Caution: Be careful to follow the battery manufacturer’s precautions when testing, as well as the tester manufacturer’s directions, and read the vehicle (or other system) manual before doing any test, or even disconnecting the battery.
But before testing a battery, you should know a little about how batteries work, and why they degrade over time.
Why Batteries Get Old
- Electrolysis
- Sulfation
- Stratification
Batteries store energy chemically, by using a series of plates arranged in a liquid or gel called an electrolyte. Rechargeable batteries store energy by dissolving an acidic or basic compound in the electrolyte. The battery releases energy by allowing this chemical to solidify onto a plate. In the case of a lead-acid battery, the acid ‘lead(II) sulfate’ (PbSO4) is dissolved in sulfuric acid. In the case of ‘gel’ batteries, the electrolyte contains sulfuric acid and fumed silica, which become a gel when mixed.
Overcharging can cause problems with a lead-acid battery, including loss of capacity and even explosions. This is because overcharging causes electrolysis, also known as ‘gassing’, which is the separation of water into hydrogen and oxygen. The loss of water can cause problems with electrolyte levels, and if the surrounding air is stagnant, the hydrogen and oxygen gas may explode.
Over time, as the battery is charged and discharged, the lead sulfate (or other compound) starts to crystallize on the plates, forming a stable solid mass in the battery; this is called ‘sulfation’. The crystallized compound cannot be dissolved and solidified by the battery, so the battery loses ‘capacity’.
Some batteries also lose capacity because of a process called ‘stratification’, which is when frequent discharging (and charging) of the battery causes the acid (or base) to concentrate around plates, and sink to the bottom of the battery. Stratification can cause an increase in Stratification can be mitigated by mixing the electrolyte (while the battery is charged); a vehicle in motion usually mixes the electrolyte by itself. Be careful to follow the battery manufacturer’s instructions and precautions if you plan to mix the electrolyte yourself.
Carbon Pile Battery Testers
- Attach the carbon pile battery tester leads to battery terminals.
- Press the test button or switch for the required amount of time.
- Observe how much the battery voltage drops under load.
Carbon pile battery testers are the simplest type, are intuitive, easy-to-use, and can be very effective. The carbon pile is a large (low-value) electrical resistor, and the ‘tester’ works by measuring how much current the battery is able to discharge through the large resistor. Carbon pile battery testers usually have markings on their gauge which indicate what is expected for a given battery size or rating, and some can be adjusted to provide different loads. This is a very effective means of testing batteries, especially when trying to measure cold-cranking current, because the battery is subjected to a load almost similar to the one it will experience in service.
The carbon pile battery tester must be sized to correspond to the battery under test, and too small of a carbon pile will give incorrect results; some testers have a variable resistance, allowing you to test a range of different battery sizes. Another disadvantage to carbon pile battery testers is that you must be able to load the battery, which may require disconnecting it from the circuit it is attached to, and could damage the battery or system/vehicle. The carbon pile in these testers will eventually crack, either due to a shock load (from being dropped or hit), or from fatigue due to heating and cooling during regular use.
Hand-Held Battery Load Testers
- Connect the hand-held load tester leads to battery (observing electrical polarities).
- Turn on tester, and input battery specifications (e.g. size, capacity, cold cranking amps).
- Press the test button to perform the test.
- Read the results from the tester’s screen or print-out.
Small and light portable battery load testers work very similarly to carbon pile battery testers, by measuring a battery’s response to a large current draw, but these do it for a short amount of time. Some hand-held battery load testers also feature more advanced mathematical analysis of the battery’s response to the sudden load. These devices are usually digital, they will may give you a pass/fail indication, a reading of the cold-cranking amps, or the capacity of the battery.
These hand-held testers are very light and portable, making them useful for technicians on-the-go, but they often require that a battery be disconnected from its vehicle/system, and many of these testers give inconsistent results depending on the battery’s temperature and other factors.
Battery Hydrometry Testers
- Open the battery (carefully).
- Insert the hydrometry tester’s tube into the electrolyte.
- Transfer the required amount of electrolyte into the tester (usually via a pump or syphon).
- Observe the tester’s reading.
- Return the electrolyte to the battery.
- Calculate the amount of energy stored in the battery (if necessary).
