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The question I have has to do with supplying 12 volts to the existing car’s circuitry. Why can one not use voltage drop across the first battery in the battery pack to get the required 12 volts?

If the high voltage ground is kept isolated from the rest of the circuits, then we should have no problem using the voltage drop across the first battery OR any one battery for that matter in the battery pack?

This isn’t normally suggested, for a couple of good reasons:

1. it disproportionally drains one battery more than the others in the pack, leading to a mis-balanced pack. Depending on your car’s 12v load you could end up with one battery in the overall pack getting vastly out of sync with the others.

2. safety: typically the high voltage pack isn’t tied to ANY of the car’s electrical (not even ground) so as to keep it isolated. Let’s say you tap off the first battery in a 144vdc system. Now your car’s “ground” is part of the pack, so if you (or a wrench or a frayed wire) touched ground and somewhere along the pack you’ll be exposed to up to 122vdc and all of the current the pack can provide: quite a bit!

A DC-DC converter isn’t all that expensive and it will turn the high voltage into an isolated 12vdc which can feed the rest of the car’s electrical system and charge a small accessory battery. Other than the DC-DC hookup you’ll want to keep the two power systems completely separate and this includes NOT hooking the battery pack’s ground to the chassis of the car.

Comments 2
  1. — Greg Coleman    Apr 10, 2007 00:51 AM    #

    Using the original battery for the “car needs” was the best bet. I put a small 12 volt charger in just to charge the single battery. Also your pack will go down long before the “car needs” battery will. Both chargers can run at the same time.

  2. Gavin Shoebridge    Apr 10, 2007 01:53 AM    #

    I like that idea Greg. I might use that when Treddles is running while I save up for a converter.
    I have access to lots of smaller sealed 12V batteries which is handy, and considering that your travel batteries would go down faster than your “car needs” battery, I could afford to use a small, light 12V battery for the “car needs” part.