#Custom lithium batteries

Capacity grading, matching, welding, fitting the BMS, heat-shrink wrapping — five steps. For the welding step I used the UK1, 0.15 nickel-plated at gear 60, just right for full penetration. The one thing I most want to say: matching matters ten times more than welding. If cells from the same batch differ by more than 5mΩ in internal resistance, no amount of good welding afterwards will save it. The most-asked question in the comments, "how much per pack?", is answered too — in step 4 of the project.

Building a 21700 five-series pack with Samsung 40T cells (INR21700-40T); the welder is an AWithZ UK1, 5V (2.4A) input, 8.4V output voltage, 1380A current, Shenzhen Juequi Technology; on power-up it has adjustable mode, gear, trigger and continuous; its pen doesn’t weld the moment it touches, you also have to press a switch — he’ll show that when welding. The kit is a Dayi A3 pin-terminal one, open it, there’s nickel inside, tip it out. How the cells go: the bent part faces this way, this is the main −, so the negative goes on this side; once in, locate the main − and main +, this is positive; join the main − first, the small nickel piece goes here; unlike the previous pen that welds as soon as it touches, this one only starts when the switch is pressed, very convenient; the large nickel goes on; flip it over, weld the main + first, then put it in; this pack is now connected. Gauge: by the way the case sits, the gauge goes at the back, red to L+, black to L−; set it in; pack welded, lid on, there’s a spring that must go in or the button won’t come back up, four screws in; find an A3 tool and try it; have you learned to weld a pack now? 159 comments: where to buy brand-new genuine cells, JD / Taobao / Pinduoduo are full of fakes (3); are 5 × 4.0 reliable, EVE 40P for about ¥100 (2); how much is the welder; tabless vs Sony C6; which welder for copper shims.

Jidian Research Institute builds a battery for his own Lujun Yihao electric impact wrench (case rated 21V 6000mAh). Lay out the cells (10 purple cells, 5 columns × 2 rows); spot-weld them with the AWithZ H2 welder; place the BMS according to the cell polarity; tin the BMS, attach the leads, tin the wire ends; tin the nickel strips on the cells and solder the matching leads to each; add insulation (green tape); measure the BMS output (multimeter −19.86V); tighten the four case screws and it is done, fit it to the wrench. Comments ask what the welder is called, how much it costs, whether it is a Quanyou pin-type; ‘welding is silky smooth, clean welds’.

Jidian Yanjiusuo (Electromechanical Lab) builds a 16.8V screwdriver drill pack. The BMS negative is at the top-left, so the cells are laid out in the BMS’s order; spot-welded with an AWithZ H2 (screen reads 25°C), the excess nickel strip trimmed off, negative and positive welded; the cells are tinned; a small piece of nickel strip is welded on to make soldering the leads easier; fish paper stuck on; the BMS + and − soldered; the two leads at the bottom of the pack soldered; foam stuck on; into the case for a test, a ‘high-power power Li-ion battery, charge once every two months’ sticker applied, test fine, done. Comments ask which welder / how much the welding pen costs, whether the nickel strip is 0.1 and too thin, whether four cells in series is enough capacity; one suggests just welding on a small piece of nickel without tinning.