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Discussion Starter #41
I guess I won’t be getting any orders from Arizona. While some Li-ion chemistries do not do well with charging faster than 1C, but some others aren’t as sensitive; such as this A123 26650 formulation. A123 is suggesting a max of 10A at 3.6V for fast-charging this cell (4C), which is 30A for a 3P. I now see where AZ got all burned up about; where I said “This A123 will not likely do 120A on a regular basis for too long…”, but I think it was interpreted as me saying 120A is OK for charging; I was thinking peddle-to-the-metal when I was writing that. Sorry for the sloppy writing.

Since I still don’t know what the max regen current is, I would not have said 120A recharge is OK. Sorry to have not make it clearer. An earlier posting did say he was seeing 50A at 192V. Since recommended max charge rate is 10A at 3.6V, then we should be looking at 54P5P; which is a 2.16kW pack if we wanted a pure A123 pack.

However, I haven’t mentioned my full plan yet; which is to have a few LTO cells on the front end of this pack to absorb the inbound and outbound current spikes. This multi-chemistry pack was first considered when I was trying to improve Tesla’s “supercharging” for their NCA packs. By adding some LTO cells (e.g. 5-10kWh), one can max out the LTO cells in 5-10 min. during supercharging, while NCA can take a more relaxed pace to not reduce its cycle-life. If the guy only had 5-10 min. for a quick boost, then that might get him home or to the next supercharging station.

I’m also designing an LTO/NCA pack for some eFormula 1 racers (college teams). This would allow fast acceleration and lots of regen stored without stressing out the high specific-energy cells (NCA). I can get 18650 NCA cells from one of my China sources at a good price, while the cells are only at around 3.3Ah at 3.6V; but that's still around 246 Wh/kg. So, that was Plan-B for the Insight Gen-1 power pack:

192V/4.2V = 46 cells, 1.5C cont., 3C peak. So, 46S3P would give us around 1.6kWh at 2.16 kg (cells only). While only capable of 14.8A cont., and 29.7A peak on discharge, the LTO cells step in to assist. As you know, LTO can do 10C without batting a wink. So, to do 200A at 10C, one would need 20Ah cells; which I can get readily in prismatic form. Since they’re only at 2.4V, we’ll need about 71 of them (2.7V max). That will really take up lots of space (22x106x116mm ea. cell). Nevertheless, it’s a potential solution if one wanted high current during acceleration and regen.

I’m going to try a pure LTO pack using 20Ah cells, which can charge at 200A and discharge at 300A without sweating. So, 71S1p will weigh 36kg (cells only). I will test these packs on my own car first, and then present the field test data.

Look forward to more critique.
 

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If we are intending for little to no knowledge / experience / thought , type usage approach for the driver .. Than I would also suggest including some type of thermal management.

For cooling in hot weather , for a reasonable cycle and calendar life , in most places .. I think Peter's previous advise on ambient air fan cooling is adequate .. Such an approach will see some faster degradation in hotter summer locations .. however , with well matched cells in the pack , and regular balance & fault monitoring BMS .. I think (with such) it would be fair to expect the degradation rate bell cure to be near the OEMish range .. roughly 8-10 years for majority ~1σ , 6-8 years for a few ~2σ, 4-6 years to a tiny ~3σ.

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A123 style LiFePO4 cells have proven (NASA) , they can tolerate super cold storage conditions.

Operating / usage of the battery is significantly different .. Cold weather (overnight lows in winter), however has a significant effect .. It can significantly reduce the Capacity 'C' , and increase the cell's internal resistance .. both of these effects compound .. a lower 'C' itself reduces the amps for the same effective C-Rate .. and higher resistance lowers the C-Rate .. all combined for significantly reducing both charge and discharge rates that produce effectively the same wear and tear rates on the cell as would at higher (near room temperature).

I'll go into some examples of the magnitude of this effect for my 20Ah A123 pouch cells shortly .. but 1st ..

Without including some compensation for this in the design , it will accelerate the degradation rate of the battery significantly.

This can be compensated for 4 main ways:

#1> Environment / conditions it is used in doesn't see this cold weather often enough , that the overall reduced useful life of the battery (from such) , is considered minor/insignificant .. either warmer winters, or cars stored in garages, pack insulation.

#2> Oversize the pack enough to reach condition #1 above for a wider range of environment / climate conditions / locations.

#3> Provide means of more actively heating the battery before it is used .. if this option is used, I would recommend some type of direct (inside the pack) PTC based low power heating element .. any such device needs to either , not be used unless the pack is 'plugged-in' , or has some type of low voltage/SoC cut off that prevents it from over discharging the pack.

#4> Include some low temperature compensation .. ie reduced IMA power when the pack is very cold .. for both assist (discharge) and regen (charge).

