YAPiP - recreating pEEf's approach

Regarding the PiP not recharging the Li battery on regen, I think this was only in the early prototypes. I don't think this was a feature, just a side affect of the prototype architecture (basically a "hack" on standard prius). Based on the link below, both the 2010 pre-production and 2012 production PiP have only a Li battery and certainly regen to that battery on breaking.

https://techinfo.toyota.com/techInfoPortal/staticcontent/en/techinfo/html/prelogin/docs/priusphvdisman.pdf

I think jdh is right on when he says that if anything regen is much less stressful on the Li pack, due to the much lower relative rate of charge. Those panasonic NimH cells were pretty special beasties to repeatedly handle those big surge currents in such a little battery and still last so long. IMHO the A123 pouch cells seem similarly special in their ability to handle very large discharge and recharge currents relative to their capacity. Of course the grey market A123s have their own issues. It will be interesting to see how "lesser" batteries of a larger capacity hold up over the long run.

Rob

 

Hi miscrms! I think that the cycle life versus dod of NiMH is a function of it's chemistry. Most other chemistries (lead, li-ion) seem to experience a linear change in lifecycle wrt dod. For instance the Winston LFPs I linked earlier will store roughly the same amount amount of energy independent of dod, but according to ovonics/cobasys/ecd NiMH will store substantially more energy (1000kWh@100% dod versus 20000kWh@20%dod) if the dod is kept down, which is why hybrid vehicles almost exclusively use NiMH packs onboard AFAIK and the PHEV Prius doesn't recharge the Li pack.

Why Can't Toyota's Plug-In Prius Recharge Its Larger Battery?

jdh2550, my WAG is that a Prius normally uses something like .5kWh to 1kWh of battery capacity per 10 miles, so someone who goes 7500 miles all EV in a custom gen2 phev and also has 7500 miles of normal operation would only incur ~$25 to $50 per year of additional costs by using the more expensive (per kWh stored) Li pack in the manner that the NiMH pack is currently used in (which isn't just due to regen, but also due to the manner in which the Prius charges the pack so the engine can always run at an efficient load). I suppose the best bet would be a contactor so that the driver could selectively enable/disable charging (through regen or otherwise) of the larger Li pack. That said, if an owner could save that much by ditching the NiMH pack in favor of an extra 4kWh or so of capacity then it's a moot point, but probably applicable for some.

 

I think it's my turn to confess being too vague. When I'm talking about energy use I'm referring to the energy the car generates and stores in the pack during normal operation, not how much it would use in EV mode. The basic idea is that once the large Lithium pack has discharged in a longer trip, isolating it and using the stock pack may be cheaper because the stock pack can store more energy for less given the way it's managed. Regen doesn't have to be disabled, but it may cost less to isolate the stock pack if it's still there after the bigger Li pack has been discharged. It technically doesn't matter how its done, but since I've taken a couple ee classes I think of a contactor to separate the packs.

 

Prius ICE runs at startup(charge nimh hv pack./heat cataylic converter) neither are helpful to EV fans..
ICE will also run above/below 35mph again not a status EV people are desiring. The nimh pack is a step up from lead acid for charge/discharge/and regen however, LifePo4 and is the new storage tech is now the current cutting edge. Even so their are pros/cons cost/benefits and other considerations to this quest for more EV function !!!

 

All the different Li batteries are newer, but only better in specific applications. For large discharges, ala EVs (I'm guessing ~50%-100% of battery capacity), a Li battery will last longer and store more energy than a NiMH pack, but for shallow discharge cycles of ~10%-30%, a NiMH battery will still store more energy than a Li battery.

It's up to the owner whether or not keeping or ditching the stock pack is worthwhile in a conversion. If someone can consistently do more EV miles with more Li cells, then the energy savings of PHEV versus gasoline might be greater than the energy costs of using the Li pack versus the NiMH pack for normal (non-EV) use, so it really depends on the specifics.

