Is adding a second battery possible?

Yes that's true you shouldn't charge a lifepo4 at freezing temp or below. But you can discharge no issues. Lithium titanite would be suitable and lasts for many more cycles then the already good lifepo4, but the energy density is a fair bit less.

 

Fair call. Well i will make sure to document the process.

 

Thanks Rmay635703, yes all valid points there too. Well i will start a new thread on the build pretty soon and see how it goes i guess. Biggest obstacle is creating the BMS2 board from plans on this very forum.

 

If I plan to change car with lower mileage lets say another gen 2 due to my familiarity with the car and the driving purpose which its a "must to own a hybrid" like I do. Then that will be the time I would upgrade my battery to a lithium and maybe I want to add another small 12 volt battery so I can listen to your radio longer when parked.

 

I want to use the Gen 2 trans-axle and not welding the gearset so MG1 and MG2 can function as normal so reverse still works without the need to reverse the polarity of MG2. The added torque of MG1 and its high speed and gearing added to MG2's torque make it quite nippy with the correct battery to hold the voltage high. By spinning the planetary carrier at as little as 1,000rpm, the Gen 2 trans-axle can handle 110km/h (70mph) without MG1 suffering over speed. When the Gen 2 Prius goes into "scream my tits off" mode under heavy throttle up a hill, the ICE only seems to reach 5,500 RPM max, so a 3 phase motor wired to spin to 6,000rpm could be used to replace the ICE. Just because an electric motor spins at high rpm, it doesn't mean it is using a lot of Wh from the battery, so it could still be all electric and efficient. Maybe even using the throttle position actuator to drive the pot for the controller on the added electric motor so it can be speed and output controlled by the Prius CPU ...... just thinking out loud here, but it might be possible to actually use most of the original sensors and keep the same driver feel as a Gen 2 with the ICE still in there .....

T1 Terry

 

Not that it wouldn't work; it just doesn't seem ideal.

The ICE replacement motor would run at low RPMs most of the time. That might not affect it's efficiency that much, but it would affect how torque and power work. Electric motors have more torque and power at lower RPMs but have less torque and power at high RPMs, which is opposite of an ICE.

It also means you're feeding electricity into an electric motor which runs splits half it's power to a generator and then that gets put back either into the battery or into another motor.

 

Ya. The thing is also the price. I don't want to spend $10,000 on batteries to save $500 on fuel.

I do wonder what options would be best.

I think the Lithium Project one makes the most sense. If I had a way to charge the batteries via an outlet I could have enough juice to make it to my work and enough to make it back (1 miles there and 1 miles back), at least in the winter when riding a bike is like suicide nuts.

 

Its only about $2000-3000 for 7-10kw of quality lifepo4 cells. (headway cells or even CATL as used in base model Tesla model 3 but smaller version)

 

I'll look into it. That's a good price and just enough for what I would need. I'd also like something that could assorb enough juice going down mountain passes that I wouldn't need to engine brake.

 

Yeah that's going to be interesting. As i think the Prius can pump back in 10KW into the batteries pretty easily down a hill. Definitely monitoring of the battery pack will be important mainly heat, and voltage balance of all the cells. Shouldn't be an issue as long as the battery pack isn't too undersized.

 

Interesting thinking there, i also was thinking the same, removing the ICE and having an electric motor take over its duties. But it did start to get too complicated in my mind how this would work - Thinking would need to know the deep secrets within the prius ecu on how it controls the ICE. But your line of thinking it may not be out of the question. For me with fuel prices the way they are and having 2 prius's. The cheapest/ quickest and simplest option seems to be making norms BMS2 board and making a LFP battery pack of sufficient capacity plug and play.

 

I haven't done the math recently but it's not impossible to calculate. The thing would be to apply it to the longest passes around here and go from there. 1

I wonder if it would be worth it making a 70S2P 8Ah (16Ah total) Headway pack. The cells themselves are $1,900 on EVcomponents. Max charging current is 10C (160A) according to some. Discharging is up to 20C. Even without spoofing the BMS from what I understand the NiMH charge and discharge curve translates to about 80% (10% to 90%) of a 5S LiFePO4 block instead of about 50% for the 12S NiMH block. Any ideas for balancing and temp management?

