You will need more than just a voltmeter to understand what is going on with your vehicle and its battery, and the system that charges it. But you don’t need much, and you don’t need the latest-and-greatest, most-expensive tools. Figures 1 and 2 show images of the very simple, and rather inexpensive, tools that you need.
Figure 1 shows a small, inexpensive battery charger nominally rated at 6 amps, and it also shows a very simple and inexpensive battery load tester. Figure 2 shows a small generalized wire brush, and a battery terminal-type of wire brush, plus a commonly-available chemical that has proven effective for reducing corrosion on the steel nuts and bolts that secure your battery clamps.
Figure 1 – Small, Inexpensive Battery Charger and Load Tester Devices
Figure 2 – Two Styles of Wire Brush, and a Common Chemical That Cleans Corrosion From Nuts and Bolts
The chemical was intended to clean hard water deposits and corrosion from household plumbing items. However, it does serve our purpose here, which is to clean corrosion deposits from the nuts and bolts of our battery cable clamps. And it works rather well for that. It is a poisonous chemical, so be sure to take care not to expose yourself, anyone else, and especially children, to it.
Problems Usually Encountered
What usually draws one’s attention to batteries and charging systems is a failure to start when you turn the key to “start”. When it works, you never give it a thought. When it doesn’t, you are stumped, unless you have the right tools, and you also know how to use them.
There are two reasons that a battery might not start an engine: (1) it is discharged, and (2) it is “bad”. Those two are not the same thing. Get out your load tester and hook it up. If the battery tests “weak”, it could be either bad or discharged, you don’t know which (yet). If it tests “good”, your problem lies elsewhere.
You do this initial test by hooking the load tester positive cable lead to the positive pole on the battery, and the negative lead to the negative pole. Immediately, you will see the needle read a voltage. This is because the load tester is a voltmeter, wired in parallel with a load resistor sized to draw a large current from the battery. You have to activate the test switch to hook-in the load resistor, however.
A 12 volt battery ought to read 12.0 to at most 12.6 volts at no load. A 6-volt battery ought to read 6.0 to at most 6.3 volts at no load. An 8-volt battery ought to read from 8.0 to at most 8.4 volts at no load. The usual no load that values I see on otherwise-charged batteries are about 12.2 volts, 6.1 volts, and 8.2 volts.
If you see a no-load voltage 2 or more volts below what you should see, you are “done”: you have a bad cell in your battery. Go get a new one. Simple as that.
If you see a voltage that falls in the range of what you should see, then hit the load test switch on the load tester, and hold it there for 5-10 seconds. The voltage will drop as long as you hold the load switch, but hopefully it will not drop too far. The scale on the load tester dial in Figure 3 shows how much voltage drop you can tolerate. It has “good” and “weak” ranges marked, for both 6 volt and 12 volt batteries. The red “replace” range is also shown. “Weak” means you replace it very soon. “Replace” means you replace it right now.
That loaded-voltage drop is almost 2 full volts on a 12-volt battery, about 1 full volt on a 6-volt battery, and although there is no indication on the load tester scale shown, about 1.3 volts on an 8-volt battery. If your voltage drop is bigger than what you should see, either your battery is discharged, or it is no good. You must now find out which possibility is true.
Figure 3 – The Scale on the Battery Load Tester
If you have been running the engine within the last few hours, and you also know your charging device (usually an alternator these days) is good, then you know your battery should be charged as much as it can be. If it has failed the load test under these circumstances, then it has to be “bad”. Replace it.
Otherwise, drag out your small battery charger, and put the suspect battery on “charge” at 3-4+ amps for about half an hour. Put the positive lead of the charger on the positive pole, and the negative lead on the negative pole. Be sure the volt switch is set to the appropriate 6 or 12 volt setting for that battery. Then plug in/turn on your charger (as applicable). Before you walk away, look at how much current it initially draws.
If the initial current is near the nominal max for the charger, you have at least some confidence the battery might not be totally “bad”. That current should drop a little bit (from near 6 amps to near 3-4 amps) during your half hour of attempted charge. However, faster current drops are more likely to indicate that the battery really is “bad”.
8-volt batteries are a bit odd, and most cheap battery chargers are not set up for them. Initially, start out using the 6-volt switch setting, to keep the initial charging current within the operating range of your charger. It will start out a bit low on current. Later, when the current has dropped significantly further, go to the 12 volt setting, which will increase the charging current. As long as it is within an amp or so of the nominal current rating of the charger, you are OK.
If you have a battery with filler caps, loosen them before charging, so that the produced gases get to leak easily out of the battery, without pressurizing the battery. If not, you need to stay at charging currents under about 3-4 amp, no matter what. You set the battery voltage lower with that switch, to control that charging current to tolerable values. That 3-4 amp current restriction applies to all of the sealed battery types. You risk a battery explosion, if you fail to heed this warning.
If your charging current starts out low (2-3 amps), and the battery either holds that charging current, or increases charging current very slowly , as time goes by, your battery is most likely totally “bad”. That’s not to say you can’t still charge it up (over several hours) and successfully use it for a little while yet, but its useful life is now quite limited! Start looking for a new one!
Once you have about half an hour of charge, at 3+ amps, into the battery, try the load test again. If it fails, the battery is “bad”, just replace it! If it passes, you can try to start your engine and go about what you were trying to do, except that you have to worry about why your battery was discharged! And it WAS discharged! Now you know that!
