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This is a design I did using (2) Tesla model S batteries in a late model 19' Airstream RV upgrade.
  • (2) Model S Batteries were wired in Parallel 24v system
  • Victron Inverter, MPPT Solar Charge Controllers converter (for 12v) all components Victron
  • About 500 watts of solar panels
  • I fused all of the connections and wired 1/0 from the batteries to the Buss
Initially I used two BMS’s that connected via a Ribbon to each of the Tesla Tap Cards that I got on ebay. My intent was to use a third party BMS for individual cell balancing on each of the Batteries, and also for backup HVD and LVD events should the primary Victron components fail to do so. After a lot of testing I found some problems that I never seemed overcome.
1. Wiring the ribbon cables with to the tesla batteries was buggy, I never got them right to work consistently using different methods wire sizes and connection methods.
2. The BMS would then at times fault by attempting to push more power into one of the cell groups (the BMS thought that one of the cell groups was out of balance) I always checked manually with the volt meter and always found the cell balanced. Random volt level checks on the cells always show very near balanced.
3. The BMS had issues with when it “acts” on the LVD and the HVD events. In a HVD event, since it used the single group highest value/lowest Value for HVD and LVD .. this didn’t quite test out either.
Because of the way the BMS seemed to introduce issues I elected to remove it from my design and have since used a “supervised” approach. My rate of discharge is low and charge rate I can easily control to a slower charge setting on my inverter when on shore power. I have also found that the Victron components work flawlessly at HVD, temperature and LVD shutoff events as primary protection. We have used it for one full season and we hardly even monitor anything while boondocking (1 week offgrid) The batteries stay very cool even when we occasionally use the AC.
Since Cell balancing seems always very much in check just using the native Tesla batteries I am thinking of sticking with a design that does not leverage a BMS. Is this a bad idea or should I pursue another BMS solution.
 

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Is this a bad idea or should I pursue another BMS solution.
You can skip a BMS, especially on a 7S system.

But there's no need to skip it. It's only 7s. 24v BMS's are so cheap and simple you might as well slap one on. You don't need a big massive one, just something that will correct any minuscule, gradual imbalances that may happen over months. You can grab one for like, $20, that's solid insurance on a pack that expensive. Go with a nicer one if you want, cheap ones will be more prone to fail.

I would even consider having 7 panel voltmeters on a little ribbon cable you can plug in to see the voltages at any given time.
 

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I personally would never use any LI chemistry other than LFP or LTO for that use case,

House bank storage in or near a small mobile living space.

As the cells age and wear - much more quickly than they would with those two

the risk of something going wrong leading to thermal runaway is too high.

Boom bad!
 

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Discussion Starter #4
I personally would never use any LI chemistry other than LFP or LTO for that use case,

House bank storage in or near a small mobile living space.

As the cells age and wear - much more quickly than they would with those two

the risk of something going wrong leading to thermal runaway is too high.

Boom bad!
 

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Discussion Starter #5
you mentioned in one of your previous posts Note that "a BMS" is just a collection of functionality: Maybe balancing, depends what you want, make sure high balancing current and all setpoints fully adjustable HVC, LVC might be based on per-cell or whole-pack voltage maybe OCP but fuses are better IMO, no need for current to flow..."

I like the idea of fuses, how did you do that for HVC and LVC?
 

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Fuses and breakers are for OCP not voltage cutoffs.

High-current quality versions get expensive.

Cooper Bussmann or Blue Sea Ignition Protected ANL type are good.

Cheap Chinese need calibrated testing, and even then performance may vary from one run to the next.
 

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1. Wiring the ribbon cables with to the tesla batteries was buggy, I never got them right to work consistently using different methods wire sizes and connection methods.
2. The BMS would then at times fault by attempting to push more power into one of the cell groups (the BMS thought that one of the cell groups was out of balance) I always checked manually with the volt meter and always found the cell balanced. Random volt level checks on the cells always show very near balanced.
Is this a bad idea or should I pursue another BMS solution.
It's never a good idea to use these modules without a BMS. Unless, you are willing to accept the substantially increased risk of ruined modules, or a catastrophic fire. Have you visually inspected all of the cell whisker wires? Broken or loose wires could throw off the BMS logic.
Are the ribbon cables you're using the stock Tesla ones?
 

