Last update: 21 Jan 2019

12 Volt Power

DC to DC Converter

12V power comes from two DC-DC converters, which are Mean Well HRP-600-15 power supplies. The HRP-600-15 is isolated and specifies a DC input range of 120-370VDC. Together the two power supplies provide 86 amps. The open circuit output is set at 14.7V. Operating in the car, the voltage drops to around 14.2V.


12V Battery

A 22Ah AGM lead acid battery supplies 12V power when the ignition is off. With the key out of the ignition switch, my multimeter shows the car draws just 9mA, which is unchanged from before the conversion. This battery is about half the size and capacity of the original. Universal Battery model UB12220PDF.

12V Lead Battery

Protection and Disconnect

The outputs of the DC-DCs are tied together in parallel, then pass through a 100A MIDI fuse and then a 100A relay. The relay disconnects the DC-DCs from the car when the ignition is off to prevent them from draining the 12V battery when the car is turned off. The DC-DC outputs exhibit 60Ω each when powered down, which corresponds to a 400mA load. Without the relay, this load would deplete the 22Ah battery in 2 days. The fuse is Waytek parts 46386 and 46008. The relay is Picker Components PC7150-1C-C2-12C-N-X.

Fuse Fuse Holder Relay


The two DC-DCs and lead acid battery are mounted in the original battery location using the original battery hold down bracket. A structure made of wood holds the battery, the fuse and the relay. This structure spaces the components to the width of the original battery and the height of the stacked DC-DCs. A few coats of polyurethane protect the wood from moisture. Everything is protected from the elements by a cover made from ABS plastic sheet and Permatex 84115 Plastic Weld. Two copper spacers glued into the cover transfer power to the outside. The DC-DC fans draw air up from the bottom on one side and eject air down and out the other side.

Spacer Spacer Cover Plastic Mounted

Charge Warning Light

Charge Warning Light Charge Warning Light Module

A custom device operates the instrument cluster's charge warning light so that it performs a similar function as before the conversion, which was to indicate the alternator is not operating. This device illuminates the warning light when the car's electrical system drops below 13.2V, indicating that the DC-DC is not operating. To operate the warning light, the circuit controls a normally closed relay connected between ground and the warning light wire that originally ran to the alternator. With this particular car, the brake warning light illuminates along with the charge warning light.

Charge Warning Light Schematic

The heart of the circuit is a voltage comparator chip, IC2. This comparator has two inputs, one fixed and one variable. The fixed input is from a 5V reference voltage chip IC1. The variable input is from a voltage divider R1 that reduces the car's 12V system voltage down to near the reference voltage. This divider is a potentiometer calibrated in situ to reduce by a ratio of 5.0/13.2. For example, 12.4V is reduced to 4.7V and 14V is reduced to 5.3V. When the variable input rises above 5.0V, we know the 12V system is above 13.2V. When this happens, the comparator output energizes the relay coil to open the contacts and turn off the warning light. The TVS diode D1 protects the two chips from spikes coming from the car's 12V system. The TVS diode D3 protects IC2 from relay coil surge. The diode D2 prevents any surge below D3's clamping voltage from feeding back into the car. Two status LEDs show whether the device is powered (green), and whether the warning light is lit (red).

Source code is here.

Digital Voltage Display

A DROK 100123 digital voltmeter shows the 12V system voltage. It is located on a panel that replaced the ash tray.

12V System Voltage Display

Voltage Monitor

An automotive battery charging indicator shows whether the 12V battery is charging or discharging, which indicates whether the DC-DC is operating. It is the Gammatronix Ltd 12v LED Battery charge / level indicator monitor - E, installed in one of the unused switch blanks on the lower left dash.

Gammatronix Battery Charge Indicator Gammatronix Battery Charge Indicator

The indicator is connected to a conveniently accessible ignition switched 12V source, specifically pin 36 of the relay block B225. This relay position is empty in the sedan without heated seats. The indicator normally lights steady yellow or green when the DC-DC is off or on, respectively.

High>15.2V Flashing red and green
Normal charging13.2-15.2V Green
Normal discharging11.8-13.2V Yellow
Low11.5-11.8V Flashing yellow
Denial11.2-11.5V Red
Depression<11.2 Flashing red