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Forum Index : Electronics : Making a battery bank from 18650’s
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bigmik Guru  Joined: 20/06/2011 Location: AustraliaPosts: 2914 |
Hi All, I have picked up a bunch of 18650 batteries that I would like to combine to make a pack of say 6 batteries in parallel to make a 10-12 Ahr bank of the nominal 3.7v I have some management strips that I intend to use on each battery to stop overcharge etc.. Can I just parallel the outputs from the management chips or should they be combined a bit smarter? Any ideas from the experts here? Regards MICK Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1024 |
If you intend to parallel all of them, you would treat them as one cell and use 1 cell management strip to take care of it. Cheers Aaron Off The Grid |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2914 |
Hi Aaron, Thank you for the reply, These are the protection strips I have   As you can see there are 2 nickel straps that go to the batteries one neg pole the other pos pole, one strip per battery, I intend to fit one to each battery then link all the P- pads together and the B+ pads and use this as my bank so higher capacity but still nom. 3.7v. Do you think this is the correct method? I have been using an expensive 20000mAhr battery bank but I picked up the 18650 batteries and thought I could use them and maybe increase the capacity in the process. It’s all fun and experiments.. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1024 |
Ok so everything is parallel, it may be ok, provided the components on all of the strips are identical or they might sort of, fight each other. I have never used balances before on any of the (2000 or so) 18650 cells I have in use. They will help increase the capacity of the 20000mah pack, make sure the voltage is the same when connecting them together. I agree, its all fun and experiments. I'm no expert either.  Cheers Aaron Off The Grid |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2914 |
Thanks Aaron, I will build it as I thought.. what have I got to risk? Besides burning the house down?  I will try with only 3 or 4 before I go to 8 or 10. Regards, MICK EDIT *** I just re-read your post.. Do you mean you only ever soldered the batteries in parallel, I have seen in laptop battery packs they seem to have them in pairs then series... Hmmm but 10 batteries in parallel?? I am worried that if one battery dies, heaven forbid in a short circuit, then all the other cells will try to feed that short.. and I will lose the house.. if it dies by losing capacity I think the others will try to charge it so the banks capacity will suffer to an extent but that isn’t as bad.. I will need to use thermistors like those in laptop packs to test for high temperatures.. All of the bms setups I have seen advertised have higher output voltages as they series connect them.. I suppose I could then use a switch mode down regulator.. that is not a bad thought.. Mik Edited 2020-10-18 09:49 by bigmik Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You could always fuse each individual cell. If one cell does go short circuit, it should then gracefully disconnect itself from the rest of the battery. Cheers, Tony. |
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| astroboy Newbie  Joined: 28/12/2014 Location: AustraliaPosts: 39 |
Hi Mick If its any help... Some time ago I got something like what is shown here I put eight 18650 cells that I had scavenged from laptop batteries (unprotected cells)in and it seems to work okay. Interestingly, some of these cells I gently brought up from less than one volt, let them sit and found that they had some usable capacity. It has two USB A and one USB C outputs and a "fuel gauge" operated with a button. Sorry, I can't find a link to the one I bought. I had a quick look on ebay and Banggood and have seen some that accept 10 cells and also support QC. The plastic case is quite good but the "fuel gauge" is not accurate. I've read quite a bit about LiIon batteries and have never seen mention of a cell failing short (short circuit) but maybe it happens? Regards John |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I have had two Winston cells fail dead short, and Klaus has had the same experience. The cell voltage goes completely to zero. Not 1mV not 2mV but ZERO. Fortunately these were in a series string of thirty cells, so no real harm done. Both cells swelled visibly, so there was some heat and pressure involved during the "event". I have no experience with smaller cells, but I would be afraid to connect a large number of cells directly in parallel without fusing. Cheers, Tony. |
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| astroboy Newbie Joined: 28/12/2014 Location: AustraliaPosts: 39 |
Hi Tony I'm glad to have found out about this. So the cells were short circuit after the "event" not open circuit? Regards John |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458 |
My experience learned me you need to be careful with those cells, most of the Li-ion 18650 I tested out of cordless drills (Santo, Samsung...) were not shorted but were open circuit. I think failure because of to long at a too high state of charge. I think they go shorted when pulling them to low or abuse them. My experience after testing thousands of lifepo4 18650 cells that were pulled to low is that they go short circuit most of the time. That's also the case when charged/discharge to quickly. So when combining parallel cells I recommend to use a fuse to a central busbar/ conductor. And if using paralleled groups in series, use one BMS. Because otherwise if 1 dedicated cell BMS shuts off, you will have mismatched capacity in the paralleled/series.groups. As a fuse you can use a thin wire on 1 side of the cell but be aware it can start a fire, maybe there are safer options. |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1024 |
connecting them with a fuse wire is probably good practice, some or most laptop batteries have a fuse inside the cell, I have blown an internal fuse when when charging cells that have been less than a volt, I hit them with a charger and it give them 10-15amp the rest of them survived as it was just momentary. If one cell is playing up it would be quick and easy to cut the little wire or unsoiled the tab and remove the cell. What you could do, make a fireproof box for your battery bank with a vent, then all inside can go crazy and would not have to worry about it, anything bad would be safe and contained, they did something like that for aircraft batteries. You could put in 2 temp sensor's in side the pack, just for curiosity, they should have only a small temp change during use if everything is good and treated well. Its not uncommon to have a number of 18650's in parallel, the early notebooks had 3 parallel and 4 series unlike the new ones. Powers walls have many cells in P, EV's have a large number in parallel, One set I have is 100p and the other is 80p or 90p. I have never had a cell burn in flames, however I have had 2 cells (of the 2000 or so) fail and generate heat, It was left like that in the pack for 3 months draining power from the other 80 or so cells, not enough power to blow any fuse, after removing that cell it took most of the day to run it self dead flat. (I have written this somewhere before). Have fun with it Mick, a lot is learnt by DIY experimenting. Cheers Aaron Off The Grid |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2914 |
Hi All, Thank you for your advice.. I will parallel them using a fuse of around 2A so if one does go short I can’t have 100A pumping into it.. The drain max on my pack I guess would be 2A and I would like to get 8-10hr from it. @astroboy, That looks pretty cheap for what it is. @Revlac, Yes I agree with experimenting but I didn’t want to cause some real damage because of a newbie mistake.. Thank you all. Regards, MICK Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Just rechecked a few minutes ago. Both cells, 0.00 volts and 0.00 ohms (with an ohm meter). Dead shorted. These were both 60Ah Winstons, Klaus has had the exact same thing with at least one of his 150Ah Winstons. Whatever happens, its sudden and without any warning. Cheers, Tony. |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
The way Tesla and many others are doing it is to fuse each cell, if one goes bad you don't have a fire risk.  There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458 |
Were they new and traited well? Shouldn't have happened with such expensive cells.  |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Both about 2.5 years old with full battery management and individual cell monitoring. One day they were working, next day zero volts. It's happened to me twice now, several months apart. Klaus had one of his cells do the very same thing, and he asked Trev about it when he bought a replacement. According to Klaus, Trev said he had never known it to happen. I really have no idea what happened or why. One day I plan to buy another thirty cells to double my battery capacity. The original plan was to parallel pairs of cells to do that, using the same battery management and cell monitoring system I have now. Revised plan will use two entirely separate thirty cell strings, each with its own solar controller, management, and cell monitoring. I can then run off either, or both, feeding my inverter via diodes. Its not difficult to do, I just need to double up on circuit boards I already have, and it will add a lot of redundancy. Cheers, Tony. |
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