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Discussion Starter #8
It's never a good idea to use these modules without a BMS. Unless, you are willing to accept the substantially increased risk of ruined modules, or a catastrophic fire. Have you visually inspected all of the cell whisker wires? Broken or loose wires could throw off the BMS logic.
Are the ribbon cables you're using the stock Tesla ones?
Thanks for responding. I am using a third party tap card not the tesla card.. From the tap card extends a ribbon to the BMS the ribbon comes with the bms and needs to be wired manually to the tap card. I measure the voltage at the tap card and find consistent balanced cells always. i have not gotten into the cell groups and the connections the battery itself. as the cells always are balanced at my tap card.maybe i should , The problem seems to be introduced via the BMS ribbon connection and or something downstream of that, or maybe component "incompatibility" I'm thinking. The connections are VERY sensitive.
 

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Thanks for responding. I am using a third party tap card not the tesla card.. From the tap card extends a ribbon to the BMS the ribbon comes with the bms and needs to be wired manually to the tap card. I measure the voltage at the tap card and find consistent balanced cells always. i have not gotten into the cell groups and the connections the battery itself. as the cells always are balanced at my tap card.maybe i should , The problem seems to be introduced via the BMS ribbon connection and or something downstream of that, or maybe component "incompatibility" I'm thinking. The connections are VERY sensitive.
Do your modules use wires or ribbons (Tesla has used both) to connect the BMS to the 6 cell groups? I don't understand what you mean by "VERY sensitive". If you shake the modules, do the circuits open. As in bad connections? Are there any signs of damage from coolant leaking onto the cells? Stains? Corrosion? Some good pictures of your set-up might be helpful.
 

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This is a design I did using (2) Tesla model S batteries in a late model 19' Airstream RV upgrade.
  • (2) Model S Batteries were wired in Parallel 24v system
  • Victron Inverter, MPPT Solar Charge Controllers converter (for 12v) all components Victron
  • About 500 watts of solar panels
  • I fused all of the connections and wired 1/0 from the batteries to the Buss
Initially I used two BMS’s that connected via a Ribbon to each of the Tesla Tap Cards that I got on ebay. My intent was to use a third party BMS for individual cell balancing on each of the Batteries, and also for backup HVD and LVD events should the primary Victron components fail to do so. After a lot of testing I found some problems that I never seemed overcome.
1. Wiring the ribbon cables with to the tesla batteries was buggy, I never got them right to work consistently using different methods wire sizes and connection methods.
2. The BMS would then at times fault by attempting to push more power into one of the cell groups (the BMS thought that one of the cell groups was out of balance) I always checked manually with the volt meter and always found the cell balanced. Random volt level checks on the cells always show very near balanced.
3. The BMS had issues with when it “acts” on the LVD and the HVD events. In a HVD event, since it used the single group highest value/lowest Value for HVD and LVD .. this didn’t quite test out either.
Because of the way the BMS seemed to introduce issues I elected to remove it from my design and have since used a “supervised” approach. My rate of discharge is low and charge rate I can easily control to a slower charge setting on my inverter when on shore power. I have also found that the Victron components work flawlessly at HVD, temperature and LVD shutoff events as primary protection. We have used it for one full season and we hardly even monitor anything while boondocking (1 week offgrid) The batteries stay very cool even when we occasionally use the AC.
Since Cell balancing seems always very much in check just using the native Tesla batteries I am thinking of sticking with a design that does not leverage a BMS. Is this a bad idea or should I pursue another BMS solution.
 

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I did something similar with a 1978 GMC Motorhome I'm restomodding. I built everything in a container well away from my shop and anything flammable with four panels on the roof to test extensively while I did other work. When I was satisfied I put the modules into a box I welded up from 1/4" steel plate, lined with Rockwool and put the box into the frame, between two ladder frame rails.
Here are a few old blog posts on the project:
There are a few more related posts, but they are probably not useful. And here's a video on the control system, which I based on the EV.TV controller. I built a different controller that works identically but found it to be a bit harder to tune. So far this setup has been rock solid, running the loads without shore power for five months. I don't use the roof-mounted AC anymore--too inefficient and too loud. I'm running a single mini-split in the back bedroom--1/3 the energy consumption, no noise, and it cools the coach better.
 

