Just google Joule Thief. It's a brilliant little circuit that can suck a battery so empty that you should only use it with disposable batteries.

 

I was just thinking that its simple to build small and light,,,and since he metioned the goal was as many laps as possible in a set amount of time wins that the joule thief would just give him the extra juice from the batteries to get the max laps..

 

The only electrical tip I can think of is to make the wires from battery to switch to motor as short and as big as is reasonable. This will give you maximum voltage to the motor and reduce power loss in the wire.

 

I was thinking a lm317 after the joule thief to regulate voltage until the battery is down as low as it can get.. and if you get a big enough torroid and wrap it with a heavy gauge wire like maybe 10-12 gauge, its power capabilities would be out standing.. of course I think he would need a power transistor to handle current, the lm317 can only do like 2a max but with a pass transistor or even 2 he can get 4-6 amp constant..It might damage the battery from pulling it completely dead but he said he can use 4 at pit stops.. sounds like he could get a lot of laps with a pretty simple circuit.. and really light too so it wouldn't add much weight to the car..

 

here is a basic joule thief circuit. you could easily modify it for the lm317 and pass transisitor to maintain 12v and maybe add a couple large capacitors to keep the power consistant.. http://www.instructables.com/id/Joule-Theif/

 

Serious doubts about the LM317 idea. The small scale Joule Thief depends on inductive kick to get AS MANY VOLTS AS NECESSARY to discharge something like 20 ma. Some people have put several LEDs (in series) on a simple Joule Thief circuit and they ran correctly. If you try to limit it's output voltage with a blocking switch, all you're going to get is transistors with holes in them.

You could do this with a regulator circuit to make the Joule Thief run weaker or slower, but it's not an LM317 style circuit that will do this.

Remember, it's only my opinion, but I think I'm right. Go with ronv and use proper size wire and nice, clean connections.

Edit: Yep. Just double checked and I still think I'm right. That transformer is going to kick out as many volts as necessary. The best you can do is give it a capacitor to charge and use the capacitor voltage to defeat the positive bias on the switching transistor.

 

You probably are right sir.. my knowledge is limited but im learning more everyday.. I just used 4 317s with 4 pass transistors to make my variable votage 6a psu with an ols Marantz power transformer I had... can go from 2v to 25v with a max of 6a, but the heatsinks get pretty damn hot at lower voltage with high current draw, but I fixed that making the whole back of the psu a huge heatsink from an old car audio heatsink and fan.. pretty awesome psu though..

 

making the whole back of the psu a huge heatsink from an old car audio heatsink and fan.. pretty awesome psu though..

There is something awesome about a 5 pound brick of a heat sink.

 

There is something awesome about a 5 pound brick of a heat sink.

you would be correct sir... looks awesome and I mounted every 317 and pass transistor to it.. then I have an old cpu processor blowing across it..

 

There might be one area when you can save energy. The on/off switch control is very wasteful. When you close the switch while at a standing stop you draw a very large amount of current getting up to speed. A lead acid battery will provide significantly more amp-hours of power at a lower current than at a higher current.

Consider this, when you push the button, a buck switching regulator ramps the voltage up keeping the current low. At a certain point the buck regulator is bypassed with a relay. This method will leave you with more energy in your battery than simply "dropping the switch".

If you look at lead acid battery discharge rates verses load current you will see this to be true.

Mark

 

How about two switches/relays? The first to use the batteries in parallel for high amps when starting from a stop. The second would have the batteries in series for a higher voltage/speed when the racer is moving. It would almost be like a two speed transmission that way, low and high.

 

Good point Mark.
If this is the rule, http://forum.allaboutcircuits.com/showpost.php?p=736314&postcount=11 Then a motor controller is ok. In that case one with regen would be the way to go. Joule thief has way to low of current and efficiency to be of value I think. The motor looks pretty big.

 

How about two switches/relays? The first to use the batteries in parallel for high amps when starting from a stop. The second would have the batteries in series for a higher voltage/speed.

Top quality wiring practices are mandatory for that arrangement, considering that this motor can pull 100 amps from a dead start.
Do a good job on the wiring and it should work nicely.

 

Continuous rating 240watts 1.15Nm.
Max.

 

100 amps is a lot of juice..lol..forget the joule thief unless you can find a ferritr torroid the size of Detroit.lol.. are you limited to type of battery? like a blue top optima battery is designed for deep cycle purpose,, then maybe get on in the group34 class...idk

 

100 amps is a lot of juice..lol..

Can you spell 'De-magnetization'
Max.

 

How about two switches/relays? The first to use the batteries in parallel for high amps when starting from a stop. The second would have the batteries in series for a higher voltage/speed when the racer is moving. It would almost be like a two speed transmission that way, low and high.

This sounds like a good idea and not overly complicated - its a shame such a big torroid would be required for a joule theif as it seemed like something that could really improve the car. Well, things that seem too good to be true usually are.

 

how about a non eloectrical suguestion? narrow tires have less rolling resistance, try to avoid the current popular super wide tire loss.

 

I suggest a new motor. It wasn't clear to me whether this is an option or not, but your motor's efficiency is rather pitiful and it deserves an upgrade. If a new motor isn't an option, then I would suggest optimizing the gearing to keep the motor in its underwhelming butter zone of Torque between 1 and 4 nm and speed above 1500rpm.