 

Thanks rofl. I think part of the confusion is indeed that the article you linked is related to the 2009-2010 early prototype PiP. This was basically just a stock 2010 Prius with two extra lithium packs added on for testing, much like many of the hacker community conversions. At that time, they just disconnected the lithium packs when discharged and reverted to stock Prius mode to avoid having to protect/deal with the Li pack. I believe this was simply out of convenience in the testbed. In the pre-production 2010 and production 2012 PiP Toyota did exactly what peef and jdh are working on, and replaced the NimH pack completely with a large Li pack. Even with two packs, most hackers and third party converters have discovered its far batter to leave the Li pack connected in parallel as the greatly lowered internal resistance improves the efficiency of capturing regen energy into the batteries. This was even true on the early PbA cal-cars kits, even though that was hell on the over-discharged batteries.

NimH may be superior for shallow discharge, but this is of little relevance to PHEVs and EVs where by definition you are nearly always using the pack over a large range of its capacity. A 10% discharge from 100% to 90% and recharge back to 100% is also much different than a mid-range discharge/recharge, from 60% to 50% and back to 60% for example. As mentioned before, particularly with Li batteries my understanding is most of the "bad" stuff that happens that reduces cycle life happens at the extreme ends of the capacity range. From 0-10% and from 90-100% for example. In general a mid capacity charge/discharge is much less of a big deal for the battery. Avoiding those extremes of range on charge/discharge is just a matter of software in the operating controller and charger.

Add to that that the same regen event will be much more stressful on the small NimH pack than on the larger Li pack and I would expect any advantage would be lost. Lets say an unusually large mountain regen event will produce maybe 1kWh of energy at 60-100A. That's enough to run the NimH pack over its entire charge range, and likely fill it resulting in regen energy being thrown away. The same event will only result in a 10-20% recharge of the Li pack, is more likely to be completely captured, will be captured and stored more efficiently due to lower resistance, and as mentioned before will be at a much lower charge rate relative to battery capacity (2-5C vs 10-16C) which is much less stressful on the battery. For a smaller typical 50-100Wh the same would hold true, but I doubt either battery would much care or notice.

Its also worth noting that the Ovonics batteries you mention were a much different animal than the small NimH cells used in the stock Prius pack. They are an order of magnitude bigger in capacity per cell, and of a 90s technology recipe. They were a great battery in their day, and would have been fantastic if patent holders GM/Chevron had allowed anyone to use them in EVs. At this point, even when the patent exclusivity expires its unlikely large capacity NimH can compete with the Li cells that now have a 15 year development lead and significant production volume driving down prices.

Sorry, this is getting pretty OT for the thread, but an interesting topic none the less

Rob

 

I don't know if its possible (or legal) but I always thought it would cool in a PHEV conversion to commandeer the speedometer readout to display other data. Its by far the most visible feedback display when driving, and there are often parameters that might be of more interest than speed For example, during charge depletion mode maybe having drive or regen amps displayed, or maybe even better drive/regen current as a percent of CDL/CCL as I think the CANView can display. In charge sustain mode, maybe viewing instantaneous mpg or for acceleration instantaneous mpg/speed * 100, or maybe even ICE RPM / 100? I don't have either a scan gauge or CANView, so maybe there are other parameters that would be more valuable for realtime driving feedback, but you get the basic idea Having a way to cycle through different parameters on the speedo might be cool too. Maybe fuel gauge and odo could even be used for more useful info. Maybe engine rpms on the odo, and amps/CDL on the fuel bar graph?

Rob

 

It may be a bit off topic, but I think for some users who could use the vehicle in all elecric mode but could also completely discharge the Li pack and use the car like a normal hybrid, it could be an issue because AFAIK, the Winston/TS batteries don't exhibit an increase in overall energy storage capacity with relatively shallow cycles like NiMH cells do. I agree that looking at regen is moot, but the biggest concern I have is the normal operation of the hybrid system. Depending, on the voltage ranges of the packs, it would be more costly if the charge on the Li pack fell enough for the hybrid system to start charging it primarily, rather than isolating it and letter the engine/MG2 charge the stock NiMH pack, since the NiMH pack can still store energy for less, although it is a lesser amount of energy.

 

Two thoughts on ROFLs comments:

1) As jdh's project represents what is in essence the first diy/open project to attempt to replace the stock pack I think it has enormous potential value to the collective understanding of the diy conversion community. This has been the preferred approach of most commercial conversions, including Toyota's PiP, so its a great chance for the diy community to explore why that seems to be true. Regardless of whether its more cost effective to run one pack or two, there is a great deal to be learned from jdh's experiment and I'm really grateful he's doing it and sharing it with all of us.