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As was previously mentioned .. there are some variations from cell to cell .. even in the same A123 LiFePO4 chemistry .. this bellow (and attached) is for the A123 20Ah pouch cells I've been using in my car .. it might not be a direct (linear) correlation to the specific ~2.2Ah A123 26650 cells being suggested .. but it's an example / starting for for the concept of designing around cold weather usage.

Where I live (RI - Option#1 above) overnight winter low temperatures outside can often get down to single digit F (-10C range) temperatures ( very rare / short to see -F / -20C range ) .. while driving fan from cabin temp warms up battery in an hour or two , but it can start in the morning within about ~10F of outside ambient low air temp .. I over sized the battery to help compensate as per Option #2 above , and I've previous winters experimented with the reduced power usage rate option #4 above , and this winter I will be experimenting with a PTC high voltage heating pad (with low voltage disconnect) I installed as option #3 above.

The reduced capacity 'C' as it gets cold is seen in attached .. if I started with room temperature ~20Ah , 1C is ~20Amps .. that is down to ~75% of C at -10C temps means that ~20Ah C is now ~15Ah , and likewise ~1C rate is now ~15Amps .. down to ~55% of original C at -20C temps means that ~20Ah is now ~11Ah , and likewise ~1C rate is now also ~11Amps.

The increased cell resistance (mOhms) at different SoC and Temperature is also seen in attached .. This means the C-Rate itself (for whatever the current C is) reduces as well as the cell gets colder .. down at -10C temperatures the cell's internal resistance is about 4x higher than that of room temperature ~25C temps , likewise a roughly ~4C rate at 25C room temps is roughly as hard on the cell as a ~1C rate at -10C temps .. the gap between different SoC is also significantly increased .. by -20C temps the internal cell resistance and the gap between SoC is as much as 7x higher than it was at 25C room temps, likewise a 7C room temp C-Rate is about as hard on the cell as a 1C Rate rate at -20C .. Keeping in mind the C itself is also decreasing.

The combination of smaller C itself , and lower C-Rate produces the combined maximum Amp rates .. again this is for the ~20Ah A123 Pouch cells I use .. the 26650 ~2.2Ah cells at about ~1/10 the capacity might not be a linear correlation.

What was a a max of ~200 Amp 10second pulse charge rate at room temps (any SoC) , is down as low as 42Amps into a 90% SoC -10C temp battery .. and down as low as 16Amps into a 90% SoC -20C temp battery.

What was a max ~600Amp 10second pulse discharge rate at room temp (any SoC) , is down as low as 123Amps from a 10% SoC -10C temp battery .. and down as low as 68Amps from a 10% SoC -20C temp battery.
 

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Since I still don’t know what the max regen current is
Even if you didn't want to do the searching to find it .. I did explain this already in post #37 ;) .. but sure , I'll try again.

OEM pushes about ~10kw Assist (discharge) and Regen (Charge) .. a ~25% safety buffer would mean a battery that can in all such conditions do about ~13kw in both directions.

Some people are running mods that allow them to go up to 15kw .. a ~25% safety buffer on that would mean a battery that can in all such conditions do about ~19kw.

Watt = Volt x Amp

Personally I would include not only a ~25% buffer on what the max of what could be asked of it side .. but I would also include a conservative buffer of about ~25% on the max of what a given battery can do as well.

However, I haven’t mentioned my full plan yet; which is to have a few LTO cells on the front end of this pack to absorb the inbound and outbound current spikes.

the LTO cells step in to assist. As you know, LTO can do 10C without batting a wink.
Mixing chemistry has pros and cons .. it can help as you suggested .. but it does also add complications as well .. more pack break points , fuses, etc .. it also means more complicated top and bottom SoC/SoE detection during charge and discharge use.

The interaction between the two different chemistry packs can also get complicated.

I am doing some LTO battery testing now .. but .. without explicit data showing otherwise, I would still be cautious in what I would expect from them down in the -10C and -20C temps .. most Battery OEMs are if anything overly optimistic most of the time about what they do and don't tell you and publish the battery can do .. If they were really confident with the battery taking 2C or 4C (40A or 80A) charge rates at -10C temps they would show it doing just that .. it would be a selling feature .. if they don't , there is usually a good reason they didn't.

I’m going to try a pure LTO pack using 20Ah cells, which can charge at 200A and discharge at 300A without sweating. So, 71S1p will weigh 36kg (cells only). I will test these packs on my own car first, and then present the field test data.

Look forward to more critique.
Thanks .. I look forward to seeing the data :D
 

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I think I am going to let you settle on your own design/path and see how it turns out.
Good luck..
 

