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Old 04-14-2009, 10:46 AM   #919 (permalink)
Roger Heuckeroth
EV Converter
Join Date: Mar 2009
Location: Saugerties, NY
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Originally Posted by Wherewolf View Post
So say you have a 144v pack (24) of 6 volts and you parallel every two, and tie them to a common 12v bus, + and - . And then use that bus to power your 12 volt systems ( fused of course ) - you would be drawing from all 24 equally - wouldn't you? Why is a dc-dc converter better than that?

There are several reason why not to get your accessory power from your traction pack batteries. The big one is safety. For safety purposes you don't want your high voltage traction pack to be grounded to the car. If any terminal accidently touches the body, and the traction pack is grounded to the body of the vehicle, you get plasma balls of molted metal flying about. It is much safer to keep the traction pack completely isolated from the body of the vehicle. Note that Paul's design has an isolated 12V DC/DC converter. This is specifically to keep that isolation in tact.

The other reason as you may have guessed is that you will be pulling more amps from that one set of batteries that you are connected to. In my case, I'm running a vacuum pump for the brakes, power steering pump, and cooling pump for my controller. That is a lot of power, and it would lead to a great deal of imbalance in the battery pack. When you charge an unbalanced battery pack in series without a means to actively balance voltage, you do damage to all batteries in the pack.

Paul has individual isolated 12V chargers on his EV Beetle. That is actually a really good way to go if you have the room. It becomes impractical on larger battery packs, so you need battery balancers on larger packs if you want to maximize the life of the pack.

Hope that helps.
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