The Short Answer
BCM's brush life percentage is calculated from a fixed physical range:
| Physical Brush Length | BCM % Reading |
|---|---|
| 3.0" | 100% |
| 2.25" | 50% |
| 1.5" | 0% |
The sensor tracks a 1.5" range (3.0" minus 1.5"). That means each 1% of displayed brush life corresponds to 0.015" of physical brush length.
Worked example:
if a brush measures 2.055" with calipers, that is 0.555" above the 1.5" floor. Dividing 0.555" by 0.015" gives 37%, which is what BCM should display for that brush.
Why 1.5" Is the Floor, Not 0"
A carbon brush has 3.0" of material, but only 1.5" of that is usable. The remaining 1.5" is held in reserve and is not meant to be run down further. When BCM shows 0%, the usable portion of the brush has been fully consumed, and the brush should be replaced immediately.
Running a brush past this point risks two failure modes:
- Reduced spring pressure. Past 1.5", spring pressure drops, which degrades the contact between the brush and the ring. That degraded contact can lead to selective action and arcing.
- Copper lead contact with the ring. Past 1.5", the brush's copper leads can come into direct contact with the ring surface, which can cause arcing and damage the ring.
Note that 0% is not your only warning. BCM also supports a separate, user-configurable alert threshold that triggers earlier, giving you time to plan a brush change before you reach 0%. That threshold is for planning ahead. A 0% reading itself means the brush needs to be replaced now.
Does the 3.0" / 1.5" Range Apply to Every Brush and Holder Type?
Yes. The 3.0"/1.5" range holds across every Cutsforth EASYchange brush holder. There is no per-SKU variation to account for, so the table and formula above apply universally, regardless of which EASYchange holder is installed.