Subscribe to a magazine

Auto Battery Charging & Battery Test - Willie's Workbench

Willie Worthy | Writer
Posted June 1, 2009

Battery Charging and Testing Tips

The voice on the other end of the phone had that "upset sound" to it that you just knew that he had messed something up and was going to ask me to bail him out.

It came out, "Willie, I think that I really screwed up! I was trying to jump-start my Jeep and hooked the battery connections backwards--like man, I got the negative to the positive and the positive to the negative. It sparked a lot, but I thought that was just because one battery was dead and the other was charged, then I realized my mistake. What kind of damage did I do?"

Well, I told him there were three things that could have happened: (1) the battery could have exploded, but obviously that didn't happen or he would have told me he was calling from the hospital with chemical burns; (2) it could have fried the computer, and (3) killed the diodes in the alternator. I went on with the fact if he had done any damage, the easiest way to find out would be hook up the jumper cables correctly and try starting the Jeep. It started right up, so that was a good sign that the computer was not fried. I had him disconnect the jumper cables and check the voltmeter to see if it was showing a charge. It was, so he got very lucky.

This is a good opportunity to review Battery Jump-Starting 1A. Get out your jumper cables. Don't even bother with those that you can buy for 13 bucks at your discount store. The cables may look big and heavy duty, but most likely they're nothing more that 10-gauge wire with a lot of stiff plastic insulation around them. A good pair of jumper cables will cost you around 25 to 30 dollars. They should be at least 8-gauge wire. My choice is at least 6-gauge, and I have a set that is 2-gauge. (When it comes to wire size, the smaller the number, the bigger the wire.) Eight feet or longer of cable length is a good idea because you may not always be able to get right up against another vehicle. Remember, the longer the cable, the larger the cable size should be. Think for a minute--the cable going from your battery to the starter is about three feet long and is generally either 4- or 6-gauge. Why would you want to try and start a vehicle with a set of jumper cables that had a longer length and half the wire size?

On to the hookup: Shut off the starting vehicle. Grab the clamp with the red end and put it on the positive battery post of the dead battery. Find the red clamp on the other end and hook it to the positive battery post of the starting battery. Make sure the teeth bite in and make a good contact. Pick up the black clamp and hook it to negative post of the starting battery. Actually, you can hook it to any place that has a good solid ground, but the battery post is convenient. Now this is the important part: double-check that the connections are right, and then hook the other black clamp to a clean solid metal ground that is connected to the stalled vehicle's engine block. That means the exhaust manifold, or a mounting bracket for one of the accessories. It doesn't mean to the radiator or an A/C metal hose. You want to be standing as far away from the battery as possible now. Why? When you make a connection, you may get a spark. That's the current trying to flow from the good battery to the bad battery. Make sure the jumper cables are not laying on the exhaust pipe, or can get caught in the fan or the accessory pulleys and belts. Start the vehicle with the booster battery and make sure all accessories are off--lights, heater, A/C and such. Run the engine at a fast idle--okay, a bit faster than a fast idle: About what the engine speed would be when driving, let's say 2,000 rpm. This is to make sure that the alternator is putting out maximum power. Wait a few minutes before trying to start the stalled vehicle, as this gives the battery a chance to recover a bit and store up some electricity so all the starting current doesn't have to flow through the jumper cables. This is especially important it you're still trying to use those cheap cables.

Once the stalled vehicle is running, carefully remove the jumper cables. Remove the last cable clamp you hooked up first--that would be the one to the engine block; then you can remove the others. The reason for this is kind of old-school, but it's still a good safe habit to get into. When a battery is being charged, especially at a high rate like our dead battery, it gives off explosive hydrogen gas, which being slightly lighter than air, "floats" just above the battery. Removing the battery jumper cable clamps will produce a spark that could and has resulted in an explosion (ever heard of the Hindenburg?). This is no way near the problem that it used to be before sealed batteries, but there still is the possibility.

OK, you might think that a battery is a pretty simple thing, but in reality it's one of the more important items on your vehicle. When it fails, you're S.O.L. or "simply out of luck." I have seen what amounts to plain old abuse to a battery and the owner still expected it to perform--things like missing hold-down brackets that let it move around in its holder, using an undersize battery for the demands placed on it, or letting the posts and other electrical connections get corroded.

