Re: (ET) Shunts in for NiCD batteries

["David Roden" <etpost drmm net>]

On 5 Sep 2005 at 21:00, Christopher Zach wrote:

> Maybe 100-120 amps while chewing.  ... Still, I only burned about 
> 20 amps total plowing a few rows, not that bad.

> The lift motors *really* throw packs out of balance. To the tune of 
> half an amp for 10 amps drawn. 

OK, there seems to be some confusion here between amps and amp-hours.  Of 
course I don't know your background, but this is an error I see fairly 
often 
among auto mechanics, some of whom have training in automotive technology 
but perhaps rather less education in basic electricity.

I'm going to take a flyer at clarifying the difference.  Understand that 
I'm 
a EE dropout, so if I mess this up I hope that those here who completed 
their degrees <g> will straighten me out.  

Amps (amperes) are a measure of current.  You could compare this to the 
flow 
rate of water in a pipe (gallons per hour).  

Amp-hours are not exactly a measure of energy - as I understand it, that 
honor goes to kiloWatt hours.  Rather amp-hours (abbreviated AH or ah) are 
a 
measure of what we might call electrical storage capacity (I don't mean 
capacitance).  They are a mathematical construct, effectively the constant 
product of current multipled by time.  (I am treating the battery as a 
theoretical device and ignoring Peukert's effect here.)  You could compare 
this to the capacity of a water tank (gallons).  

A couple of notes:  

1.  Amp hours says no more about voltage than gph says about pressure.

2.  Amp-hours are written that way to indicate that they are a product.  
This makes equations using them work right.  If you write amp/hours, you 
will screw up any math you try to do using amp-hours.  You will 
^certainly^ 
screw up the math if you write just amps when you mean amp-hours.

To demonstrate this second point, let's go back to the water analogy for a 
moment.  I compared amps to gallons per hour, and amp-hours to gallons.  
There's a good reason we write gallons per hour gal/hr.  That means that 
division is involved; you can write 10 gallons per hour as 10 gal / hr.

Doing the math with gph, gallons, amps, and amp-hours, we get

(gallons / hour) * (1 / gallons) == 1 / hours (the inverse of hours)

gallons / (gallons / hour) == hours

amps  * (1 / (amps * hours)  == 1 / hours (the inverse of hours)

amp-hours / amps = hours

amp-hours / hours = amps

To apply this to the example at hand, if your tiller requires 100 amps, in 
one hour it will have consumed 100 amp-hours; in 12 minutes, it will have 
consumed 20 amp hours.  

Did I get that right?


David Roden - Akron, Ohio, USA

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