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Discussion Starter · #1 ·
Is it possible to wire up 2 of the "Tesla Smart Battery 57 Volt 3kWh" with a single BMS?
I require a 7p2s configuration with these batteries, and just the 7p uses up one BMS.
I would like to avoid using 2 BMS.

I would also like to avoid opening the modules or breaking any seals, as I want to be able to use the watercooling.
 

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it’s not exactly clear what you are asking.

each battery group needs to be monitored. every group in series. recommend to use oem bms systems. It is the safest, and you are less likely to mess something up
 

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If I understand correctly, you are trying to use 14x modules in a 2s 7p configuration? Each of these modules has 15 series cell-groups, so what you wind up with is something like 30s 7p. That means you have 210 cell-groups to monitor, and since 96 cells is a common limit for a single BMS, it does seem likely that you will not be able to monitor that many cells even with 2 BMS systems.

If each of the cell-groups in the 7 parallel modules was connected with large enough conductors, you could treat them as a single cell-group, and you would then only need to monitor 30 points. These modules look like they are functionally identical to the ones I have, only higher voltage, so I will say that connecting them would be possible, but not fun. the individual 18650 cells are grouped together onto current collector plates which (at least on mine) are 2mm aluminum. I had no success soldering to them, and drilling them and bolting on wires will be a chore that needs to be repeated 15 times per module on each of the 14 modules.

I wouldnt advise trying to parallel these modules. You would be much better off going the route of a very large BMS.
 

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Discussion Starter · #4 · (Edited)
Each module is a 22s15p configuration of 18650 cells.

If I understand it correctly, each module has the following features:
  • 2 thermistors (total of 4 wires)
  • water cooling
  • each parallel grouping has a wire attached that can go to the bms (total of 16 wires)

With 7 of these modules in series, to generate the 400 volts I need, I would end up with 105 cell groups for a BMS.
The Orion 2 BMS seems to be able to handle this no problem, The Orion 2 BMS can also go up to 180 if needed.

The problem arises when I try to put 2 of these 7-groups in parallel.
Considering batteries in parallel self balance themselves naturally, I was thinking I could somehow wire the voltages together by using the BMS cell taps to create a "44s15p module". My reasoning behind this was that, so long as the voltages are the same when I connect them together, there would be no current flow, and this small wire would be able to handle it. When the voltage deviates even a tenth of a percent, it should self balance itself quickly with very little current on this wire.

I'm not sure what the OEM BMS for these would be, and it seems pretty common to use the Orion BMS with tesla modules.

Having to connect up 2 Orion BMSs isn't the end of the world, but I would like to avoid it, especially since I'm not sure how to designate one as the 'master' on the CAN bus for when I'm charging.

At the end of the day I need a battery system that runs at 380-400 volts nominal, has over 20kWh, and can output at least 300 amps (for a total power output of at least 120kW). This setup is just on the edge of that, so that does worry me. I love the idea of watercooled batteries for overall lifetime health and fast charging/discharging. However, if anybody has another suggestion, I'm all ears.
 

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You are mixing up Series and Parallel. You said you want 7p, 2s (in modules) but you are actually talking about 7s 2p. 7 modules in series would add the voltages, putting them in parallel adds their capacity.

That aside, you are correct on the theory, I believe. Electrically connected cell-groups in parallel should be able to keep each other balanced. I considered going down this route, but ultimately decided to just use 2 BMSs. For me, the problem is that you are building in a failure point on your safety system. That BMS wire is only 20 gauge, and IF, while you are trying to pull the max current, a cell does dip lower than its nieghbor, you could theoretically cause an eddy current that was large enough to burn out that thin wire. (I looked up how much current those wires should be able to handle, but I dont remember now what it was) Also, If the top half of one pack is slightly higher resistance than the top half of the other, current is going to cross over that wire in the middle to find the path of least resistance. Maybe those currents will always be small, or short enough in duration and the system will work fine. I dont know. Really, it comes down to how big a safety margin you are comfortable with. If you wired them with fuses, and were able and willing to check the fuses now and again, that might give you a degree of confidence that the system was working. I think there is a lot of interest in this topic, though, so if you do decided to try a BMS wire paralleling, document your experience.
 

