Volvo 850 alternator mod for EFB & standard batteries

Some time ago I bought a new EFB battery for my Volvo 850. This type of battery is charged in the same way as a standard lead-acid battery as it originally belongs in an 850.

The end-of-charge voltage should be 14.4 volts, according to the theory. But if you take a closer look, all 850s have a Bosch alternator with a 14 volt regulator installed. The alternator delivers an output voltage of max. 14.10 volts, measured at the alternator connection, while approx. 13.85 volts are applied to the battery itself.

This is not enough to fully charge the battery and ultimately results in a shortened battery life. Replacing the alternator is useless, as it is completely in order and functions correctly within its parameters. So what can be done to get more out of the alternator?

I decided to convert the alternator to 14.50 volts. Excluding the line losses, the battery then has an end-of-charge voltage of 14.35 volts, i.e. 0.05 volts less than specified for a standard or EFB battery. This means that the battery is always full, but does not gas too much. As a positive side effect, the voltage in the entire on-board network is increased by 0.5 volts, which is also reflected in tex. more light output of the main headlights.

The following parts are required for the conversion:

A standard relay with base, several meters of cable, a 10mm and a 6mm soldering eye and a 14.5 volt regulator for Bosch alternators, e.g. 1010699

The new regulator has 3 connections (the old one has 5), these are the two sliding contacts (D-, DF) and a contact via a contact tongue of the diode plate (D +). This contact via the contact tongue must be interrupted by bending the contact tongue back until it no longer has any contact with the regulator.

A warning in advance:
The problem with the 14.5 volt regulator is that it has no connection for the speed signal and the L connection for the battery light is also missing, as this type of controller is built into alternators that are controlled by an ECU. If you simply install the new regulator without changes, you will find that everything works as it should, but after several hours the battery is dead, as the regulator continuously draws 50 watts when the engine is off.

The extra relay which is connected to the fuel-pump relay ensures that the alternator regulator does not draw any current when the engine is off.

Just reassemble and rewire the generator according to the plan above in the picture.


Wiring plan