How to connect a BMS to 48 Nissan Leaf Modules in a 24V configuration

[Castro975]

Hi guys,

I recently bought a Titan solar generator and wanted to use Nissan Leaf battery modules to expand the capacity of my system, but ran into a problem due to ignorance on how to install a BMS. If the modules only had 2 terminals it would be a cakewalk, however there are 3 terminals because each Nissan battery module has 4 cells. 2 in parallel and 2 in series, which confuses things. When I saw the few videos on YouTube with a Nissan Leaf example, the general concusses was to add 2 red wires per module since they were already running in series. So since the Titan is a 24v system I needed a 24v configuration, I originally thought I would need a 6s 24v BMS, but after seeing people on YT only configure a BMS in 7s or 8s configurations I thought I would copy that (7s configuration) since I have no clue what I am doing or how they came up with wiring the way they did. However a 7s configurations come with some negatives, I would have to buy 6 7s 48v BMS's and a 48v to 24v Buck converter which has 95% efficiency. All of this would end up costing me $700+ dollars that I do not have, so if someone has a way to connect all 48 modules in a 24v system with 1 or 2 BMS's please share! Your wisdom would be greatly appreciated.
P.s. Can someone please tell me why when he attaches the 8th red wire on module 4 he moves the 6th red wire from module 3's positive terminal to the negative terminal of module 4 @ 7:25?

 

[GXMnow]

Nissan Leaf Cells are a cobalt chemistry, so the cell voltage is about 3.6 volts nominal. But they are in series pairs for 7.2 volts per module. If you use 3 modules, you get just 21.6 volts, full charge will hit 4.15 per cell or 24.9 for the pack. This is on the low side for most 24 volt inverters. Fully discharge can go down to 3.0 volts per cell, or just 18 volts. If you go to a 4th Leaf module, you end up at 8 cells in series. Nominal voltage is then 28.8 with full charge going up to 33.2 which is getting a bit too high for most 24 volt systems. Fully discharged goes down to 24 volts, which is okay, but your inverter will need adjustable voltages to work, as the common trip off for a 24 volt inverter is just 20 volts, that is too low for 8 cells (4 leaf modules) in series.

The most common setup for Li Cobalt cells for 24 volt is 7 cells in series, and that is a problem with the Leaf cells. You could just abandon one cell at the end of the string, but it is not good to let a cell just die. And the center terminal to get the the odd numbered cells is not meant to carry current. That is why it is smaller, it is just for the BMS to monitor and balance between the two cell groups. For these reasons, not too many people like using the Leaf cells for 24 volts. For a 48 volt system, you can use 7 module groups which then give you 14 "cells" and you get very good 48 volt levels. That is what is shown in the video you linked. 14 x 3.6 is 50.4 volts nominal, which is just a little higher than a lead acid bank. Full charge goes up to 58.1 volts, which is still below lead acid equalize charge, and fully discharged drops to 42 volts, which works well with many inverters.

The BMS sense/balance wires need to connect to the full pack negative and positive connections and also to every cell junction between. This is how it measures and balances each cell. It does not matter which end of the buss bar it goes on. The positive of the lower cell, or the negative of the next higher cell are connected with a high current plat, bar, or cable, and it should make little to no difference unless you are at a very high current on the pack. So moving that 6th wire makes no real difference. Maybe the wire routing looked better. It is just measuring the voltage of the whole copper plate. In an ideal situation, it should actually be in the center of the plate, but we are talking a few millivolts of difference at most.

As for the Titan solar generator... What are the full charge and low battery shut off levels? You may not be able to get the Leaf packs to get in the correct range using an even number of cells. Full charge must not exceed 4.2 volts on a single cell and fully discharged shut down, should not go below 3 volts per cell. The is from 6 volts to 8.4 volts on a leaf 2S2P module. And 8.2 is safer on the top side, 8.4 is an absolute limit. Some say you can pull them down to 2.6 volts, on the bottom, but that will certainly shorten the life. I almost bough 28 Leaf modules to build my 48 volt battery bank. I would have done 4 parallel groups of 7 modules in series. That would have only gotten me 220 amp hours. The Titan is most likely using LFP or LiFePo4 cells. They are just 3.2 volts nominal per cell, so then you do use 8 in series for 24 volts. Their max voltage is just 3.65 and full discharge is about 2.6 or so. So 8 cells would go from 20.8 dead to 29.2 maximum at full absorb charge. At rest, they will drop back a bit to 3.45 volts or so, or 27.6 volts for the whole 8S pack.

