Most heated clothing is bought for how it looks. A base layer is bought for what it does to the outfit on top of it. If the pouch shows through a fitted shirt, or the cable ridge is visible across the back, the garment fails regardless of how well it heats.
Bulk is the specification that decides the product
Buyers comparing heated base layers tend to mention battery size before anything else, and the complaint is almost always the same: the pack is too bulky. That single constraint drives the whole design, because a base layer is worn under a garment that already has its own fit allowance. Every millimetre added at the lower back is a millimetre taken out of the shirt above it.
The practical answer is a flat pouch rather than a box, positioned at the lumbar area where the body's own curve hides it, and dimensioned around the pack you intend to specify rather than around a generic pocket. This is worth stating as a dimension in the tech pack, not leaving to the pattern room.
Position matters more than size
A pack at the small of the back disappears under a jacket and stays comfortable when the wearer sits. A pack at the hip catches on waistbands and rides up. A pack on the chest is visible through any fitted layer. The lumbar position also keeps the cable run short, which is the second largest contributor to visible bulk.
Category listings describe heated base layers as heated long underwear and place the heat across the back and core. That is the right instinct: the core is where a base layer can make a measurable difference, and the extremities are better served by gloves and socks.
Zone placement across the back and core
Two or three concealed zones across the upper back and the chest cover the area a wearer notices first, and they keep the element away from the shoulder blades where a backpack strap would press. Zone count is worth publishing, because buyers use it to compare products, but the more useful line in a tech pack is the zone map: where each panel starts and ends, and which seam it follows.
The knit has to carry the element
A base layer is a fine-gauge stretch knit, and stretch is the enemy of a bonded element. The carrier has to move with the fabric without delaminating, which means the bonding method and the knit's recovery behaviour matter as much as the element itself. A garment that passes a static test can still fail after twenty wear-and-wash cycles if the knit has relaxed around the panel.
Cable routing along a seam
A cable crossing the spine is a comfort fault before it is a cosmetic one. Routing the lead along the side seam or the shoulder seam hides it and protects it from the abrasion of a backpack or a chair back. The connector at the pouch end should have a moulded boot, because that is the point that flexes every time the wearer bends.
Washing a stretch knit with a bonded element
Base layers are washed more often than outer layers, and they are washed with the pack removed. The care instruction has to describe one sequence: remove the pack, close the pocket zip, wash on the stated programme, dry flat or on a low setting. High tumble heat relaxes the knit and stresses the bonding, and it is the most common cause of a panel that starts to lift at the corner.
What to write into the tech pack
Fix the pouch dimensions and position, the pack format you intend to build around, the zone map with start and end points, the bonding method and knit recovery target, the cable route and connector type, and the wash and dry programme with its temperature limits. Add a wear-and-wash cycle check to the approval process so the sample is judged after repeated cycles rather than once.
For a garment worn against the skin, the honest measure of quality is that the customer forgets it is there. Bulk, seams and cable routing decide that, not the heat output figure on the label.