Question about Low Voltage Side of the System

Hello, I am a Prius driver who wants to get rid of my other car (Buick Regal) and build a plug-in EV commuter. I am in the beginning planning stages now and am learning a lot!
I notice most conversion “packages” and descriptions call for a DC-DC converter to provide charging for a separate battery for lights and accessories. My question is why can’t you simply parallel across the first battery in your HV battery set and use the 12 volts from that battery to drive the lights and accessories?
Is there an important reason to keep the LV side completely separate?

The dc-dc converter is used so that an equal amount of capacity is taken from the main batteries. Draining a single battery would result in a decreased system voltage and removing the isolation that the converter gives would cause the accessories battery to be discharges by the motor.

Good question.
Like Huronlad said your low voltage or accessories battery to be used also by the propulsion motor would drain it.
Also some points

  1. Your low voltage 12 volt system will be very demanding. Lights, blower, heater (heater can be high voltage) vacuum pump, cooling fan for controller, cooling fan for battery boxes, relays and contactors.
  2. If you did pull out one of the series batteries for LV system, the pack would be unbalanced with charging issues.
  3. Some EVers have used an alternator or DC/DC without an accessory battery, most later add the extra battery.
  4. I used a tiny motorcycle battery (5.5" X 3.5" X 6") to start up the relays and contactors then the dc/dc keeps all the 12 volt system going and charges battery. You can adjust the DC/DC output voltage. BTW I added a toggle switch to isolate the little 12 volt guy because the normal parasite drain would cause a no start some mornings.

    Bigger image:


It is much safer to have a completely isolated high voltage and low voltage system. any short in your 12v system could get the entire pack voltage in case of a malfunction. Also your range is determined by the weakest battery, so discharging 1 battery will reduce range. Just get a 12volt battery to run the low voltage, and put in a Dc-dc converter to charge it. OR I just charge the low voltage seperately with an off the shelf 12volt car battery charger.

Running down the road you will quickly exhaust your 12 volt battery and be stranded on the side of the road you so desperatly want to be driving on. Unless you run a DC to AC converter to run the battery charger to replace the current you use on the low voltage circuts in the car. What you don’t realize is that your contactors are run on 12 volts as is the pb6 that connects to your controller. Various systems in the car take quite allot of juice to run or are you limiting your driving to daytime with no brake lights or turn signals? As for me I love to hear my premimum sound system in my quiet car it sounds so much better than in a gas car. I highly reccomend you taking some serious thought on this subject as it could be a highly embarassing situation when you need a jump for the 12 volt system when you have a nearly fully charged main battery pack. Plus the time you do run low on the main pack and are sitting on the side of the road waiting for the pack to recover you can still listen to your music, take it from me I have been there and done that. LOL Good luck, but I consider the DC to DC converter as standard equipment on my car. Oh yes, most cars today have power brakes and need a source of vacuum for the booster, the pump can be quite demanding for DC power.

new dawn, as in the dawning of a new era