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Discussion Starter · #6 · (Edited)
OR-Carl,

Thank you very much for the detailed reply! Yes, you're correct, I switched up the 7s2p with 7p2s. Don't know what I did that. Luckily it looks like you understood what I meant.

After reviewing your reply I did some research.
With the 22p15s configuration for a single module, and a 2p7s configuration for the module-to-pack, I calculated the following which addresses this thread:

TLDR;

I've decided to go through with this process. I will definitely document my findings and post more here.
Until then, here are my pre-calculations.

Using the BMS wires to balance cells in parallel:

A 20 gauge wire that is 2 meters long (That's the upper end it could possibly even be, but I wanted to use 'worst case' in these calculations) has a resistance of 0.16 Ohms. The wire is likely not solid-core, and is probably braided. I didn't know how many braids, so I just went with worst case. The current these wires can handle asymptotically approaches a single amperage as number of strands increase. This is about 43 total strands in one 20 gauge wire. The table I used documents this as being able to support up to 2.5 amps through. Even more before it would 'blow' (like a fuse). Again, we want to use the worst case, so I'll even round down and call it 2 amps.

2 Amps * 0.16 Ohms = 0.32 Volts
This means that between 2 cell groups, that were connected together in parallel over a BMS wire, a voltage difference of 0.32 would be needed before the wire starts to break down.

Below I do some calculations for the entire pack, and individual cell voltage. I found that for a cell-grouping to be different enough to make this an invalid solution, the capacity would need to be over 50% different from the cell-grouping it's connected to in parallel. Considering I will also be wiring up the entire module in parallel (and not just the BMS wires), this isn't really a concern of mine.

Addressing the fact that there could be large current spikes during periods of high current draw, even if only one module is taking the full load, which is next to impossible, the voltage difference does not rise above the 0.32 volt limit in order to cause damage. This assumes similar state of charge, which I will be closely maintaining with the use of the Orion 2 BMS.

Heavy(er) math for my specific setup:

Assuming I keep each cell grouping in parallel at the same temperature, I won't need to worry about how the temperature affects the voltage. This will be relatively easy with the use of the water-cooling (or heating) built into the modules.

The cells in this module have a 4.2 volt cutoff on the upper end, and a 2.5 volt cutoff on the lower end, but realistically you never want to run these below 3.3 volts. Between these values the state of charge and the voltage are fairly linearly related. Each cell holds 2.7 Amp hours, which would result in 2.7*22 = 59.4 total Amp hours. However, the whole module only advertises 57 Amp hours. This is likely due to both safety factors, and losses due to balancing between the cells. This leaves us with 346.5 volts on the low end and 441 volts on the high end. Because of the boost in voltage during regenerative breaking, the boost in voltage from generator based charging, and the 450 volt limit on my inverter, I don't want to go over a total voltage of about 420 volts. Extra safety calculated in. For a bit of extra margin in battery health, and to run my motor more optimally, I don't want to go below a total voltage of 357 volts. This leaves us a per-cell-grouping voltage range of 3.4 volts to 4 volts. Which is about 66% of the original range of the pack. However, this is much less on total power in the pack. When charging and discharging, the voltage curve ramps up quickly between 4 and 4.2 volts, and then linearly falls down between 4 and 3.2 volts, before dropping off sharply. So, liberally, we would be loosing about 10% of our total pack capacity. If each pack has a total original capacity of 57Ah or 3kWh, then the new capacity would be 51.3Ah and 2.7kWh. With my 2p7s configuration, this would result in a total (conservative) capacity of 37.8kWh. Generally, as a rule of thumb we can take 10% of the weight of the vehicle in lbs to get watt hours/mile. We estimate about 3000lbs, but I conservatively will say 3500lbs. So 37.8kwh/0.35kwh/mile = 108 miles on a single charge. For those that don't know, I plan to drive the whole Rubicon with this, which is about 22 miles. So even if we only get 1/4th of that, we could still make it on a single charge. However, as backup, we have a charging solution. I understand rock crawling will use much more power than normal driving, but I believe this estimate will be suitable. I'm not too worried about it, especially since we have an on-trail charging solution.

