The Prius PHV (prototype at least) has 3 identical battery packs, each rated to 2.6kWh, to give 7.8kWh total capacity. The two EV packs (5.2kWh total) are rated just enough to qualify for minimum requirement for tax credit in the US. The main pack (HV pack) does not count as there is a risk it will not qualify for tax credit purposes due to its different function. Each pack consists of 96 individual 3.6V cells for a total of 345.6V, so each cell is 7.52Ah (2600 divided by 345.6) as compared to about 6.5Ah cells in the Prius HEV. Each pack weighs 50kg so its energy density is 52Wh/kg. When fully depleted, it takes 3.56kWh to recharge the two EV packs from the wall socket, allowing for about 10% charging losses, the two packs receive 3.2kWh so the usable charge is about 62% of rated charge (3.2 divided by 5.2). The discharge rate of the EV packs is kept low to moderate, allowing for low to moderate acceleration, any further demand by the driver will result in kicking-in the ICE. This is done for the sake of packs’ life. The HV pack works differently, its usable charge (HV mode) is kept low (40%, same as in the HEV) however high charge/discharge rates are allowed. In EV mode, the HV pack is controlled to stay at 50% charge as much as possible, when regen opportunity comes (in EV mode) it will capture it, at the end of the opportunity it will give power to the wheels until discharged back to 50% then, the EV pack on line will take over. Its capacity is twice the capacity of the HEV battery. At the end of the EV session, you are left (in most cases) with 50% SOC HV pack ready for the HV session. In summary: EV packs: deeper discharge cycles, low to moderate discharge rates, do not recharged by regen. HV pack: shallower discharge cycles, high recharge rates for short periods (regen), does not recharged from the socket. I have no link to back-up the above statements, it’s my perception of the system. Do you have links to contradict any of them?
nice write up, looks reasonable. will it qualify for the $7500. tax credit? that would really make it poplar!
I too agree. The only thing I can't figure out is why they isolate the 2 EV packs during operation? This doubles the battery I2R losses over what they would be if both packs were online at the same time, yet the descriptions we read indicate that they are used sequentially. Makes no sense! Maybe we will get a surprise in 2012 and they will unveil how the system really works like GM did on the Volt with the clutch fiasco. I replaced the OEM NiMH pack with a 6.5kWh A123 Li-Ion pack in my PHEV, and I can eeke out about 40 miles city or 20 highway and still leave 15% SOC for Hybrid use. If I were to split my pack into 2 equal packs and switch them out, I would probably only get 35 miles city and 10 highway! The rest of the energy would be lost as heat in the pack.
Would like to add an additional point: EV packs' charging rate of about 1.6C i.e. low charging rate. Based on 3.56kWh drawn from 220V wall socket in 90 minutes assuming power factor of about 0.9
i also dont understand wy not use 3 packs! less resitance and with regen more energy captured also when in EV and acelerating the presure is over 3 packs instead of now 2? or i m i mistaken?
Might be because they can moderate the heat generation from a single small pack while discharging over a shorter period of time better then both over a longer period of time. I would have liked a super EV button to have some control over when I'd like to discharge the batteries. If I know I'm going to take a highway trip of like 40 or fifty miles, to me it would seem better to use them in area where they would make the most sense like city driving where regen would keep them from discharging as rapidly. Or, for instance, turning them off when in a parking lot so there's no need for a "noise maker" to be used.
But there will be more overall heat created this way, so that doesn't make sense. The big doozy is the hit to the range! Why put in 5.2kWh of EV pack when because of isolation, you actually only get 4.8kWh when you try to use it?
I'd have to disagree. I think a single EV pack will definitely put out less heat than both EV packs. Plus they are only going to supply electricity for ~6.5 miles before switching out. My question would be, if you made it home before both EV packs discharged, would they recharge the EV pack that cooled down first and not the partially discharged EV pack? Or would they recharge the partially discharged EV pack first? And then again they haven't really made up their mind about the final configuration have they? Maybe they'll bring both packs into play on the final version.
You disagree with Peef on an engineering issue? I am sorry, but he is correct (as usual). Power is given by I squared times R. Each pack has approximately the same R, but when 2 packs are in parallel each gets one half the current. This reduces overall energy losses (by a factor of two) and the heat associated with it. JeffD
Electrically speaking, you are both correct indeed. However as i was reading the summary, giora did point out a very limited range of I which would have limited heat build-up in any case. I'm as flummoxed as you two are except in the design it may be the current design has the HV pack sandwiched by the 2 EV packs. I'm not entirely sure of the thermal characteristics but if the HV pack is always in use and an EV pack is being discharged it might be too much heat in certain circumstances. In any case maybe they'll redesign the entire battery to allow both EV packs to be in use for the final design.
Actually, the following calculation shows the charging rate of the EV packs is less then 1C: About 3.2kWh of net charge in 90 minutes that's 2.13kW charge rate, on 345.6VDC it is 6.17A, divided by 7.52 (Ah) gives 0.82C.
The charging rate is nothing compared to the discharge rate. That's where the real stress is! Even my relatively low powered PHEV conversion (~20kW) can still heat up my stiff A123 pack if I drive like a madman. Has anyone seen any spec for what the car can pull in EV mode? I figure it's probably at least 30kW. That would make a nice EV mode that can run w/o needing the ICE in most situations.
I also agree with you guys it is true that "Power is given by I squared times R. Each pack has approximately the same R, but when 2 packs are in parallel each gets one half the current"
I will be surprised if they are stressing the EV packs for 30kW or even 20kW power. Good indication to the max. power allowed may be the power needed to keep steady speed of 96mph (EDIT: 96km/h - 60mph) on a flat surface (say about 13 to 15kW?) any extra demand will have to come from the HV pack or the ICE.
