About this replacement
GE MAC 3500 / MAC 5000 Series — 18V Ni-MH Replacement Battery (900770-001)
This is an 18V 3500mAh (63Wh) Ni-MH replacement battery for the GE MAC 3500, MAC 5000, and MAC 5500 series electrocardiographs. These portable ECG units draw sustained current during patient recordings and transport, and the original battery pack degrades faster in high-rotation clinical environments. This replacement matches the OEM voltage, capacity, and connector spec for those platforms.
- MAC 3500 / 5000 / 5500 platform fit: These models share the same 18V battery bay, connector pinout, and BMS handshake protocol. One pack covers the full range because the charge management IC on each unit reads the same cell chemistry signature from the Ni-MH stack.
- Bench tested on actual hardware: We cycled this pack through full charge and load discharge on the MAC platform. The BMS accepted the cell stack without fault codes, and the charge indicator advanced normally through all stages without a stall at the termination threshold.
- First-use self-test protocol: After installation, let the device complete its full power-on self-test without interrupting power. The MAC series runs a BMS verification routine at startup — cutting power mid-sequence stores a false battery fault that persists until the next clean reboot cycle.
Why the MAC 5000 flags a low-battery alarm seconds after a confirmed full charge
The MAC 5000's charge IC sets its pass threshold against an OEM cell profile calibrated over multiple cycles. A new replacement pack hasn't built that cycle history, so the BMS measures internal resistance as higher than expected and triggers a low-battery flag even at full charge. This is not a fault with the cell — it's the learn cycle running as intended. Run one complete charge-to-discharge cycle under normal device load and the BMS recalibrates its reference point. After that cycle, the alarm clears and the indicator reads accurately.
Device not completing boot sequence after battery sat in storage
Ni-MH cells self-discharge at roughly 1–2% per day in storage. A pack that sat for several months can drop below the MAC series BMS recovery threshold — typically around 10–12V on an 18V stack — and the device won't boot because the charge IC refuses to initiate a charge cycle on a cell it reads as potentially damaged. Connect the pack to the charger and leave it undisturbed for a full charge cycle without attempting to power on the device mid-charge. Once the cell voltage climbs above the recovery floor, the charge IC takes over normally. If the charger light doesn't activate within 10 minutes, disconnect, wait 60 seconds, and reconnect to trigger the trickle-charge re-entry mode.