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Old Aug 14, 2012, 10:00 PM
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United States, AZ, Tucson
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Lipos for auto racing use

Yes, you read it right.

Long story short I race karts. No not your average pay $10 and go around the track go kart. We are talking advaced 120mph+ karts. The kind that compare closer to F1 than any other vehicle in the world.... Anyways enough about that.

I race in a class of Rotax 125cc karts. They have digital ignition powered by a 2.5 amp hours lead battery which weighs 6 pounds. Doing some quick calculations I could either have a Lipo battery that weighs 10oz, and the same capacity, or a Lipo battery of the same weight with 25ah!

The lead battery is 12v so my only option would be to use a 4s battery with a voltage regulator! These are very common and used all the time such as in DLG or for RX packs. My problem is I need a voltage regulator, that can regulate at 12volts but can still put out up to 15amps.. I assume that's plenty, as I have never really checked the current used... I assume realistically quite a bit less as the lead battery lasts for about 3-4 hours of driving.....

So any help either creating my own, and or where I can purchase a voltage regulator?

I have pondered the idea of a simple resistor... But would like something that would more reliably stay at 12.5 volts
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Old Aug 15, 2012, 12:04 AM
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The ignition should be fine on the 14.x volts that you normally get out of an alternator. I would not use Lithium Polymer soft packs, I would use Lithium Iron Phosphate cells (A123 style). You may want to look at Shorai batteries and their balance charger, there are many others that you should also look at and you can probably find a large selection at Summit Racing or one of the other racing shops.

Otherwise pick up a 4s LiFePO4 pack with as many cells in parallel to make the current you need. Probably a 4s2p pack should do the job (around 3.6 amp hours if my memory is correct).

You could also maybe do NiMh in 10s and 2p with AA cells if the draw is somewhat small, might want to go with 10s2p in A or B cells to make sure. A 10s1p in sub C or full C would probably do the job too. All of these are more stable in a wreck than the LiPo sack will be which is why I don't suggest the soft pack LiPo. Last thing you need is a lipo fire along with a fuel leak.

One thing that bothers me is a 2.5 amp hour lead acid should not weight 6 pounds. The battery for my motorcycle was somewhere up around 18 amp hours and weighed around 7 to 9 pounds. The Shorai I just installed weighed around 1 pound and claims 21 amp hours (LFX21 series).
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Old Aug 15, 2012, 12:07 AM
"Simplify, then add lightness"
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Don't know why you would need 15 amps. If you can run off a 2.5 Ah battery for 3 hours, then average current would be under 1 amp. Surely something like this would run it. http://www.hobbyking.com/hobbyking/s...y_12_23v_.html
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Old Aug 15, 2012, 08:01 AM
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Edit, double post
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Last edited by Jwmflying14; Aug 15, 2012 at 08:56 AM.
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Old Aug 15, 2012, 08:55 AM
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Hey guys, sorry I just caught my mistake in the first post. The stock battery is a 6ah battery.

My concern with amp draw is simply the starter, I don't think the ignitio takes mire than approx an amp or so.

These karts do not have alternators or magnetos, so I am not concerned about the battery charging. They DO however have a digital timing system which changes timing throughout the rpm range, extending the length of powerband. Not only is the digital ignition sensitive to voltage input, but the batteries are a Spec item and need to be 12.5v at races.

I hope this explains why I want to use 4s lipos, regulated to 12.5. The battery isn't my concern, just finding a regulator that is able to take the amp draw of the starter or is able to regulate at 12.5 volts.

As far as the concern with amp draw to the starter I am not too concerned. I think I would just wire in a bypass and use a straight connection to the battery for the starter, however when the switch is open the voltage is forced through the voltage regulator to regulate for the timing system.
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Old Aug 15, 2012, 09:40 AM
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Castle Creations CC BEC Pro is adjustable to 12.5V output and good for up to 20A. Input voltage up to 50V.

Might be worth a look.

http://castlecreations.com/products/ccbec.html

http://castlecreations.com/support/d...user_guide.pdf
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Old Aug 15, 2012, 11:52 AM
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I didn't even know about the pro. I've always used the 10amp!