 

First of all sorry for a long post.

That's interesting yeah those headway cells have been popular with the EV DIY crowd. And that's a good price! i have been looking on aliexpress etc, I see they also come in 16ah cell size. 70x 16ah headway cells for a 3.5kw pack @10C 35kw max charge / @20C 70kw max discharge for short bursts sounds like it would be a good fit for the gen2 prius!

That's a good point about the NIMH voltage ranges and how much of a LifePo4 pack that translates into. That's the biggest challenge i think, to try and keep a lifepo4 pack in balance and in a SoC range in the middle to extend cycle life - they have such a flat voltage curve through most of the range makes it hard to know state of charge without a amp counter. I do wonder if the Prius ECU will make use of that extra capacity, it should if it thinks the battery is charged as it tries to bring the charge down by using the MG2 more. I think definitely the MPG would have to improve regardless.

Just some rough back of envelope numbers here
70s2p (or 2p70s i was thinking) 16ah headway cells for a 7kw pack @10C charge = 70kw / @20C discharge =140kw max discharge - Way overkill but wouldn't break a sweat with the Prius, an easy life!

From what i understand the Prius can manage approx. 13-14kW per 100km/60miles range at highway speeds (and much better at slower speeds). Which is pretty darn incredible as that's what a Tesla model3 averages roughly.

7kw battery - approx. 50km/30 miles pure EV range
3.5kw battery - approx. 25km/15miles pure EV range

As for battery monitoring, I can only speak as an off grid person using lifepo4/LFP for 9 years now using various battery management systems, cell balancing systems, diy Arduino setups etc. All the below work really well for 16S 48V lifepo4 solar setups and would also work for EV in my view:

For balancing inexpensive Heltec 5amp balance boards (but need to see if they are able to be used with cells in series up to 70S using multiple boards - eg. a board for every 8 cells)

Otherwise maybe top balance when batteries get up to 3.5v/cell using evpower cell balance modules(based in Australia), maybe only need to do a full charge once a week to let balancing keep things inline. These modules also have a way to control a relay if a cell voltages get in the danger zone (high cell and low cell voltage cut off). Or could just be to let the driver know with an alarm that something is getting out of hand - pull and turn car off in a rare event.

Temperature are few options i know of, easy enough to have a temperature controlled fan with a set temperature to turn on (mini-kits sell a simple device that would work). Id also have to see what is out there for monitoring a large number of cells, its probably way overkill and not necessary but to fit a temp probe to every bus bar, if a battery is getting too hot or there is a bad connection it should show up. Perhaps a breakout board and Arduino running a simple program to poll every second or so, a simple display or again could just be a simple alarm or idiot light when temps are in the danger zone. That's if no product already exists, I'm sure the EV diy crowd have already solved a lot of these issues.

 

Apologises in advance for the very long post, thinking while posting helps put things in perspective.
70S2P for a total of 140 cells. The cell capacity is measured at 0.5CA, or 8 amps, so as long as you only plan to draw a 16 amp load from the battery you can get the 7kwh capacity. reference used https://www.lifepo4-battery.com/Products/Cylindrical-battery-cell/Headway-40160-16Ah.html
The load current for a Gen 2 Prius is roughly 150 amps (reference Toyota Prius Gen 2 Battery - Battery Design ) so the actual load would be close enough to 10CA or 20 times the load the capacity was measured at .... so forget about using the 7kwh as a capacity reference.

