Boston 5300 Li-ion 24V 8P6S pack picked up at a scrapyard, repaired successfully — but no BMS; ordered one online and will test capacity when it arrives

Repairing a ¥30 scrapyard Boston Swing 5300 8P6S 24V pack: two dead cells swapped for new, each parallel group 2.9–3.5mΩ, nickel strip welded with a portable welder (the pen tips need regular cleaning of oxide), total 24V at 19.8mΩ; the salvaged BMS was water-damaged and broken so a new one is on order; two H4 bulbs pull 10.8A / 261W fine, capacity test waits for the BMS

Bought a complete Boston Swing 5300 Li-ion pack at a scrapyard for ¥30 (a single cell normally goes for ¥8–9); 5 of the series groups are fine, each parallel group about 3mΩ, one group has two dead cells; bought two replacements on Douyin and there is a complete 24V pack; it is no good without a BMS, and he happened to pick up another pack with a BMS to fit to this one. Repair: weld the nickel strip on with the welder (a bit crooked; the welder is very easy — press it down and one click, bang); push the cells into the holder in order, insert from the front, flip it over and weld the underside too, leave some room to move; very solid; the portable welder is just like a power bank; not a neat job but it works; the welding pen tips need regular cleaning, they oxidise and cause poor welds (the pen got hot); ugly after welding but very solid. This parallel group measures 3.5, the others are all 2.9–3.5; three are welded, keep welding the rest into one strip, rough but solid. Total voltage 24V, internal resistance 19.8mΩ, nearly 20. Fit the BMS: the salvaged one, opened up, was cracked and water-damaged, no good, buy another. Discharge test: multi-function tester clipped on at 25V, two H4 bulbs (yellow + green together is 24V, black-yellow / black-green is 12V, don’t mix them up or they burn out), sparks flying, discharge current 10.8A (about 261W), no problem; the voltage drops fast because one group isn’t full — charge it up and test total capacity when the BMS arrives. Comments: with hands like that, thumbs-down for not building your own welder; AC or DC resistance meter (seems to be AC); it’s got some years on it (five or six); what resistance and what C rate (single cell 22mΩ, new 13mΩ, 3–6C discharge, mine I reckon 1C is fine); what is it for (storage / backup power); this kind of welder won’t hold on these cells, goes intermittent after a while; chicken-poop welds.

Outcome

Steps6 steps · click a frame to jump

1. 
   1. ¥30 scrapyard Boston 5300; swap two dead cells; scrounge a BMS

   5 series groups fine at about 3mΩ each; one group has two dead cells, bought two on Douyin to make a complete 24V; no good without a BMS, picked up another pack with a BMS and fitted it.

   00:00 – 00:33
2. 
   2. Spot-weld the nickel strip: press and click; into the holder, flip and weld again; leave room to move

   Weld it on with the welder, a bit crooked; very easy; push the cells in in order, insert from the front, flip over and weld the underside, leave room to move; very solid; the portable unit is like a power bank; not neat, but it works.

   00:33 – 01:23
3. 
   3. Pen tips oxidise and need cleaning; the pen gets hot; ugly but solid

   The welding pen tips need regular cleaning, oxide causes poor welds; this one is scorching hot; put it back and keep welding; the pen is a little hot after finishing; ugly but very solid.

   01:27 – 01:57
4. 
   4. Resistance 2.9–3.5mΩ; keep welding into one strip

   This group is 3.5 mΩ, the others between 2.9 and 3.5; three are welded, keep welding here and here into one complete pack; forgive the roughness.

   01:57 – 02:25
5. 
   5. Total 24V at 19.8mΩ; the salvaged BMS was water-damaged and broken

   Measure 24V, resistance 19.8 mΩ, nearly 20; fit the BMS but it doesn’t work, inside it is cracked and water-damaged, buy another.

   02:25 – 02:45
6. 
   6. H4 bulbs discharge at 10.8A / 261W; capacity test waits for the BMS

   Multi-function tester clipped on at 25V; two H4 bulbs for discharge (yellow + green 24V, black-yellow / black-green 12V, don’t mix them up); sparks flying; discharging at 10 A, voltage drops fast because one group isn’t full; charge it up and test total capacity when the BMS arrives.

   02:45 – 03:17

Bill of materials

Measured on camera

Creator’s tips

• Clean the oxide off the welding pen tips regularly, otherwise the welds are poor 01:27
• Leave room to move when pushing cells into the holder; flip and weld the other side 00:55
• H4 bulbs as a discharge load: yellow + green is 24V, black-yellow / black-green is 12V, don’t mix them up 02:53
• Open up a salvaged BMS first to check for water damage 02:39

Pitfalls

• Crooked, ugly welds; the pen tips get hot 00:28
• The salvaged BMS was water-damaged and broken 02:39
• One group not fully charged makes the voltage drop fast 03:07
• Comment: this kind of welder won’t hold on these cells, goes intermittent after a while

Li-ion safety

• Mis-wiring the bulbs burns them out; high-current discharge throws sparks 02:53

Quotes

Cost & time