18650 Battery Pack build

Good afternoon,
I have been scouring the net for hours trying to piece together and determine how to build my battery pack/bank. This isn't for an EV but this is the only place I've found with a forum that seems promising.

I am building a bluetooth speaker which can accept a power supply up to 25v. I want this to be portable and wireless so I can take it where ever, which is why I wan't a battery bank and not hardwired to an outlet.

I have 17 ICR18650 batteries in good condition and want to make a chargeable battery bank to power the Bluetooth amplifier https://www.amazon.com/gp/product/B01BTJZFY6/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1.

I have 8 Samsung ICR18659 26F and 9 Samsung ICR18650 30A batteries. I believe using these in a 7S2P configuration will get me where I need to be. I'd like to get as close to 25V as possible while having a high mAh. I've also found this PCB which looks to be what I need.https://www.amazon.com/Balancing-Li...-32&keywords=Battery+Protection+BMS+PCB+Board. I've also found this one that could be a possibility http://www.ebay.com/itm/7S-Li-ion-L...Board-PCB-25-2V-25-9V-29-4V-BMS-/321128915784.

Now the question is how do I put all this together? I would assume I should use 8 30A and 6 26F batteries to make each cell the same and not 9 30A and 5 26F. Or does that matter? And how would I connect the batteries to the PCB?

Lastly, is there a way to connect another circuit board or something that would allow me to use standard power adapter to charge this pack? So many questions. I apologize ahead of time for not being super knowledgeable about this but this is why I'm here. I don't want to burn my house down!

Please help! Thank you in advance.

If using two different types of cells in a 2p setup, then you want only one kind in each series string, then parallel those two strings.

For actual pack build info, including BMS and charging options, I'd recommend looking thru the many 18650 pack build threads, as there are a lot of ways to do it, and each has it's good and bad points. Once you see how they did it, and how it turned out for each, then you can decide which way to do it for yours.

It's a lot of reading, but it is also a lot of information that will help you understand the process and reasons better.

Because of the way people choose to word things, there's no single search that will find just the pack builds, but this one
https://endless-sphere.com/forums/search.php?keywords=18650&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search
is most likely to find most of them, even though a lot of other less relevant threads will also come up (but most can be dismissed based on titles without reading any of the thread, if you don't need that info).

Thanks amberwolf. I will take a look and see what I can find.

I was thinking of using a config like this but with one more in series. So in that case would this configuration work? . Purple would be the 30A 18650's and the pinks would be the 26F 18650's. Or did I completely misunderstand?

I disagree with amberwolf for once, two strings without cell level connectivity will lead to odd balance issues due to minor cell differences.

The second diagram you've posted is NOT how to do it, this will result in large imbalances over time due to the differing capacities of the two types of cells you have.

Your upper voltage limit is 25v, this means you cannot use more than 6s in series for a total of 6x4.2=25.2v. Each cell grouping needs to made up of an identical mix of cells, given your numbers the maximum you could assemble that meets your voltage limit would be 6s2p with one of each type of cell in each group. Pair them up, then weld in series.

For charging I would recommend terminating the pack with a standard type of connector used in the RC world like an XT60. Wire in a 6s balance connector and then use an RC charger to ensure it's safely charged and balanced.

Do not solder to cells.

Thank you Ohbse. I felt the second wasn't right. Glad I asked.

So the first diagram will work with one cell type of each (30A and 26F) correct? Do you recommend a PCB be used? or is using the XT60 and Balance connector replacing this. Do you have any recommendation for a particular balance connector? And lastly you state to weld, not solder. What is the benefit/reasoning here?

Soldering damages the cells, importantly it damages them varied amounts, this leads to uneven aging and pronounced balancing issues in particular on smaller packs like you're talking about. Spot welded pure nickel is the correct way to terminate

Ohbse said:

I disagree with amberwolf for once, two strings without cell level connectivity will lead to odd balance issues due to minor cell differences.

Click to expand...

I didn't say not to connect them between cells, only to not use different cells in the same string but to use each cell type in it's own series string.

The rest of the pack construction I left up to the OP to read about to determine the best way for his uses to do it (because there's a lot of discussion on which ways to use, and I can see the merits of each, but havent' tested any of them other than the "standard" way commercial packs are often connected up for BMS-monitored use..