Home >> News >> Battery Knowledge

What does a battery load test determine?

Mar. 15, 2022

What does a battery load test determine?

A battery load test is used to determine the condition of a battery by applying a load to it for a specific period of time. This is usually done at an auto parts store, garage or battery store. During the test, a technician checks the voltage of the battery and checks for any leaks.

If your battery fails the load test, you may need to replace it. If you're not sure how much your car repairs will cost, we can help you get a free estimate from a local mechanic.

What is the battery reload test?

A heavy-duty test is a test in which the battery is discharged at a rate that can cause an excessive increase in battery temperature. The purpose of this test is to determine the battery's ability to deliver high-rate current without damage, and to determine the effect of this discharge on the battery's capacity and performance.

The load used for this test shall be capable of dissipating half the ampere-hour capacity (C) within 15 minutes. The temperature rise should not exceed 40° C (104° F) while maintaining the ambient temperature at 27° C (80° F). If the ambient temperature cannot be maintained, the temperature rise should not exceed 50° C (122° F).

In this case, the time it takes for the battery to discharge should be between three and four hours. If the time exceeds 4 hours, a 1/3rdC load should be applied. When discharging with a 1/3rdC load, the allowable temperature rise does not exceed 50°C (122°F). At an ambient temperature of 27° C (80° F), this discharge rate corresponds to approximately 25 amps per 100 amp-hours of capacity.

This test is designed to show the amount of gas expelled from the battery under heavy discharge conditions. It also shows the splitter in action. If more gas is produced during heavy loads than at medium loads, it means there is some short circuit inside the cell and the separator failed to separate the plates.

The heavy duty test provides an indication of the battery's ability to deliver high current under adverse conditions, and if combined with the readings obtained from the low temperature test, can give a rough indication of its ability to start the engine, which shows its ability to deliver current at low temperatures.

The heavy load test also indicates the possibility of an internal short circuit in the battery, which will be indicated by very low voltage and excessive current.

To perform this test, apply a load equal to 1/2 the CCA rating of the battery for 15 seconds. For example, if the CCA rating is 400, apply a 200 amp load for 15 seconds. When performing this test, make sure the battery is adequately ventilated. The voltage should not be lower than 9.6 volts. Readings between 9.6 and 10 volts are acceptable.

Do you have to disconnect the battery to load test it?

Will not. Both the starter and alternator will work with the load tester connected as long as the battery is fully charged.

Load testers are designed to test batteries that are charged by an alternator or that have been charged using a car charger.

The load tester does not need to pass directly through the battery post, as long as the first clamp of the load tester is between the positive battery post and the second clamp of the load tester is between the engine block (or the negative battery post).

You can load test it without disconnecting the battery. You can use a digital multimeter (DMM) with duty cycle capability and calculate the duty cycle of the battery under load.

If you have an old analog multimeter that doesn't have this feature, you can measure the current draw and use Ohm's law (V=IR) to convert it to duty cycle.

Apply a load and take a voltage measurement, then apply a load and take an amperage measurement. Divide the voltage by the amperage to get the resistance (R), then divide the voltage by the resistance to get the current (I). Multiply this current by 100% to get the duty cycle percentage.

For example, if you have a 12V battery and a 5A load, you will draw 60W. If you get 10.5V under load, you'll be running at about 85% capacity, which is still fine for most batteries.

As for what load to use: the more accurate the better. If you want to be completely accurate, multiply the battery's amp-hour rating by 0.05 and use that as the amperage; for example, if you have a 120Ah battery, put a 6A current on it (120 * 0.05 = 6 ). You can also just use a multimeter to measure current draw if your device can draw constant current or approximate measurements from a lamp or any other device that draws power directly from a battery and is not regulated by a computer or similar.

If you don't have any way of measuring current draw, then just put some kind of resistive load on it and see what happens after 15 minutes or so; for example, if your headlights draw 30W each and they run at 15 minutes After it starts to dim, then you can assume your battery is continuously delivering about 30W / 15min = 2W. Then divide your amp hour rating (let's say 20Ah) by 2W to get 10 hours as your expected run time under such loads.

How can I load test a battery without a load tester?

If you don't have a load tester, you can use a multimeter. Connect the battery to the multimeter and set it to amp mode. Start with a low setting (like 10A), and if your meter can handle more current, turn it up until you reach the maximum.

If your battery is in good condition, you will see one of two things when you turn up the amperage:

1 - The voltage will drop, then stabilize at a certain level. This is similar to what happens when testing a battery with a load tester. The battery will get hot but not (unless abused).

2- The voltage will drop and continue to drop until below 10V. This means that your battery has too much internal resistance or is overcharged and needs to be replaced.

You may have seen people test the battery by turning on the headlights. This works because the headlight draws about 5A, which is enough to disable the problem battery, but not enough to disable the good battery. Something is better than nothing, but if a guy sells you something and asks you to test his battery that way, he's probably trying to do a quick test on you.