# Self balancing tesla modules by connecting them in parallel for a few days???



## stealthE (Jan 31, 2016)

I have 6 modules sitting there for the next couple weeks till I can install them. Well... If I connect them all in parallel few a couple days..., they will self balance...correct?


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## GoElectric (Nov 15, 2015)

Yes, but don't blow any fuses when you connect them?


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## stealthE (Jan 31, 2016)

why would I risk blowing fuses? I mean....why would they blow from the process?

Sounds like your going to be bottom balancing, right? Would this method not be just as good...until the next time I need to balance?


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## GoElectric (Nov 15, 2015)

Hi. Unless you have all of the cells ALREADY at the same voltage, current will rush from one cell to the other. You could do it slowly with a resistor between 2 cells, then tie those together, move the resistor and add one more "cell" (74P). That should work. Worst-case they are 2 volts apart, I would GUESS a 5W 1 ohm resistor would do it.

You should read the thread on here about bottom-balancing. It is tedious, but reading all opinions and having things said in different ways is worth it. If you top-balance, you still don't know what the capacity of your cells is, so if one is low capacity, you can drain (and damage) a cell. You can get the most out of your pack, but there is a danger of damaging a cell. I guess it would be a bad cell anyways, so there is that, but you may damage the other cells.

The main thing is to stay above 20% SOC (state of charge). With Tesla Cells, I think top-balancing is impossible, as each cell is 74 little cells in parallel. You cannot address individual cells. 

I will be doing lots of testing but think I will find insignificant differences between "cells." I also have 5 single Tesla 18650s, so will be comparing them.

Have you taken my advice on keeping the modules flat, or....? Mine will fit fine flat, although it was tempting.


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## Ampster (Oct 6, 2012)

stealthE said:


> I have 6 modules sitting there for the next couple weeks till I can install them. Well... If I connect them all in parallel few a couple days..., they will self balance...correct?


Yes they will all equalize. I have done it numerous time with prismatic cells. Unless one module is way lower or higher that the rest you might see a little spark from the current. You could charge them as a group and that would be the same as top balancing. You could discharge them to 20% and that would be the same as bottom balancing. It is very easy to tell the capacity of a pack that is top balanced or bottom balanced. Just run a discharge test and measure the amphours.

NOTE; Corrected to read equalize instead of balance. Also the equalizing would only be between modules. The bricks within the modules could be out of balance.
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## Ampster (Oct 6, 2012)

GoElectric said:


> ........ With Tesla Cells, I think top-balancing is impossible, ............


It is common knowledge that Tesla uses a BMS and that presumes top balancing. 


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## Duncan (Dec 8, 2008)

NO they will NOT balance

You can only balance at the top of a charge or at the bottom

In the middle the voltage slope is simply too shallow so you can't get them all to the same state of charge

Connecting them in the middle will give a very very rough balance but you will still be unbalanced at the top and at the bottom - where it is important


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## Tomdb (Jan 28, 2013)

Gentlemen,

Everyone is missing the main point. A Tesla module contains cells built up in Series, 6 in series. 

By connecting the modules in parallel the only thing you will be balancing is the total module voltage. Not the cell voltage.

When connecting any battery in parallel, I use the practice of using a resistive load to limit the inrush current. Before connecting a bus bar.


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## Ampster (Oct 6, 2012)

Tomdb said:


> Gentlemen,
> 
> Everyone is missing the main point. A Tesla module contains cells built up in Series, 6 in series.
> 
> ...


Maybe balancing is the wrong term. When connecting modules in parallel, the modules will all end up at the same voltage. (Approximately 22 volts) within the modules are groups of parallel cells in bricks. The cells within the bricks are the same voltage. The bricks are connected in series and the voltages between the bricks would not be the same. It is at the brick level that the Tesla BMS operates in order to balance each brick to the others.



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## GoElectric (Nov 15, 2015)

Hi. That sounds right Ampster. I guess the cells are then equalized, not so-much balanced. But that is a good starting point for subsequent charging/balancing.

Question: I was actually assuming he wanted to connect each of the 74P "bricks" in parallel. The wires are there for each brick. I think this would be the ideal starting-point for charging/balancing? But probably this is a non-issue: on one pack I have measured, the bricks differ in voltage by only a few 100ths of a volt, so maybe just equalize the modules, as you say. And point-taken on the need for a resistor doing this - as long as the modules are not too far-apart.