It is not always viable to tie together different battery chemistries.

 

[Ampster]

I am not going to watch the video but i can try to give you a high level overview. First the voltage is nominal. Since the Leaf modules are pairs you can only do 3 or 4 modules. Assuming each module voltage is 8.2 volts max, a 3 module pack would be 24.6 volts. Is that okay with the voltage of the Titan? If too low you would have to go with 4 modules which is 32.8. It is not possible to split a module because the center terminal is not designed to carry any load. If either of those voltages work we can then design a pack and plan the BMS wires.
In the meantime @GXMnow gave you a good over view about why mixing chemistry has issues.

 

[Castro975]

Nissan Leaf Cells are a cobalt chemistry, so the cell voltage is about 3.6 volts nominal. But they are in series pairs for 7.2 volts per module. If you use 3 modules, you get just 21.6 volts, full charge will hit 4.15 per cell or 24.9 for the pack. This is on the low side for most 24 volt inverters. Fully discharge can go down to 3.0 volts per cell, or just 18 volts. If you go to a 4th Leaf module, you end up at 8 cells in series. Nominal voltage is then 28.8 with full charge going up to 33.2 which is getting a bit too high for most 24 volt systems. Fully discharged goes down to 24 volts, which is okay, but your inverter will need adjustable voltages to work, as the common trip off for a 24 volt inverter is just 20 volts, that is too low for 8 cells (4 leaf modules) in series.

The most common setup for Li Cobalt cells for 24 volt is 7 cells in series, and that is a problem with the Leaf cells. You could just abandon one cell at the end of the string, but it is not good to let a cell just die. And the center terminal to get the the odd numbered cells is not meant to carry current. That is why it is smaller, it is just for the BMS to monitor and balance between the two cell groups. For these reasons, not too many people like using the Leaf cells for 24 volts. For a 48 volt system, you can use 7 module groups which then give you 14 "cells" and you get very good 48 volt levels. That is what is shown in the video you linked. 14 x 3.6 is 50.4 volts nominal, which is just a little higher than a lead acid bank. Full charge goes up to 58.1 volts, which is still below lead acid equalize charge, and fully discharged drops to 42 volts, which works well with many inverters.

The BMS sense/balance wires need to connect to the full pack negative and positive connections and also to every cell junction between. This is how it measures and balances each cell. It does not matter which end of the buss bar it goes on. The positive of the lower cell, or the negative of the next higher cell are connected with a high current plat, bar, or cable, and it should make little to no difference unless you are at a very high current on the pack. So moving that 6th wire makes no real difference. Maybe the wire routing looked better. It is just measuring the voltage of the whole copper plate. In an ideal situation, it should actually be in the center of the plate, but we are talking a few millivolts of difference at most.

As for the Titan solar generator... What are the full charge and low battery shut off levels? You may not be able to get the Leaf packs to get in the correct range using an even number of cells. Full charge must not exceed 4.2 volts on a single cell and fully discharged shut down, should not go below 3 volts per cell. The is from 6 volts to 8.4 volts on a leaf 2S2P module. And 8.2 is safer on the top side, 8.4 is an absolute limit. Some say you can pull them down to 2.6 volts, on the bottom, but that will certainly shorten the life. I almost bough 28 Leaf modules to build my 48 volt battery bank. I would have done 4 parallel groups of 7 modules in series. That would have only gotten me 220 amp hours. The Titan is most likely using LFP or LiFePo4 cells. They are just 3.2 volts nominal per cell, so then you do use 8 in series for 24 volts. Their max voltage is just 3.65 and full discharge is about 2.6 or so. So 8 cells would go from 20.8 dead to 29.2 maximum at full absorb charge. At rest, they will drop back a bit to 3.45 volts or so, or 27.6 volts for the whole 8S pack.