Vehicle life
Imagining for a moment that I made this my commuter vehicle as well:
On average I drive about 100 miles per week, but lets round that to 108 to make the math easy. This would result in 52 full charge cycles per year and a 20% pack degradation over the course of 9 years (referenced from the NCR18650 datasheet). This is very good! Meaning if I use this car 24/7/365 for 9 years, by the end of that 8 years I would still have 30.24kwhs in my pack.
All of this assumes I'm doing full charge cycles, which I'm not. Likely I would plug it in whenever I'm at home to keep the pack voltage as constant as possible, as this is what's best for long health in a lifepo4 battery. I would be comfortable doubling that number to 20 years before the state of health reached 80%.

Gas is cheap
I would also like to comment on how great gas really is, as electric car enthusiasts we often don't give it enough credit. Assuming all the same values above, very good gas mileage of 30mpg, and cheap gas of $2 per gallon, this leaves me spending about $350 per year on gasoline. Assuming we compare this to the 20 year lifetime noted above, that would be about $7,000. This entire battery setup will cost about $15,000 and doesn't include the cost of the electricity used to charge it. The motor/inverter combo would roughly be the same cost as the engine. However, this doesn't take into account yearly gas engine maintenance, and because of this, the electric vehicle would probably end up ahead. But my point: Gas is cheap for how much energy we get out of it. More so my point: Battery technology really needs to improve for this to be adopted globally. This mere fact alone makes me worried that in 2 years my batteries will be completely obsolete.

Off-grid solar
If you wanted to live completely off grid, and use your car to double as a battery storage system for the power to your whole house, you would need about 520 square feet worth of solar panels. That's about half of (if not the entire because of bad angles) roof of an average middle class home in America. That's about $5000 in just solar panels. Probably another few grand to set it up.

All these things are pretty fun to think about.

For the individual modules, and more specifically individual cell groupings, I remembered some of the fail-safes that tesla designed in:

Each individual cell has a fuse in these tesla batteries. I'm assuming they are the same, or at least close for the car batteries as they are for these "smart" modules. Each cell in a group has what is roughly a 25 amp fuse attached. This is a safety measure put in by tesla in case there is some kind of short, the rest of the pack would be able to handle it.

So an entire cell grouping would be able to "take" 22*25=550 amps before the batteries would be totally busted. My goal is 300 amps total, or 150 max per cell grouping. So this seems to check out. Even though these batteries say 100 amps continuous max current, and 150 peak current (10 seconds max), I believe that 150 amps could be run for longer with proper thermal management. That will probably never happen, but it is an edge case that I wanted to address in case my car ever draws that much for longer.

Initial setup safety concerns

This setup requires that each cell-grouping and each module be nearly perfectly balanced with each other before connecting! If they are not I risk damaging an entire module. These batteries are not cheap so I will have to make sure to do this! Also with these high voltages, I will have to be careful to isolate everything so there are no accidental shorts anywhere.
 

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I feel like that all sounds pretty plausible. Fuses might not be a terrible idea, if you have space for them. That way if you get an eddy current that is unsafe for the wire, you will see it by checking the fuses now and again instead of having a break somewhere that you cant see. Like where the wire connects to the current collector plate. Also, check those connections - mine corroded and came loose, which was a pain to deal with. I suspect those modules were built with a similar process.

Anyway, I am looking forward to seeing your project, keep us posted.
 
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