BMW Voltage System Test

Our standard BMW voltage system test includes testing of the battery, starter and alternator

A vehicle’s electrical system basically consists of an energy storage device (battery), an energy generator (alternator) and a large number of electrical loads (electrical/electronic devices). Electrical energy is taken from the battery in order to start the engine via the starter (electric load). Once the engine is running, the alternator supplies current which, in the ideal case, is sufficient to supply all electric loads and charge the battery. If the current requirement of the connected electrical loads is higher than the current supplied by the alternator, the system voltage will drop to the level of the battery voltage and the battery will be discharged.

A breakdown due to a drained battery or problems in the vehicle energy system can have a wide range of causes which, in most cases, are not caused by the battery itself. For this reason, replacing the battery will only rarely provide a sustained solution to the problem.

Energy Diagnostics

The electrical system of BMW vehicles has been subject to an ongoing development process over the years. This has led to increased demands being placed on the battery but it has also provide new power and energy management systems. The power supply of most current BMW vehicles is a combination of hardware and software that ensures the necessary power for all the vehicle systems. Essentially it can be divided into two major functions:

  • Energy management
  • Power management

The energy management system ensures that sufficient starter motor current is always available and monitors the vehicle even when the engine is off. Energy management includes all the components in the vehicle that generate, store and consume energy. The data for the energy management is distributed across a number of control modules.

Power management is a subsystem of the energy management. The power management is run from the engine control module Digital Engine Electronics or Digital Diesel Electronics (DME or DDE). While the vehicle is being driven, the power management regulates the power output of the alternator as well as the battery charging.

BMW Energy Diagnosis
BMW Energy Diagnosis – State of Charge (SoC)
BMW Energy Diagnostics – Battery Status
Battery Results
Midtronics Battery Test Results Information Breakdown

If the vehicle comes to a standstill and will not start during or shortly after a journey, the cause probably lies in the charge balance of the alternator. A requirement here is that the charged battery was still able to deliver adequate current for the starting operation.

Midtronics Alternator Test Results Information Breakdown
Starter Results Example
Midtronics Starter Test Results Information Breakdown

1996 E36 318Ti with S52 engine swap

Customer’s complaint; Intermittent no start, car kept on maintenance charger when not in use.

Standard System Test Results

battery test
Midtronics Battery Test Results – Battery OK
starter test
Midtronics Starter Test Results – Starter OK
alt test results
Midtronics Alternator Test Results – NOT OK

As you can see in the above alternator test results the voltage is OK but there is a problem with the current, 0.2amps with no load and a -1.4amps loaded.

If the test results for the starter or charging systems indicate that there may be a problem, you may want to perform the Cable Drop Test to determine if it is due to worn cables or bad connections between the battery and the alternator or starter.

Cable-Drop Test

voltage drop test
Midtronics Cable Drop Test – Battery to power supply box in engine compartment
Voltage Drop Test
Midtronics Cable Drop Test – Power supply box in engine compartment to alternator – NOT OK


Bad ground cable – 6.80v drop; The ground cable from the body to motor mount bracket was bad, even though it looked good and the connections were secure.

Worn cables or bad connections create higher resistance, which causes a drop across the circuit. The voltage drop reduces current carrying capability that displays the same symptoms as a weak alternator or starter and causes premature battery failure. There is no need to run the engine. Bimmerscan’s Cable Drop Test uses Midtronics’ conductance technology to send a signal through the circuit to the component under test. The test then simultaneously calculates voltage drop on the positive (+) and negative (–) sides of the circuit as well as the total voltage drop.

Test Results after cable replacement

Results after cable replacement
Results after cable replacement