It is not always viable to tie together different battery chemistries.

I sent an email off to point zero to see which combination would work best for their system, but lets just say both options don't work and I use your 28 leaf module 48v configuration with a 48v to 24v buck converter. Would the Titan be able to charge up the leaf battery's through the buck converter or will power just run one way?

 

[GXMnow]

A Buck converter is a one way device. If set for the right voltage and current limit, it could take the 48 volt battery pack power and feed current into the 24 volt Titan to help keep it's battery charged up.

Where is the Titan be getting it's charge power from now?

It may be possible to use a CC CV boost type converter to use the 24 volts from the Titan to charge the 48 volt pack, but it would not be very efficient. A better option would be to charge it directly from solar panels with a 48 volt charge controller, or you could use grid power with a CC CV charger set for the 48 volt pack.

 

[Castro975]

-

 

[Castro975]

A Buck converter is a one way device. If set for the right voltage and current limit, it could take the 48 volt battery pack power and feed current into the 24 volt Titan to help keep it's battery charged up.

Where is the Titan be getting it's charge power from now?

It may be possible to use a CC CV boost type converter to use the 24 volts from the Titan to charge the 48 volt pack, but it would not be very efficient. A better option would be to charge it directly from solar panels with a 48 volt charge controller, or you could use grid power with a CC CV charger set for the 48 volt pack.

The Titan is getting its charge from a 1500w solar array through the two integrated charge controllers. I found a 1500w 30a CC CV boost type converter that has a 97% efficiency (posted below), how would I wire the buck converter and the CC CV boost type converter into the Anderson port which then connects to the Titan?

Amazon.com: Boost Module - DC-DC 10-60V to 12-97V 1500W 30A Voltage Step Up Converter Boost CC CV Power Supply Module: Home Improvement

Buy Boost Module - DC-DC 10-60V to 12-97V 1500W 30A Voltage Step Up Converter Boost CC CV Power Supply Module: Power Converters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

[GXMnow]

The one problem you will have is making it switch between charging the 48 volt battery bank, and using the 48 volt bank to then power the Titan.

The converter you linked can do the job, but keep in mind it is not isolated, and I'll bet the buck converter is not either. So it would be best to switch both the + and - leads so that you never have both converters connected at the same time. If one is a common negative, and the other is a common positive, it would basically be shorting the full 48 volts to your 24 volt system. Without having full schematics to ensure it is a safe connection, keep them completely separate.

Probably the easiest solution would be doing it on a time basis. When the time is right for good sun on the solar panels, have a relay connect the 24 volts from the Titan to the input of the boost converter. The output of the boost converter will charge the batteries. The current setting will need to be chosen to get enough charge into the large 48 volt battery without starving the 24 volt system in the Titan. Your 1,500 watt array could make about 7 KWH's of energy with decent sun. 28 Leaf battery modules can hold up to about 11 KWH's depending on their condition. Some Leaf cells I have seen for sale are only promising 70% capacity, which would only be about 8 KWH's then. Still a good sized bank. How much energy are you using from that 1,500 watt array? Do you have enough left to charge up the large 48 volt 11 KWH battery bank? If you just want the extra storage in case of a power failure, you could charge them fairly slow, like 6 amps for the 5 hours the sun is up. But then only draw 1 or 2 amps back each night to help out the Titan to run loads. After just a few days, the battery will be reaching full charge, and not taking much from the Titan at all, but if you lose sun, or grid you have the battery near full to run your loads.

When the sun starts to fall off, the timer then shuts off the charging to the 48 volt bank so that it does not just pull the energy from the Titan's battery.

When you need the energy back from the 48 volt bank, another relay can connect the 48 volt battery to a buck converter that is set to feed from the 48 volt pack into the 24 volt in the Titan. This could be another timer, or a manual switch, but make sur both can't be on at the same time.

The current you pull from the 48 volt system can be set so that it just keeps the titan battery floating, or you could even charge it. But you should not pull more than was put in that day unless you are working through a blackout. Keep some reserve and don't run it dead. You should have a BMS that will shut it down if it drains any cell too low.

 

[Castro975]

The one problem you will have is making it switch between charging the 48 volt battery bank, and using the 48 volt bank to then power the Titan.

The converter you linked can do the job, but keep in mind it is not isolated, and I'll bet the buck converter is not either. So it would be best to switch both the + and - leads so that you never have both converters connected at the same time. If one is a common negative, and the other is a common positive, it would basically be shorting the full 48 volts to your 24 volt system. Without having full schematics to ensure it is a safe connection, keep them completely separate.

Probably the easiest solution would be doing it on a time basis. When the time is right for good sun on the solar panels, have a relay connect the 24 volts from the Titan to the input of the boost converter. The output of the boost converter will charge the batteries. The current setting will need to be chosen to get enough charge into the large 48 volt battery without starving the 24 volt system in the Titan. Your 1,500 watt array could make about 7 KWH's of energy with decent sun. 28 Leaf battery modules can hold up to about 11 KWH's depending on their condition. Some Leaf cells I have seen for sale are only promising 70% capacity, which would only be about 8 KWH's then. Still a good sized bank. How much energy are you using from that 1,500 watt array? Do you have enough left to charge up the large 48 volt 11 KWH battery bank? If you just want the extra storage in case of a power failure, you could charge them fairly slow, like 6 amps for the 5 hours the sun is up. But then only draw 1 or 2 amps back each night to help out the Titan to run loads. After just a few days, the battery will be reaching full charge, and not taking much from the Titan at all, but if you lose sun, or grid you have the battery near full to run your loads.

When the sun starts to fall off, the timer then shuts off the charging to the 48 volt bank so that it does not just pull the energy from the Titan's battery.

When you need the energy back from the 48 volt bank, another relay can connect the 48 volt battery to a buck converter that is set to feed from the 48 volt pack into the 24 volt in the Titan. This could be another timer, or a manual switch, but make sur both can't be on at the same time.

The current you pull from the 48 volt system can be set so that it just keeps the titan battery floating, or you could even charge it. But you should not pull more than was put in that day unless you are working through a blackout. Keep some reserve and don't run it dead. You should have a BMS that will shut it down if it drains any cell too low.

I am planning to use this system 24/7 on a Bus used as a tiny home for the next year and don't like the idea of having a manual switch or timers to turn on/off the charging system. What if I arranged the leaf batteries to a 48v configuration and used a Buck converter to drop the voltage to 24v and used an external 48v charge controller to charge the leaf batteries directly with my 1500w solar array so power will only run into the titan expansion battery port?

 

[GXMnow]

I am planning to use this system 24/7 on a Bus used as a tiny home for the next year and don't like the idea of having a manual switch or timers to turn on/off the charging system. What if I arranged the leaf batteries to a 48v configuration and used a Buck converter to drop the voltage to 24v and used an external 48v charge controller to charge the leaf batteries directly with my 1500w solar array so power will only run into the titan expansion battery port?

That would work. But once you have your whole array just charging the new 48 volt battery bank, why not just use a 48 volt inverter from there?

 

[Ampster]

That is a simpler solution. The thing you will have to determine is what voltage the Titan needs or wants and the Amperage. Then finding a buck converter to do that. The cost of converters goes up with Amperage so the sweet spot will be finding an Amperage output that can put in a few more Amphours (including overhead and headroom) than your Titan needs to stay full. Do you have any details from Titan.
Once you get that figured out assembling a Leaf pack and wiring a BMS will seem like a piece of cake. I would wait on the decision whether it would be 6 or 7 modules until you find the correct DC to DC converter. You will need to be very specific about voltage. As you have discovered 24 and 48 volts are generic terms.
I have configured and reconfigured my Leaf pack and have plenty of extra buss bars as long as 6P.

 

[Ampster]

why not just use a 48 volt inverter from there?

That was the nagging question I had. I will let the OP respond with specifics. My guess is the investment in the Titan is already significant and there may be some comfort with the features and functionality.

 

[Castro975]

That is a simpler solution. The thing you will have to determine is what voltage the Titan needs or wants and the Amperage. Then finding a buck converter to do that. The cost of converters goes up with Amperage so the sweet spot will be finding an Amperage output that can put in a few more Amphours (including overhead and headroom) than your Titan needs to stay full. Do you have any details from Titan.
Once you get that figured out assembling a Leaf pack and wiring a BMS will seem like a piece of cake. I would wait on the decision whether it would be 6 or 7 modules until you find the correct DC to DC converter. You will need to be very specific about voltage. As you have discovered 24 and 48 volts are generic terms.
I have configured and reconfigured my Leaf pack and have plenty of extra buss bars as long as 6P.

Point zero emailed and said that it would work if I used a step down converter that could handle 150a, however the highest amp converter I was able to find was 60a. Do you know why the Titan would need 150amps?

 

[Ampster]

Do you know why the Titan would need 150amps?

No, not in a Tiny Home unless you have a small battery in the Titan and want to run everything at once for a long time. That is what I was implying earlier. Now is the time for you to get into the numbers. Do you know the size in kwhs of the Titan battery? Do you know how many kWhs you use in a day? I suspect some of those numbers are why you concluded you wanted to supplement your Titan, correct?

 

[Castro975]

No, not in a Tiny Home unless you have a small battery in the Titan and want to run everything at once for a long time. That is what I was implying earlier. Now is the time for you to get into the numbers. Do you know the size in kwhs of the Titan battery? Do you know how many kWhs you use in a day? I suspect some of those numbers are why you concluded you wanted to supplement your Titan, correct?

The titan has a 2kwh Lithium battery (not sure exactly which chemistry that's all they had in the manual), not sure exactly how much we will use in a day. I suspect 6-10kwh on a high usage day.

 

[Castro975]

No, not in a Tiny Home unless you have a small battery in the Titan and want to run everything at once for a long time. That is what I was implying earlier. Now is the time for you to get into the numbers. Do you know the size in kwhs of the Titan battery? Do you know how many kWhs you use in a day? I suspect some of those numbers are why you concluded you wanted to supplement your Titan, correct?

I am still confused about using different amperage's for the step down, is it simply that the higher the amperage is the more continuous power the Titan inverter can produce?

 

[Ampster]

It is confusing, especially when comparing Amperages at different voltages. That is why I asked the question in terms of Watts. As long as we are talking about different voltages, have you committed to the Nissan Leaf modules?

 

[Castro975]

It is confusing, especially when comparing Amperages at different voltages. That is why I asked the question in terms of Watts. As long as we are talking about different voltages, have you committed to the Nissan Leaf modules?

I have committed to the leaf modules, already disassembled it and not returnable at this point. So I'm stuck with 48 modules and I hope this 1440w step down converter has an output the Titan can handle.

https://www.amazon.com/Cllena-Converter-Voltage-Waterproof-Transformer/dp/B07TYFVB9X?th=1

 

[Ampster]

I hope this 1440w step down converter has an output the Titan can handle.

That DC to DC converter does have a good range of input voltages. It does say not to be used to charge batteries and it is not specific about the exact voltage it outputs.

• - Do you know the specifics about what the charging voltages the Titan needs or what range of voltage it can receive?
• - You have still not answered my question about the capacity of the Titan battery in kWhs and your daily consumption in kWhs?

Those numbers are critical for any further discussion or decision about a DC to DC converter. That decision may determine whether you put six or seven modules in series.

 

[GXMnow]

By the math, that converter is actually 24 volts even. 1440 watts and 60 amps out. That will likely run the inverter, but it won't charge up the battery. It comes out to just 3 volts per cell, which is a safe float level.

Allowing for efficiency, you can probably still get over 1200 watts out of the Titan inverter.