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Discussion Starter #45
Ian, thanks very much for sharing your cold weather operations experiences. I didn’t think these little Insight Gen-1s made it that far north. That reminds me of living in Lake Placid, NY with my 1986 BMW 524td. Average Winter night temperatures are in the -20F. At least once in the Winter, we’d see -50F on the digital thermometer. Even at that temperature, I was able to get my diesel started only after 1-2 cranks.

I did what any simple-minded DIY guy would do to start a diesel engine in cold mornings:
1. Make sure all glow-plugs work
2. Add engine block heater
3. Add magnetic oil pan heater
4. Add oil-dip stick heating rod
5. Add 10% unleaded gasoline to diesel fuel in tank
6. Wrap heating tape around lead-acid battery with fiberglass cloth tape over that
7. Wrap heating tape around the 1-liter fuel bowl with fiberglass tape over that
8. Wrap heating tape around the fuel line from the fuel bowl to all the injectors with fiberglass tape over that
9. Wrap heating tape around the fuel line from the fuel bowl to the fuel tank with fiberglass tape over that
10. Bond heating pad to outside bottom of fuel tank with fiberglass cloth covering the heating pad
So, with this experience, I later designed BCPs (Battery Comfy Packs), which would keep Li-ion batteries in a school bus at around 60F when it got below that temp. When it exceeds 90F, the fans turn on. In hot climates, a liquid cooling pad is placed between each prismatic cell to turn on above 95F. School buses are plugged in at night, so the batteries are always ready to rock in the mornings. If it gets too hot during the day while driving around in hot climate areas, the fans turn on and phase-change pads absorb the excessive heat. Then if the batteries go above 95F, then the liquid cooling system comes on.

However, with LTO, life is a little easier, since it tolerates the cold and the heat a little better than its cousins, but still don't charge below 32F. So, for the Gen-1 power pack, I will design cooling/heating accommodations for those who are in extreme climates. In the Seattle area, you just need a small fan if you park the car in the sun all day during the Summer months. Maybe I’ll build an ice chest in the rear sunk-down space for cold drinks and a coil to cool the pack off ��.
 

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However, with LTO, life is a little easier, since it tolerates the cold and the heat a little better than its cousins, but still don't charge below 32F.
I know people talk about LTO being better charging in the cold .. I just haven't seen actual data actually showing such yet .. That cold usage will be one of the tests I'll eventually do in my own LTO testing I'm doing now .. but I probably won't have that for a little while yet.

My shoot from hip guess is that the LTO should be at least equal in cold weather performance (charge, discharge, capacity, power, etc) as the A123 style LiFePO4 are .. If they do turn out to be at least equal , than they too will not be a black and white line in the sand at 32F .. but instead will continue to offer lower and lower performance in colder and colder conditions .. less capacity, slow charge rates, etc.
 

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Discussion Starter #47
The lowest operating temperature for LTO that I've ever heard of is -30C, but I've never tested it myself. For charging, I've read -10C was OK, but I've never tried that either. I'm sure you already know that an LTO cell still has either LFP or NMC for its cathode and LTO for the anode (in place of graphite). So, that means you don't really want to deviate too far off the temperature scales.

Another note of caution for LTO pouch cell users. LTO expands more than other Li-ion chemistries; therefore, you need to make sure you minimize any pouch skin displacements. Meaning, the more you let the pouch change shape, the more internal movement the electrode material will experience. If this occurs too often, then there is the possibility of micro-cracking in the active materials; which means potential increase in internal impedance. This could introduce hot spots or overall cell temperature increase more than usual.

Excessive heat accelerates electrode degradation and eventually could lead to runaway reactions. Much less likely in LTO than in LCO, however; for readers not familiar with LTO chemistry. LCO is in cell phone batteries. Lot of energy per pound, but also gets hot easier.
 

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Does that mean deleting this thread?
NO. We never delete any threads which are viewed as constructive and we view almost all threads as constructive:D

Sometimes we merge threads if we can do it conveniently. If you start a new thread, we may merge the two?
 

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"Bump" means to move it back up to the top of the list of threads. It can be helpful to go back to original info to better understand 'new' info... Basically, reading the old helps me better understand what you're trying to do, and I figure it might help others, too. So moving it to the top will make it more visible.
 

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Discussion Starter #52
OK, got it. Well, since both LTO and LFP are of the Li-ion family, can we rename the merged threads to another name? If yes, maybe something like LFP vs. LTO (?) or something you feel will be a better topic name for all the combined postings. How about Drop-in replacement kit: LFP vs. LTO ? I'm new to this, so feel free to rename the way you prefer. I'm OK either way.
 

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Super moderators do moving, merging, etc. I'll take a look and see if there are "natural" break points which could be used to repackage the design discussion
 

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Discussion Starter #54
"Design and Development of a Drop-in Lithium Replacement Battery or Kit"

Works for me!
 
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