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Discussion Starter #1
Here is a graph of battery voltage against current (I hope):
Edited by Pete to link to local copy, was only able to get to Yahoo hosted picture if you log in to Yahoo.


What I think this shows clearly is that the internal resistance of my Prius battery is about 0.46 ohms, the slope of the dashed pink line.

I captured this data as I started the car and began my commute this morning. The orange section of the line shows the part of the data that was before I moved the car. The big swoop down and to the left is, I presume, the pulse of power taken to start the engine. Obviously, voltage drops as current is drawn and increases when charge current flows in. It takes a little while for things to settle down and there are a few small loops.

The blue section of the line is me accelerating along my (short) street and then braking to a stop at the end. I used up to 50 amps and caused the battery voltage to fall by about 18 volts by the time I backed off on the pedal to zero current again. I got back up to 33 amps by braking, but only about 7 volts. This section was uphill.

The red section takes over now and this is a second, shorter but harder acceleration, followed by a short coast and then again braking to a stop. Following the red line down and left this time I use over 60 amps at the peak. Going around to the right and up, while I coast I get about 15 amps, falling off as I slow. Then it's up and to the right further as I brake, peaking at just over 50 amps. By coincidence, this swoop ends up where it started and the battery voltage is the same as at the end of the blue section. This section was slightly downhill.

Movement along the diagonal is due to internal battery resistance, that is instantaneous voltage change with current. Movement of the lines up and down the plot are due to charge and discharge of the battery, that is slow change of voltage over time. I have matched the dashed pink line's slope up with the swoops as they cross the vertial axis at zero current. This is where the change of voltage due to charge/discharge will be minimum and so the change of voltage due to current (resistance) will be most apparent. At several points, the lines move across the vertial axis quickly with time as my foot came off the accelerator and moved to the brake. I have taken particular notice of the slope here as change of voltage with time is again minimal.

I hope someone finds this interesting. The colored sections are in fact one continuous sample set. I only colorized the line to aid discussion. If anyone is confused by the large current draw, I should point out that the car was cold so the engine just sits there warming up and leaves all the work to the battery and MG2.
 

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Graham,

Good job. 0.46 ohms is in agreement with Toyota. You may have discovered a way to check the health of your HV battery. The paper available at the following link has a graph of battery resistance versus mileage driven.

http://groups.yahoo.com/group/toyota-pr ... es1/files/
-> SAE articles -> newbattery.pdf

The graph of figure 8 shows a new battery measuring a little under 0.50 ohm. This is in line with your measurement.

At 250,000 km (155,000 mi) figure 8 shows maybe 0.55 ohm. So I guess as far as battery resistance goes your HV battery is still young.

It seems as the internal resistance increases the maximum charge or discharge current the battery can handle will decrease. I wonder if the resistance says anything about the charge capacity of the battery?

Toyota says there are 38 modules connected in series. Each module is rated at nominal 7.2 volts for a total of 273.6 Volts. It is interesting to note in your plot that voltage at zero current ranged from about 276 Volts to 300 Volts. Seems the Prius does not let the modules drop to nominal 7.2 volts very often. At 300 Volts the module voltage is about 7.9 volts. Also interesting that module voltage started at 7.9 volts cold but then was never that high again (at zero current).

Ed
 

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Graham,

You've evidently found a way to measure the output voltage of the traction battery. Where are you getting it from? Is it from those wires adjacent to the yellow current sense wire, or somewhere else?

Regards,
Julian.
 

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Resistance

Graham,

May I assume that your calculations were the result of measurements made at the battery (ECU)?

I wonder what it looks like at the entry to the Inverter/Converter: what is the resistance including the wiring?
Is that the effective power limiter rather than a system programmed limit?
 

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Resistance - where measured

Bill, yes, the voltage was measured close to the battery, at the output of the System Main Relay behind the back seat. The details are currently here: http://www.channel1.com/users/graham/Prius/DataAqu.htm (but I need to get this stuff more organized, so it will probably move).

I don't think I'm going to be the first to remove the cover to the Inverter/Converter box and stick wires in there. I might do it if someone else blazes a trail and posts pictures. What a lot we could find out, though, if we instrumented that!

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