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Discussion Starter · #1 ·
Not sure if this is the right place to post this, but . . .

I have a few observations, hypotheses and ideas concerning the Prius being operated in a sort of Prius Plus mode.

First my observations/guesses -

The Prius uses a gas-gauge algorithm of some sort to tell you what's left in the traction battery. It also uses this information when deciding to start the ICE, stop regen braking etc. I hypothesize that this 'gas gauge' is charge-accumulator based; that there is a controller somewhere looking at current into the battery and current out of the battery, and keeping track of how much has gone in and how much has come out. If this is true, trying to charge the traction battery when the car is off won't work well because the charge accumulator won't notice the charge, and it will always report the wrong battery capacity after that.

The Prius senses current into and out of the battery at the battery itself; I have seen several references to a current sensor near the battery breaker. I am guessing that this is the same sensor used to report current direction and to determine capacity remaining.

I am _guessing_ that if current were applied to the input of the battery, therefore, the gas gauging algorithm would still work OK. In other words, if you attach a battery charger to the battery while the car is in Ready, you would see battery current flowing in as if you were in Regen and the battery charge would be seen to increase.

If all this is true, then the idea is:

Design a Prius Plus with an external battery bank that feeds the internal traction battery at the battery terminal. Start with a 48 or 96 volt SLA battery bank (easier to charge than a 360 volt bank) and then use a boost converter to boost the 48/96 volts to the 300 or so volts you need to charge the traction battery. Regulate the boost converter so you never charge the traction battery. So if the propulsion system is drawing 20 amps, the boost converter supplies 20 amps to the battery, powering the motor without draining the traction battery.

The purpose of all this is to allow extended driving in EV-only mode via a battery pack that can be charged externally. My assumption is that if I supply enough power, the Prius will be happy to stay in EV mode as long as none of its normal criteria (34mph, throttle position etc) are exceeded. This would also require minimal mods to the Prius; basically one connector to the battery pack and a current sensor around one of the power lines.

So am I missing anything here?
 

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I believe that for safety reasons, the Prius computers will open the HV battery relay if the current measured at the battery does not match (to some tolerance), the currents measured at other parts of the HV system.

So any current you add into the system must be less than the tolerance. I think other Prius+ groups may have published what this tolerance is. Also, be very careful to keep both sides of the battery floating w.r.t. ground or you'll trip the GFI.
 

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Two points:

1: While there may be some "charge accumulator" calculation going on, this sort of integration would be fatally flawed over a long period. Charge in does not equal charge out for various reasons, including poor efficiency caused by heating and self discharge. The main determinant of the battery state of charge would have to be the battery voltage. (In fact, the battery is acting as its own integrator in this case.)

2: I've experimented with taking current directly from the HV battery apart from any actual use in the vehicle (see http://www.PriUPS.com). At least at the 2.4KW level the battery seems happy to supply current and goes through a normal charge/discharge cycle while the car is stationary.
 

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Not only does the Prius monitor current, it also measures voltage at each module. I believe this, as well as temperature, determines SOC. This would explain why some have observed a decrease in SOC while in Ig-On mode, where the relays are off. The batteries probably warm up, since now there is no AC, so charge capacity of the battery increases, thus relative charge decreases.
It's like how you see the humidity increase when the air temperature decreases. The same amount of water is in the air, but the air can't hold as much, so the air feels more humid.
 

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Discussion Starter · #6 ·
RSnyder said:
So any current you add into the system must be less than the tolerance. I think other Prius+ groups may have published what this tolerance is.
Interesting. Do you know which groups would be discussing this?
 

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See also my flcooper post [Comments on current Prius+ article in HOMEPOWER] under Prius Q&A for a reference in the current HOMEPOWER.COM magazine that features a good article on Prius+ by CalCars.

Unfortunately, I found out that the magazine is delayed posting by one month (da!) so they can sell them on the news stands.

Fortunately, the editor (Michael Welch) wrote me back with the following:

If you want, you can let the chat room know that if individuals send me a personal email with the words "PHEV article request" for the subject, I will send them a copy of the article in Acrobat format.

But when you post my address, in order to not get on spam lists, please post it this way:
[email protected][remove this]power.com
Redundant NOTE: delete the text [remove this] from the above e-mail address such that it reads [email protected]

GOOD ARTICLE!

Frank
 

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I fly Electric RC Airplanes, and one thing I've noticed is that most battery chargers I've owned use voltage to determine SOC. I know this has been stated once, but other than Delta Peak detection, you would never want to determine SOC based on current out vs. current in. Voltage is a much better way.

So, if you're interested in extending your EV travel with a supplimental battery, all you need is a way of plugging in your battery in a paralelle configuration. But, to me your robbing Peter to pay Paul. Peter bing the coal buring elecric generators that you will use to charge your battery, along with lower gas millage when the ICE is running because of the extra weight from the additional battery.
 
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