ripple4
Posts: 9
Joined: Tue Sep 18, 2018 1:45 pm
Delivery Date: 18 Sep 2018

Re: Replace individual battery cells to renew/increase capacity

Thu Oct 04, 2018 10:13 am

Looking further into this i found that there is a cell chemistry called Lithium Titanate in the marketplace. these batteries can have even higher current ratings than LiPo, they also have a much wider temperature operating range, up to 50C, and down to -30C. They also have 10x the cycle life of LiPo, with LTO batteries a pack would last 20-30,000 cycles.

https://www.alibaba.com/product-detail/ ... 5fefVVxGYE


https://www.ebay.com/itm/Lithium-Titana ... SwgGBbp984

But they have the lower cell voltage, so how can we work around that? The idea to have 6 cells in a module instead of 4 can help with this, running the numbers a 2012 24kwh pack has 96s Lipo max charge volts are 4.0 and the cells is flat at 3.3v that works out to 384 max volts, 360 nominal (@3.75v), and 316v minimum with a total of 65ah. A module made of 6x 55ah LTO pouches in a 3s 2p arrangement would have a max pack volts of 396v (144s*2.75v max charging), nominal of 345v (2.4v nominal) and minimum of 316v and have a capacity of 110ah (36.4KWH and 54-87 years life!!!)

While the 2012 nissan leaf BMS obviously cannot measure two taps inside of each module it could be worked around by putting a high impedance voltage divider across the center LTO cell in each module, that way it would report to the BMS nearly the same “cell voltage” it would expect from a center tap in a 2s LiPo module. But doing that will prevent any balance current coming into the cell. How big of a problem would that be for LTO chemistry?

Also the turtle mode would be activated with some power still left in the LTO pack, but would the BMS have the ability to learn to allow lower voltages before turtle mode kicked in over time? some of these questions can only be answered with experimentation, since its never been tried. sort of like how the BMS learned to allow extra capacity when G2 cells are put into a G1 pack DIY. What about completely fooling the BMS and having an elaborate voltage supply board that scaled the LTO voltages to expected LiPo voltages for any given state of charge at each of the 97 BMS tap locations to allow the LTO pack to go down to its minimum? if the BMS is fooled anyway, then the pack could be made into any arrangement less than the max inverter volts and work with any chemistry! with 100,000+ leafs on the road and near the end of their packs lifes its worth at least talking about it. would this LTO chemistry finally shoot down the main complaint about EVs, that the battery is short lived, expensive and over its life cycle worse for the environment? with a 54 year life, a car would be multi generation, my grandkids could still be cruising around in my leaf in 2072!
Last edited by ripple4 on Fri Oct 05, 2018 6:13 pm, edited 1 time in total.

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Re: Replace individual battery cells to renew/increase capacity

Fri Oct 05, 2018 9:39 am

ripple4 wrote:Looking further into this i found that there is a cell chemistry called Lithium Titanate in the marketplace. these batteries can have even higher current ratings than LiPo, they also have a much wider temperature operating range, up to 50C, and down to -30C. They also have 10x the cycle life of LiPo, with LTO batteries a pack would last 20-30,000 cycles.


You would want a completely revamped BMS. Not only because of different cell voltages, but also would need to handle cell balancing, charge tapering and more. Reverse engineer or license from Nissan the communication protocol to the rest of the car.

The Honda Fit has a Lithium Titanate battery. People seem to mostly like it, other than at low temperatures. Cold weather range seems rather worse than other BEVs. A more aggressive battery heater and a larger pack size might make it acceptable in cold weather. However, the energy density isn't as good, so a larger pack size would be more mass and volume, making for an engineering challenge.
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ripple4
Posts: 9
Joined: Tue Sep 18, 2018 1:45 pm
Delivery Date: 18 Sep 2018

Re: Replace individual battery cells to renew/increase capacity

Fri Oct 05, 2018 7:20 pm

I made a mistake with my pack voltage calculations. different manufacturers have different cell volt info and I got in the weeds. so I reworked it and the 144s LTO pack is perhaps a very close match to the 96s LiPo pack at important voltage states. The LTO idea I'm talking about is replacing the 48x 7.5v, 65ah 2s2p modules with 48x 7.2v, 3s2p modules of nearly equal physical size. the cells I link to are not the right shape, I was just trying to show the market availability. From a packaging perspective the stock 2012 modules are 303x213x55 which is 3716cc per module and there looks to be space to expand the cell foot print a little, maybe an inch longer. the amp hour capacity of a 6 LTO cells that could fit into the 4100cc maximum envelope for each module would be about 48 amp hour, for a pack power of 33kwh.

the primary idea of this thread was to talk about replacing the cells in the battery with anything else. I still like the new 37ah LiPo plan with liquid cooling. the prototype 2s3p module is going to happen.

You would want a completely revamped BMS. Not only because of different cell voltages, but also would need to handle cell balancing, charge tapering and more. Reverse engineer or license from Nissan the communication protocol to the rest of the car.


As I corrected myself the overall pack volts are perhaps closer than I first though, so would a new BMS really be easier? nearly all market-available BMS' are less than 100a and don't get close to 144s without a scary stack of daughtercards daisy chained together. are we talking about a custom designed BMS? know an EE? also, do other new Nissan cars have the ability to be interfaced with non OEM canbus parts? I was operating on the assumption that Nissan will never help a person or a business interface with older leaf's, because they want to sell new leaf's, or that's the vibe I get reading this forum. while I've never worked with Nissan, my experience in automotive engine control systems is that even the larger tuning houses, AVL, Roush, FEV and the like cannot interface with OEM computer system without inside help. the people who do get stuff to work use open ECUs and toss the OEM computer.

I understand the risk of not using a cell balancing BMS for LiPo cells, as they burst into flames at the slightest provocation. hoverboards right! but even in the stock 2012 2s2p LiPo pack there is not protection for the 2 parallel cells, that's less than ideal for sure and we accept that risk. according to the web sources I've read, LTO cells do not explode and burn when crushed or short circuited and can recover from a 0v discharge. so the risk of a dead cell would be reduced capacity until the faulty cell was replaced and it would show up on LEAFspy as one or two low cells before it got there (the BMS would think its 96s so it would group up to 2 serial cells together in the readout). and the 144s arrangement of 48x 3s2p modules would still allow balance current every three series cells. just brain storming here with the LTO stuff, thanks for the good leads!

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