Nimh HV battery and discharge 40 amps

[Flying White Dutchman]

"YOU_ need to make sure that you know what you're doing"

you now that there is the opensource plugin prius project?
a lead acid battery packs are already being put in paralel to the prius battery!!

and i already have the 500volt dc protecting gloves

[archae86]

Originally Posted by Flying White Dutchman

you now that there is the opensource plugin prius project?
a lead acid battery packs are already being put in paralel to the prius battery!!

I seriously doubt it is directly hooked in parallel--the characteristics are seriously mismatched for charge and discharge to the NiMH installed traction battery.

I'm not a car guy, but I am an electrical engineer and also know something about batteries. From the content of your posts so far and the nature of the problem, I think this endeavor is not likely to go well for you. A poor relation of cost and risk to reward seems quite clear.

[kevinwhite]

Originally Posted by archae86

I seriously doubt it is directly hooked in parallel--the characteristics are seriously mismatched for charge and discharge to the NiMH installed traction battery.

I'm not a car guy, but I am an electrical engineer and also know something about batteries. From the content of your posts so far and the nature of the problem, I think this endeavor is not likely to go well for you. A poor relation of cost and risk to reward seems quite clear.

I was surprised but that is exactly what they do here in this link from Calcars:

see PriusPlus-Theory - EAA-PHEV
PriusPlus-RawData - EAA-PHEV

This uses a 240V 20Ah lead-acid pack directly paralleled with the NiMh pack - I guess it works but I would have thought the surge currents would be rather high. If a lower resistance battery is used a 0.3Ohm resistor (1800W!) is inserted in series to reduce the peak current.

kevin

[kohnen]

Originally Posted by Flying White Dutchman

"_YOU_ need to make sure that you know what you're doing"

you (k)now that there is the opensource plugin prius project?
a lead acid battery packs are already being put in paralel to the prius battery!!

and i already have the 500volt dc protecting gloves

Sorry to seem anal, but I'd rather not anyone get hurt.

[miscrms]

Well I just erased this post before I could publish it, so here goes second time around. Probably won't be quite as detailed this time, sorry.

The cal cars design is about as simple as you can get (and hence about as cheap as you can get). At its essence it really is just as simple as paralleling a big lead acid battery with the built in oem battery.

Of course things aren't really that simple. The Prius' battery controller functions as a Coloumb counter, meaning it derives SOC by measuring how much charge comes in and out of the battery not by voltage. There is a good reason for this, as voltage is a very poor indicator of NimH SOC. The upshot is, you can park a big battery in parallel all day, but the HV controller won't use any of the extra charge available from it. It already "knows" how big the battery is supposed to be, and only lets out as much current as that battery can handle and then tells the HV controller to put it back.

Fortunately it was discovered that there is a built in "recalibration" routine that kicks in at 242V. The OEM battery is nominally 201.6V (1.2V/cell*168) but at full charge its more like 1.4V/cell or 235V. At 242V it assumes things have drifted over time (since coulomb counting is sort of a "dead reckoning" approach) and it revises its SOC value upwards to compensate. By using a secondary battery with a voltage higher than 242V, you can trigger this routine and get the controller to use the extra charge.

Of course this creates additional challenges. Cal-cars uses a 20 battery pack, that is nominally 240V, but more like 260V at full charge. This is more than enough voltage to cook the oem battery if you were just simply to parallel the two together. In addition, it turns out that you can't just keep the oem battery "full" and have everything work well. There is a "magic" SOC range at which the HV controller will use a lot more electric drive at all speeds, which is key to getting good mileage out of a conversion. If the SOC is a little lower, it will just drive like a normal Prius and you won't see much benefit. If the SOC is a little higher, the HV controller will start burning off charge in inefficient ways like spinning the ICE. Whats worse is that the magic number charges with temperature and driving conditions.