Awesome thanks a bunch. I think that will work fine as long as I make sure to wire in a bypass when using the starter also I could wore in an 18-20 amp circuit breaker right before the CC BEC just incase it tries to blow up the BEC, or I forget to bypass when starting.

I'll give this a shot!
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Old Aug 15, 2012, 12:02 PM
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I wouldn't run the starter though the regulator, I'm sure there is no rule on what the started gets. The solenoid that triggers the started can get the regulated voltage since it isn't high current.

One thing to check... Are other racers using lithium batteries? If not there is probably a reason, and the reason is the rules.

I would probably try a 10s Nimh pack with really good sub C cells (about 5 amp hour) and see what happens. My concern would still be the starter since it probably uses 30 or 40 amps to turn it over, this might be beyond what a good NiMh pack can supply and may require a 10s2p pack to handle the draw. Still going to be a lot lighter than lead/acid.. Is it possible to use an external starter or do the rules say you must have an onboard starter?

If required to have a functional starter, then you could use any light weight battery you want for the starter and use any other battery for the ignition as long as the voltage doesn't exceed the maximum allowed in the rules. You would still be able to save around 4 pounds. You should look at the lithium motorcycle batteries something like this would probably work once you regulate the system power, I would still run directly to the starter solenoid to make sure it gets the high current supply it needs. http://www.shoraipower.com/p-146-lfx07l2-bs12.aspx
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Old Aug 15, 2012, 12:33 PM
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A fully charged 3S lipo is 12.6 volts, the same as a fully charged lead acid after it has had time to stabilize. 3S lipo doesn't work with normal automobiles because the charging system voltage is too high, but you don't have an on board charging system, so it should work as a direct replacement without a regulator. The discharged voltage is about 11 volts, roughly the same as lead acid also.

Dan
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Old Aug 15, 2012, 01:40 PM
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Hey guys, thanks for the input. As far as the racing spec is concerned it is a very spec class and has regs all the way down to fuel filter that can be used. I will be consulting the organization about checking voltage (12.5) at tech rather than only allowing one specific lead battery.

So as of right now I am making a battery for practice that doesn't need to be charged in lesst than 6 hours of driving.

Once more, digital ignition, no magnetos, no alternator, no charging. The battery is simply depleted by starting, and through ignition.

I imagine ignition only draws between 1-3 amps max. This is where I came up with the idea to Bypass the BEC with a switch. When the switch is closed it can provide full current draw, and voltage to the starter motor. When the switch is open, it can no longer bypass the BEC and it is forced to be regulated at 12.5 to the ignition.

I will also experiment with a direct 3s as well and see how it works.
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Old Oct 12, 2012, 05:14 PM
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I'm a little late to the game here. I don't look at this forum very much.

There should be a starter solenoid someplace. The battery will be hooked directly to the one side of the solenoid and the other side will go to the starter. The solenoid is controlled by your regulated supply through a common switch. The solenoid should draw no more that a couple of amps when engaged. There is no need to regulate the voltage to the starter. There should be no need for any other switch in the system other than is what already there.

How did you fair on the rules question?

Ken
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Old Oct 13, 2012, 02:10 PM
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Quote:
Originally Posted by kenh3497 View Post
I'm a little late to the game here. I don't look at this forum very much.

There should be a starter solenoid someplace. The battery will be hooked directly to the one side of the solenoid and the other side will go to the starter. The solenoid is controlled by your regulated supply through a common switch. The solenoid should draw no more that a couple of amps when engaged. There is no need to regulate the voltage to the starter. There should be no need for any other switch in the system other than is what already there.

How did you fair on the rules question?

Ken
Thank you for the input but these karts aren't like normal vehicles. It is an all loss battery and ignition system. There is no alternator, magnetos or anything maintaining ignition or charging battery. The motor uses a built in mechanical fuel pump, and water pump. The digital ignition runs off the battery and works by changing the timing throughout the rpm range to maintain a strong and longer lasting powerband.

It is otherwise simple. There is a pushbutton that powers starter, and an ignition switch which essentially arms the system and turns ignition on. These both Y off of the same power, which is why my plan doesn't consist of any relays, there are none!
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