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Get yourself a SimpBMS, and a few big contactors and you can use the existing balancing while having overcharge / undercharge protection (a fully protected battery). This is really the only way you should use those modules. Here is the best part, The SimpBMS is already designed to work with the Victron inverters! You can get state of charge and other great info as though it's a native Victron battery. Its not a super simple plug and play, but not altogether that difficult to assemble. Carel Hassink [email protected] was super to deal with.
 

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Get yourself a SimpBMS, and a few big contactors and you can use the existing balancing while having overcharge / undercharge protection (a fully protected battery). This is really the only way you should use those modules. Here is the best part, The SimpBMS is already designed to work with the Victron inverters! You can get state of charge and other great info as though it's a native Victron battery. Its not a super simple plug and play, but not altogether that difficult to assemble. Carel Hassink [email protected] was super to deal with.
I have the SimpBMS as well, I tested it for a while and found it quite good--that's the other controller I referred to above. It's a LOT cheaper than the EVTV controller, but since I already had the EVTV setup I used it.
 

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No matter what controller you use, you have to get properly sized contactors to completely disconnect the battery in case of over-discharge or overcharging. You also have to protect the contactors from welding their contacts by soft-starting with a resistor bypassing one contactor. You then close the second contactor first, give the system time to come up to voltage, and then close the bypassed contactor. you also should have auxiliary contacts in the contactors so you can detect welded contacts. The wiring diagram for either the SimplBMS or the EVTV system shows how to do this. And unless you want to deal with recovering from an overvoltage situation with the battery completely disconnected and no power, you need a charge enable circuit that cuts off well before the overvoltage happens. A temperature cutoff that prevents charging when the battery temp is below 0C is also a good idea.

These batteries are as safe as other chemistries if they are operated within their specified ranges, but if you overcharge or over-discharge, they can and probably will run away. Not pretty when that happens.
 

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For those of you not familiar with the Rich Rebuilds video of just 2 Tesla modules in full TRA mode, here it is:


And check out his follow-up video too:

 

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I have the SimpBMS as well, I tested it for a while and found it quite good--that's the other controller I referred to above. It's a LOT cheaper than the EVTV controller, but since I already had the EVTV setup I used it.
The EVTV solution is great but expensive. I was hoping to design a solution for more family and friends to get them into the world of battery-backed solar. In my case full AC coupling to grid-tied inverters and such. My comment was really directed at RVUser who was only doing balancing, which is NOT ADVISED. You really reallly reallly want to control the input/output if things get out of spec.
 

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No matter what controller you use, you have to get properly sized contactors to completely disconnect the battery in case of over-discharge or overcharging. You also have to protect the contactors from welding their contacts by soft-starting with a resistor bypassing one contactor. You then close the second contactor first, give the system time to come up to voltage, and then close the bypassed contactor. you also should have auxiliary contacts in the contactors so you can detect welded contacts. The wiring diagram for either the SimplBMS or the EVTV system shows how to do this. And unless you want to deal with recovering from an overvoltage situation with the battery completely disconnected and no power, you need a charge enable circuit that cuts off well before the overvoltage happens. A temperature cutoff that prevents charging when the battery temp is below 0C is also a good idea.

These batteries are as safe as other chemistries if they are operated within their specified ranges, but if you overcharge or over-discharge, they can and probably will run away. Not pretty when that happens.
In my case, I went with a solid-state precharge relay that feeds via resistor then TWO big traditional contactors for the main (overkill but as you said what if one welds itself closed). The simpBMS has that capability standard. Totally agree that this is something everyone should do. Have other logic that also talks to my charging circuits. If the BMS does not think the Tesla modules are happy and wanting to charge, nothing will be allowed to send charging power.
 

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I personally would never use any LI chemistry other than LFP or LTO for that use case,

House bank storage in or near a small mobile living space.

As the cells age and wear - much more quickly than they would with those two

the risk of something going wrong leading to thermal runaway is too high.

Boom bad. . .
 
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