Performing the diagnostics; programming, encoding, or even simply calibrating the sound equipment (for custom projects), the car has to be supplied by electricity.

E6X/9X generation cars in “Ignition ON” mode can consume current even till 35 .. 40 A, the voltage maintenance tool has to be powerful! “Simple” chargers will not fit, the current is insufficient. Also, the old school boosters will not be the right choice – the voltage has to be “high quality”, it means, with small pulsations, stabilized.


Choice of the voltage maintainer

There are several maintainers, which are officially recognized by BMW AG. For example:

Deutronic DBL 1200

Eltek Multicharger 1500


Solid maintainers. Also, their prices are solid.

From the special features of Deutronic, I would like to remark reverse polarity protection – they can be very useful. Both maintainers are equipped with automatized charging programs – this time it’s not topical for me.


Deca 330600 life 120

Unfortunately, it looks that long-term current is only 30 A, but 120 A – available only in short-term. In addition, even Power supply mode has time limitations (counter) and other features, which can not be disconnected.


Telwin Doctor charge 130

Solid maintainer.


And finally – my attention was drawn to Manson SPS-9600


This is a power supply module, intended for laboratories. Accordingly – it has no programmes. It’s been sold by several/different brands.

However, this module has:

a) voltage and current indicators – large and visible very clearly;

b) limiting type (not trigger) of current overload – critical requirement;

c) compensation of wire resistance;

d) small pulsations (40 mV max);

e) voltage regulator of “analog” type (it means – I can easily rise the max voltage by 10% if there will be such need).


Why the limiting type of current overload is such important? In case, if for a short moment (for example, second or two, when some powerful consumer turns on) the “appetite” of the car will overreach max available current, the power unit will “do what it can”, missing current will be taken from car’s battery. Instead, if the supply source has a trigger type protection, in such moment it will turn off, and the problems are guaranteed!


Basic parameters of the power supply unit:

  • voltage: till 15.0 V (inquired); to 15.3 V (real)
  • current(24/7): to 60.0 A


This power supply unit has a shortage: it has no reverse polarity protection. If I manage to mix up the terminals, “puff” is guaranteed!


The unit has 2 stage RF filter in the inlet – it’s well thought of reducing interference in the electrical network. The unit has a correct PFC. PWM (output) voltage regulator realized on the old school IC TL494 – no MCU.



I choose 50 mm2 copper wires. That was the largest crosscut, which was available without special order and waiting. Maybe someone will say – this man is out of his mind, such wires are used to start the car, not to maintain the voltage!

The wire is compared to the AAA type battery.


But – let’s calculate. If the power supply unit is placed 2 meters from the car (for convenient access), the length of wires has to be around 5 meters. 0.5 meters will be “lost” because the connection socket is placed on the back of the unit, 0.5 m will be “lost” in the engine hood. 1.0 + 1.0 m will be lost, when the wire “drop” to the floor from the engine hood and “rising” to the shelf, where the power supply unit is placed.

The distance is 5 meters, but – the total length of wires is 10 meters (!) because there are 2 wires!

The resistance of 10 meters of 50 mm2 wires is around 0.0035 Ohm or 3.5 mOhm. It looks very less, but the voltage drop (when current around 60A passes thru) will be already around 0.2 V (!)


Note: for example, the typical sets for starting the car, which are sold in the gas stations, the crosscut of wires is 10 mm2 or even only 6 mm2! With such “wires” you will be able to start the car… Only in case, if you will manage to charge the battery. It means, they are charging wires, not starting wires!


The connection of the terminals

Terminals have to be connected by soldering the wire. No impression – such connection will oxidize. BOTH sides of terminals have to be soldered! Otherwise, only the side with soldered wire will work.


Compensation of wire resistance

Professional large power supply units offer to compensate for the wire loss. This unit is not an exception. Accordingly – from connectors, we have to take extra wires (with small crosscut), to ensure, that the unit will maintain the voltage exactly on connectors.



Testing the unit in field conditions

Here, E6X car, with “Ignition ON”

The main challenge – “give” the electricity, generated by the unit, to the car. The weakest spot – terminals. Unfortunately, BMW AG has not intended any modern wire terminals. The ground has to be connected to the screw (which oxidizes by time), plus connection – a simple metal plate in the engine room.

Here, the measurement: voltage loss, if the current is around 40.0 A:

0.3 V or 12 W in warmth! Not a surprise, because the screw starts to warm up.


Cleaning the screw, choosing optimal connection of the terminal, these losses were successfully lowered to 0.1 V.

Approximately the same voltage loss also in connection with plus wire.

So, in these connections we lose around 0.2 V. It’s no small amount!

Voltage loses has to be reduced as much as possible. One of the reasons – when the voltage drops, the consumable current increases – the “chain” reaction starts: when the current increases, voltage losses increase, even more, the current rises, etc. This is SMPS – Switching Mode Power Supply (which are used in electronic modules of the car) work nuance. The explanation is simple – electronic modules (and sensors, actuators – valves, bulbs, etc.) consume energy (power) – it’s unchangeable. When the voltage drops, the current increases in such a way, that total energy stays the same.


Many are asking – what crosscut of wires have to be chosen? My logic is following – all charging components have to be chosen in such a way, that there are no pronounced “weak” spots.

Losses consist of: losses in wires, losses in connection “power supply unit/wire” and connection “wire/car”.

Connection “power supply unit/wire” don’t give huge problems. Directly opposite: connection “wire/car” is a much weaker spot.

In my example, voltage losses in the wires (current: around 60 A) are around 0.2 V. Real losses in connection “wire/car” also 0.2 V (by 40 A).

These both losses are quite similar – it means, that the crosscut of wires has been chosen correctly.

If the wires are shorter (for example, 2.5 meters to the car), by such voltage you can choose wires with crosscut around 25 mm2.

Or, looking from another point of view – if any stage dominates strongly (its losses are significantly higher than for others) – the problem of losses in this stage have to be solved.