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What does a Battery Load test Determine?
A battery load test is used to determine a battery's condition by applying a load to it for a specific period of time. This is usually done at an auto parts store, garage or battery shop. During the test, technicians check the voltage of the battery and also check for any leaks.
If your battery does not pass a load test, you may need to replace it. If you are unsure of how much your car repair will cost, we can help you get free estimates from local mechanics.
What is Heavy Load Test in Battery?
Heavy Load Test is a test in which the battery is discharged at a rate which may cause an excessive temperature rise of the battery. The purpose of this test is to determine the ability of the battery to deliver high rate currents without damage and to determine the effects of such discharges on the capacity and performance of the battery.
The load used for this test should be capable of removing a current equal to one-half of the ampere-hour capacity (C) in 15 minutes. The temperature rise should not exceed 40° C (104° F) with an ambient temperature maintained at 27° C (80° F). In case, when it is not possible to maintain an ambient temperature, the temperature rise should not exceed 50° C (122° F).
The time required to discharge the battery under this condition should be between three and four hours. If it takes longer than four hours, then 1/3rdC load shall be applied instead. When 1/3rdC load is used for discharging, a temperature rise not exceeding 50° C (122° F) will be permitted. This rate of discharge corresponds to about 25 amperes per 100 ampere-hour capacity at 27° C (80° F) ambient.
This test is meant to show how much gas comes out of the battery under heavy discharge conditions. It also shows how well the separators are functioning. If there is more gas generated during heavy load than medium load, it means that there is some internal shorting in the battery and that the separators are failing to keep the plates apart.
The heavy load test provides an indication of a battery's ability to deliver high current under adverse conditions and serves as a rough indication of its ability to start an engine if used in conjunction with the readings obtained from a low temperature test, which indicates its ability to deliver current at low temperatures.
The heavy load test also indicates the likelihood thata cell will have an internal short circuit, which would be indicated by a very low voltage and excessive current flow.
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. Make sure the battery is adequately ventilated when performing this test. The voltage should not drop below 9.6 volts. Readings between 9.6 and 10 volts are acceptable.
Do you Have to Disconnect a Battery to Load Test It?
No. As long as the battery is fully charged, both the starter and alternator will work with the load tester connected.
The load testers are designed to test batteries that are under a charge from the alternator, or have been charged with an automotive charger.
The load tester does not need to go directly across the battery posts, as long as there is a good connection between the 1st clamp of the load tester and the positive battery post, and between the 2nd clamp of the load tester and engine block (or negative battery post).
You do not need to disconnect the battery to load test it. You can use a digital multimeter (DMM) with a duty cycle function and calculate the duty cycle of the battery while under load.
If you have an older analog multimeter without this function, you can measure the amperage draw and convert that to duty cycle using Ohm's Law (V=IR).
Apply a load and take your voltage measurement, then apply your load and take your amperage measurement. Divide the voltage by the amperage to get resistance (R), then divide the voltage by that resistance to get current (I). Multiply that current by 100% to get your duty cycle percentage.
For example, if you have a 12V battery with a 5A load, you will be consuming 60W of power. If you are getting 10.5V under load, you will be operating at about 85% capacity, which is still good on most batteries.
As for what load to use: the more accurate the better. If you want to be totally accurate, multiply the amp-hour rating of your battery by 0.05 and use that as your amperage draw; for example, if you have a 120Ah battery, put a 6A draw on it for an hour (120 * 0.05 = 6). You can also just use a multimeter to measure current consumption if you have a device that draws constant current or get an approximate measurement from lights or anything else that draws power directly from the battery without being regulated by a computer or similar device.
If you don't have any means of measuring current consumption, then just put some kind of resistive load on it and see what happens after 15 minutes or so; for example, if you have headlamps that draw 30W each and they are starting to go dim after 15 minutes, then you can assume that your battery is delivering about 30W / 15min = 2W continuously. Then take your amp-hour rating (let's say 20Ah) and divide this by 2W to get 10 hours as your expected run time under this type of load.
How to 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 amps mode. Start with a low setting (like 10A) and if your meter can handle more current turn it up until it reaches the maximum.
If your battery is in good shape, as you turn up the amperage you'll see one of two things:
1- The voltage will drop and then stabilize at some level. This is similar to what happens when you test a battery with a load tester. The battery will get warm but not hot (unless it's been abused).
2- The voltage will drop and continue to drop until it's below 10V. This means that your battery has too much internal resistance or has been overcharged and needs to be replaced.
You've probably seen people test batteries by turning on their headlights. This works because headlights draw about 5A which is enough to make the problem batteries fail but not enough to make good batteries fail. It's better than nothing, but if a person is selling you something and lets you test his batteries that way, he's probably trying to pull a fast one on you.