Help needed for my first DIY 13S7P build

[soulaiman]

Hi Guys,

As the tittle suggest this is my first battery build that I have ever done, so please feel free to educate me on the faults I might have made on a educative manner.

So I'm using LG B4L cells which are 2.6Ah and are currently all at 3.65v.

I've connected 7 cells in parallel and 13 in series, however I'm only getting 43.7V ?

Can you guys have a look to see if I have messed up somewhere ? I'm thinking maybe on the first parallel group I did something the opposite way...

https://drive.google.com/file/d/0B-U9UK-jkOttZzBDYlhRZFFPTjQ/view?usp=drivesdk
https://drive.google.com/file/d/0B-U9UK-jkOttSFdJVDFrb2VDdHM/view?usp=drivesdk

Second q: I've got this BMS:
Free Shipping 48V 40A BMS 13S 48V(54.6V) li-ion battery BMS PCM 40A continuous 100A peak discharge With balance function
http://s.aliexpress.com/VNbQvmmI
(from AliExpress Android)

But the wiring for the signal wires is not clear to me. So can anyone draw over my pictures to indicate where B1 until B13 have to go ?

 

[amberwolf]

I can't access your images (would be better if you attach them to your posts), but usually the cell monitoring wires (Bxx) go to the individual cell group positives. Usually they will go lowest number to most negative group first, moving up toward the positive end of the pack.

So I'm using LG B4L cells which are 2.6Ah and are currently all at 3.65v.

I've connected 7 cells in parallel and 13 in series, however I'm only getting 43.7V ?

Keep your black voltmeter lead on the most negative strip of the whole pack.

Move the red voltmeter lead to each successive positive strip and note the voltage down.

It should increase by 3.65v on each one, if the cell groups are each charged to 3.65v.

Wherever it does not, you have a problem.

 

[soulaiman]

Hi amberwolf,

Sorry for the very late reply but I had put the project on hold for a couple of months due to more important things in life.

Anyway, I figured the BMS part out and it is, as you said, very easy. Just the first one to the negative of the first set and de rest to the positive of each set.

However I still don't know why I get only 44v... The voltage goes up 3.7 volt but stops at the last group at 44v. It's like I'm missing an entire series group ?!

Also, when connecting to the BMS I only got 40 Volta out of the discharge connector, and the BMS got warm with a burning plastic smell. This really scares me and I disconnected the discharge wires.

I'm trying to find my fault so that I can learn. I don't mind making mistakes, but they should be useful ones.

Here's a pic attached of both sides.

 

[DanGT86]

I see it. Your main discharge leads appear soldered to the wrong end of the cells. The main positive and negative wires should be on the 2 furthest ends of the pack or the locations with only 1 nickel strip. Any time you have a main discharge lead soldered to a location where positive and negative cell ends are bridged then you are effectively ignoring a whole cell group because the electricity does not need to flow through that cell. You must create a path from the most negative location on the pack to the most positive location.

What you have built is a really nice 11 series pack. 3.65*11=40.15v

Does that make sense?

 

[d8veh]

That's it in principle, but something else must be wrong.

The negative pack power wire will always be fixed to a row of cells that are not connected to any others on that end, and the positive pack power wire will also always be soldered to a row that isn't connected to another row that end.

Both of your power wires are connected to rows that are 100% connected to the next row. At the other end of the pack, you can see the unconnected row in both photos, which is where they should be soldered. Therefore I'd say that you have both power wires connected to the wrong end of the pack, but they are on the correct side.

You might have to reconfigure your sense wires too. Without seeing how they're connected, it's not possible to say.

If they're wired correctly and it's only the power wires in the wrong place, there's a fair chance that you wiped out your BMS.

 

[soulaiman]

Thanks guys for the information. I will have a look at the positive and negative wires later.

So just to recap: the two positives (P+ & C+) are on the correct side but should be moved to the left side of the picture ?

And then the negative on the other side should be moved to where now the positives are (I mean on the opposite side of course but to that set of cells).

