{"id":1861,"date":"2022-05-09T17:04:12","date_gmt":"2022-05-09T14:04:12","guid":{"rendered":"https:\/\/yo9eat.com\/log\/?p=1861"},"modified":"2022-05-09T17:07:14","modified_gmt":"2022-05-09T14:07:14","slug":"4s2p-power-pack-with-18650","status":"publish","type":"post","link":"https:\/\/yo9eat.com\/log\/4s2p-power-pack-with-18650\/","title":{"rendered":"4S2P power pack with 18650"},"content":{"rendered":"\n

Having a few leftover 18650 batteries (about … too many), I decided to put them to use instead of recycling them, so let’s build a power pack.<\/p>\n\n\n\n

DISCLAIMER<\/strong> don’t do this unless you know what you’re doing. I’m not responsible for anything you might do wrong.<\/strong><\/p>\n\n\n\n

<\/p>\n\n\n\n\n\n\n\n

First, had to decide the voltage. Considering I wanted to use it for portable operations and most of my stuff runs on 13.8V, having a 4S pack made the most sense (as it’s easy and cheap to drop voltage). <\/p>\n\n\n\n

18650 crash course:<\/em> each of these “batteries” is considered a 1S element. Fully charged, it puts out 4.2V; it needs to be stored at around 3.7V and can be discharged down to around 3.2V-3.15V. 18650 elements can be put in series (1S would be one battery in series, 2S would be two batteries in series, etc) and\/or parallel (2P means two batteries in parallel, 3P means three batteries in parallel and so on). Of course, one can mix series and parallel connections to achieve various configurations; I decided on an 4S2P (meaning 4 in series, 2 in parallel, total of 8 batteries). <\/p>\n\n\n\n

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The connection diagram I used (the bottom PCB is a balance charger, which I won’t use – the pack will be recharged using an external charger)<\/figcaption><\/figure>\n\n\n\n

It’s very important to keep the elements “balanced” (you don’t want them to have very different voltages because too many reasons which I won’t get into); enough to say each pack needs to have a balance cable which gives the charger a way to measure the voltage for each element individually (for 4S2P, the 3 yellow wires in the above schema + 2 other wires for the + and -). The physical layout I used is “bend in the middle and bring over the ends” \ud83d\ude42 which in real life looks like this:<\/p>\n\n\n\n

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You cannot solder on 18650 batteries (well, you can, but it sucks and you risk damaging the battery), so perfect excuse to get a spot welder as a new toy as well.<\/p>\n\n\n\n

For the pack, after considering a lot of designs and even making one parametric one in OpenSCAD, I eventually settled on this one: https:\/\/www.thingiverse.com\/thing:4628783<\/a> … and let’s get welding!<\/p>\n\n\n\n

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Top welds<\/figcaption><\/figure>\n\n\n\n
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Bottom welds<\/figcaption><\/figure>\n\n\n\n
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Bottom balance cables (that would be wires 2 and 4 of the balance cable)<\/figcaption><\/figure>\n\n\n\n
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Top balance cables (1, 3, 5 in the balance connector) and the power wires (AWG 18 fine stranded silicone, enough for 20A which this pack won’t ever do)<\/figcaption><\/figure>\n\n\n\n
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Top welds again (use flux, added one more spot weld on each 18650)<\/figcaption><\/figure>\n\n\n\n
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Preparing the XT60 (best connector!) with shrinkwrap for reinforcement<\/figcaption><\/figure>\n\n\n\n
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Nice and shiny<\/figcaption><\/figure>\n\n\n\n
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This is the top cap, 4S balance wire and power connector are out<\/figcaption><\/figure>\n\n\n\n
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Bottom view<\/figcaption><\/figure>\n\n\n\n
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Side view<\/figcaption><\/figure>\n\n\n\n
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Elastic helps holding things together<\/figcaption><\/figure>\n<\/figure>\n\n\n\n

All good, nothing blew up, let’s test.<\/p>\n\n\n\n

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Nicely balanced charging<\/figcaption><\/figure>\n\n\n\n
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Decent (not great) discharging<\/figcaption><\/figure>\n\n\n\n

From the above discharge analysis, you can see element 2 is the worst (it also has the highest internal resistance) and is the first to reach 3.15V. This is perfectly fine to use, as long as YOU STOP USING THE PACK <\/strong>when this happens (even if the other elements would have more juice available). Once done, put it back on balanced charging.<\/p>\n\n\n\n

Overall, this pack gives me about 2.2 Ah – not bad at all for something that would have went into the trash!<\/p>\n","protected":false},"excerpt":{"rendered":"

Having a few leftover 18650 batteries (about … too many), I decided to put them to use instead of recycling them, so let’s build a power pack. DISCLAIMER don’t do this unless you know what you’re doing. I’m not responsible for anything you might do wrong.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[29,13],"class_list":["post-1861","post","type-post","status-publish","format-standard","hentry","category-hardware","tag-29","tag-hardware"],"_links":{"self":[{"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/posts\/1861","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/comments?post=1861"}],"version-history":[{"count":16,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/posts\/1861\/revisions"}],"predecessor-version":[{"id":1895,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/posts\/1861\/revisions\/1895"}],"wp:attachment":[{"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/media?parent=1861"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/categories?post=1861"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/yo9eat.com\/log\/wp-json\/wp\/v2\/tags?post=1861"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}