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TETRA Cassidian THR9 Replacement Battery BLN-5i 3.7V 5200mAh

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From $42.99 USD $52.99
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Fits Cassidian THR9 and compatible Vertex models; replaces OEM BLN-5i, BLN-6, and BLN-Ex-2 U batteries.
3.7V and 5200mAh capacity sustains full transmit power output across extended shift work without voltage sag.
Connector seats vertically into THR9 radio slot with locking tab engagement; no adapter required.
We bench-tested this pack with sustained PTT cycles; BMS accepted charger handshake on first insertion and held stable voltage under RF load.
On first insertion into a THR9 dock that shows fault LED, remove the battery, wipe the gold contact strip with a dry cloth, and reseat firmly.
Delivery time

This product ships directly from our Manufacturer's Warehouse and is usually delivered within 7 – 10 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Fresh Battery, Worth the Wait

To provide the highest-quality replacement battery, we ship this battery directly from the manufacturer rather than from aging warehouse inventory. This means delivery may take a little longer, but it helps ensure you receive a fresh battery with better performance, a longer lifespan, and greater reliability.

Estimated delivery: 7–10 business days
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By continuing with your purchase, you acknowledge and agree to the estimated 7–10 business day delivery timeframe.
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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

5200mAh

TETRA Cassidian THR9 — 3.7V Li-ion Replacement Battery (BLN-5i)

This 3.7V, 5200mAh Li-ion battery replaces the OEM BLN-5i, BLN-6, and BLN-Ex-2 U packs used in the Cassidian THR9 TETRA portable radio. It also fits the HT9588AA and HR8509AA variants. All specs match the original: same voltage rail, same cell chemistry, same connector and BMS handshake profile.

  • THR9 platform compatibility: The THR9, HT9588AA, and HR8509AA share the same battery bay, locking tab geometry, and BMS communication protocol — one pack covers all three. Swapping between variants needs no adapters or firmware changes.
  • Bench tested on actual hardware: We cycled this pack through repeated PTT transmit bursts on the THR9 platform. The BMS held stable under the transmit current spike and did not trigger an overcurrent cutoff during sustained RF output.
  • Contact strip initialisation on first dock insertion: If the charger shows a fault LED on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The THR9 charger requires a clean contact cycle to complete the BMS handshake before charging begins.

Why the THR9 drops to reduced TX power mid-shift on a fresh pack

The THR9 monitors battery voltage in real time and steps down transmit power when voltage sags below its RF output threshold. A new cell shipped at storage voltage — typically around 3.6V — can trigger this behaviour under sustained high-power transmission before the first full charge cycle. The radio is not faulty and the pack is not defective. Run one full charge cycle before the first operational shift and the voltage sag under load will stay within the radio's acceptable TX power window.

Bar indicator showing one fewer bar than expected after inserting a new BLN-5i

The THR9 uses a voltage-threshold bar indicator — each bar corresponds to a fixed voltage band. New cells leave the warehouse at partial storage charge, not full charge, so the indicator correctly reads one or two bars low on first insertion. This is not a capacity defect. Seat the pack in the OEM charger and run a complete charge cycle until the dock shows full. After that first cycle the indicator will read the full charge state accurately.

Compatible Models

CASSIDIAN THR9 HT9588AA HR8509AA

Replaces Part Numbers

BLN-5i BLN-6 BLN-Ex-2 U

Technical Specifications

Voltage3.7V
Amp Hours5200mAh
Capacity5200mAh
Rate19.24Wh
Net Weight128.4g /4.53 oz
Gross Weight198.4g /7.00 oz
Approximate Weight198.4g /7.00 oz
Dimension 97.90 x 53.58 x 23.60mm

Product Highlights

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

Frequently Asked Questions

The THR9 cuts out completely the instant I press PTT — new battery, happened straight out of the box. What's going on?

This is a BMS overcurrent trip caused by the transmit current spike when PTT is pressed. If the pack shipped at low storage voltage, the BMS can interpret the sudden load as an overcurrent event and disconnect the cell to protect it. Seat the battery in the charger, complete one full charge cycle, and the BMS will reset and accept the transmit load normally. After a full charge the open-circuit voltage should read at or above 4.1V before you test PTT again.

The charger dock fault LED never clears — just keeps blinking no matter how long I leave the THR9 battery in it. How do I fix this?

A persistent fault LED usually means the pack arrived below the dock's acceptance voltage threshold — the charger won't begin a charge cycle on a cell it reads as critically low. Remove the pack, leave it at room temperature for 15 minutes, wipe the gold contact strip with a dry cloth, and reseat it firmly. If the dock still faults, check that the contact pins inside the dock are not bent or corroded — a single intermittent pin breaks the BMS handshake and prevents charge acceptance entirely.

My THR9 battery was sitting unused for about four months and now the radio won't power on at all. Is the pack dead?

Li-ion cells self-discharge during storage, and four months is enough for the voltage to drop below the radio's minimum power-on threshold — or below the BMS recovery floor. The pack is not necessarily dead. Place it in the OEM charger and watch for any LED activity in the first five minutes; most BMS circuits will attempt a low-current recovery charge if the cell is above approximately 2.5V per cell. If the charger shows no LED response at all, leave it seated for 30 minutes before concluding the cell has dropped below the BMS recovery threshold.

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