About this replacement
Brady BMP21 Plus Series — 7.4V Li-ion Replacement Battery (170420)
This 7.4V, 2400mAh Li-ion battery replaces OEM part 170420 in the Brady BMP21 Plus, BMP21-Lab, M210, and M210-Lab portable label printers. It matches the original cell format and connector, and slots directly into the battery bay without modification. Capacity figures come from the product data sheet, not third-party estimates.
- BMP21 Plus, BMP21-Lab, M210, M210-Lab compatibility: All four models run on the same 7.4V two-cell Li-ion pack with an identical connector and BMS handshake protocol. Brady cross-references this under part numbers 170420, BMP21-PLUS-BATT, and G6120161 — all map to the same physical cell configuration.
- Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on a BMP21 Plus unit. The BMS accepted the new cell without fault flags, thermal head voltage held steady across the test prints, and the paper feed motor drew current within the expected range at both full and mid-charge states.
- First-install print sequence: After fitting this battery, charge it to full and run five consecutive test labels before field deployment. The BMP21 Plus thermal head calibrates its temperature profile against available voltage — skipping this step can leave the head running a slightly off baseline, which shows up as faint or uneven print density on the first real job.
Why the BMP21 Plus thermal head prints faint even on a charged battery
The BMP21 Plus thermal head draws a short high-current pulse to heat each dot element during printing. If the battery's internal resistance is elevated — common after long storage or heavy cycling — the voltage sags during that pulse and the head temperature drops below the threshold needed to fully activate the label media. The result is faint, washed-out text that gets worse toward the end of a print job. A new cell with low internal resistance restores the pulse voltage and brings the head back to its operating temperature band.
BMP21 Plus won't power on after sitting unused for several months
Li-ion cells self-discharge during storage, and if the pack drops below approximately 2.5V per cell (5.0V total), the BMS enters a deep-discharge lockout state and blocks normal startup to protect the cells. Plugging into the charger may show no charging activity for the first few minutes — this is the BMS running a recovery trickle charge before it allows a full charge cycle. Leave it connected for at least 30 minutes on the first charge attempt. If voltage does not recover above 6.0V at that point, the original cell has degraded past recovery and a replacement pack is the correct fix.