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Old 12th February 2009, 04:25 AM   #1
star882 is offline star882  United States
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Default 11.5-12.8v to 13.8-14.4v boost

A friend of mine, who is a hypermiler (a motor sport about saving gas), has the idea of substituting a deep cycle battery (recharged from AC power) for the alternator in order to reduce fuel usage. The problem is that 12v lead acid batteries (both starting and deep cycle) charge at 13.8-14.4v and discharge at 11.5-12.8v. Therefore, if the batteries were simply connected in parallel, the starting battery would run down with the deep cycle and wear out early. A solution is to use a circuit to boost the voltage, but I'm not sure exactly how it would work.

What I'm thinking is:
* Start with a push-pull topology, with the center tap connected to the input and the ends connected to MOSFETs going to ground.
* The output rectifiers tap between the center tap and the ends to operate as sort of a double ended boosted current boost converter. The winding would obviously need larger wire near the center to handle the current. Maybe use MOSFETs as synchronous rectifiers to increase efficiency.
* The controller can be a common UC2525A. It will try to hold a user-set output voltage of 13.8-14.4v while staying within current limit.
* I'm not sure how much current it would need to handle, but probably something around 25A would be the minimum. Much higher if the A/C has to be used. At high currents, several such boost circuits can be connected in parallel with the feedback coordinated and the clocks staggered to reduce ripple.
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Old 12th February 2009, 06:36 AM   #2
es44 is offline es44  Denmark
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I will follow this thread with great interest. I need to charge the battery in my trailer from the car, and the voltage drop makes it impossible to just run a wire.
The project you are thinking of is just the one i'm looking for.

Best regards
Ebbe
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Old 12th February 2009, 06:58 AM   #3
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Default it wont work

The engine get the power to run the engine and accessories from the alternator the battery's main purpose is to just get the car started and be recharged by the alternator. There is a rating called the reserve charge state in VA so if the car draws so amps and divide the reserve volt/ amps you will know about how long your car will run should the alternator. What your friend should be doing is looking at driving habits, best freeway speed to obtain best milage, areodynamics with best cooling combination.
plus the extra weight does not help
I have a friend who is into electric car conversion they only cost around 100,000 for a Corvette but at each wheel 275 hp and will get 300 miles on a charge and will out accellerate and regular gas engine
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Old 12th February 2009, 01:38 PM   #4
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A battery with enough amp hours to satisfy the electrical needs of the car long enough to use an entire tank of gas would be VERY large and VERY heavy.

I don't see how adding weight would improve gas mileage.

There is much more energy available from the equivalent weight of gasoline.
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Old 12th February 2009, 05:29 PM   #5
albin is offline albin  United Kingdom
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Default totally irrelevent I know

Why not add lightness.May be capacitance instead of battery.
regards
Max
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Old 12th February 2009, 06:00 PM   #6
star882 is offline star882  United States
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Quote:
Originally posted by theAnonymous1
A battery with enough amp hours to satisfy the electrical needs of the car long enough to use an entire tank of gas would be VERY large and VERY heavy.

I don't see how adding weight would improve gas mileage.

There is much more energy available from the equivalent weight of gasoline.
The point is not to last an entire tank of gas. The point it for it to last for about 90% or more of trips.
Quote:
What your friend should be doing is looking at driving habits, best freeway speed to obtain best milage, areodynamics with best cooling combination.
He's already doing that. He just wants to eliminate the alternator load.
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Old 13th February 2009, 01:16 AM   #7
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This isn't all that uncommon among drag-racers and such.They commonly remove the alternator,to reduce weight,and load on the engine.(a couple more HP available to the wheels instead of spinning the alt.)

What about using some of the smaller 2V cells? You could put 7 in series (~14V+),and not need to worry about the DC-DC converter.
Just a thought.
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Old 13th February 2009, 03:48 AM   #8
star882 is offline star882  United States
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Quote:
Originally posted by DigitalJunkie
This isn't all that uncommon among drag-racers and such.They commonly remove the alternator,to reduce weight,and load on the engine.(a couple more HP available to the wheels instead of spinning the alt.)

What about using some of the smaller 2V cells? You could put 7 in series (~14V+),and not need to worry about the DC-DC converter.
Just a thought.
The 2v cells are pretty rare and expensive. Since saving money is part of the objective of hypermiling, the batteries would have to be relatively cheap for it to make sense. 7 cells would also have an initial voltage of 16.8v, which would need a buck converter to reduce it to safe levels. The switching transistor can simply be left on once the voltage drops below 14.4v, however. But I think it would be much cheaper to build a boost converter and just use a 6 cell deep cycle battery.
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Old 13th February 2009, 09:24 PM   #9
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I believe that 7 cell (16 voltish) deep-cycle batteries are available now for golf-carts. People use the golf carts as on the road cars in gated communities. You would need a 16 Volt charger though.
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Old 13th February 2009, 10:08 PM   #10
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Default Miles Per Gallon

Hi:
What all seem to be missing is that while the alternator does keep the battery charged while running , the alternator in fact supplies the electrical system of the car and the battery is there for smoothing out , or filtering the power , while the engine is running.
The alternator only supplies the required amount of power to run the electrical requirements of the car and to recharge the battery after starting . The car is also designed to run with the alternator putting out 13.8v to 14.4v . A smaller and lighter alternator is available but would still be required to put the same amount of power out.
Without the alternator the car may not start after a long trip that would run the battery down. Just try to run a car with the alternator not working. Eventually the car will stall for lack of proper voltage for the electrical system , how far would be a result of how large and HEAVY the battery is. Then you would have to have someone help you with a jumper battery and jumper cables to start the car and you would STILL need to have the car battery charged to drive further.
All in all VERY impractical.

Ebbe , try to find a multiple battery charger block from an RV dealer that has two big diodes on a heat sink. Each battery can be connected to the battery terminals of the diodes , eg. the cathodes . Where the two diodes are connected together , eg. the anodes , gets connected to the alternator output terminal. The alternator voltage sense terminal or lead then would be connected to the battery in the car , if possible , and this would compensate for the diode loss and the diodes would isolate the two batteries. Then run a 10 gauge wire to the remote battery and it will work ok. As the trailer battery charges , the current thru the 10 ga wire will drop as will the voltage drop thru the wire and the trailer battery will be fully charged.
Hope this helps
Ed
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