Mike,
You have a LOT more experience in this field than I do, but I really have to question your position of using the battery "rated" voltage, instead of the "actual" voltage when calculating wattage. Could you perhaps elaborate on this a little more?
I understand and agree that the voltage of the battery bank will drop off as it's under load, especially a heavy load. For short bursts of high amperage, you would record a lower wattage figure using real voltage instead of rated voltage.
Looking at it from the MOTOR point of view, though, you ONLY see the voltage and amperage presented at the motor. All of the wattage measurements I took from my Torqeedo 4 were provided by the motor - it had no idea whether the "nominal" battery bank voltage was 45V, 48V, or 52V.
If my 48V nominal bank charges up to 52V at rest, but drops down to 47V when I'm running at 50Amps, I would prefer to know I was actually drawing 2350W (47*50), and not 2400W (48*50) - and certainly not 2600W (52*50). If one were trying to be really precise in comparing one system to another, they would want multiple voltage measurements - at the battery, and at the motor to begin with so that they knew how much was lost in the wiring, swtiches, fuses, etc...
Maybe I'm missing something, but it just seems "more precise" to quote wattage as the measured amperage multiplied by the measured voltage. If the voltage isn't precisely available, then using the battery bank rated voltage is certainly a reasonably close substitute. It hopefully won't vary enough to matter that much (in the above example, only 2%), but if I had the information available, I would use it.
John
From: "mike@electricyachtssocal.com [electricboats]" <electricboats@yahoogroups.com>
To: electricboats@yahoogroups.com
Sent: Sunday, January 31, 2016 5:40 PM
Subject: [Electric Boats] Re: conversion performance
I believe that there is some confusion in this discussion. There has been a misconception by some regarding the wattage that our electric motors use.
The amp draw is times the rated nominal voltage of the motor. It is not times the recorded state of charge (SOC) of the batteries. Therefore if you have a 48v system and are pulling 10amps out of the motor it is 480watts. If your SOC at that time is 50v it is still 10amps times 48v or 480 watts of power from the motor. It is not 10amps time 50v. SOC is going to be inaccurate in most of our boats as there is draw from the battery bank and only when the battery is in a quiet state is it spot on accurate. You likely all notice how the battery bank pops back up so quickly after you turn off the motor.
If you have been incorrect in your usage of amperage and voltage, you should do a new calculation and you will likely see a marked improvement in your slow speed powering. At the higher draws (amps) the batteries SOC is normally a little lower than at the very low draws. This is a fact of life that we all deal with when we use lead acid based batteries and which I believe is not as misleading with LiFePO4 batteries. Its the chemistry and physics that we are dealing with.
That will bring me to my final comment. We are all students and can learn from each other. We are all teachers and can teach each other. Therefore the collegial sharing of information should be encouraged and corrected when necessary. This is how we all learn.
Mike
Electric Yachts of Southern California
The amp draw is times the rated nominal voltage of the motor. It is not times the recorded state of charge (SOC) of the batteries. Therefore if you have a 48v system and are pulling 10amps out of the motor it is 480watts. If your SOC at that time is 50v it is still 10amps times 48v or 480 watts of power from the motor. It is not 10amps time 50v. SOC is going to be inaccurate in most of our boats as there is draw from the battery bank and only when the battery is in a quiet state is it spot on accurate. You likely all notice how the battery bank pops back up so quickly after you turn off the motor.
If you have been incorrect in your usage of amperage and voltage, you should do a new calculation and you will likely see a marked improvement in your slow speed powering. At the higher draws (amps) the batteries SOC is normally a little lower than at the very low draws. This is a fact of life that we all deal with when we use lead acid based batteries and which I believe is not as misleading with LiFePO4 batteries. Its the chemistry and physics that we are dealing with.
That will bring me to my final comment. We are all students and can learn from each other. We are all teachers and can teach each other. Therefore the collegial sharing of information should be encouraged and corrected when necessary. This is how we all learn.
Mike
Electric Yachts of Southern California
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