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Fujitsu Loox 400 S26391-F2607-L50 Compatible Battery 3.7V 1250mAh

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Fits Fujitsu Loox 400, Loox 410, Loox 420, Loox N500, and nine additional models; replaces OEM part S26391-F2607-L50.
3.7V, 1250mAh lithium-ion cell delivers 4.63Wh — adequate for full-day PDA operation on moderate use.
Connector seats flush into the battery slot beneath the device back cover with a single locking tab.
Bench testing showed normal BMS voltage ramping on first charge; no protection circuit faults detected through discharge cycles.
Sync all open files and calendar data to your host computer before battery removal — the Loox 400 RAM loses power immediately when the cell disconnects, erasing unsaved application state.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

Delivery and Shipping

🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

Disclaimer

⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.7V

Amp

1250mAh

Fujitsu Loox 400 / 420 / N500 Series — 3.7V Li-ion Replacement Battery (S26391-F2607-L50)

This 3.7V, 1250mAh Li-ion battery fits the Fujitsu Loox 400, 410, 420, and N500 series PDAs and Pocket PCs. It replaces OEM part numbers S26391-F2607-L50, PL400MB, PL400MD, and PL500MB. The cell slots into the same battery bay as the original and connects to the same BMS contacts on the device board.

  • Loox 400 / 410 / 420 / N500 compatibility: These models share the same 3.7V battery rail, physical form factor, and BMS handshake protocol, which is why one cell covers the full range. The connector pinout and thermistor line are identical across all variants listed.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on a Loox-class device, confirming the BMS accepted the cell without fault flags and that charge termination triggered correctly at full capacity. Voltage under load remained stable across the discharge curve.
  • RAM data protection before swap: The Loox series holds open files, calendar entries, and application state in volatile SRAM powered directly by the main battery. Sync all data to your host PC or ActiveSync connection before pulling the battery — anything unsaved to flash will be lost the moment power drops.

Why the Loox 400 won't boot after a new battery is installed

The Loox boot sequence checks for a minimum voltage threshold before the CPU initialises. A new cell shipped in storage can arrive at 3.0V or lower — below what the device needs to complete a cold boot. If the unit shows nothing when you press the power button, the battery is not flat or faulty; it just needs a charge cycle first. Connect the device to the cradle or AC adapter and let it charge for at least 30 minutes before attempting to power on.

Battery capacity dropping faster than expected during wireless sync sessions

Wireless radio and backlight together pull significantly more current than the Loox draws in standby or basic PIM use. If the battery appears to drain quickly, the cause is almost always an active sync session running with full backlight brightness — not a cell defect. Check that auto-backlight timeout is set to 30 seconds or less in Power Settings. For scanning workflows, disabling Wi-Fi between sync windows can extend active use time without replacing hardware.

Compatible Models

Loox 400 Loox 410 Loox 420 Loox N500 Loox N520 Loox N560 Loox N560c Loox C500 Loox C550 Loox N520c Loox N560p Loox N520p Loox N560e

Replaces Part Numbers

S26391-F2607-L50 10600405394 PL400MB PL400MD S26391-K165-V562 PL500MB

Technical Specifications

Voltage3.7V
Amp Hours1250mAh
Capacity1250mAh
Rate4.63Wh
Net Weight24g /0.85 oz
Gross Weight49g /1.73 oz
Approximate Weight49g /1.73 oz
Dimension 51.54 x 38.98 x 6.64mm

Product Highlights

  • Brand: Fujitsu
  • Manufacturer: CS
  • Series: X-Longer
  • Color: Black
  • Product Type: Li-ion
  • Battery Type: Li-ion
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

My Fujitsu Loox lost all my contacts and calendar after I swapped the battery — is that normal?

Yes, and it's a known behaviour of this hardware. The Loox stores contacts, calendar data, and open application state in SRAM, which is powered directly by the main battery. The moment the old cell is removed, that memory loses power and clears. To prevent this, always perform a full ActiveSync or backup sync to your host PC before pulling the battery — data written to ROM-based storage or a storage card survives the swap.

The Loox screen stays blank after installing the new battery and pressing the power button — what's wrong?

The device requires a minimum voltage to complete its boot sequence, and a battery shipped in storage may sit at 3.0V or below — too low for the Loox to initialise. This is not a fault with the cell or the device. Connect the Loox to its AC adapter or cradle and charge it for at least 30 minutes before pressing power. At 3.5V or above, the boot sequence should proceed normally.

Wireless sync keeps dropping on my Loox 420 since fitting the new battery — what's causing it?

The Loox 420 radio is voltage-sensitive, and a partially charged cell can cause the Wi-Fi module to reset under the current draw of an active sync session. Charge the battery fully before running a wireless sync — the radio stabilises once the cell is above 3.7V resting voltage. If drops continue after a full charge cycle, check that the battery contacts on the device are clean and making firm contact with the cell terminals.

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