Cal-cars addresses this problem by putting a switch (contactor) between the primary and secondary packs (told you it was simple). Then you need a controller to decide when to open and close the switch to keep the SOC where you want it. Currently this is done by the CAN-View. It monitors the CAN bus traffic, and opens and closes the contactor as needed. This is a pretty brute force approach. The oem battery is physically dragged around to each SOC rating, which takes time. The CAN-View can also only target two fixed (though programmable) SOC values based on two driving states (hwy, city). The next gen controller, which is currently in beta will be open source, and much more sophisticated. It uses an SOC "spoofing" approach, whereby the real SOC CAN data is read from the battery, and then the ideal SOC for the current conditions is transmitted to the HV controller. In this way the battery is simply maintained at a good level, and the HV controller is always told what it wants to hear almost immediately. This allows for much faster response times, and a much more sophisticated algorithm.

The batteries currently in use are like these:
http://www.bb-battery.com/productpages/EVP/Evp20-12.pdf
They are AGM type SLAs capable of >100A sustained output, and are designed for use in UPSes and wheelchairs/scooters. At this capacity and weight you end up with a ~300 lb pack for ~10-12 miles EV range which seems about optimal. In practice the observed discharge current is about 40A-60A when fully charged and discharging into a low SOC pack or under heavy load conditions. There are other batteries available, as this is a standard form factor, these ones just seem to be above average. I have done a survey of other options on the "medium" tab here:
Google Docs - phev batteries

The biggest disadvantage to the cal-cars conversion at this point is battery cycle life. Part of this is inherent to lead acid, and part of it is exacerbated by the lack of battery management. Currently their is no balancing done on charge or discharge, and the batteries are run to very low SOC. When they are depleted they are currently left in parallel with the oem pack in whats called charge sustain mode. This has the benefit of providing extra regen charge storage, and lowers the effective resistance of the battery pack improving performance and efficiency somewhat even when the pack is dead. I believe both of these things take a toll on the batteries, and contribute to the observed lifespan of ~1 year. All tought the upfront cost of the cal-cars conversion is low (~$3-4k), this recurring battery cost (~$1k/year) keeps the conversion from really being financially viable. A BMS would probably help, but would add cost.

There is work going on to expand the basic cal-cars method using Nilar nimH, or Li-ion. Both of these require custom BMSes, but the NimH is simpler and farther along. NimH adds to the upfront cost, (maybe $6-7k total) but should bring the EV range up to around 15 miles and improve cycle life greatly keeping recurring costs low. Li-ion is farther out, but should improve both again, but probably at higher cost.
http://www.nilar.com/be/__media/page...024V%209Ah.pdf

You can find a lot more on all things PHEV related here:
Main Page - EAA-PHEV
technical details on the Cal-car Prius+ here:
PriusPlus - EAA-PHEV
and an excellent mailing list where many of the open source contributors hang out here:
Maillist - EAA-PHEV

Lastly, these things have to be treated with a lot of respect. You have ~260V DC at hundreds of Amps available in a short circuit situation. Thats more power than most people have coming into the main panel on their house. A short under these conditions will create a self sustaining plasma fire that will make an arc welder look like a 4th of July sparkler. The recent Hybrids plus fire is a good reminder of what even a simple mistake like a loose HV connection can lead to.




Rob

[Flying White Dutchman]

i am not and never was going to put it in parallel without a contactor
if you just parallel it then it wil cook the prius battery

[miscrms]

Originally Posted by Flying White Dutchman

i am not and never was going to put it in parallel without a contactor
if you just parallel it then it wil cook the prius battery

I think you'd seen this all before Dutch, some of the others on the thread seemed to be wondering about the cal-cars conversion.

The spreadsheet I linked above has a pretty exhaustive list of batteries if you want to check it out. The medium tab are the 17-22 Ah units usually used by cal-cars. The small tab are more in the 8-15 Ah range, but still capable of putting out 100A. This list is not as fleshed out as I'm mostly looking at the medium sized ones.

Rob