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Old 04-13-2009, 11:30 PM   #911 (permalink)
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I have a question, being a noobie at this.....I've always heard that you shouldn't tap 2 of 6 volt batteries from the pack to provide 12v, presumably because you will draw them down from the pack, and the recharge won't happen correctly - a point I disagree with, because I think the electron flow will equalize all the batteries in series when the load is removed...but I could be wrong - and I digress.....

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?


The same configuration would apply to every scenario of batteries in multiples of 12 v, except the oddball 8v golf cart systems....

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Old 04-13-2009, 11:35 PM   #912 (permalink)
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Old 04-13-2009, 11:53 PM   #913 (permalink)
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Quote:
Originally Posted by Wherewolf View Post
I have a question, being a noobie at this.....I've always heard that you shouldn't tap 2 of 6 volt batteries from the pack to provide 12v, presumably because you will draw them down from the pack, and the recharge won't happen correctly - a point I disagree with, because I think the electron flow will equalize all the batteries in series when the load is removed...but I could be wrong - and I digress.....
I'm no expert on this, just typing out loud! haha!

Let's imagine each battery is a waterfall. Let's also pretend there are 12 12v batteries in series.
Some of the waterfalls are 11 feet tall (11v), some are 12 feet tall (12v), and some are 13 feet tall (13v).

You are up on a mountain. You come to the first waterfall. It falls 11 feet, and then levels out. Next you hit a 13 foot waterfall after walking for a while. After that, you come to a 12 foot fall. etc...

Each waterfall doesn't care how tall the other waterfalls are around it. So, if the analogy holds, then batteries can get unbalanced, and stay unbalanced.

But how does the more current flowing through 1 waterfall than the others happen? It can happen in batteries. Hmm...
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Old 04-14-2009, 12:05 AM   #914 (permalink)
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You can't put batteries in a series string and tap parallel at the same time, even with diodes it is always a short.. If you could do this, then you could also charge your series string from a single 12V charger, which you can't. You CAN tap off each battery or pair but you can't connect the taps..
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Old 04-14-2009, 12:19 AM   #915 (permalink)
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Ummmm. I accidently rigged my 144V pack in series and parallel at the same time last week.

I am glad there was plywood between me and the melted battery cable!

At least now I know better!
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Old 04-14-2009, 08:51 AM   #916 (permalink)
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Hey Paul -

How about a look at that sketch? I am just so curious.
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Old 04-14-2009, 09:11 AM   #917 (permalink)
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Yeah, you actually could string 12 12V batteries together to get 144, then attach the 12-volt system across the terminals of the first battery. You'd get your 12 volts. However, as the motor is cranking ripply amps out of that battery, your tiny electronics would likely be getting thrashed. You'd also be drawing more current out of that one battery, which apparently is bad for pack balancing and life. No explosions here.

I'm sad that the controller blew up. I am looking forward to your tear-down diagnostic. Don't take it all appart till some electronics experts have walked you through a few trouble-shooting methods to determine the first point of failure.

I'm glad that you're taking the time to beef it up. Your controller design would likely be perfect at 12V and 1A, but when you start really pushing power there becomes a bunch of periferal requirements to the basic pwm-mosfet system.

Paul, does that new AT16 chip have more ADC's? Could you watch pack volts and post-pre-charge voltage? Then you could automatically close a contactor that has the precharge in series with it and monitor the battery and post-pre-charge voltages till they are equal, then automatically flip the main contactor. This would eliminate timing and issues of pack differences, pre-charge resister differences, etc. No guessing, no goofing up.
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Old 04-14-2009, 10:44 AM   #918 (permalink)
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With regards to sketch:

Fran is looking at it, and there is at least 1 serious error, and he's checking for more. I am not to build this control board until he's done looking at it. I don't know what the error is yet. He's very mysterious! haha! You guys can look at it if you want, though!

Oh dear, the PDF is too big!

-Paul
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Old 04-14-2009, 11:46 AM   #919 (permalink)
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Quote:
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?
Wherewolf
Andrew,

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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Old 04-14-2009, 12:05 PM   #920 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
With regards to sketch:

Fran is looking at it, and there is at least 1 serious error, and he's checking for more. I am not to build this control board until he's done looking at it. I don't know what the error is yet. He's very mysterious! haha! You guys can look at it if you want, though!

-Paul
Paul,

Do you think after Fran completes his review, we should go about getting the PCBs made? Or, do you want to breadboard everything and test it out first? What controller are you going with now?

I was thinking, what we may want to do is get some additional beta testers on board. Not to leave out Ben. Definitely get Ben back up and running with version 2, but maybe a few others as well. What do you think?

How much do you think it would be to buy populated control boards with all the components professionally soldered in place? Most of the money is in the power section, right? Under $100?

Then all we would have to do is work out the power section. I assume the power section could vary depending on the pack voltage and motor it was controlling.

Anyway, I might be getting too far ahead. I have a tendency to do that.

Roger

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