This past weekend I was out doing some errands and I could not "start" my truck because my 12V battery went dead. This is kind of a funny situation to be in because the vehicle has a large traction battery with plenty of charge to power the vehicle, and yet I can't use it because the 12V lead acid battery is dead.
I need 12V in order to turn on the contactors (which are a kind of giant relay). Once the contactors are turned on, then the traction battery is connected to the DC-DC converter which will then supply 12V to the 12V system while also charging the 12V battery. So I had a chicken and egg situation. I needed 12V to get the 12V system turned so that I could "start" the car.
I was at a Lowes so I thought surely Lowes must have a way to deal with this. Maybe they have one of those portable jump-start kits or have a guy on a kart who comes out to the parking lot to provide a jump start. But I went in the Lowes and asked and they weren't able to help me. So then I was back at my truck with the hood up, thinking about how I can start it and a nice guy asks if I need a jump start. I said yes but I had no cables. He looked around in his trunk but didn't have any cables either so he left.
I knew that I could get 12V from the traction pack but I wasn't sure what kind of electrical problems I might cause to try and use the traction pack for 12V. The traction battery is floating relative to everything else. So it is not attached to chassis of the truck and should not cause a short to attach any part of the traction battery to the 12V system. Next I thought about the DC-DC converter. It supplies 12V once the truck is "turned on". I thought about what would happen once the DC-DC comes on and is connected in parallel with 12V worth of traction battery. The unloaded cells are about 3.3-3.4V which means that 4 cells are from about 13-14V. This is really close to what the DC-DC converter produces when it is charging the 12V battery. So I decided that it was close enough to not cause a problem and tried it out.
I went back into the Lowes and bought some jumper cables. I connected the most negative end of the traction battery (0V) to the 12V ground, and the positive terminal of the 4th cell to the positive of the 12V battery. There was a little spark but that was it. Then I was able to turn the key and the contactors clicked on. As quickly as I could I disconnected the jumper cables. Once the DC-DC is turned on then it continues to supply 12V.
Later after I was home and thinking about this, I realized that a different and probably better thing to do would have been to just use the jumper cable to jump past the contactor, as if the contactor were closed. Then this would have turned on the DC-DC and I could proceed from there. However it would have been a lot harder to get to the terminals of the contactor since it is buried in a sealed up junction box.
I knew my 12V battery needed to be replaced so I guess this will finally motivate me to get a new one.