These tools help the user to measure the density of the electrolyte in a battery, and if you also know the volume of the electrolyte in the battery, you can calculate the acidity (or basic-ness) of the electrolyte, which indicates the amount of energy stored in the battery. If the battery is fully charged, it is usually assumed that the difference between the amount of energy stored in the battery and the battery’s original specified energy capacity is due to sulfation. Many battery hydrometers have markings which indicate what the relative state of a (certain chemistry of) battery is; these testers can give you an idea of the ‘health’ of a battery without requiring math.
Battery hydrometry testers can be dangerous and confusing to use, but are often the most effective tool for large lead-acid batteries, such as those found in forklifts, scissor lifts, and backup power supplies. Since the test requires opening the battery, and removing electrolyte, it only works on non-sealed, liquid-electrolyte batteries.
Battery Conductance Testers
- Connect the battery conductance test leads to the battery terminals (observing polarity if necessary).
- Turn on the tester and press the test button.
- Read the results from the battery conductance tester’s display or print-out.
Testing with a battery conductance tester differs depending on the tester and the electrical circuit connected to the battery. In most cases, the test takes a few seconds, after which the tester will display either a pass/fail result, or an estimate of the battery’s capacity and capabilities.
Battery conductance testers work by sending a small current through the battery, and measuring its electrical resistance; they then use this information along with a mathematical model of the battery (, and any data the user may have input) to calculate the battery’s performance parameters. These are often the easiest battery test tools to use, and can usually perform tests without disconnecting a battery from its vehicle or system. These tools are called ‘conductance’ testers because the inverse of resistance (one divided by the resistance value) is called conductance, and it is a much larger number than the resistance in the case of a reasonably-sized battery, which makes it easier to talk about.
Different conductance testers use slightly different methods to compensate for the inductance and capacitance of a battery, so different testers can give different results, depending on how they were designed and calibrated. Conductance testers must also make complicated calculations, which may not give accurate results, because battery resistance is affected by its electrolyte/dissolved compound condition, the condition of the plates, the size of the plates, the plate layout, and the battery’s temperature. Given that most users do not have access to detailed design information about a battery (aside from rated capacity, size, and cold-cranking current), you should be careful in selecting a conductance tester, and even more careful in interpreting the results.
Testing Battery Open Circuit Voltage with a Multimeter
- Turn on the multimeter, and select the ‘DC voltage’ mode.
- Connect the multimeter test leads to the battery terminals.
- Read the voltage off the multimeter display or gauge.
- Be confused by the ambiguous result.
Checking a cell’s voltage is as simple as turning on the multimeter, switching it to the ‘DC Voltage’ setting and connecting each of its test leads to a battery terminal; the battery’s voltage will be displayed on the DMM, and all you have to do is interpret that number…
This is the most common method of testing a battery, being simple and inexpensive, but the results are usually inconclusive. The biggest problem with using a multimeter to check a battery’s condition is that you will only spot complete failures, where there is little to no compound to react, or almost no plate area to react on. Even an almost completely ‘dead’ battery can produce enough current for the multimeter to detect a ‘good’ cell voltage; this is known as ‘surface charge’. The surface charge produces this result because multimeters have very high input impedance (>1 MOhm), thus requiring very little current (<12 uAmps for a 12-volt reading).
Testing Battery Voltage Under Load with a Multimeter
- Turn on the multimeter, and select the ‘DC voltage’ mode.
- Connect the multimeter test leads to the battery terminals.
- Read the open-circuit voltage off the multimeter display or gauge.
- Load the battery with the vehicle’s starter motor, headlights, or other loads.
- Read the voltage under load off the multimeter display or gauge.
- Compare and interpret the results.
By checking both the battery’s open-circuit voltage (with no load), and the battery’s voltage under load, you can detect large problems with the battery, such as high levels of sulfation, which will result in a large drop in voltage when the battery is loaded. This test can also help you catch under-sized or badly-selected batteries, which drop below their safe minimum voltage when loaded; an example of this might be a deep-cycle battery being used as a starter battery. If you have a clamp-meter, you can also measure the battery’s current output to gain some additional insight into the battery’s current-voltage relationship.
This test is easy to do, and digital multimeters are much more common than battery testers, but it requires you to interpret an ambiguous number. It is also difficult to detect batteries which are failing due to electrolysis with this test. Most importantly, not all situations allow for this type of test, and you have to be careful to follow the device or vehicle manufacturer’s precautions, which may not allow the battery to be tested for a sufficient amount of time. Another problem with this test is that it has a very limited range (as you usually cannot vary the load by much).
That’s All!
If you’re looking for an easy-to-use and reliable, clamp meter, check out our CM100 1mA to 100A AC/DC Clamp Meter.
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