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Help with some doubts. Replace cells for Li-ion battery.

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This battery is a Panasonic 25.2V 13.2Ah Li-ion battery for an electric bicycle. It is 7 or 8 years old. When new, a full charge was good for 70km and now just 20km. Buying a new battery from Panasonic costs 304 USD and sending it to a third party to have the Li-ion batteries inside costs 210 USD and 290USD. And they only guarantee 60 to 80% or the original charge in the cheaper option and 80% in the more expensive one. So it is better to spend the money on a new one or so I thought . The “cells” inside the battery are normal 18650 Li-ion batteries and the are pretty cheap, even the Sony branded ones. 20USd for a pack of 4 I think and there are some even cheaper on amazon-jp


I found a video in youtube for the exact model as my battery, the guy just tears the battery apart and doesn’t show how he changed the li-ion batteries or how he welded them together ( watch?v=yH_ZpWyQtzY ).

So my question is, why are the batteries in the center mixed with negative and positive and the side ones just positive or negative. I will just follow the same pattern to avoid any issues, just wondering WHY??!!.

Checa the before and after from that video and see how the center batteries are mixing + and - on the same side (same circuit?).

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I am also buying a cheap spot welder from amazon to weld the batteries to my DIY metal board, the original was torn apart in the process of freeing the batteries inside. Like in the image below.

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Any help, regarding the correct way to handle Li-ion batteries will be much much appreciated, by me and my wallet.

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@ecolmi the original 18650 Li-ion batteries is a 3.7V and somewhere between 1800mAh and 3500mAh (mili-amp-hours).  In order to get the voltage and the amperage needed by your bike (25.2V 13.2Ah ), the batteries are connected in series and some in parallel. It’s called series-parallel. Take a look on here for an indepth explanation. We would have to see a picture of exactly they individually are connected to do the math :-)

So, when you rebuild yours, make sure that you absolutely follow the old one for your pattern . Li-ion batteries are finicky and you would not want anything to go off in smoke. I’ve rebuild battery packs for power tools etc. and have not had any big issues. I do not have a fancy spotwelder so I use a soldering iron :-)

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Thank you very much for your kind reply. Your link was very helpful and now understand a little bit better about circuits. So Li-ion packs are made of both of them.

About how they are individually connected, do you mean under the nickel plate?, which ones are connected together in parallel / series. The 2 images in the middle are screenshots of the guy opening his battery (my exact same battery)... what else is there to know about it to make the math you talk about?.

One more thing, the third party company also offers to upgrade the battery and make it something like 15Ah, don't remember the exact number... is it possible to use higher capacity Li-ion batteries to accomplish this?.

And finally.. thanks to you, I won't be buying the spot welder thingy... I have a soldering iron and if that work like you say, then I don't need to spend more money :D

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@ecolmi ye sit is possible to push it to 15Ah but it can also put a strain on your charger and sometimes even make the battery unsafe. I'd stick with what is known to work. Yes, as in which cell connects to pos-pos and which to pos-neg and so on. You will have to really draw it out so you get the same voltage/amperage. Just replace them the same way you remove them

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@oldturkey03 I don’t know what happened but the reply i wrote yesterday doesn’t show anymore… It was about the image below being the connection from both sides of the battery. The + in the middle of the circles was just to help me copy paste and doesn’t represent the positive pole, but the green and red colors do.

So according to the image i made based on the images from the video I concluded that I need to buy 42 18650 3.6V 2200mAh batteries as this is a 7s7p configuration. Am I right in this one?….. to be honest, it is still a little bit confusing. Im also having trouble finding 3.6V 2200mAh batteries, but there are some 3.7V available locally (amazon jp)? is that too much of a diference?.. my only issue is that they are Chinese brands and the ones from Samsung and Panasonic cost WAY WAY too much per piece and ends up being almost double what the new battery would cost me. Any advice on the batteries?

EDIT: the diagram of the batteries is on top, in the original question.

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@ecolmi that looks awesome and it looks right (42 bat.). You should get away with the 3.7V. Once it is build, you need to measure the voltage anyway and if you find it should be above what it needs to be, add a resistor to it to limit the voltage. Personally, I'd go with what I can afford so for me it would be the Chinese brands.

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@oldturkey03 i have been reading about those Chinese brands from aliexpress and..... it doesn't look promising haha. You never know what you're gonna get. :S

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