Properly checking the battery with a load plug is the easiest and fastest method to assess its condition. The device is widely used by professional auto electricians, car battery dealers, battery workers (specialists servicing batteries). Non-professionals - that is, ordinary motorists - also often have a load fork in their garage arsenal. And all because the device is relatively inexpensive, extremely easy to operate, and quite accurate. Even if it was bought for cheap.

Why do I need a battery test with a load plug?

Offhand, you can cite several quite everyday situations when the ability to check the battery with a load plug will be very useful:

  • A test before buying a new battery allows you to avoid buying a frank marriage or a battery that the seller did not care for during storage (did not regularly recharge).
  • When the starter turns poorly, the check makes it possible to understand whether the battery is able to provide the necessary power for it.
  • The starter turns for two seconds and stops.
  • When buying a used car with a load fork, you can find out if you also have to spend money on a new battery.
  • Infrequent but regular testing during the operation of the machine allows you to determine in advance the moment when the battery will require replacement, and thus avoid the "morning surprise".

Also, if you are, for example, a seller of car batteries, the skillful use of a load fork will make sure that the return of the purchased product under warranty is justified. As for accumulators and auto electricians, for them a load plug is the same as a computer for a programmer. Just the right tool.

What is a load fork and how does it work?

Battery tester
Battery tester

In its simplest version, a load plug for testing car batteries consists of the following mandatory five elements:

  1. Voltmeter.
  2. Load.
  3. Power contacts.
  4. Test button.
  5. Frame.

The scheme of the device is as follows.

Tester circuit
Tester circuit

The voltmeter performs only two functions. First, it displays the voltage at the battery terminals without load. Secondly, it does the same, but with a load. The first function allows you to preliminarily estimate the battery charge level. The second - serves to assess the ability of the battery to hold the load.

Actually, the load itself is a resistor of a certain resistance, made of refractory metal. Most often it is a plate folded in a zigzag (to fit a long part in a compact case). Its parameters are selected depending on how much current is required to be “squeezed” out of the battery. It is very easy to do this. Knowing the nominal voltage of the battery under test and the load current, according to Ohm's law, the appropriate resistance is selected. In general, the task of the load is to simulate the energy consumption of the starter.

Power contacts - as a rule, one thick (and short) wire with a crocodile clip, as well as a metal probe mounted directly in the body of the load plug. By the way, tools of this type are called a fork because there were two power probes before, and they vaguely resembled this cutlery in appearance. The contacts are connected directly to the voltmeter, and also through the button to the load.

Old-style battery tester
Old-style battery tester

The load fork works as follows. As soon as both power contacts are connected to the battery terminals, the voltmeter shows voltage without load. Further, if you press the test button, a power load is switched on in the circuit, simulating a car starter in operation. The voltmeter, at the same time, shows the voltage at the battery terminals, and from these values it is possible to determine its drawdown.

How to use the load fork

Checking the battery with a load plug is performed in just two steps:

  1. Voltage measurement without load.
  2. The same, but with a load.

Extremely simple. But there are a few features that must be taken into account. Otherwise, the test will either be biased or simply confusing or misleading. These are the features:

  • The first stage of testing will give the correct results only if the battery has not been charged for at least 8 hours before the test (including from the generator), and also “rested” for at least 15 minutes after a serious load.
  • The objectivity of measurements is strongly influenced by the ambient temperature, where +25°C is ideal, and below zero is unacceptable.
  • During the test, right on the machine, you should turn off the engine (mandatory), turn off all consumers, and it is better to remove one of the terminals altogether (optional).
  • You can measure the voltage without load for as long as you like, but you can’t keep the battery under load for more than 5-8 seconds.
  • If the first test under load did not give clear results (yawned, distracted, scared of sparks and did not note the voltmeter readings), then the next test is performed no earlier than after 5 minutes.

We return to the two stages mentioned above. If at the first stage the voltage at the battery terminals is not lower than 12.7 volts, then it is 100% charged. If less, then every tenth of a volt (0.1) is minus 10%. That is, if the voltmeter without load shows, for example, 12.2 volts, then the battery is about 50% charged. It is worth emphasizing once again that this principle only works with a "rested" battery.

The second stage is nothing more than an imitation of the starter. We press the button, hold it for 5-6 seconds, and follow the readings of the voltmeter. Depending on how much the battery is charged, the voltage under load will vary from 10.2 volts to 7.8 volts. If it is the maximum, it is 100%. Minimum - the battery is discharged to zero.

And here the fun begins, if the battery is not new or defective. The fact is that even being 100% charged, a faulty or dead battery will not hold the load. That is, the voltage during testing at the second stage will drop below 7.8 volts. Accordingly, if you conducted a test, and the voltmeter needle under load “fell” below 8 volts, the battery is potentially defective.

It has lost its capacitance, cannot give out a normal starting current, and therefore the voltage sags so much. There may be several reasons. Among them are sulfation of plates, shedding of the active mass, short circuit in the cells, and so on. In any case, if the battery (even incompletely charged) so clearly does not hold the load, it is time to change it. She, sooner or later, will let you down, and one morning the engine will not start.

Conclusion and how to check the battery without a load plug

As a result, checking the car battery with a load plug is a very simple, affordable, fast and very accurate diagnosis. It allows you to assess the state of the battery - the level of charge, the ability to hold the load, overall performance and potential resource.

By the way, it is not always necessary to have a load fork for this test. Sometimes a simple voltmeter is enough. The battery is checked right on the car. To do this, one measures the voltage at the battery terminals under the hood, and the second (assistant) “turns the starter” in the cabin, thereby loading the battery.

The only important condition is that the voltmeter must be “fast”, that is, update the readings with minimal delays. For this reason, expensive microprocessor-controlled multimeters are not suitable for these purposes. But the Chinese ones, which are sold today even in grocery stores for a couple of dollars, react to a voltage drop quite quickly. This is how you can check the battery without a load plug with a sufficient degree of accuracy.

Comprehensive battery check.

Detailed material about the internal resistance of the battery

VIDEO: about testing a car battery