 Hi Folks.. I currently use a standard starter attached to a 12V 7Ah SLA battery but I would like to move away from this setup to a self contained unit (cordless). What I have in mind is two racing battery packs mounted in a holder and connected in series which is then attached to the bottom of the starter . I intend (or intended) to use 2 x 7.2V (3Ah) batteries to give 14.4v but am unsure if there would be enough 'Ummph' in the setup to turn over engines up to size 60. I heard somewhere on the grapevine that people were using 9.6v batteries (19.2V) to offset for lack of current as the starter would only be used in momentary bursts but is this true. What do folk suggest?? Regards Declan
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 3 cell 4000 lipo? Have you tried measuring the current drawn when spinning up a .60+ motor? I'm guessing less than 30 amps so a 10c lipo would do the job easily, but I could be miles out so I'd check the current on a pb battery before commiting to a lipo.
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Cheers Bob.. I've just asked the science technician here where I work if he had an ammeter that would cover up to approx 30amps. At the moment, all he has is ammeters that work up to 10A but he thinks he has something somewhere. Unfortunately we just moved into phase 1 of our new build so everything is all over the place. Its interesting to read that a 7Ah battery can punch out in short bursts up to 3 or 4 times its rated capacity Declan
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on my Aprilia V twin (998cc) a 12 Ah battery punches out enough amps to shove the 1/2 litre pistons over tdc (most of the time  ). I think the cold crank amps are rated around 200 for these batteries, so I'd guess a 7Ah should be good for 50-100 amps for short period.
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Hi Bob.. Checked on the powersonic website. The 12V 7Ah battery has a short duration burst of 21A. According to overlander batteries, some of their 7.2v batteries have 25 -35A bursts. Declan
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typical 7 ah lead-acid motorcycle battery will give 74 amps max (cold crank amps) see here Thinking about using lipos again, I guess the only thing to watch is that you don't run the batteries down too far - ni-cads should recover from a certain degree of abuse, lipos won't.
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 Using my Whattmeter in series with a sullivan "hi-tork" 12 v starter,I have just measured the following. Unloaded spin up of the starter shows approx 5 amps. Gripping the cone lightly pushes this up to around 15A, and gripping firmly shows 30A plus. Turning over a 90FS gave 45A, and stalling the starter gave 60A plus. These were all off a 12V leisure battery showing 12.9V off load ( so not really "full" ) but gives an indication of likely current draw. Increasing the voltage will of course increase the current, and have not much effect on the starters ability to turn over the engine.
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 Bob, don't forget that your bike starter motor is geared down by about ten or fifteen to one! It doesn't need a very fast spin, because the spark fires the mix, not the compression temperature.
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I remember a 'hints and tips' entry from years gone by where someone was using a 7.2 v nicad and 540 size buggy motor wih drive taken through a model car rear axle as a starter. it was all built into a ply box about the size of a nomal 12v starter. I still can't get over a 90 needing 45 amps - about 500 watts or 2/3 hp to spin it!
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| Edited: 10/05/08 10:56 |
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