BMS for Tesla

I have searched through half a dozen pages of tesla bms results on the whole forum there wasn't anything that stood out or conclusive in regards to what is the BMS option for a tesla s module in 2020.

I know the consensus now is becoming not to use tesla modules to begin with, but for those that already have batteries, like myself, its better to be safe with a bms than not.

Backstory: A bit over a year it ago, it was the hype to use tesla batteries for DIY solutions, so I purchased two s modules. I am now building my camper-van which I think can have up to 4kw+ power draw through the inverter from the batteries. With that draw I am a bit concerned with the stories I am hearing about cell imbalance.

I understand the idea is to get a bms that does top balancing.

What are my options for available bms' ?

I would like to use a 24v inverter since most of my non inverter equipment is 24v and so I would need two or more 6s bms units. Is the option of having multiple 100amp bms' in parrallel to keep cost down an option? would that be an issue for balancing or shut off?

I am even thinking of making my own with an esp32 module that will monitor each cell and handle shutoffs with maybe a switch relay, handle balancing and report every figure over wifi as well. Has anyone attempted this?

There are a couple Tesla module discussions underway. Do a search for "Tesla" and you will find lots of questions and answers to things that you mentioned.

Here's one:

alexzan said:

I have searched through half a dozen pages of tesla bms results on the whole forum there wasn't anything that stood out or conclusive in regards to what is the BMS option for a tesla s module in 2020

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I believe that module consists of over 70 cell bricks in 6 or 7 serial groups. You just need a 6 or 7S BMS with adjustable set points for voltage. You should check my assumptions. The cells are NMC which have a nominal voltage of 4 volts. 6 times 4 is 24 volts. Some of the 8S BMSs can be configured for 6 or 7 Serial cells.

On this thread the poster used Tesla modules and maybe he describes the BMS.

I'm working on a DIY BMS using a Teensy 3.2 to talk to the board that's built into the battery module and control a relay to disconnect/reconnect the battery. This is dirt cheap. It can also instruct the board to balance at whatever SoC I choose.

I'm basing it on the code from here:

markb said:

I'm working on a DIY BMS using a Teensy 3.2 to talk to the board that's built into the battery module and control a relay to disconnect/reconnect the battery. This is dirt cheap. It can also instruct the board to balance at whatever SoC I choose.

I'm basing it on the code from here:

GitHub - collin80/TeslaBMS

Contribute to collin80/TeslaBMS development by creating an account on GitHub.

github.com

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Wow thats very cool! How is it coming along?

MisterSandals said:

There are a couple Tesla module discussions underway. Do a search for "Tesla" and you will find lots of questions and answers to things that you mentioned.

Here's one:

New Member Struggling To Put Together Previously Reccomended Tesla Module S Solar Set Up

Hello there! I made the decision last year to pull the trigger and order the list of recommendations by Will for the TESLA model S solar set up that Will was himself using. A year later I have still yet to hook it all up because life happened. Here I am now trying to hook it up and I cant find...

diysolarforum.com

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Yes that was one of the first I came across, I didn't find anything helpful there in regards to BMS options.

Ampster said:

I believe that module consists of over 70 cell bricks in 6 or 7 serial groups. You just need a 6 or 7S BMS with adjustable set points for voltage. You should check my assumptions. The cells are NMC which have a nominal voltage of 4 volts. 6 times 4 is 24 volts. Some of the 8S BMSs can be configured for 6 or 7 Serial cells.

On this thread the poster used Tesla modules and maybe he describes the BMS.

DIY Tesla powerwall

Time to install a critical loads panel! The demonstrates the value of being adaptable. What are you planning on doing to switch between your inverter and the grid in case you need to work on the inverter? Eisenhower said, on the eve of D day, " Plans are nothing, planning is everything." I...

diysolarforum.com

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I also came across that one, but searching that thread for BMS came up with 0 results

alexzan said:

Wow thats very cool! How is it coming along?

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I have the teensy talking to the battery module and logging the cell voltages and temperature. It should be a simple matter to connect and control a relay, which is the next step. After that, I will have it control the charger and the inverter.

It’s all pretty straightforward. I just need to get around to working on it!

Daly just introduced their 6s BMS for about $12. I also have two Tesla batteries but don't run a BMS (also per Will's initial advice). I'm not entirely sure of the benefits or efficacy of a BMS. We say 6s but that battery is actually 444 cells split into 6 "cells" with 74 batteries each! Treating 444 cells as one cell or treating 74 cells as one cell, I don't see much difference there. How are those 74 cells going to get balanced? They are not. Trying to balance 6 groups of 74 cells each sounds like a pipe dream to me.

sed6 said:

How are those 74 cells going to get balanced? They are not.

