The converting, maintenance, and operation of a Suzuki Samurai conversion to Electric power.

Tuesday, December 25, 2007

More on the DC/DC

The way it was set up, the DC/DC was always on floating the batteries unless I turned off the main breaker. This caused the main pack to have a parasitic loss. Some discussions on the EV discussion list made the point that floating a 12v battery at 13.5v will cause it to gradually lose capacity.

I decided to use the optional feature of the Iota wherein it will produce 14.5v and only charge when the car is being driven. In order to do this, I installed a second contactor in the negative line to the controller and motor. When the ignition key is turned on the negative contactor will close which will supply power to the DC/DC whose negative lead is on the controller negative bar. This also allows the controller to precharge. Once the precharge is complete, the contactor in the positive lead is closed and the car is ready to go.

This contactor also isolates the controller from the charging voltage which it would ordinarily see via the precharge circuit. It also means that there are 2 contactors that can be opened in an emergency. It is possible that trying to open a contactor under a load could cause it to be welded closed. The emergency kill switch on the dash is designed to open the contactors.

A downside to this arrangement is that there is a path for the high voltage to the negative side of the motor through the DC/DC. Since motors can leak to their case which is connected to the chassis through accumulated carbon dust, there could be a path to the chassis while charging. It is not a concern to me as the liklihood of any significant amperage through this path seems small.

Finding space to mount the second contactor was difficult. It ended up above the main contactor. I really would rather have the positive leads separated more from the negative. I did mount the new contactor so that its terminals are on the other end rather than right above the terminals on the main contactor.

The top picture shows how the 2 contactors ended up on top of each other. The second picture is a wider view showing the blue controller and the batteries. The firewall is on the left.

I am also finding that the cold weather has halved the range. Coming up the hill to my house at 5 MPH is not so much fun. When the pack is low, I watch the voltmeter and try to keep it above 120v for my 144v pack. This means that the amps might have to be kept below 25! When the pack is more than 1/2 full I try to keep the ammeter below 200. I am also going to do a thorough equalization charge on the batteries. There is a chance they are not being fully charged.

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