The Yinlong 55Ah LTO cell is sold as a 66260, but in reality, after the studs are welded on each end they measure 66mm diameter x 320mm long. The studs themselves are roughly 25mm long on each end and could be shortened a bit depending on the nut and busbar used.
84 of these cells would give a capacity of 11kwh nom. the capacity is tested at 2CA or 110 amps for these cells, they have a max charge rate of 4CA (220 amps) and max discharge rate of 10CA (550 amps) so both peak regen and peak discharge is well inside the load window for these cells .... but they are seriously big.
Laying on their side, they equal 3 modules wide, the length of a module (plus the studs) and stacked 2 high is roughly the same height as the NiMh traction with its 2 rods on top and 2 rods on the bottom for compressing the modules.
Roughly 20 cells will fit where the original traction pack was mounted.
The box that held the 74 x 40Ah LFP cells that were part of the extended range kit, with a bit of modification will hold 63 cells ..... still gotta find where to put the remaining cell
Another alternative:
With a bit of modification I could fit 35 cells where the original traction pack was and 16 cells in the spare wheel well and 1 layer of 15 across the top of that backed up against the traction pack and another layer 18 wide between that and the rear of the inside panel and that would give me the required 84 cells ...... some oxy work and panel beating coming up I think :lol:
The bonus is I could then modify the plastic insert that was originally in the space between the traction battery and rear hatch so I could put the jack and spare tyre in there ..... Sounds like a plan .....

T1 Terry

 

I don't know why you keep talking like LiFePO4 batteries are the worst, only good for 0.5C or 2 hour charge rate, when there are people using them without a problem getting triple the range of NiMH batteries for half the weight. Of course longevity is a metric that hasn't been thoughrouly prooven, but I haven't heard of any LiFePO4 failures yet that died before NiMH would have with the same mileage.

Yes, LTOs are the superior battery in everything except bulky size and weight. But if bulky size is what I'm going for, I don't see what difference there is between going with LTOs or additional OEM NiMH packs as the best NiMH batteries actually have more energy per kg than the best LTOs.

Not that I wouldn't consider LTOs, but I like to go off of what other people have had success with, and I don't know of a single person who has had success with LTOs yet. LiFePO4 has been done and there doesn't seem to be any reason it won't work except for balancing and freezing weather.

 

LIFEPO4 on the market today do suck because the 3rd and 4th party makers of them don’t have technical expertise of auto LION makers. Their life and balancing issues are currently terrible, the set I had even developed self discharge.

Let’s face it Lion is ancient and mature technology with the bugs worked out, once first tier makers dabble in Lifepo4 beyond hobby quality cells they might then become a real competitor.

But beyond Tesla doing a name drop nobody is biting

 

Ummm...... try asking anyone who has used them in a DIY EV, they are better than lead acid, but that's not a very high bench mark to beat, the cycle life and voltage sag after suffering high rate discharging and recharging creates a high rate increase in the internal resistance causing the voltage sag and poor recharge characteristics. I do have over 11 yrs experience using this chemistry, or actually the improved one created by Winston Chug at Winston batteries that have yttrium mixed into the compound and a better electrolyte chemical mix. They do perform better than LFP, but the LYP chemistry still falls well behind the ability the LTO chemistry offers.

I guess if you use the 520wh the Prius extracts from the NiMh modules, the LFP is way in front, but without a real contender challenging them it's a bit like the comparison of LFP to lead acid ...... but the NiMh does have a better charge and discharge rate than both of them ....

T1 Terry

 

I've assembled the cradles for the cylindrical cells and very little modification will be required to fit all 84 cells in very close to the configuration I posted before.
The weight of the 84 cells with their cradles is roughly 168kg.
The original traction battery comes in at 37.2kg according to the battery design link. the 74 GBS 40Ah cells weigh in at around 120kg, , then the weight of the aluminium battery box at 10kg = roughly 167kg, so close enough to the same weight as before.
The stored nom. energy in the 74 GBS cells comes out to roughly 9.4kwh and the NiMh traction battery is 1.3kwh according to battery design people = 10.7kwh roughly, but not all of that would be available.
The 84 Yinlong 55Ah cells come in at 11kwh nom. but a lot more of that would be available along with the bonus of utilising the fast charging networks with the cells being able to cope with up to a 52kW charge rate ..... not that I'd subject them to that, even at 44kW the recharge time would be 15 mins for full discharged to fully charged, so 22kW for 30 mins would be quite acceptable and gentle on the cells.

Time to build this battery, so I'll start a new thread with the progress and results, drawbacks and all.

