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From: "toast" <toast-at-shadowsofchange.com>
Subject: Car Starters [TANKS]
Date: Fri, 3 Mar 2006 09:55:47 -0600
Reply-To: tanks-at-rctankcombat.com

I’ve searched the threads, I’ve read forums and other websites about using car starter motors, I’ve talked with people in the know about automotive components.  Some of the mysteries at this point will only be solved by testing so I guess I’ll throw some ideas out and see what you guys think about my proposed drive train setup.


I think I can use the Anvilus RC Mini Relay Controller (http://www.anvilus.com/m_050321_manual.pdf) which to my understanding is the same as the bigger Anvilus SSR (http://www.anvilus.com/h_040305_image.jpg) but has no relays.  Lets assume that the starter in question is rated at 900 watts.  On a 12v system like the car it came out of then then starter would draw a max of ( 900 watts / 12 volts = ) 75 amps.  I would have to find heavy relays able to handle very high current.



I’ve heard/read somewhere that the average automotive starter requires 30 or 40 amps just to get it moving.  If this is true then I think this makes it unsuitable for rc tank combat applications as the battery draw before movement would be wasted.  If they do require so much power to get them moving initially then I thought about using some type of kicker motor.  This kicker would be a small, very low speed high torque (through gearing probably) electric motor that would simply push the starter around once or twice to get it moving and then disengage.  The starters bendix is a good example of how to make this other motor disengage.  Some custom circuits would need to be in place to cause the kicker to stop running but I doubt this would be hard either as I do have some electronics skills.


At this point I’m wondering if the 900 watt rating means that even if they work, I’ll be drawing so much power that my batteries will only last 5 minutes.  I seem to remember most of you guys using 24-26ah batteries and I’m guessing that these allow you to run about 30 minutes (my target).  Let me do some math on that.  To get a 24ah battery to last 30 minutes you have a steady amp draw maximum of ( 24a / 0.5h = ) 48 amps.  A 24ah battery weighs about 19-20 pounds.  If I’d need 2 of them to have 96 amp maximum current capacity at 12 volts then I’m talking 40 pounds in batteries. 


Is this reasonable?  Is 40 pounds in batteries reasonable?  Are these calculations reasonable?  Should I consider the the ah rating on these batteries soft(overrated 24ah really means more like 20) or firm, or even underrated?  If I get the batteries and the relays needed for this and find that car starters aren’t a good choice, can I still use the other parts with other motors proven to be suited for the rc tank combat environment?


Please keep in mind that I understand the issues with cooling and bearing problems with starters and am not currently considering them as factors as to whether or not to use an automotive starter.  Cooling and bearings have been thoroughly discussed other places and I’m trying to look at this from another angle.


Kevin “I write long emails” Adams