell and must be replaced.
As batteries age, the lead flakes off the plates and reduces its storage capacity. Voltage will still be unaffected, but the length of time the battery can run anything will become increasingly shorter. This is where it loses its ability to dampen the harmful voltage spikes '87 and newer GM generators develop. That results in repeat generator failures and elusive engine running problems that defy diagnosis. The voltage spikes can damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals. To reduce the numerous repeat generator failures, always replace the battery at the same time the generator needs to be replaced, unless it is less than about two years old.
When charging a battery, expect to see the current shown on the meter to be close to 0 amps if it is totally discharged. It takes time to fill the electrolyte with electrons so it becomes conductive. That takes as much as five to ten minutes, then you'll see current begin to increase. Ten to fifteen amps is a good charge rate for an hour or two. The easiest way to know when charging is complete is the battery's voltage will rise and oppose that of the charger, so current will decrease. When current drops to five amps, the battery is charged and current will not drop any further. Continued charging will just run more electrons through the plates and electrolyte, without any of them being absorbed into the plates. That will generate heat which can boil the water out of the acid, and promote that lead flaking off the plates.
As a side note, all AC generators, ("alternator" is a term copyrighted by Chrysler), develop three-phase output current which is very smooth and steady. Home battery chargers rectify 60 Hz house current and put out a heavily-pulsing 120 Hz DC current that goes from 0 volts, up to around 20 volts, back to 0 volts, then starts all over again. The current switches between maximum and no current 120 times per second. That on and off pulsing will cause the battery's plates to vibrate, and that helps to shake off some of the lead. This is why charging an old battery aggressively can destroy it. As more and more lead flakes off, the storage capacity is reduced, as I mentioned previously. When enough of that lead collects in the bottom of the case, it builds up high enough to short the plates together. There is no solution for that other than to replace the battery. Any older battery, or one you don't know the history of, should only be charged at a slow rate, ... No more than ten to fifteen amps. Slower for longer time is better than a quick, high-current charge. That will reduce the intensity of the vibrations the plates are subjected to.
For one more point of great value, we read way too often here what happened after someone connected a battery or jumper cables backward. When using jumper cables connected to a second car, "you want more of the 12 volts", so the donor battery's positive is connected to the dead battery's positive, and the negative is connected to the negative. If the jumper cables are reversed on one battery, both batteries will be "in series", which is the same as having a dead short across a 24 volt battery. Sparks are going to fly like from an arc welder, and something is going to melt. Be aware too that batteries give off explosive hydrogen gas, and sparks can ignite that. To avoid that possibility, it is standard practice to connect the positive jumper cable first to the two batteries, then to connect the negative cable to one battery, then the other end to a paint-free point on the engine. The spark, regardless how small, will occur where the last connection is made, and we want that to be not near the battery. That clamp on the engine should also be the first one removed. Once it is removed, no sparks will occur at any of the other clamps.
When you connect the car's battery cables to the battery, a real lot of damage can be done if they're connected backward. The negative cable will almost always have a second, smaller wire that bolts right to the body sheet metal. Since the early to mid '90s, the positive cable will have a second, smaller wire that bolts to the under-hood fuse box. There will always be a large "+" and "-" sign next to the positive and negative posts. Some battery manufacturers still use a dot of red paint on top of the positive post, and a green dot to identify the negative post. Most importantly, the positive post is always larger in diameter than the negative post. If you find yourself reaming out or otherwise resizing both clamps to fit, you're doing something wrong.
For my final parting thought, do not waste your money on "juicy rings" or spray-on chemicals that are supposed to prevent corrosion on the battery posts. As the battery ages to the point that half of the lead has flaked off the plates, you have a battery half the electrical size as you originally had, but the charging rate stays the same from the vehicle's charging system. Only half of the electrons can be put in storage. The rest generate heat, and that results in excessive gassing and bubbling of the electrolyte. The bubbles splash up onto the bottom of the top cover, and to the joints between the cover and the posts. That is where the acid sneaks out and collects under the cable clamps, and is what causes the white corrosion. Any time I saw that corrosion, such as during other routine services like oil changes, it was safe to warn the owner their battery was going to fail within six months. You will not find that corrosion on newer batteries that are still in good condition, and therefore there is no need for those acid neutralizer products.
Monday, February 6th, 2017 AT 5:16 PM