Now this still doesn't explain why even measuring directly on the nickel I get 44v instead of 48. Have I done something wrong in connecting ?

 

[DanGT86]

You can verify with a volt meter but assuming the cell ends with the white ring are + and the large grey end are neg then this is how it should be connected.

If you volt meter between the 2 single nickel strips there is no way you will not get 13 X cell voltage. In this case that should be 13*3.65 or 47.45v total. The only way you could be getting anything other than full pack voltage is if a whole string of cells were dead. That would be highly unlikely on the verge of impossible. You can check for this by using a meter to measure voltage between the 2 ends of every group individually.

The BMS wires are just like the volt meter you use to check each cell group. So the black wire should go on the most negative location. It will be on the same single nickel strip as the main NEG discharge lead. The red wire next to it in the connector goes on the positive end of cell number 1. There should measure 3.65 (one cell voltage) between these 2 points. The next red wire goes on the positive end of Cell 2 and should read 2x cell voltage or 7.3v (2 x 3.65v). Follow this pattern until the last red bms wire is sharing the same location with the main discharge positive lead. Measuring between any 2 ADJACENT BMS wires will give you one cell voltage if its done correctly.

Unfortunately the BMS is probably dead. They are very sensitive to being hooked up wrong. I would connect the main discharge leads as we suggested and volt meter around until you understand exactly how it works. Once its clear to you then try to hook up a BMS according to the instructions that come with it.

You have already done the hard part by soldering them all together.

 

[soulaiman]

Dan, Thank you for the clear explanation. I was worried that I made a mistake when putting my parellel groups in series and that's why I was confused.

I was just measuring wrong, like a real professional noob
I was measuring the last positive cell to the last negative one on the SAME side. When I got my leads on the most negative and most positive of the WHOLE pack it measured 47.3v.

Yeah probably all the BMS wires are the opposite way around (13+ is connected to 1+ and so on). I have and EM3EV bat pack in order but will buy a new BMS as they are cheap.

Can it hurt to connect the one I have now safety wise to test ?

So to recap: where I now have the positive wires should be the negative on the negative side of that cell group, because that is the first group of cells and NOT the last, as I thought.

 

[DanGT86]

So to recap: where I now have the positive wires should be the negative on the negative side of that cell group, because that is the first group of cells and NOT the last, as I thought.

That sounds correct. To be sure hook up the main leads the same way you did the volt meter. If you have the pos leads of the meter in the correct locations the voltage will read positive on the meter. If you have them backwards then it will read the correct number but negative 47.3. What you get on the meter is what you will get in the real world.

This is probably a valuable lesson for you to see how the positive end of one cell is the negative of the next cell. Voltage is potential difference between points in the circuit so each battery only "feels" its 3.xx volt contribution. It can be hard to visualize it this way when you are first learning. Intuitively it would feel like each cell is experiencing the total voltage of the whole pack but in actuality it is not. Electricity is strange like that. Its this same reason that BMS's are so easy to damage. Each balance lead circuit is only designed to handle a small balancing current at approximately the voltage of 1 cell per wire.

The BMS is almost certainly dead but probably not unsafe to hook up and try it. I wouldn't do it next to your pile of oily rags or anywhere you cant afford a fire but that's a pretty good safety rule for messing with batteries in general. I usually hook up my last connection on a new project with some leather gloves on or with pliers just in case. Lithium batteries can unleash an incredible amount of current in a dead short. Search KFF or Kentucky Fried Finger if you aren't familiar with the term.

 

[soulaiman]

Thank you. On further inspection I will need to order a new BMS as it smells kinda funny, will not risk it.

Also I don't want to resolder All the sense leads with extension cables and stuff.

The funny thing is that somehow I had first soldered the uneven side of the cells the correct way to the sense wires, and somehow then decided that that was wrong and did it the opposite way.

 

[d8veh]

Double check that the sense leads count up from ground. Before you connect the multi-pin connector, always check that the voltages on the pins from B1 to B13 (or whatever) go up in incremental steps of approx 4v.