So let's back up a bit and talk about batteries in general. Consider the battery a storage tank full of electricity. When fully charged, with the temperature around 80 degrees Fahrenheit, the voltage reading across the two posts will be about 12.6 to 12.7 volts and at a 75-percent charge about 12.4 volts. For those of you who know what a hydrometer is, and have non-sealed batteries (that is, having removable caps), the specific gravity of the battery acid should be at 1.250, again at 80 degrees. Yes, there are still batteries with removable caps, and yes, you periodically have to do some maintenance on them such as adding distilled water to bring the level up to above the plates, usually to a ring in the fill tube. What happens to the water? Heat from charging, as well as the ambient temperature, causes the water to evaporate but leaves the acid behind. With a sealed battery, there is a special material that absorbs the water and then recycles it back into the acid. Ok, batteries like Optima and Odyssey have acid in them too, but in an entirely different form which is beyond the scope of this month's "Workbench."

Remember I used the temperature number of 80 degrees, as that is the standard that all battery charges are measured at. This means that for us who live in the frozen lands of the Northwest, at zero degrees the battery has lost 40 percent of its cranking power. This means that 2.5 times as much power is needed to crank an engine at 0 degrees as is needed to do the same job at 80.

OK, now why did the vehicle fail to start in the first place? Has the battery reached the end of its useful life? Did you do something stupid like leave your lights on? Are the cables corroded at their mounting locations to the point where there is not a good transfer of electricity? Even if the cable-to-post connection is tight and clean, what about the starter connection or the ground to the frame or engine block? These are just as important. Is the alternator charging?

Here is a quick test of the alternator to see if it is charging. On the engine side of the alternator, in the middle of the case, there is usually a steel cap or plug. This is what supports the rear bearing of the armature. Carefully take a screwdriver and touch the cap while the engine is running. If you feel a slight magnetic pull, then the alternator is charging. If you have a hand-held volt meter, hook it up to the posts of the battery and you should have somewhere between, say, 13.9 to 14.7 volts. This figure may vary a bit, depending on the design of the alternator and the operating temperature. If the outside temperature is, say, 20 degrees, then the alternator may put out as much as 15.9 volts, and if it's really hot, then it might be as low as 13.3 or so. Remember, we're just troubleshooting, not looking for exact numbers. We just want the voltage to be above the 12.6 of a fully charged battery.

A quick check of cable connections is to measure the voltage at the battery and then off the alternator's positive post. A difference of, say, a half volt indicates a problem either with the cable or the connections. Perhaps you have a circuit that is putting a drain on the system. This would be the equivalent of a slow leak in a water pipe. Again, think of your battery as a storage tank and the alternator as a well pump that resupplies it. When the well pump is off and you have a leak in the system, the tank will eventually go dry. It could be as simple as some lights being left on, such as a glove box light, or a wire going to ground. Some people may call this a "short", which is short (pun intended) for a short circuit or electricity taking an easy path to ground. Things like this can really try your patience.

A quick way to find the problem is to disconnect the negative battery cable, then place a 12-volt test light (everyone should have one of these in their toolbox) between the cable and the negative battery post. If the light comes on, you have a drain. How do you find where it is coming from? This is the fun part. Start pulling fuses, one at a time. When the test light goes out, you have found the source of the power draw, or at least the circuit that it is being drawn from. Keep in mind that things like a clock or radio constantly have current to them, so you need to pull those fuses first.

There are some ways to test your battery to see if it is up to par. It's called a variable load test, and for it you need a special tester--yep, you're right, it's called a Variable Load Tester. However, a quick and easy test can be carried out with a simple volt meter. Make sure the temperature is somewhere in the 80-degree range. Charge the battery up to 12.6 volts and then crank the engine over for 15 seconds. OK, you can't do that without the motor starting, can you? Forget that, and turn on the headlights, heater, driving lights and anything else that draws electricity for three minutes or so. Now have someone turn the starter on while you have the volt meter connected. The voltage across the battery terminals should not drop below 9.6 volts. There is one slight problem testing it this way. If your starter is on the downhill side of its life, it may be drawing an excessive amount of current and will give you a false reading. (Conversely, if you know your battery is good, it's a quick way to tell the condition of your starter or the cables and their connections.)

About those corroded connections. Use the old standby, baking soda. Just make up a watery paste and brush it on the connections. It's even better if you disconnect them when you do it. Just like you learned in ninth-grade chemistry class, the soda is a base, and the corrosion was formed by an acid, so it bubbles. A small wire brush helps things along. In fact, they even make a special brush for cleaning off the posts and interior of the battery clamp. Wash everything off with fresh water, lots of water, and reinstall. (It's not a bad idea to wear some safety glasses when you do this.) When they are bright and shiny again, put a few drops of oil on the terminals.

Comments

Advertisement