greatest invention i am tyrying to build

Morning Peter, evening Kevin, I'm just going to bed, had some lovely blizzard driving today, then trying to find a minor electric fault and discovering the Transit Connect's lighting switch whilst looking the same as a Transit is wired differently! All at -10c with the snow going down my neck so I'm a bit grumpy. we get this sort of question come up every so often. Please there really is nothing new in the automotive world. Most ideas are stolen or hand-me-downs from the avionics/computer worlds. Yes I do know* Kevin could make something but I agree with Peter, install a voltmeter and I agree with me, carry a power pack, fully charged, especially with the current low temps. in the the Northern Hemisphere. *Been on your web site Mr. P, seen your wonderful devices, to me your mirror aids are a lot more useful than most things.

Sorry KP - I missed your reply.... Excluding "in principal energy discussions", it is a great idea, but ultimately IMO not practical. Starting from its reverse.... Say you have a dead battery - what is the solution? (IE, a spare) Say you have a flat battery - what is the solution? (IE, a spare, or a charger like solar or generator) Those are simple solutions with the spare battery being the ultimate including piece of mind. You (KP) hit on some of the base constraints - no restarts etc. The system will not fix a faulty battery. And if the alternator is faulty.... (That's what a dash voltmeter is for. Or a spare battery.) The start-to-charge system is simply a self-start system (how cheap are they?) with the added trigger - like self-heating cars etc - and the smarts. But monitoring a battery as in trying to judge its condition or capacity.... no. I would assume a simple voltage monitoring only - eg - if battery voltage measures under 12.2V for 5 minutes (which equates to an OC voltage of 12.4V @ 25C; ie, ~30% discharged), then start and charge until battery current drops to 3A or for 30 minutes (whichever first) and stay off for 24 (or 12) hours. I would also add thermal sensors so the battery temp above ambient can be monitored and cause shutdown if required. But to test for battery condition other than its (apparent) OC voltage - nope; too difficult, and I argue, there is no point. To release this as a product IMO would be fraught with claims etc. But maybe do as the little "emergency cig-socket battery chargers" manufacturers will do (it has a small AGM battery probably equivalent to 6V-4AH or less) - be gone before people start their claims.

I get what you guys are saying.I don't want to build a permanent battery monitor as a serious project for mass production.What i would like to achieve is have a electronic box, tell my client that this can be used for starting his car when the battery runs down for example he is at a barbeque and the car is right next to him(pumping the sound system).This should never be a permanent thing for his car. You guys have given me lots of advice and i will take heed of all the dangers.I am much more aware of this little project of mine.I have to correct many errors in my project and you have helped me see this. Well let me get going with this project and i will keep you updated. I see there are many electronic engineers on this forum.In cape town where i stay it seems i am the only guy that thinks out the box:D the only interesting answers are the one that destroys the question.

Well in the example you give, it's simply a (low) voltage sensor to trigger the autostart, and a battery current sensing to tell when it's full; and a timeout timer. But that could also be done with the voltage sensor (or $20 MW728 "battery protector" etc) triggering a lamp or klaxon or audio to signal manual starting is due, or turn off the audio etc. And yet again, I recommend a dual-battery circuit anyhow. What if the main battery is weak and won't crank? Or the circuit fails? That's simple. It's only the autostart that is complex.

What i find difficult is some cars need a certain amount of amps for one full crank and start,and others like jeeps with a v8 needs more amps to crank.How could i really set up for what is maximum.i would have to have a serious over the top amp meter across the battery in series. Tell me this http://www.conrad-international.com/BATTERY-GUARD-MW-728.htm?websale7=conrad-int&pi=851744&ci=SHOP_AREA_14724_0312032 is 20 dollars.So that means i will have to x7.5 for our currency which works out to R137.This looks cheap but shipping cost is gonna be heavy. Let me order one and see what mods i can perform.the only interesting answers are the one that destroys the question.

What has cranking amps got to do with it? If wanting to modify the voltage switch, I recommend the Oatley kit K227 - 12-24V Dual Battery Adapter For A Vehicle instead for AUD$22. I'd suggest a spare relay in case... (see here; AUD$6.50) The K227 is a typical "smart battery isolator" (but cheaper!). But since it describes the circuit operation and needs you to construct it, it may be more versatile. You may be able to have it provide the maximum timeout (though the RC for the off delay may be excessive). And it is voltage adjustable (unlike the MW728 in standard form) with your choice of housing.   

The answer to your rather obvious question is the size of the engine you're trying to start such as its inertias.

It's not only size of motor, it's compression ratio as well.  I would be willing to bet a 13:1 four cylinder would take more power to crank then an 8:1 V8.

If all you want is simple voltage monitoring the entire thing is easy.  Auto starts, in general, are pretty straight forward.  The only thing that requires any thought is the tach threshold programming and even that isn't too bad.  Past that, it's just writing the logic for max restarts and safeties.  Most of those, even, can be copied from remote starts (hood pin shutdown, brake pedal shut down).  It would be awesome to somehow work a CO3 detector in as well just in case someone forgets to shut it off and parks indoors.  I've never seen that in a car before!

Kevin Pierson

CO3 Kevin? Carbon trioxide?

Check this out!

http://www.sparkfun.com/products/9403

Kevin Pierson