Prius Gen 3 HV battery replacement - NiMH or ?

So, for a while I’ve been lurking around on here with a few posts here and there. I put on the self-folding side mirrors with puddle lights thanks to lost of help from Sfv41901 and his support and write-ups.

Now, a harder task. First some observations and background so others can understand my motivation.

The Prius uses a NiMH set of battery cells. NiMH is cheap and not bad as a battery. We’ve all had these in older cameras and cam corders and lots of other devices. Laptops on the other hand, needed a bit more oomph so turned to the higher cost Li ion cells. Why? shear energy storage and since they demanded higher prices anyway, they got away with the higher priced battery. As did mobile phones and now other devices (think Tesla, Leaf, Bolt, etc.). Yet. Toyota has sagged far behind still using the NiMH batteries (but there is an option now in 2017 to get a Li battery but it’s not the staple stocked battery).

Some characteristics about NiMH which may help answer some questions out there on “should I replace my HV battery or rebuild with a few replacement cells or ?”

All batteries gradually self-discharge even if not connected and delivering current. That’s why Toyota tells you to do a few things if you’re going to leave your Prius/Camry for a long period of time (e.g. months).

References:
https://en.wikipedia.org/wiki/Nickel–metal_hydride_battery
NiMH cells historically had a somewhat higher self-discharge rate than NiCd and Li cells. The self-discharge rate varies greatly with temperature, where lower storage temperature leads to slower discharge and longer battery life. The self-discharge is 5–20% on the first day and stabilizes around 0.5–4% per day at room temperature (around 60 °F). But at 45 °C it is approximately three times as high.

So, your Prius HV battery after say a few years is loosing without you doing anything between 0.5-4% every day. Toyota does a great deal to mitigate the battery life (and their exposure to having to pay for new HV batteries under warranty) by keeping the battery in it’s “sweet spot” which is about 40-80% charge and NEVER letting it get fully discharged or over charged. Here’s some observations with a UltraGauge monitoring the Prius. Not there’s some difference in ECU behavior if the engine temperature is above about 140° F and below that value (having exactly narrowed it down but it seems to be around that number).

If engine temp > 140°F AND HV Battery SOC > 45% charge, you can drive along up to 25 mph (in EV only mode) before it will force start the engine.

If engine temp < 140°F AND Battery SOC > 45% charge, you can drive along up to 11 mph (in EV only mode) before it will force start the engine.

If the Battery SOC drops below 40%, it appears it force starts the engine no matter what is happening except possibly if sitting still (zero velocity) with foot on the brake. This behavior varies a bit depending on engine temp (obviously above 140°F) and other HV battery drains (AC, etc.). (related Prius chat thread:
Low Battery engine behaviour | PriusChat)

Your battery icon, from what I can gather:
Full equals 80% SOC roughly. It varies a little bit based on HV voltage and amps going into the HV battery from regeneration or engine charging.

half way full equals 50% SOC roughly plus or minus say 5%

two bars to empty equals 40% SOC roughly plus or minus say 5 %

So, how would this all be affected by a switch to a Lithium ion HV battery?

a Li+ battery will provide the same performance EXCEPT after a few months, years, that performance will remain where as the NiMH HV battery slowly looses it’s capacity (0.4-4% per day) which only marginally affects the Prius because of the way the Battery ECU uses and limits the HV battery cycles and depth of charge/discharge. So the Battery ECU in some ways hinders the performance but it protects Toyota from early < 100,000 mile battery failures. Had Toyota enhanced the ECU logic, they could easily get closer to 75 mpg but at the cost of lower mileage on the HV battery. In some respects, the Feds caused this by their requirement that automobile EV batteries last a minimum of 100K miles (California upped that to 150K miles as I recall). It’s about money and market (lower sales price) in the end. They’ve sat on their laurels sucking $$$ out of the old design as a safe bet for income. They did look at Li+ batteries for the 2010 model but opted for the tried and cheap NiMH batteries (30 Days of the 2010 Toyota Prius: Day 12, Battery Pack).

Li+ Batteries, why not?
First, here’s some characteristics about Li batteries:
References:
Lithium-ion battery - Wikipedia
Types of Lithium-ion Batteries – Battery University

Li+ rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5-2% per month. The rate increases with temperature and state of charge which is why most new EV’s limit repeated charging to around 80% unless a significant need (e.g. hurricane escape or something).

So, Li+ batteries loose approximately 1/30th of that of NiMH batteries - approximately. What does that mean to the Prius driver in terms of MPG gain/loss.

It’s very complicated to actually determine the change in mpg if switching from NiMH batteries to Li+. However, if there is significant local driving with moderate speeds, the Li+ configuration could easily result in around 5-10 mpg increase. It would also depend on if it was charged the evening with a home charger. The reason for this is that the Prius battery and main ECU, attempts at ALL costs (this means your $$$ (or your country money) being spent on fuel to charge the battery when you get in the car and it’s say below 45% SOC no matter what you’re doing in your drive. This only gets worse if the engine temp is below 140° F.

Here’s a graphic comparing various batteries from batteryuniversity.com. If you’re interested, I advise you start there and read. They have some excellent educational information.

