Whats the power requirement of starter motor?

Thinking once more to switch to a Li-ion battery.
Only thing is I dont fancy paying £150 (shocker), I've heard some statesiders have been known to extract the battery out of a garden stimmer and they've modded this to work on track bikes.

I know the voltage is 12v
the Amps are around 20A

But IIRC there are 2 other factors which must be considered and its not just as simple as volts and amps.

Esentially once the engines running then power comes from the R/R anyway so running power isnt a problem, in my head logic dictates that all the battery needs to do is provide enough juice to fire the starter motor.

Someone tell me what inevitable facepalm Im heading for.

[Shawthing]

Though you get the weight advantage, Don't these new cells have issues to do with the avoiding over charging and operating temperature needing management electronics ( or is this already built into the batteries you are looking at?).

li-ion batteries are memoryless ~~~ I dont know the ins and outs of it but from what I gatehr they cant be over charged and generally come with about a life cycle of 2000 (guestimated figures based on various sources).
I've just been reading on SVrider that they've been using 9000amh and thats been a sucess but once again these are bike specific batteries not general use ones.

[Shawthing]

Originally Posted by Owenski

from what I gatehr they cant be over charged .

Yes, but only if they have apropriate protection circuitry.
As i rememberred:

Wikki quote :
If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. In extreme cases this can lead to combustion. Deep discharge may short-circuit the cell, in which case recharging would be unsafe. To reduce these risks, Lithium-ion battery packs contain fail-safe circuitry that shuts down the battery when its voltage is outside the safe range of 3–4.2 V per cell. When stored for long periods the small current draw of the protection circuitry itself may drain the battery below its shut down voltage; normal chargers are then ineffective. Many types of lithium-ion cell cannot be charged safely below 0°C.

http://en.wikipedia.org/wiki/Lithium-ion_battery



9000amh (or 9Ah) is close to the lead acid std YTX12-BS capacity of 10Ah.
If 20A is the drawn current calculates to about 10minutes of turning engine over ( so plenty )

Most Li-ion (and Li-polymer) battery packs are supplied with the relevant control circuitry built onto the battery, even in really tiny batteries. In the case of a rechargeable strimmer battery I'd expect the controller to be in the pack - if you can get one for cheap, experiment... outside If you've never shorted a LiPo battery my geeky side can highly reccomend it

But seriously, take apart the battery pack and see what's inside! Might give it a bash myself...