T1 Terry

 

But doesn't everything, even Li-ION fall well below LTO? Even one of the selling points for Tesla's LiFePO4 cars is that LiFePO4 charges faster than Li-ION!

Have you used Headway or Nexcell cells? Even with lead acid, there's a huge difference between a deep cycle Trojan battery and a red top Optima battery, even though they're the same chemistry.

Telsa uses LiFePO4. Project Lithium uses LiFePO4. And Project Lithium is a DYI of sorts project with many people doing the same mod to their Toyota hybrids. Shouldn't they be suffering left and right from the problems you've described?

Not that I'm defending anyone here, but if LiFePO4 cells suck and it's LTO or bust, then for most of us it's a bust as we don't have an economical means of obtaining LTOs, and even if we did, they still have limits too.

___________________________
Do any of your LTO box designs fit into the original space the OEM NiMH box does? Personally I don't want a battery that takes up a bunch of cargo space. The LiFePO4 kit from Project Lithium, using Nexcell cells, fits into the original box and gives about three times the usuable energy without any BMS spoofing.

Personally I also don't want to add a fast charger to my car either. For one, there aren't any for hundreds of miles from here and two, I need somthing somewhat economical. If a Prius is going to cost the same as a Ferrari but still drive like a Prius, what's the point?

 

:lol: The point for me is fuel costs. I couldn't afford the fuel for a Ferrari, nor the insurance and it couldn't move the amount of gear around that the Prius does, so there is no comparison there.
Personally I don't want a battery that takes up a bunch of cargo space.

If you use the plastic tray under the floor as part of the cargo space, then a long range battery isn't for you. We needed a battery that could handle the high regen charge rate and have the capacity to store all of it on the long down hill runs travelling from Mannum to Adelaide in South Australia. The longest continuous section is just over 10km and deemed steep enough to add two truck arrester beds and a low gear restriction on all heavy vehicles and limit of 40km/h.
With the NiMh traction battery, the regen cuts out about halfway down and so far has cost me a 3 mth loss of licence and around $600 in fines ... naturally the point the regen lets go and a speed camera are very close together The speed limit for cars drops from 110km/h (70mph) to 100km/h (60mph) a few kms before the speed camera and there is tunnel through the mountain along this stretch, not much further down the speed drops to 90km/h, then 80km/h then 60km/h and then a set of stop lights at the bottom of the hill ...... if the weather is bad like fog or rain or icy conditions, or a vehicle breakdown, the speed drops a lot earlier. depends where the problem section is .... the whole 40 km run on the freeway has quite a few down hill runs, with speed cameras at the bottom, followed by steep uphill runs.
A traction battery with 520wh of usable capacity isn't much value on this section of road, but it's a lot better than none at all, so the test is to see how the LTO cells handle the job.

As a side argument, the cost of the LTO cells isn't near as bad as you seem to think, using this as an example China Rechargeable LiFePO4 Battery Cells Headway 40152s 15ah 3.2V for Solar Energy Storage - China 40152 Headway, 40152 Headway Cell
70 cells are required to to make up the required voltage and 4 in series is required to match the usable LTO capacity, 280 cells @ USD $!8.21 each = USD $5098.80 plus the plastic frames to build them into a battery pack.
As a comparison Osn Power Original Yinlong Lithium Titanate Battery Lto Cell 55ah Lto Battery Solar System Lto Battery 2.3v 55ah - Buy Yinglong,Lithium Titanate Battery 55ah,Lto Cell Product on Alibaba.com LTO cells,
84 x 55Ah cells @ USD $47 each = USD $3948 plus the plastic frames to make them into a battery pack ....... now compare the cycle life, 1500 for the headway cells and 30,000 for the Yinlong LTO cells .... you tell me which is a better bargain?

Just as an aside,
But doesn't everything, even Li-ION fall well below LTO? Even one of the selling points for Tesla's LiFePO4 cars is that LiFePO4 charges faster than Li-ION!
Li-ion is the battery cell chemical family, just like lead acid covers the family of flooded cell, AGM, deep cycle, GEL and lead crystal with all the different combinations of cell construction thrown in.

T1 Terry