Toyota Aqua New Hybrid Battery Project (hybrid battery refurbishing)

[nomi01]

Last year I had been involved in some EV Bike projects and got the idea of designing a solution for failed battery pack problem common in Aqua. Initially the idea was to design a pack using Lithium Iron Phosphate (LiFePO4) cells as we were using the same for our EV bike project. NiMH and LiFePO have different cell chemistry and hence different charge/discharge characteristics which means we cannot simply swap the cells. LiFePO4 is newer technology and better than NiMH in many aspects. The Battery Management System (BMS) needs to be modified to accept the different cell profile while retaining all the original functionality of the BMS. An initial proto design was made (for tech geeks ; the differential op-amp input multiplying factor feeding analog-digital converter was modified) but abandoned since the printed circuit board (PCB) was very dense and modification didnt seem reliable enough.
Ultimately we settled to use new NiMh cells to avoid any modification in BMS. The amp hour capacity and charge/discharge rate for the cylindrical cells are same as the original cells. The original prismatic module is 7.2v and there are twenty such modules in series for Aqua but the BMS monitors them in a pair, so the ecu sees "ten" modules of 14.4v. In our case, twelve cells were spot welded to make a module of 14.4v nominal and hence ten modules in total and it doesnt make any difference to BMS.
The thing different here is that the new cells are cylindrical while the original cells are prismatic (rectangular). This required few small modifications/custom designed components but was not a problem but did take some time to perfect the design.

Result
The first proto pack was installed last year and has accumulated around 20k kms and running good. We are monitoring the cell relative volt difference and its within good limits. Average is between 20~26km/l depending on driving conditions. Few more packs were installed in Aquas in my close circle after we were satisfied with the results.
I can be reached on WhatsApp https://wa.me/923345596797

LiFePO4 Cell

NiMH Cells Spot welded

Battery Pack

[capsat]

your write up make sense. please carefully share your contact details and location map.

Also let us know if you use, new cells to rebuild battery packs or you obtain cells from scuttled hybrid vehicles.

What will be approx. life cycle warranty if & when you rebuild a battery pack.

Try not to skin the customer reaching you through Pakwheels. Feed back from PW customers will help you build up future tracks.

@nomi01

[hash4u]

[rajaatta]

Its an good initiative but heat management in longer term needs to be rectified more for abusive driving and maintanence schedule and behaviour in pakistan followed by people here .
Overall good experiment.
Keep it up

[capsat]

I will wait for the gentleman to respond.
I assume the battery bank casing is used for cell replacement / refurbishing while using the same duct cooling fan as stock there is nothing extra added.

[rajaatta]

Actually these hybrid batteries are specifically designed in a way that it is compressed so that if it heats up still it is concealed in a manner that it doesnt blasts up.
These cylindrical cells have special casings as we had witnessed the same cylindrical desighn in old prius models and honda vehicles with short life span as compared to the square desighn

[nomi01]

The cells used are brand new, imported on demand. Yes, abusive use is an issue, under hard accelerations, cuurents can peak as high as 60amps. The cells are designed to handle those currents but such frequent use shortend the pack life, be it toyota original or these cylindrical cell.
Regarding heat management, the toyota design didnt work as they intended it to be. The idea was to compress the pack and have tight spaces inbetween cells but still the pack had non uniform heat distribution and cells followed sagging wire failure mode(more heat and hence failure as u move to middle of pack )

[nomi01]

Yes, you are right, the existing casing with cooling fan is used.
The older cylinderical cells used in early honda hybrids and these cells have different characteristics, these are improved version. I would add here that some toyota hybrid vehicles for specific regions are using cylindrical cells. There is a reason for toyota to adapt them again. cylindrical cell technology varies slightly from manufacturer to manufacturer.