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It depends on what you define as balanced. Because they are in parallel they will all be at the same voltage. If they are well matched, they will be very close to the same SOC. If they are not well matdhed then during charging the weak ones will discharge into the strong one. During discharge as the weak ones head south the stronger ones will prop up the weak ones. The same thing happens in a prismatic which consists of numerous pouch cells in parallel. The Ahr capacity determine the number of pouches in parallel. I have notice a trend with some prismatics that the height and length are the same for a range of Ahr models. Only the width (thickness) changes as they add pouches. Makes sense from a manufacturing efficiency standpoint.

markb said:

I have the teensy talking to the battery module and logging the cell voltages and temperature. It should be a simple matter to connect and control a relay, which is the next step. After that, I will have it control the charger and the inverter.

It’s all pretty straightforward. I just need to get around to working on it!

Click to expand...

After reading up on teensy 4.0 and comparing it to what ive been using (ESP) I think I am going to grab a teensy 4.0 and join in. Is your project open source?

sed6 said:

Daly just introduced their 6s BMS for about $12. I also have two Tesla batteries but don't run a BMS (also per Will's initial advice). I'm not entirely sure of the benefits or efficacy of a BMS.

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The benefit of a BMS on your Tesla batteries is that your house does not burn down when you have thermal run away.
BMS is a safety device.
@Will Prowse initial advice on Tesla batteries has changed drastically as you probably know.
Your home insurance probably will not cover non UL approved batteries causing a fire.

Yeah this is why I took those videos down six months ago. Too dangerous. LiFePO4 way safer. I agree with MBR. I sold my tesla module

6 foot high blazing fireball in an instant.
Guy plugged his new high Ah Lithium ebike battery in backwards with spectacular results. Luckily it was outside on paved asphalt and a fire extinguisher eventually knocked it down.

MBR said:

The benefit of a BMS on your Tesla batteries is that your house does not burn down when you have thermal run away.
BMS is a safety device.

@Will Prowse initial advice on Tesla batteries has changed drastically as you probably know.
Your home insurance probably will not cover non UL approved batteries causing a fire.

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Let me start out by saying that I have always used a BMS and most likely continue to use a BMS. That being said a BMS will not prevent thermal runaway. It is just a component in a system that can be designed to reduce the risk of thermal runaway. Some thermal runaway in Tesla batteries is cause by puncture of the pack by a collision with a sharp object and those are infrequent and not normally a risk in a stationary setting like storage.

The other factor often cited in thermal runaway is over charging and the first line of defense is the charger. Both the charge cutoff voltage and the charge rate are important When the modules are cooled as they are in the car the rate can be as high as 2 or 3 Cs. In a stationary application charge rates can be controlled by the charger and are usually below 0.5C. The charge cutoff voltage of the inverter should be the first line of defense against over charging. The BMS or any voltage programmed relay opening up a contactor is a good risk management step to ensure there is no likeliehood of over charging.

MBR said:

6 foot high blazing fireball in an instant.
Guy plugged his new high Ah Lithium ebike battery in backwards with spectacular results. Luckily it was outside on paved asphalt and a fire extinguisher eventually knocked it down.

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If that is the case we should all go back the Lead Acid and get rid of our cell phones too.. There was a spectacular fire in a garage in a nearby County that actually sent a couple of Firemen to the hospital. In that case it was a guy selling ebike batteries that were LiPo AKA LiPoly. LiPoly were the cause of a lot of Hoverboard accidents. Tesla batteries are NMC same as in Nissan Leafs.
I don't disagree with Wills decision to take down his review. The question for me is how to help people who already have a Tesla module understand the risks of a Tesla module and how to manage those risks.

MBR said:

Your home insurance probably will not cover non UL approved batteries causing a fire.

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I hear this so often, as an insurance agent I can say that's 100% false. Your home insurance covers stupidity completely. As long as you don't purposely burn your house down, you're covered!

sed6 said:

as an insurance agent I can say that's 100% false.

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.....and you undoubtedly have a good sense for risk management.

[QUOTE="
I don't disagree with Wills decision to take down his review. The question for me is how to help people who already have a Tesla module understand the risks of a Tesla module and how to manage those risks.
[/QUOTE]

Yes this what im looking for, support for those who already have it.

Another thing to consider is Canadians like myself have a very hard time getting any lithium batteries as they are banned at the border. There are very few good comercial options, especially now with covid, but there are plenty of salvaged tesla batteries.

alexzan said:

After reading up on teensy 4.0 and comparing it to what ive been using (ESP) I think I am going to grab a teensy 4.0 and join in. Is your project open source?

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Not yet. Once it's a complete BMS, I might do that, but right now, it's more like a proof of concept. And really all I've done is taken the original code (linked in my previous message) and made a few hacks to it.