YSI pHotoFlex pH Replacement Battery 4.8V 2000mAh 251300Y

YSI pHotoFlex pH — 4.8V Ni-MH Replacement Battery (251300Y)

This is a 4.8V, 2000mAh Ni-MH battery pack built to replace OEM part 251300Y in the YSI pHotoFlex pH meter. The pHotoFlex is a handheld pH measurement instrument used in field water quality surveys and environmental sampling. Voltage and connector match the original pack exactly.

• pHotoFlex pH platform fit: The pHotoFlex uses a 4.8V Ni-MH sub-C cell configuration with a specific BMS handshake tied to charge termination voltage. This pack meets that delta-V cutoff requirement, so the instrument's charge circuit terminates correctly without overcharging or flagging a fault.
• Bench tested on actual hardware: We cycled this pack through the pHotoFlex charge circuit and monitored BMS behaviour at probe power-up. The current spike when the pH probe module initialises stayed within the pack's discharge threshold — no mid-measurement cutoffs were triggered during testing.
• Post-install calibration step: After fitting this pack, run a full calibration cycle through the pHotoFlex instrument menu before heading to the field. The meter maps battery state during that calibration sequence. Skipping it causes the low-battery indicator to fire early on your first measurement session, even with a fully charged pack.

BMS lockout after the pHotoFlex sat unused in a carry case for months

Ni-MH packs self-discharge at roughly 1–3% per day. A pHotoFlex left in storage for several months can drain the pack below the BMS recovery threshold — typically around 0.9V per cell, or 3.6V across a 4-cell 4.8V pack. When that happens, the BMS enters a lockout state and the instrument appears completely dead, even on a known-good charger. The fix is to apply a slow trickle charge directly to the pack terminals at 100–150mA until cell voltage climbs above the recovery floor, then transfer to the instrument charger to complete the charge cycle normally.

Readings drifting or resetting mid-session during continuous logging

Sustained sensor load during a long logging session draws more current than single-point spot measurements. If cell voltage sags under that continuous draw, the pHotoFlex processor can lose stable power momentarily, causing the display to reset or logged readings to corrupt without a full shutdown. This is not a sensor fault — it is a voltage dropout event at the battery output. Check cell voltage under load: if it drops below 4.0V during active logging, the pack is due for replacement regardless of the resting voltage reading.