Fish Paper vs Kapton Tape: Which Insulation Your Pack Needs
Three materials turn up in almost every battery build — fish-paper insulator rings, fish paper sheet, and Kapton tape. They get used interchangeably all the time, and they shouldn’t be. Each one solves a different problem, and substituting one for another is how packs end up shorting months after they were built.
First: where shorts actually happen
On an 18650 or 21700 cell, only the small raised button in the centre is positive. The flat shoulder ring around it — and the entire can, and the crimp bead at the edge — are negative. They’re the same piece of metal.
That means the positive end of every cell has a live positive terminal sitting a couple of millimetres from a negative surface, separated only by a thin plastic gasket and the cell wrapper. Push a nickel strip slightly off-centre, nick the wrapper while trimming, or let a stray whisker of nickel bridge that gap, and the cell shorts through itself. It doesn’t trip the BMS — the BMS is downstream. It just gets hot.
That single geometric fact is why insulator rings exist, and why they go on the positive end only.
1. Insulator rings — for the positive end of every cell
An adhesive fish-paper ring covers the shoulder and crimp bead while leaving the positive button exposed. It gives your weld a clean target and makes an off-centre strip harmless instead of catastrophic.
They are not one-size-fits-all. An 18650 ring on a 21700 cell leaves a visible crescent of bare shoulder exposed — which is precisely the surface you were trying to cover. Match the ring to the cell format.
How many you need: one per cell. A 5S2P tool pack is 10 cells, so a 200-pack covers twenty rebuilds. A 13S4P e-bike pack is 52 cells — one build eats a quarter of a 200-pack.
2. Fish paper sheet — for the barriers between things
Fish paper is vulcanised cellulose fibre. It has high dielectric strength for its thickness, it resists abrasion, and — unlike PVC — it doesn’t soften and flow when it gets warm. At 0.2 mm it’s thin enough to fold and slip into tight spaces but tough enough to stop a nickel tab that’s been vibrating against a wall for six months.
Cut it to size and use it:
- Between the cell stack and the BMS. This is the one people skip. The board has exposed solder joints on its underside sitting directly against welded nickel. One sheet here prevents the most common catastrophic failure in a DIY pack.
- Between the pack and the housing wall, especially where a screw boss or moulding rib presses in.
- Over series bridges that pass across other cell groups at a different potential.
- Under the balance-lead harness, so the wires don’t chafe on nickel edges.
3. Kapton tape — for heat, and for holding things still
Kapton is polyimide film. Its defining property is temperature: it stays stable to roughly 260 °C (500 °F), where PVC electrical tape starts softening not far above body temperature and vinyl will shrink away from a soldering iron on contact.
Use it for:
- Anywhere near a soldering iron — balance leads at the BMS, main output joints, terminal tabs.
- Holding cells in position while you weld a group, then leaving it in place.
- Masking a cell wrapper you’ve nicked, as a repair.
- Bundling and strain-relieving the balance harness once it’s routed.
What Kapton is not: a replacement for fish paper sheet. It’s excellent film but it is thin — it has almost no mechanical bulk. A nickel edge under pressure will eventually cut through it. Use Kapton where heat is the threat; use fish paper where abrasion and pressure are the threat. Plenty of builds want both in the same spot.
Quick comparison
| Material | Main job | Where it goes |
|---|---|---|
| Insulator rings | Stop positive-to-shoulder shorts | Positive end of every cell |
| Fish paper sheet | Physical barrier, abrasion resistance | Under the BMS, against housing walls, over bridges |
| Kapton tape | Heat resistance, holding parts in place | Near solder joints, balance leads, wrapper repairs |
What not to use
- PVC electrical tape anywhere inside a pack. It softens with heat, its adhesive creeps and turns gummy, and it will migrate off a cell shoulder over a summer in a hot van or garage.
- Hot glue as the only insulator. Fine as a mechanical strain relief on top of proper insulation; not a dielectric you should rely on by itself.
- Cardboard or paper rings cut by hand. Fish paper is a specific material with a specific dielectric rating. Card stock is not.
- An 18650 ring on a 21700 cell — or the reverse, where the ring overhangs the button and fouls your weld.
A sensible starting order
For a first tool-pack rebuild: a 200-pack of rings in your cell format, one roll of fish paper sheet, and the Kapton twin-pack. That covers roughly twenty 10-cell builds on rings, and the sheet and tape will outlast them.
Related Reading
- What nickel strip size do you need?
- Do you need cell spacers?
- Why every 18V brand uses the 5S2P layout
- How to rebuild a DeWalt DCB205 20V MAX pack
- How to tell whether a used cell is still good
Browse all insulation, nickel and bus materials at RenewSpark.
