Before concluding that the battery is faulty, the following must be checked:
Check the density of the electrolyte. If the density values in different cans differ from each other by no more than 0.04 g/ml, then the battery needs to be charged with a normal current.
After charging, check the battery under load. If the control values are not reached, the battery is faulty.
If the density of the electrolyte in one or two adjacent cans is noticeably lower than in others (for example, in 5 cans 1.16 g/ml, and in one - 1.08 g/ml), then there is an internal short circuit in the battery and it is faulty.
The battery is self-discharging
Depending on the vehicle configuration, the natural self-discharge of the battery is supplemented by current leakage in various control units when the vehicle is idle. Therefore, the battery in a vehicle that is not moving should be charged every six weeks. If there is a suspicion of leakage currents, check the electrical system as follows:
Use a charged battery for testing.
Set the ammeter with measurement ranges of 0-5 mA and 0-5 A to the maximum measurement limit. Disconnect the negative cable from the battery.
Caution: This will erase the data from the injection control unit, the radio safety codes and the settings. See the instructions in the relevant section of this chapter.
Connect the ammeter between the negative terminal of the battery and the negative wire disconnected from the battery. Connect the "plus" of the ammeter to the wire, and the "minus" of the ammeter to the negative terminal of the battery.
Note: The test can be done with a test lamp. If the lamp between the negative wire and the battery terminal does not light, then in any case you should use an ammeter.
Turn off all power consumers, disconnect the clock (and other non-disconnectable consumers), close the doors.
Reduce the ammeter's range until a readable current value is obtained.
Disconnect different circuits one by one by pulling out the fuses. If the current drops to zero when one of the circuits is broken, then you need to look for the source of the malfunction in it. The malfunctions may be as follows: corroded and dirty contacts, frayed wires, internal short circuit in the consumer itself.
If no faults are found in the fuse-protected circuits, disconnect the wires from unfused units such as the generator and starter.
The article is copied from the resource: MAZbook.ru
If the current drops to zero when disconnecting wires from unprotected units, repair or replace the corresponding unit. If there are current leaks in the starter or ignition system, the ignition and starter switch must also be checked.
Connect the negative cable to the battery.
Set the clock, enter the radio receiver safety codes.