Accurately Measuring Battery Amperage Draw

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TonyWilliams

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Joined
Feb 19, 2011
Messages
10,107
Location
Vista, California USA
Has anybody figured a way to accurately measure battery drain? I want to be able to know exactly the changes in energy consumption with given changes over a closed course. Speed, tire pressure, lowered suspension, front air dam, reducing weight, and many other logical modifications to improve to range of the car.

A way to hack into the Nissan software might be easy for some, but I don't have that level of expertise. A simpler way might be a simple interface between the battery and the motor to measure and data-log amps.
 
If I understand your purpose, you don't need a precise objective measurement, just one that is consistent enough for quantifying the relative differences between runs (i.e. lowering 1 inch improved efficiency x.x% over stock). I would suggest that the car's miles per kwh number should be up to the task. While it would be nice to have another significant digit in the measurement, this should be sufficient over a decent sized course.
 
Remember that the drive system is controlled for 'power', so you would also need to record voltage along with amps.

(This information is on the EV CAN bus.)
 
davewill said:
If I understand your purpose, you don't need a precise objective measurement, just one that is consistent enough for quantifying the relative differences between runs (i.e. lowering 1 inch improved efficiency x.x% over stock). I would suggest that the car's miles per kwh number should be up to the task. While it would be nice to have another significant digit in the measurement, this should be sufficient over a decent sized course.


The difference between 3.9 mpk and 4.0 is 2.5 percent, but it may be as much as almost double that, or as little as almost zero, depending on how that number is actually presented.

That is a far greater tolerance than I was hoping for.
 
TonyWilliams said:
GroundLoop said:
Remember that the drive system is controlled for 'power', so you would also need to record voltage along with amps.

(This information is on the EV CAN bus.)

How can we get this CAN bus info?

There are two separate CAN busses brought out to the Data Link Connector.
The BCM CAN bus is on pins 14 & 6.
This bus connects the BCM, VCM, and Dash.

The EV Bus is on pins 12 & 13. This carries communication between the TCU, VCM, HVBAT, and OBC.

Note that VCM is on both, and bridges some messages.

(The main Nav computer is connected to the TCU by USB, not CAN)
 
So, there's your answer. Let us know when you have it done ;)

What I want to know, though, is whose MPK have we chosen to believe? After all I've read, I don't believe hardly anything I see. Certainly not Carwings.
 
TonyWilliams said:
The difference between 3.9 mpk and 4.0 is 2.5 percent, but it may be as much as almost double that, or as little as almost zero, depending on how that number is actually presented.

That is a far greater tolerance than I was hoping for.
Whoops. I should have done the math before spouting off. Yeah, that would be a bit coarse. As far as CAN bus stuff, wasn't turbo2ltr the guy doing that work...Yup, here's the big thread:
http://www.mynissanleaf.com/viewtopic.php?f=8&t=2794

He seems to have something in progress, I'd PM him and see if he is able/interested in assisting your effort.
 
gbarry42 said:
So, there's your answer. Let us know when you have it done ;)

What I want to know, though, is whose MPK have we chosen to believe? After all I've read, I don't believe hardly anything I see. Certainly not Carwings.
Well if you only want to know if you're doing better or worse than the day before, I think you can use the car's numbers. If you want an accurate measure, the only way I know is to measure wall output while recharging and divide it into the miles traveled since last charge...which would be a highly awkward for this kind of testing, but for a commuter should be pretty good.
 
davewill said:
If you want an accurate measure, the only way I know is to measure wall output while recharging and divide it into the miles traveled since last charge...which would be a highly awkward for this kind of testing, but for a commuter should be pretty good.


Too awkward.
 
Tony, I think we need this.. eventually we will get a programmable display driven by the can bus for the Leaf, in a matter of months. What you really want is a KWh meter, an instant and an integrating one to keep track of the energy going in/out of the battery pack, this meter should also keep working while the battery is charging.. an amp meter would also work since the pack voltage is relatively constant for lithium ion cells, so the error would be small.

it would be nice to display the full pack capacity with a 1% granularity, and have a couple of programmable range guesstimators driven by the data. An instant one, one that averages over the last couple of days and a best case guesstimator.
 
i would like (if there is enough memory) for a program that can determine based on NAV'd routes and previous performance on said route.

i could make it work for me (although i have zero range anxiety) since a lot of my life is routine and repeatable.

but would be a complicated program that would have to integrate tightly with the GPS...but i find that it is plausible and allows the car to "learn" our needs.
 
The LEAF's Battery Pack voltage drops from around 400v down to about 300v during usage, and more when current extraction is high.

So, measuring Pack current ONLY will not give you accurate power usage, off by as much as 25%, or even more.

(*) Each Cell-Pair is roughly 3v to 4v in normal operation, and 96 in series are used to make the LEAF's Battery Pack.
 
I have my CARWINGS data logged every 5 minutes at the moment, not that that helps. The info and limited interface of CARWINGS seems too primitive for what you describe here by far; it can't even force the car to stop charging!
 
garygid said:
The LEAF's battery voltage drops from around 4v down to about 3v during usage, and more when current extraction is high.

So, measuring current only will not give you accurate power usage, off by as much as 25%, or even more.
You must mean the *cell* voltage, not the battery voltage, yes?
 
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