2) I do not think the data really supports the claim you are making. As I've said before the lifecycle data for Li batteries you are quoting relates to shallow discharges/recharges at the very top of the pack's charge capacity. From 100% to 90% and back for example. This is fundamentally different than a shallow discharge/recharge somewhere in the middle of the packs charge capacity. There is no data I'm aware of to show how those cycles affect overall life of the battery. Based on my understanding of the chemistry involved, I believe that these cycles will have little impact on the battery's life, and so they will likely exhibit the same extension of lifetime storage capacity that you mention has been observed for NimH batteries.

In almost every case I can think of those conversions with two packs kept the PHEV battery connected in parallel during charge sustain mode, as it was observed the larger battery was significantly more efficient at capturing and releasing the small amounts of power generated and consumed during charge sustain mode. This was true even of the first cal-cars lead acid conversions. In that case it is likely that using the PbA batteries in charge sustain mode degraded their lifespan, in part because the voltage mismatch of the system caused the PbA pack to be over-discharged (often at or beyond 100% DOD) in charge sustain, and in part because the PbA chemistry really doesn't like to be partially recharged and then discharged again. It doesn't even like being left sitting partially or completely discharged from a lifespan perspective. NimH and Li chemistries are much more alike in that regard. Both prefer to sit in a partially charged state, and the damage that occurs to limit lifespan occurs almost exclusively at the top and bottom of the charge capacity range. That should make both well suited to charge sustain operation.

The only way to really find out is for people to try it. In general the result is much more likely to depend on the specific battery used and the control / bms software limits than whether the stock pack is retained or not, but the only way to find out is for people to experiment with it.

 

True, the advantage of using two packs is that the larger pack can be charged and discharged in a more controlled manner. It is possible to safely use up all the rated capacity without compromising the life span of the battery since the secondary large pack doesn't suffer from surge demand in acceleration and deceleration if a DC/DC converter is put in place. The Enginer 4KWH kit can be used 95.6% as reported by our customers with 5KWH wall consumption wattage or 3.8KWH usable capacity after deducting 15% charger efficiency loss and 10% DC/DC converter efficiency loss.

In comparison, Toyota Prius-In Prius has only usable capacity of 2.3KWH based on 2.7KWH wall consumption wattage on their 4.4KWH pack. PIS has only 2.7KWH usable capacity based on 3.2KWH wall consumption wattage on their 4KWH kit.

 

M. Chenyj

So your system have 24% losses (3,8 kWh and 5 kWh)
Add losses due to charging nimh during trip, because "low" power dc/dc converter.
Pip have 15% losses according your values (2,3 kWh and 2,7 kWh)
Pis have 16% losses (2,7 kWh and 3,2 kWh)
My plug-in that use two packs directly paralleled have 13% losses, including charger efficiency and battery Charge+discharge.
Nimh internal resistance is about 0.35 ohms. My plug-in , like Pip have 0.05 ohms.
So less battery heat, longer life, really better regen, and bigger regen possibilities.

During a trip good drivers have 10 to 15% energy regen. This regen have 7 times less losses.

 

I would say a good report can only be geven after years of 95% dod with your system.i think it will impact lifespan

-Tapatalk

 

Just an observation that we are wandering pretty far OT, and cluttering up jdh's excellent thread documenting his effort

 

A few things to consider. The stock NiMH pack is used from only 40 to 80% capacity. This is because of the primitive BMS and the desire for long battery life. If you hook up a LiIon battery in parallel, in EV mode you will be OK, but when you want to go to extended mode, after depleting the pack, the BMS will want to charge up the battery. You don't want to burn gas doing this, so you only need to add about 1 Kw of charge for the car to work properly. The complexity of doing this properly may be challenging, since you might need to do this at any state of charge in the LiIon pack. For example, if you need more HP then the MG2/MG1 combo can deliver. If you keep the original pack, it alone can be used for this mode. The extra pack will be in parallel with the NiMH pack when starting out in EV mode, or used instead of the NiMH pack. The NiMH pack already has contactors, so using two more is not a big deal and may allow some simplification..