12 V battery charger math 2

[zdas04]

I just got a battery charger/jump starter from Stanley. It says that it can deliver 450 A "cranking power". At something like 12 V, that is 5400 W. Dividing that by 120V says I need 45A breaker. I plugged the charger/jumper into a 20A breaker and cranked a vehicle for nearly a minute before it started. No problem with the AC circuit. Can someone help me understand what I'm missing in this process?

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[IRstuff]

Don't these things usually have a battery inside?

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[zdas04]

This one doesn't. It is a new (to me) design that really worked well. Here it is at Sams Club

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[IRstuff]

Reading the descriptions online, I would have to disagree; your cite shows, "Built in AC charger cube" implying that the AC is for charging the internal battery.

see:

http://www.amazon.com/Stanley-450-Jump-Starter-Compressor/dp/B00B1XCQZW,

which says the maximum current available is 900A, and that the unit weighs 17 lb. The various pictures online show the unit pumping air with no external power, which implies an internal battery.

Likewise, the "900 Battery Amps" on the cover would imply an internal battery, since that would be even more difficult to run from the AC line


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[zdas04]

I think the pictures are misleading marketing crap. The only way to build pressure in the compressor is to plug it in. When it is unplugged there is no potential difference between the red and black clamps. It could easily have capicators for storage (probably does now that I think about it), but there is just no room for a battery (most of the space is taken up by the hose real for the compressor). It is heavy, but it has a lot of stuff (compressor, hose, cables, transformer, etc), I just can't see where you would put a meaningful 12V battery, there may be one, but I can't see where it would sit.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[ScottyUK]

Breakers start timing toward a trip at a value higher than the rated current. For example, a European 10A MCB will pick up at about 13A or so..

At 2x rated current an MCB will typically trip in about 3 minutes, and at 6x rated current it will trip in about 10 seconds. The tolerances are pretty wide, so treat those values as typical rather than guaranteed. I'm not sure what the equivalent times for North American breakers are, but not too different I suspect.

 

[hacksaw]

no 12v charger that you can easily carry is going to have 450 amps cranking for one minute, basically a tranformer limitation thermally and electrically. your unit may have originally been designed with a battery on board.

even the conventionial chargers rated for 150-175 amps come with wheels and cranking assistences is only in 20-30 seconds max with cool down intervals.

let the buyer beware seems to be the rule of the day

 

[mikekilroy]

That unit is one of the many with a small sealed lead acid battery inside of approx. 10-20va - see specs on the typical battery used here:

http://www.ebay.com/itm/12V-17AH-18...906?pt=LH_DefaultDomain_0&hash=item5d45872f52

18ah or less capacity typically. They have very high short term amp capability. Yes, they can produce 400+amps for short time. You car probably required 300a for .2 sec to begin rotating the engine, then dropped to 75a for the remaining minute....

http://www.KilroyWasHere.com

 

[itsmoked]

Something to remember with those type batteries is that you need to recharge them IMMEDIATELY after using them. If they sit discharged for more than an hour they will lose their capacity in a wholesale manner.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

That is a great piece of data Keith. I bought these units for my sons for Christmas and I'll make sure they know that.

Mikekilroy,
That makes a lot of sense, and I can see how that would work. Thank you.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat