Building my own battery module PART 1

[Roeldebie]

Hi Guys,

Let me first say that I am by no means an expert in the world of batteries, voltages etc. at all. Despite this I m planning on building my own battery pack for my Torqeedo motor for my dinghy.

So, I own a Torqeedo Cruise 4 motor which consumes 4000watt at full throttle, has a nominal voltage of 48v so approx 83A.

Torqeedo sells its own battery for this motor, the Torqeedo Power 48-5000 pack (5kWh). This battery pack comes at a pretty high price, $ 5.000,00. For this money you get a battery pack with a very nicely integrated BMS in the battery which can communicate with the motor. This way you have all sort of information directly on the display on the motor like remaning sailing distance and time.

If you care about this data the Torqeedo motor can of course also be fed by a regelar external battery. Lead acid was immerdiately no option for me because of the weight. I then began to investagate which battery Torqeedo uses and found out that inside the Torqeedo Power 48-5000 housing a BMW i3 120Ah module is located.

So I bought this module this week and I m now up for the tast of making this unit ready for my motor.

Allright, let s first start with some basics that I learned. Please feel free to correct me where I m not thinking in the right direction.

1.
The most important thing is the BMS system. As you can see on the pictures there already is a nice wire harness running to all 12 cells (Samsung prismatic SDI cells). The cable has a 26pin connector on the end. My goal in to connect a BMS module directly on this connector.

2.
The BMS system itself:
I want a BMS system that can
- balance all 12 cells separately
- Also the BMS system has to take care of the charging process so I don t want my cells to overload the maximum capacity (4.19V)
- The BMS system need to check the cell voltage during sailing so it does not reach 2.79V
- Temperate will not be a problem so monitoring this is not really needed

3.
The charger:
Can I just take any common 48v charger with a maximum of around 50A (fast charging) and a minimum of around 10A (slow charging, approx one night)?

4.
Housing of the battery pack:
This will be for the last step and I think the leasy difficult part but that is for later.

5.
More about the BMS system:
- Do you guys have a recommendation for a cheap but good BMS system? Is eg. the Emus MINI bms a good choice?
- I know that the BMS system has to be connected to the cable harness that runs to all cells but how will the BMS monitor the cells while charging. What I mean is: I connect the charger to the charger to the MIN and PLUS pole directly correct? How does the BMS work then? Do I have to feed it separately with 12v?
- The same questions count for "while sailing": does the BMS need a separate 12v feed? Or does the BMS gets power from the pack itself?

6.
Connection:
I will have to take care of the right connections. The PLUS and MIN pole will have to be cut at the end. I will then have to make Anderson plugs (or something like that) on the cable to run these to my motor (which also has Anderson plugs). The cable on the BMW module is thick aluminium. Will it be attach a (Anderson) plug on this?

Please note on the pictures below that there is no cable comming from the MIN pole on the package but I ordered this cable and will have it in the next few days.

One last thing: this is not a topic for letting me know that this is dangerous stuff and that I need to know what I doing and that this is not for people like me who don t know %*&# about these things. Of course I will not do anything if I cannot do it myself. I m really looking for substansive comment on my initial questions I ask in this topic.

Thanks for all input!

Gr.

 

[eee291]

Nice I3 Battery 8)

1. These Batteries should not be charged over 4.15V! I did overcharge some SDI Cells to 4.2V just for fun and after just a few cycles they started swelling!
So that means you need a Charger with a maximum charge voltage of 49.8V.
Some of the Chinese ones have a Potentiometer that will allow you to adjust the Voltage.
Like these: https://www.aliexpress.com/item/32822336523.html?spm=2114.search0104.3.18.3b51f8dcsr2HK7&ws_ab_test=searchweb0_0%2Csearchweb201602_9_10065_10068_10547_319_317_10548_10696_10084_453_10083_454_10618_10304_10307_10820_10821_537_10302_536_10059_10884_10887_321_322_10103%2Csearchweb201603_53%2CppcSwitch_0&algo_expid=8ca8c6db-bd32-4598-87c2-16187050f3f5-2&algo_pvid=8ca8c6db-bd32-4598-87c2-16187050f3f5
There are also some Server PSUs that I use that are designed to charge Batteries, these are usually more efficient (up to 96% compared to 86%)

2. Usually Ebike BMSs have the B- go through it and it's switched by Mosfets, they also take power from the balance leads.
My personal favorites are the ones with Bluetooth: https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190705062520&SearchText=Bluetooth+BMS
Read up on them on this thread: https://endless-sphere.com/forums/viewtopic.php?f=14&t=88676

 

[fechter]

That battery should work great. I hope you can find a nice waterproof housing for it. Keeping water away from the wiring and BMS is one of the most important things.

There are many BMS units out there that could work. Look for one that has a current rating of about 2x what you expect to run. Many of the cheap chinese units can work with fewer cells. For example a 14s or 13s BMS can be made to work with a 12s pack. The "smart" ones may not be so easy to reconfigure so better to find one made for the right cell count.

The primary function of the BMS is to shunt down the charge or discharge if any cell gets out of the allowable voltage range. Beyond this, balancing is nice, but with 120Ahr cells, don't expect the balance function to be very effective. If the cells are well balanced in the beginning, the weak balance function may be enough to keep them there.

Limiting the charge current and voltage is the function of the charger, not the BMS. Get a good charger that is made for lithium batteries. If you want to charge the cells to 4.1v and it's a 12s pack, then the charger gets set for 49.2V. BMS won't kick in until it gets over 4.2v.

The battery negative wire has to pass through the BMS. The positive side goes right to the load. You will have to splice the wires on the BMS connector to the plug that comes with the BMS. Be sure to only cut one wire at a time! Double check the wiring order with a voltmeter before plugging in the BMS.

How much did you get the i3 battery module for? Those look like nice cells.

 

[eee291]

https://archiwum.allegro.pl/oferta/bateria-akumulator-bmw-i3-2017r-i7189019195.html
About 5300$ for a 94Ah 32Kwh battery

IIRC the 120Ah version goes for around $7-9K .

 

[Roeldebie]

Thanks for your input guys.

I m not sure that I understand all things yet. Some reactions on your replies. Please correct my point if I thinking wrong!

1.
The BMW i3 120Ah specs say it has a voltage of 403V at 100% charging state. The i3 has 8 modules so divide it by this gives 50,375V and 4.2V per cell. That is why I thought that I can charge the cells to 4.2V (so I use their full potential). @eee291: these specs you are showing are from a 94Ah battery from BMW. This one is 398V at 100% charging state. This indeed gives a 4.15V value. So I think I might be correct with my statement of aiming for 4.2V max voltage per cel.

2.
Do you have some examples of those server PSU's you are talking about eee291?

3.
Quote from eee291:
"Usually Ebike BMSs have the B- go through it and it's switched by Mosfets, they also take power from the balance leads."
Can you explain/tell this in a more noobish way again? I just don t understand this scentence now.

4.
Thanks for the tips on the housing. The housing will be my last step. First the connections + and -, the charger and the (most important) BMS.

5.
Fetcher, you are saying to look for a BMS that has a current rating of about 2x what I expect to run. A very dumb question maybe but how do I know/calculate what I expect to run? FYI: the motor consumes 4000watt at max throttle at 48V.

6.
So the 's' in 12s or 14s stands for cells correct? So I will have to look for a 12s BMS module?

7.
@fetcher: am I correct that I do not want BMS that can only "read"? By this I mean your sentence "shut down the charge or discharge when voltage of any of the cells are out of range". And am I correct that I want a BMS that can "read" and "write"? By this I mean not only see that a cell has a wrong voltage compared to the other cells but also ACT on it (by eg. lowering the voltage of one cell).

8.
Fetcher, I don t understand your input you are giving me on the charger subject. You are saying that limiting the charge is up to the charger and not the BMS. But you also say that the BMS kicks in at 4.2V so it won t charge over this value.
I don t understand this contradiction. Maybe you can explain it again in other words.

9.
I m not yet in the "cutting" process yet . First investigation

I bought the module for just over 1750 dollar. Completely new in box with a production date of 20 juni 2019.

 

[eee291]

I didn't know that about the 120Ah version, maybe they are a different chemistry and the Cells can take 4.2V even so I'd only go for 4.15V for longevity.

This is a pretty common BMS wiring scheme.

Server PSUs:
https://www.ebay.com/itm/1PCS-NEW-EATON-APR48-ES-C19K/172171794243?hash=item28163d1743:gOwAAOSwgn5XFIwQ
https://www.ebay.com/itm/NEW-Eaton-APR48-3G-Rectifier-Module-CVS-ship-by-EXPRESS-free-ship/183061728997?hash=item2a9f544ee5:g:MQwAAOSwgQ9VhATr
https://www.ebay.at/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Delta+ESR-48&_sacat=0
https://www.ebay.com/itm/EMERSON-NETSURE-POWER-RECTIFIER-MODULE-R48-3500e-48V-3500W/273730712464?epid=1653974947&hash=item3fbb9f3390:g:2N8AAOSw68hax~3I
https://www.ebay.com/itm/Eltek-Rectifier-Power-Supply-Unit-FLATPACK2-48V-3000-HE-DCI-241119-905-New/223214628446?hash=item33f8a0ee5e:g:9KEAAOSw3zFb3GW7
These are a bit more complicated to setup but the information is out there.
The Delta is probably the easiest, since you can adjust them by the Potentiometers with a screwdriver.
The rest need soldering and or CAN commands to change the voltage.

 

[Roeldebie]

eee291, I still need to understand the basics better.

Can you take a look at this picture I draw. Is this how the connection will be running when sailing?
And if this is correct, how do I connect a 48v charger to the battery? A charger does not have a + and - connector right?

And where are P- B- and C- stand for in your picture?

 

[eee291]

It would look more like this.

And the Charger outputs + and - how else would it close the circuit.
You can simply add another connector in parallel for the Charger.
https://www.lithiumbatterypcb.com/product/14s-bluetooth-lithium-e-bike-battery-pcb-board-with-30a-constant-charge-and-discharge-current-2/

 

[Roeldebie]

So if I understand correct:

WHEN SAILING:
- The PLUS from the battery runs directly to the PLUS of the motor
- The MIN from the battery runs directly to the BMS system
- And there is a MIN from the BMS running directly to the motor

WHEN CHARGING:
- I don't use the PLUS from the battery
- I connect a charger to the BMS only

If not, please correct where I m wrong please.

 

[eee291]

You connect the charger to Battery + and the - to the BMS just like when you discharge the battery. It's the exact same two wires.

 

[Roeldebie]

Ok, but how can I connect a charger like this to the battery?

https://www.aliexpress.com/item/3282233 ... 187050f3f5

The plug is only one plug and not a MIN and PLUS

 

[eee291]

Could you try again?
The link does not work

 

[Roeldebie]

It is the first link to the 89 dollar charger you send me. My question does not involve this particular charger but all 48V chargers in general. How am I going to connect them to my battery? Because all chargers I see do not have a PLUS and MIN cable but only just one connector.

And one more thing I don t understand:
Lets say my cells have indeed a max capacity of 4.2V. I understand you that you are saying that something like 4.15V is safe to use as maximum voltage for the cells. So if I use a 49.8V charger (4.15 x 12) they will never overload right? If so, then why do we need a BMS to check when charging? Because the charger cannot load them above 4.15 right?

 

[eee291]

EV batteries are of pretty good Quality and might not ever need to be balanced, even so in case it does happen the BMS will cut-off the charger if one Cell reaches 4.2V while the rest could still be around 4.1V.

The connector consists of two pins, one for B- and the other for B+


If you are still having issues understanding how to wire it up I could look into finding some practical examples, it will be easier to understand.

 

[fechter]

When all the cells within their normal voltage range, the BMS does nothing. If any cell gets out of range, the BMS turns off the load or charger to protect the cells.

If you charge to 4.1v/cell, the balancing function will never kick in. To balance, the charger has to be set to 4.2v/cell. You should do this initially to balance the pack and maybe once every few months or when you measure cells getting out of balance. Good cells will tend to stay in balance for a long time. You can manually check the balance by measuring and comparing all the individual cell voltages.

There are more advanced (expensive) BMS units that can balance at a lower starting point but with 120Ahr cells, any balancing will be extremely slow.

Your motor load will be somewhere around 85A. I would pick a BMS with 120A or more rating. In theory, one rated for 90A would be enough, but the Chinese tend to overstate the ratings. It won't hurt if the rating is way higher than your load.

With a single port BMS like in the diagram, you charge and discharge through the same pair of wires. Unplug the motor and plug in the charger. Or install a separate charging connector to the same pair of wires. Two port BMS boards have a separate connection for the charger negative (C-). Either type will work. A two port will have less heating under heavy load.

On the BMS, B- goes to the battery negative. B+ is battery positive. P- is Pack negative that goes to the motor. On nearly all of them, the positive side goes straight through from the battery to the output and only the negative side gets switched.

 

[Roeldebie]

even so in case it does happen the BMS will cut-off the charger if one Cell reaches 4.2V while the rest could still be around 4.1V.

So if I understand correctly you are saying the the charging process will be shut down if one cell reaches 4.2V. If so, this would make no sense to me because indeed other cells could still be at a lower level at that moment and could use more charging.

The connector consists of two pins, one for B- and the other for B+

I understand but they are intertwined in 1 cable and I need 2 right?

If you are still having issues understanding how to wire it up I could look into finding some practical examples, it will be easier to understand.

If your could do so this would help me a lot to understand things better.

Thanks

 

[Roeldebie]

If you charge to 4.1v/cell, the balancing function will never kick in.

Why not? Because eg. 10 cells could be at 4.1V while 2 other at 3.9V. When the BMS does not do anything there will be a deviation.

You can manually check the balance by measuring and comparing all the individual cell voltages.

I thought about this too. Am I correct that for measuring and comparing the cells that my motor has to be in the water and in a (hard?) working mode or can you just measure this without any load?

There are more advanced (expensive) BMS units that can balance at a lower starting point but with 120Ahr cells, any balancing will be extremely slow.

Can you explain this a little bit more? Knowning my story now and my BMW unit, could you point me in a direction for a decent BMS system?

With a single port BMS like in the diagram, you charge and discharge through the same pair of wires. Unplug the motor and plug in the charger.

I was told that for this scenario you would have to connect the BMS to a computer and change things every time your are going from discharging to charging (and from charging to discharging)

Or install a separate charging connector to the same pair of wires. Two port BMS boards have a separate connection for the charger negative (C-). Either type will work. A two port will have less heating under heavy load.

Could you send me some basic drawings (if they exist) for this?

On the BMS, B- goes to the battery negative. B+ is battery positive. P- is Pack negative that goes to the motor. On nearly all of them, the positive side goes straight through from the battery to the output and only the negative side gets switched.

Ah ok, so from the battery negative there will actually run 2 cables: one to the nagative of the motor and one to the negative of the BMS?

 

[fechter]

A picture is worth 1000 words here.



The diagram only shows 4s bu the connections will be the same regardless of the cell count.

The way most of the cheap ones work is when a cell gets up to 4.2v, a resistor on the board is switched on across the cell which slowly drains it until it gets a little below 4.2v. Only the high cells will be drained down to the level of the others. If the charger is set to a lower voltage, none of the cells (normally) will reach 4.2v.

You can measure the cell voltages with the pack on the bench with no load. They should all be within about 0.01v. Closer is better but not necessary. If all the cells are close, no balancing is needed. With most good quality lithium batteries, once the pack is balanced, it will tend to stay that way for a long time. Many people charge to 4.1v or 4.15v routinely but this won't allow balancing.
Just measure all the cells every few weeks or months to get an idea for how well they are stay in balance. A good pack might only need balancing once a year, or even less if not used much. Again, with a pack your size, most boards will take forever to balance so I wouldn't even worry about that function and just manually balance when needed.

Switching from charge to discharge does not require any reprogramming with the cheap BMS boards. Not even with the fancy ones I've see. Maybe the stock BMW car one would, but my guess is that setup will not be workable for DIY.

I hate to make a specific recommendation for a BMS that I have not actually tested or used myself. And the selection seems to change very rapidly so hard to keep up. The one eee291 posted a link to looks about as good as any cheap one.

 

[Bmw330xit]

Thanks for a very interesting discussion. I am working on a similar project using the first generation I3 modules. They come with a lower cost and I am planning to use 4-6 of them in parallel. The BMS/wiring is very well covered above, but what if the battery pack is to be based on 4-6 modules in parallel? Any inputs?

 

[fechter]

Thanks for a very interesting discussion. I am working on a similar project using the first generation I3 modules. They come with a lower cost and I am planning to use 4-6 of them in parallel. The BMS/wiring is very well covered above, but what if the battery pack is to be based on 4-6 modules in parallel? Any inputs?

You would ideally want to have the cells from each module paralleled. This means run wires from each cell junction to the corresponding junction in the other modules. The parallel wires won't normally take a lot of current, so can be fairly small. Before making any connections you want to make sure the voltages of each module are very close to avoid high currents. 0.1v difference should be OK. Once the modules are all paralleled, you they will be equalized so you can use a single BMS for the entire pack.

 

[Bmw330xit]

In other words all no 1 cells in parallel, all no 2cells in parallel.... makes sense.

Thanks!

 

[John in CR]

The most important thing is not a BMS. The most important thing since your intended use is on a boat is keeping the battery dry, especially if used in salt water. Automotive grade batteries maintain balance so well with conservative use that with conservative charge and discharge levels occasional checking of individual cell voltage is required, which means easy access through a waterproof hatch or cover is required, along with becoming familiar with pack voltage during use. That familiarity helps you identify a problem, while a BMS commonly masks and contributes to problems that kill a battery pack prematurely. In addition, the inherent complex nature of a BMS introduces hundreds of failure points any of which will lead directly to pack failure. That's why BMS's have been so commonly referred to as Battery Murdering Systems.

I an application such as yours with discharge rates less than 1C, a human BMS armed with a multimeter with a tiny bit of diligence can be far more effective and safe than any connect and forget BMS. Monitoring a 12s battery pack is a matter of simplicity. Oh and BTW it's not a 48V battery pack, but a 43V or 44V battery.

What's wrong with your old $5,000 battery anyway? If it's actually dead, then it's quite likely that the BMS killed it. Had you been the BMS for those years it's quite likely that you'd now be adding more capacity to help support the old battery that is well worn but still useful.

How is your pack used?...1 or more cycles daily...or more occasional use? What is your charge routine? That is of major importance to battery life?

 

[Jerry5984]

How is the project going? Where did you buy your battery from btw?

 

[SirWilliam]

I have a 14S/6P Battery with Panasonic 3,400mA cells
I used the Verzund system and I have it unwraped in a watertight container which works well for me
I used a Daly BMS with only a B- and P-
Generally my cells are close voltagewise but would it be acceptable to use my 8.4 Volt small charger to bring up a pair of cells.
Would attaching this power do damage to the rest of the battery.
Regards
Bill C

 

[fechter]

Should be no problem. Both cells will get changed so hopefully both need it. You can always use a resistor to drain one cell.