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## Ampster (Oct 6, 2012)

I don't know how you would do that. Aren't they wired internally, ie positive from one brick to negative on next as so on. If you diagram a series connect like that you will se that you will get a short when you add paralleling wires. That will be a big spark.
I have not physically seen a Tesla module, but others who have, can probably speak with more experience.




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## bigmotherwhale (Apr 15, 2011)

Duncan said:


> NO they will NOT balance
> 
> You can only balance at the top of a charge or at the bottom
> 
> ...


+1 finally someone is talking sense. 
why don't you just use a good hobby charger / discharger can be had for nothing compared to the cost of the batteries.


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## dougingraham (Jul 26, 2011)

GoElectric said:


> The main thing is to stay above 20% SOC (state of charge). With Tesla Cells, I think top-balancing is impossible, as each cell is 74 little cells in parallel. You cannot address individual cells.


People tend to not get close to zero by nature with a Tesla. The battery indicator on the dash turns orange when you go below 20% and I think it goes red below 10%. I haven't gone below 10% with mine yet. Range anxiety usually keeps you more charged than that. What Tesla does is try to get you to not top off the pack above 90%. They will let you charge to 100% if you want to but mark anything above 90% as a trip charge. The balancing seems to take place only if you specify a trip charge to 100%. On the supercharger the voltage kisses 405 at about the 90% point and the current tapers quite quickly from that point. If the pack is well balanced the charge stops when the battery current reaches 5 amps. The first time I did a charge to 100% it took way more than an hour to get that last percent with the battery current hovering at 3 amps indicated.

Almost all batteries are little cells in parallel, just in the same housing. Ever cut open a prismatic cell? Each pair of foils is a cell.



GoElectric said:


> I will be doing lots of testing but think I will find insignificant differences between "cells." I also have 5 single Tesla 18650s, so will be comparing them.


I would like to see a 1/2 C (~1.625 A) discharge curve for real Tesla cells. Do you know what revision of battery pack these cells came from? A, B, E?

A 1.625 amp discharge on a single cell would represent about a 43kW battery load on a full 85 kWh Tesla pack.


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## Hollie Maea (Dec 9, 2009)

GoElectric said:


> The main thing is to stay above 20% SOC (state of charge). With Tesla Cells, I think top-balancing is impossible, as each cell is 74 little cells in parallel. You cannot address individual cells.


That has nothing to do with top balancing or bottom balancing or anything else. The cells are paralleled, which makes them a single balanced cell.

I'm not trying to be rude, but that's a fundamental concept, and if you don't understand how balancing works well enough for this, I would advise against using it to keep your battery safe.


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## stealthE (Jan 31, 2016)

Hollie Maea said:


> I'm not trying to be rude, but that's a fundamental concept, and if you don't understand how balancing works well enough for this, I would advise against using it to keep your battery safe.


Here is what I find funny, a forum is a place of learning, a place where no question is a dumb question. Yet I have seen on more than one occasion where is persons knowledge/ability is 'de-rated' for not knowing enough.

A LOT of people on this site seem to be super electrical engineers. There are terms thrown around that I will never understand. Yet, I can design, blueprint, balance, and over power any ICE motor; and I will help anyone do it. I won't tell them that they should not attempt it because they don't know enough. 

I am building my project with a friend of mine, who is a journeyman industrial electrician, but even he hasn't heard of BMS.

So I regress...what is the name of a company that offers a complete BMS system that will work with a Tesla setup. If there are none available, I will bottom balance....until someone way smarter than me figures it out.


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## Sunking (Aug 10, 2009)

Ampster said:


> It is common knowledge that Tesla uses a BMS and that presumes top balancing.


Are you sure?

No commercial EV manufacture allows the customer to fully charge a battery. You can only Top Balance if you charge to 100% SOC voltage
.


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## Karter2 (Nov 17, 2011)

Sunking said:


> .....You can only Top Balance if you charge to 100% SOC voltage
> .


.?? Maybe in your strict definition of "top balancing"
But there are plenty of us who "ballance" their lipo packs at 4.1, or 4.15,or even lower..
Anything above 4.0v and you are into a pretty sensitive part of the voltage/SOC curve.