Starter motor failures I am not going to cover here, except to say that if you short the big battery wire to the solenoid start terminal with a screwdriver, and it tries to start, the starter motor is NOT your most fundamental problem! Bad connections, or a burnt/broken wire or bad starter relay (if so equipped, and most are these days) can be. And if you have to ask how to do this starter solenoid short test, you need to seek professional help anyway.
The next most obvious thing to check is the condition of the cable connections at the battery. If these are corroded (or loose), it restricts current from the battery to the starter, and it restricts current to the battery from whatever charging device the vehicle has. Enough current restriction, and you have first have a charging failure, and then a start failure because of a discharged battery. The corrosion deposits are blue-to-white crusty-looking crap on the battery cable connections.
Fixing the Problems
Loose connector bolt torque, you can check with a wrench of the right size for your connections. In a side post battery with multiple cables to a single connection, this loose connector thing is a very common problem! Everybody else, not common at all. But worth checking, in any event.
To fix corroded terminal connections, you need to remove the battery cables (negative first on negative ground systems), and wire brush them clean. The cylindrical battery terminal cleaner in Figure 3 gets this stuff off the top-mount battery posts, and the small wire brush cleans it off the lead-based top-mounted battery cable clamp assemblies, and also side-mount terminals. It’s slow, take your time.
It is wise to remove the bolts and nuts from your battery cable clamps, and submerge them in the CLR concentrate, in a small glass vessel, if possible. I usually leave them overnight, and they look pretty good the next day. Wash them with water before re-installing. Reinstall the positive cable first, then the negative (except for the very rare-indeed positive-ground systems).
If your battery and your starter both test good, then cleaning corrosion deposits off the connections on the battery usually fixes most starting problems. If not, then you may well have a bad starter relay. These are not all that expensive. Just buy a new one and install it. Most likely, that will then fix your starting problem. Every vehicle is different in what these look like, and where they are mounted. Try the internet for images relevant to your specific vehicle or machine.
About the only other problem I have ever seen is a bad wire from the key switch or starter relay, down to the starter solenoid connection on the starter. As these wires age, they can no longer carry the required current. They get hard and resistive from overheating, which limits starter solenoid current, and that causes start failures. Just replace the bad wire with a new one that is a bit larger.
Testing the Charging System
On most cars today, this is an alternator. Some very old cars, it may be an old-time generator. For most magneto-ignition lawn and garden equipment and some old farm tractors, this charger is built into the magneto that drives the ignition. Whatever it is, it has to generate a voltage that is at the very least 1 volt above, and no more than 2-something volts above, your basic battery voltage. 3 is too much.
All you need to do is use your battery load tester as a voltmeter with the engine running. Make sure that your engine-running voltage is at least 1.0 and no more than about 2.5 volts, above your charged battery terminal voltage with the engine stopped. With an alternator, this set of criteria is true, even at engine idle speed. That is what the upper increments show in Figure 3 (marked “low”, “OK”, and a “red” range). This particular tool shows only 6 volt and 12 volt ranges. 8-volt is “in-between”, and about a third of the way up between the 6 and 2 volt ranges. Just read the volts scale.
With a generator or a magneto, the idle speed charging voltage may not be adequate. Speed the engine up, into the low end of the operating rpm range (usually around 1500-2000 rpm), to test that charging voltage increment, if you have an old-time generator or a magneto system. If it still fails, you will need a new alternator, a rebuilt generator, or a rebuilt magneto, as applicable. The closer this voltage increment is to 2.0-2.5 volts than 1.0 volt (for a 12 volt system), the more confident you can be in that test result being a “pass” for your charging device.
That charging voltage increment over battery voltage also depends upon the basic battery voltage. At 6 volts, the min to max increment range is closer to only 1.5 volts. For an 8 volt system, it falls between that and the 2 volt range of a 12 volt system, at about 1.7 volts.
Time In Service
This varies strongly with whatever you paid for the battery. Longer life is more expensive. It is usually marked somewhere on the battery what the “guaranteed” life is to be. There is also usually some sort of indication of when the battery was purchased. Failing that, I usually use a paint pen to mark purchase date and expected life on the batteries that I buy. They very rarely last longer than the guarantee, these days.
If you are not sure what the load test really reveals, but you are sure the battery has exceeded its guaranteed life, then the wisest choice is to presume it has gone “bad” from old age. Just buy a new one. It is just not worth the trouble to “coddle” an over-age battery any further than when it first shows signs of failing.
None of this advice applies to anything but lead-acid batteries used to start engines. Batteries that power hand tools, and batteries used in hybrid vehicle propulsion, are SPECIFICALLY EXCLUDED from this discussion!
If you have filler caps, then open them every several months to a year, to check fluid levels in the cells. If they are low, refill them to the mark with clean, pure water. Use distilled water, if your local water supply is mineralized. If your water is “soft”, then tap water will serve as well as anything.
Otherwise, there is just not very much to do, except keeping the corrosion crud off the connections.
Be sure to put an idle battery on charge every so often, if it sits for months at a time without any use. They do self-discharge over time (3-6 months max). Keep the charging current low (under 4 amp). Charge for around 12-24 hours, but no more.