[nomi01]

Nimh battery packs hav lifespan of about 5 ~7 years, depending upon how they are used. To extend the pack life, toyota follows shallow depth of discharge. These new cell packs carry 80,000km warranty from the manufacturer. And given their association with leading oems, we dont have any doubt.
My previous association was such that I had worked on power and battery management systems for satellites, so have industry links (specifically storage batteries) which contributed in progress of this project.

[Xulfiqar]

best would be to devise a system that can cool the battery bank evenly with an ac evaporator and controlling it with a heater matrix so you can maintain the batteries at 23C under all conditions.

[nomi01]

yes,or even a liquid cooling system but cost and other factors.
A cheap solution is to run a duct from rear ac floor vent to the pack.

[Xulfiqar]

That will create moisture problem inside the pack, If it was that simple then toyota would have already extended it.

Toyota are pretty good in refinement of design.

[rajaatta]

23 to 28 c is an ideal temperature for both block type batteries as per my observation i see low battery damages in winters because of low temp.
The best operating temp is below30 in summers i had seen some packs crossing 50C too under extreme load condition.
Evaporator can help but moisture will do its work.
Liquid system is not possible but a wet type liquid cooling sytem can be maded and test trials can be done.

Toyota has shifted to more small and robust desighn of both NIMH and lithium cells which are 200% more efficient in heat handling can be seen in ZVW50 and 51. Damn those packs are more desighned for abusive drive and maintanance conditions and are working perfectly.

[uzeemujeeb]

sir if the battery bank cooling fan duct is pusher type did you tried to install Paltier in the duct. it may help keep temperature balanced. Palteir can be connected through duct fan supply and heat sink can be bolted with vehicle or batter bank frame. just an idea you may bench test it.

[rajaatta]

Heat sink is not an option as the current toyota old nimh batteries are jammed pack and already i had noticed the prius C battery cooling policy is more efficient then jdm Aquas and prius because at 30C it triggers its fan at 60 % pw and in japanes case the fan speed on 30c is only 10%
So that is another factor of damaged NIMH cells from the center of the module pack because center cells are stressed more due to heat produced from all the surrounding cell packs.
So what i recomend my clients and fellows to use Battery doctor app and triger the fan manually at 30 C and yes the cases had reduced very well.

One more hurdle in this is the cells detoriates when not charged equally and soloution to that is balance charging i usally do get the pack out connect all cells parallel and then charge them from outer source and leave them for a day or 2 and hell yes the results are amazing. Because the memory loss cells are rejuvenated through this procedure if the cells are damaged till 27 %

[naeem770]

would have to devise a system to manage the moisture condensation aswell. There are different approaches for battery thermal management and there is no limit for improvement. Its a continuous process. The F1 formula are using hybrid drive trains and utilize very efficient cooling strategies.
If aircondition is used and car is driven sensibly, there are no issues.

[nomi01]

Toyota original design uses thermistor in thermal vent whereas this setup uses thermistors spring pressed against the metallic casing of cylindrical cells, so better reading if temp by ecu and corresponding action by fan. I hav notice that the airout temp is lower in this case compared to prismatic. I also came across a study that showed if fan is triggered early, pack temp is controlled at reasonable levels, but if it is triggered slightly late by few deg, it runs hotter and cant be brought down, so thermistors can be tweaked slightly to trigger fan early.

[uzeemujeeb]

Peltier is something different then a conventional heat sink.

It is capable of temperatures that are about 40 degrees Fahrenheit lower than ambient temperature. This means, for example, that if your cooler is sitting outside on an 80 degree day, the coldest it can get is 40 degrees

this can be applied to whole battery bank in patches ?

[rajaatta]

The actual trigger mapping for jdm hybrid is far more higher then australian prius c and prius 1.8 what i do is i swap the BCM and it works well but founding the australian bms ecus are difficult and yes prius c has less battery issues then jdm here in pakistan

[capsat]

@uzeemujeeb peltier is good for gaming machines and smaller enclosed setups having multi flow cooling systems.

peltier itself has a heat sink which need heat dissipation. It won't work over any type of battery banks.