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## Ampster (Oct 6, 2012)

I "top balance" my LIFEPO4 packs at 3.40 and cutoff charging at 3.5. As far as Tesla is concerned, the balancing algorithm apparently starts at 93% SOC. 
Tesla knows more about what works for their packs than many of us. You may be correct, I have never heard them call it "top balancing", just balancing. I think I can safely say they do it near the top. In fact that is what I am trying to do with my 48 stationary storage pack. Since I don't cycle that pack very hard I often switch off the balancing.
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## Ampster (Oct 6, 2012)

stealthE said:


> ..........what is the name of a company that offers a complete BMS system that will work with a Tesla setup. If there are none available, I will bottom balance....until someone way smarter than me figures it out.


Assuming you can connect to the brick connections with the existing BMS wires you could use an Orion BMS because it is centralized. The Elithion and Elektromotus use cell boards (decentralized) and they might not work as well since the boards are designed for prismatic cells. All of them let you set custom balancing voltage and high voltage cutoffs.
I think it would be a slow process to bottom balance a pack made up of Tesla modules. The difficulty might be doing high amperage discharging of the brick within the modules because of lack of access to each brick. The only access that I know of is the balacing wires and they aren't large enough to carry many amps.



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## Sunking (Aug 10, 2009)

Karter2 said:


> .?? Maybe in your strict definition of "top balancing"
> But there are plenty of us who "ballance" their lipo packs at 4.1, or 4.15,or even lower..
> Anything above 4.0v and you are into a pretty sensitive part of the voltage/SOC curve.


That would be Middle Balance.


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## Sunking (Aug 10, 2009)

Ampster said:


> I "top balance" my LIFEPO4 packs at 3.40 and cutoff charging at 3.5. As far as Tesla is concerned, the balancing algorithm apparently starts at 93% SOC.
> Tesla knows more about what works for their packs than many of us. You may be correct, I have never heard them call it "top balancing", just balancing. I think I can safely say they do it near the top. In fact that is what I am trying to do with my 48 stationary storage pack. Since I don't cycle that pack very hard I often switch off the balancing.
> Sent from my SM-N910T using Tapatalk


I have a 48 volt LFP golf Cart, and have built quite a few Solar systems using LFP cells. None use any kind of BMS. I could use a BMS in my cart, but there is really no BMS out there for solar. 

Instead just use a different strategy mimicking what commercial EV manufactures do. The packs are Bottom Balanced at 2.4 to 2.45 volts. Charging is a simple Float Charge Algorithm CCCV to around 3.375 vpc or 54 volts. Periodic checks are made on the cells to make sure non go above 3.4 volts. If that happens a Bottom Balance will be performed
. So far non have ever gone out of balance. On the Discharge side is super simple rock solid protections. LVD or LVC is set to 48 volts or 3.0 vpc well above 2 volt danger zone. In other words, Middle Balance. 

I do have a Orion Jr I initially bought for my cart. But I only use it as a fuel gauge and cell voltage monitor. Only control it has is if one cell drops to 2.5 volts for 30 seconds or more, it will operate the LVC relay. But I have never seen a cell drop below 3 volts under acceleration.


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## Ampster (Oct 6, 2012)

I have seen your earlier posts about the 48v pack and followed your strategy for the most part. I used the Orion BMS to equalize the 3 buddy pars of cells in my 16s pack then I turned off the balancing and like you, just use it for voltage monitoring. I can't use the Orion for a fuel guage because the Outback inverter throws off too much EMI. Interestingly the Outback gives me a good fuel guage and I can access the graphs of my charge discharge cycles from my phone or laptop. 

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## Karter2 (Nov 17, 2011)

Sunking said:


> That would be Middle Balance.


"Middle" of what ?
Capacity ?
Voltage ?
The road ?
For many , 4.15v is a close to the top as they want to go.


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## stealthE (Jan 31, 2016)

so, here is what I have found: the main + and - lead on the tesla brick is showing 21.77 volts as it stands.
When I take my meter and test the + and - of each series 'cell', all 6p's are showing 3.62v. Does this mean I can drain a single 'series cell' down to 2.5v, without using the Tesla BMS? Couldn't I connect a small heater to the top and bottom plates of each cell (one at a time), to get it there without taking 30 hours to do so? Is that how I can bottom balance the pack?


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## Ampster (Oct 6, 2012)

Why bother with pulling down each brick. Try discharging module in the pack to 17v and watch cell voltages as they go below 3v. If they are close you dont need any balancing. Then you can charge them up and be sure you watch them at the top. 

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## stealthE (Jan 31, 2016)

Thank you!!


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