This figure compares the specific energy of lead-, nickel- and lithium-based systems. While Li-aluminum (NCA) is the clear winner by storing more capacity than other systems, this only applies to specific energy. In terms of specific power and thermal stability, Li-manganese (LMO) and Li-phosphate (LFP) are superior. Li-titanate (LTO) may have low capacity but this chemistry outlives most other batteries in terms of life span and also has the best cold temperature performance. Moving towards the electric powertrain, safety and cycle life will gain dominance over capacity. (LCO stands for Li-cobalt, the original Li-ion.)

So, what I’m working on is a mix between the current NiMH HV system and a drop in replacement with Li+ cells/stacks and possibly or most likely the option to plug it in for the larger Kw configurations.

Toyota currently offers the lowest number of Li+ cells in their 2017 option which is 56 cells connected in 128v strings connected serially to make 256v max. This equates to about the same as the NiMH pack but as noted above has better resiliency than the NiMH pack which will save you a small amount in the long run in mileage.

So, please stay tuned. I’m working to build something that should sell for about half the price of a new HV battery with the same or better performance. And, a larger HV battery in terms of Kw in the same footprint to give even longer EV only driving times without the engine cycling on to charge the battery. Of course, this would have to be offset by plugging it in at night or else the ECU will start charging on the next cold start and actually lower your mpg!

And for the nay sayers, yes built in BMS to keep the cells from getting too hot (similar to the Tesla amperage limiters and BMS controllers).

Stay tuned for updates as things get moving. So far, I’m building out a replacement HV battery assembly and testing it in my 2013 Prius model 5 which has almost 100K miles on it (soon out of warranty).

 

yep, seen all or most of the tries. I’m hoping to bridge the gap a bit. Most of the past stuff has focused on trying to make the Prius a PHEV which is far more difficult than just switching battery types.

If you look carefully, the Li+ and NiMH have almost identical charging algorithms up to 80% charge which is nice because that’s where the Prius ECU’s try to stop charging.

More to come….

 

so, to address a few questions/points:
@Gasper: yes very aware of the current requirements. Using my scan gauge it typically shows in the 50 amp range but can peak to around 100 amps for short times like brisk braking going down a steep hill. Once the SOC reaches around 80% (starts to soften charge in reality at about 78%), the amperage comes way down to almost zero.

@s3nfo: yep, cognizant of the difference in NiMH and LiPO battery charging. Actually, up to about 80% of charge, the charge characteristics of NiMH and LiPO are identical. Once LiPO reach 80% charge, things change to a constant voltage (3.6 - 4.2v typically) with low current (0.1C). So, if one designs the replacement LiPO Prius battery in the 80% range, the charging is identical to NiMH. The extra capacity won’t hurt because of the significantly reduced physical battery size. See attached Li charge graph.

@bisco: had a delay waiting on some parts. they’ve come in and have most of the high-end KWH pack built up but not quite complete. I’m taking some time as my current prius is still under warranty so can’t really test the new pack for a few more miles (at 92,000 miles now). I may pick up a used cheap prius to test on but not sure I want two of them. My preference is to buy a new EV (bolt, tesla, leaf, etc) to use for my day-in day-out chores and work on the prius battery replacement on the side so as not to take my only means of transportation out of the picture while I’m installing/configuring. I plan to build two basic HV battery replacements:
1) increased KWH pack with possible (not sure yet) external charging configuration because if this pack isn’t recharged from an outside source, the engine will run (and burn gas) to charge it up once it goes below about 35% SOC which is why so many PHEV conversions get worse mileage with larger battery packs. This pack will work great for trips and regenerative charging if it is moderately recharged once it goes below about 35% SOC from an outside source.
2) a small almost same size KWH battery pack as the original NiMH pack without the need for external charging.

 

@2k1Toaster: yep, it's true the Prius doesn't really care if the HV battery is 0.5 KWH or 10000 KWH. And yes you are correct that the HV Battery ECU sends out only basic information like SOC and Voltage and current in/out. However, my earlier point is that if one puts in a huge HV Battery, while that HV Battery is at an acceptable SOC (according to the Prius), yes it will favor using the battery over fuel/engine. However, once the HV battery voltage drops, the same ECU will try its hardest to charge back up the HV Battery using the ICE. Thus, if it is a 100 KWH HV Battery and it's low, the ICE will be running quite a bit to recharge it at about 7-15 amp @ 1250 RPM. That was my point earlier about needing to plug it in for a larger HV battery to reduce the ICE run to recharge phenomenon when the HV battery gets below about 35%.

@s3nfo: My monitoring of the NiMH via real-time scan gauge shows about 50-75 amps at around 240v DC flowing into/out of the HV battery. It can get up to near 100 amps inbound on long declines. Not sure on how that relates to the NiMH 'C' values. For the LiPO cells, I'm designing for about 1C current on the average and well within the rated 4-10C range for the better LiPO cells. If you look at the HV battery structure it is 14 cell groups of about 15v DC (eg two NiMH 7.5v cells to make one group). So, 100 amps roughly into 14 cells gives about a little over 7 amps per cell group (ignoring voltage changes). That's easily within the LiPO range depending on serial/parallel cell configuration.

I'm working on a two prototypes:
1) larger HV battery at about 1140 WH
2) smaller basic HV battery at 190 WH (very similar to the 2017 Li option)

 

well, it would be interesting to take one of the NiMH cells and put 100 amps on it at 6.5 volts and watch it explode