The internal diameter of 'lay flat' shrink tube is the Circumference (which is the flat dimension multiplied by 2) minus the thickness of the tubing (multiplied by 2), divided by pi.
di = (C - thickness of tube) ÷ π
di = ((flat dimension x 2) - (thickness of tube x 2)) ÷ 3.14
example: 'lay-flat' dimension is 26mm, thickness is 2 mil (0.05mm)
di = ((26 x 2) - (0.05 x 2)) ÷ 3.14
Di = 16.5 mm ← the maximum diameter of the object it can wrap
Note: if the seller does not provide the thickness of the tube in the posted dimensions, consider that most 'lay-flat' single cell PVC battery wrap tubing is between 2 ˜ 4 mil (0.05 ~ 0.1 mm) thick. Be sure to ask the seller for this dimension before purchasing. If the seller cannot provide this dimension, it may be safer to choose a vendor who can provide all necessary specs on request.
To determine the minimum external flat dimension of 'lay flat' tubing for an application, the following equation applies:
'F' = lay-flat dimension, do is the outer diameter of the object to be wrapped.
(do x π) ÷ 2 = F
example: battery diameter is 19mm, the nominal diameter of a 18650 cell
(do x 3.14) ÷ 2 = F
F = (19 x 3.14) ÷ 2 = 29.8 mm ← the minimum 'lay flat' dimension of the tube for this application
Most tubing is manufactured in integer-based dimensions, i.e. 31mm, 38mm &c. If the dimension is provided in inches, simply multiply the inch dimension by 25.4 for the dimension in millimetres.
Most importantly, measure your batteries' diameters before shopping for shrink wrap.
The shrink ratio should also be considered in all applications. Most PVC battery wrap (and most other shrink tubing) has a minimum nominal shrink ratio of 2:1. However, some types of low-temperature battery wrap has an actual shrink ratio closer to 1.5:1. When purchasing battery wrap tubing, the 'conservative' estimation of 1.5:1 should be applied. To be on the safe side, multiply the dimension 'F' from above (the minimal 'lay-flat' dimension of tubing for the application) by 1.25.
example from above: 30mm (which is 'F' rounded up to the nearest integer) x 1.25 = 37.5mm ← rounded up to nearest integer = 38mm
This allows easy insertion of the battery into the tubing, and also allows the addition of external battery protection devices.
a 18650 cell with added protection, wrapped in 38mm x 2mil 'lay-flat' (di = 24.17mm) tube: a perfect fit
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