Can you add-on 18650 cells to existing pack?

[nukezero]

I am wondering if you build a 48v 10ah 18650 battery pack 2 months ago, fresh, brand new cells,...and say after 30 cycles, you decide you want more Ah. Can you purchase the same exact battery brand, type, model, (In this case the Samsung INR-18650 25R), and then add on to your existing pack by double parallel-ing it to get 48V 20Ah. Are there any concerns/adverse effects that should be carefully noted?

 

[dogman dan]

Yes. build another pack and parallel it at the main wires. If using a bms, use some relatively beefy wire to connect the two packs parallel at cell group level. Then attach the balance wires to those wires, or leave them attached to the original connection.

The idea between slightly bigger wire at cell level than normal balance wire, is mostly for durability. Don't want a tiny wire to break a few strands and disconnect.

If running no bms, no particular need to do more than connect at the main discharge wires. Just balance the packs individually. I do this with lipo, because that way I can quickly spot a runt cell before it causes damage to good cells.

 

[nukezero]

The problem is that the Signalabs BMS 13s that I'm using is using very tiny 26gauge wires. They're so thin, I can't imagine it pulling much more than 500mA. The problem is the JST socket connector for the balance wires is only designed for these thin gauge wires, and not 18 or 16 gauge wires. So even if I upgrade those wires, they can't be inserted into this jst connector. So it looks like I may need a new BMS anyways?

Secondly, the vendor is telling me that I shouldn't mix/add-on fresh battery cells with my battery cells, even though they are the same model. They tell me it's unsafe. Hrrm... but a lot of you guys seem to use recycled laptop cells and rebuild a pack. Truly, the battery pack hasn't been cycled that much in my opinion.

 

[riba2233]

It is not safe if you connect them in series, but no problems in parallel.

 

[nukezero]

Currently they are tab-welded in 4P configurations and I have soldered them into 13s x 4P bricks.

What I want to do now is to break them apart into single pieces, but still keep them in the same parallel groups and mark them. My plan is to insert them into individual cell spacers, then re-solder the tab-welds back. Then attach new tabs along each parallel groups to the new battery cells, thereby giving me 8P.

My only concern is that, I'm not 100% positive I will get the voltage of two 4P packs right, so one may be slightly more or less millivolts than the other.. I Suppose this won't be a big problem and they should equalize each other?

 

[cal3thousand]

It is not safe if you connect them in series, but no problems in parallel.

That's right.

But I bet his issue is more voltage and not just more in parallel. More in parallel for an 18650 were the cells were paralleled first will most likely require reorganization and rewelding.

 

[nukezero]

It is not safe if you connect them in series, but no problems in parallel.

That's right.

But I bet his issue is more voltage and not just more in parallel. More in parallel for an 18650 were the cells were paralleled first will most likely require reorganization and rewelding.

Why is it not safe to stack new cells to used cells in series? is it because the IR goes up and changes differently?

Wouldn't the same be said if you were to increase the cells in parallel? which is essentially what I want to do, trying to increase just the P but maintain the S.

 

[riba2233]

Because they lose capacity, and only the cells with SAME capacity should go in series.

 

[nukezero]

Because they lose capacity, and only the cells with SAME capacity should go in series.

But if they are the same capacity cells, in the same configuration, 4P, then would it still matter? For example, I go from 13S4P to 16S4P. The 4P is still 4 cells @ 2.5Ah capacity = 10Ah capacity for each 4P.

 

[nukezero]

By the way, this is what I want to do to increase the Ah capacity. Also, will it be a BIG problem if, during the joining of the two 4Ps, that there is a potential spark or damage because the two 4P packs are not equalized exactly to the same voltage? What is the old 4P cell group is at 3.8V and the new 4P cell group is at 3.64V ? What would happen? Would the 3.8V old 4P pack try to rush current in and charge up the 3.64V 4P cell group instantaneously?

 

[cal3thousand]

It is not safe if you connect them in series, but no problems in parallel.

That's right.

But I bet his issue is more voltage and not just more in parallel. More in parallel for an 18650 were the cells were paralleled first will most likely require reorganization and rewelding.

Why is it not safe to stack new cells to used cells in series? is it because the IR goes up and changes differently?

Wouldn't the same be said if you were to increase the cells in parallel? which is essentially what I want to do, trying to increase just the P but maintain the S.

The IR being different will have the cells at different voltages and possibly imbalance them.

But since you are going to increase in parallel, you're fine. Different IRs in parallel mean each one will put out and pull in a slightly different current.

 

[cal3thousand]

By the way, this is what I want to do to increase the Ah capacity. Also, will it be a BIG problem if, during the joining of the two 4Ps, that there is a potential spark or damage because the two 4P packs are not equalized exactly to the same voltage? What is the old 4P cell group is at 3.8V and the new 4P cell group is at 3.64V ? What would happen? Would the 3.8V old 4P pack try to rush current in and charge up the 3.64V 4P cell group instantaneously?

If they are at different voltages, charge will flow within each paralleled bank until they are equal (within each respective bank). So, essentially yes if you charge them separately. Put them at near voltages before assembling them. It's not like 3.8V is that far from 3.64V that a large spark would occur. The potential difference between those 2 states is only ~.16V

 

[nukezero]

By the way, this is what I want to do to increase the Ah capacity. Also, will it be a BIG problem if, during the joining of the two 4Ps, that there is a potential spark or damage because the two 4P packs are not equalized exactly to the same voltage? What is the old 4P cell group is at 3.8V and the new 4P cell group is at 3.64V ? What would happen? Would the 3.8V old 4P pack try to rush current in and charge up the 3.64V 4P cell group instantaneously?

If they are at different voltages, charge will flow within each paralleled bank until they are equal. So, essentially yes if you charge them separately. Put them at near voltages before assembling them. It's not like 3.8V is that far from 3.64V that a large spark would occur. The potential difference between those 2 states is only ~.16V

alrighty then!

time to tear down this old pack and build a new pack up with double the capacity. I'm tired of the controller/bms shutting me down after mile 14 marker @ 32mph. Where I live, we got a lot of flat, paved, straightaways. I reckon now I can do 28 mile range @ 32 mph. Unfortunately, this means I'll have to ditch my frame mounted case design and go with something like em3ev's bag.

 

[nukezero]

I don't think it was answered above but again, how do I increase the gauge of the BMS balance wires if the jst-xh connectors are not designed to go bigger? They are currently like 24-26 gauge thin wires. Increasing them to 16 gauge would be great but they won't be compatible with the existing jst-hx connector on the BMS board. I haven't seen a BMS board that takes larger gauge wires, does Em3ev sell them?

Does that mean I need to buy a new BMS?

 

[cal3thousand]

I don't think it was answered above but again, how do I increase the gauge of the BMS balance wires if the jst-xh connectors are not designed to go bigger? They are currently like 24-26 gauge thin wires. Increasing them to 16 gauge would be great but they won't be compatible with the existing jst-hx connector on the BMS board. I haven't seen a BMS board that takes larger gauge wires, does Em3ev sell them?

Does that mean I need to buy a new BMS?

One idea about that would be to make a small adapter. JST-XH on your BMS side with small wires then soldered to larger wires, keeping the small ones as short as possible, limiting the resistance.

 

[bradleyk]

depending on the cost of your bms, you could install another one

 

[tln]

I could add a BesTech BMS onto my order if you like. What discharge rating would you want?

I'm also looking to get some Samsung INR-18650 25R -- may I ask where you got your first batch?

 

[999zip999]

I think a second 48v10ah pack with it's own ms would be the ez way to go.