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BLN-5i TETRA CASSIDIAN THR9 Compatible Battery 3.7V 5700mAh

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From $45.99 USD $56.99
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Fits CASSIDIAN THR9, HT9588AA, HR8509AA radios; replaces OEM BLN-5i, BLN-6, BLN-Ex-2 U batteries.
3.7V lithium-ion cell with 5700mAh capacity delivers sustained transmit power across extended field shifts without voltage sag.
Gold contact strip mates flush with dock terminal; battery seats straight into the slot with audible detent click.
We bench-tested this cell in a THR9 dock on full-charge and sustained PTT cycles; BMS accepted handshake immediately with no fault LED.
On first insertion into a THR9 charger dock, if the fault LED appears, remove the battery and wipe the contact strip with a dry cloth before reseating firmly.
Delivery time

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

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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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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.

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🔹 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.

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⚠️ 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

5700mAh

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

This is a 3.7V, 5700mAh Li-ion replacement battery for the Cassidian THR9 TETRA portable radio. It also fits the HT9588AA and HR8509AA variants. OEM part numbers covered: BLN-5i, BLN-6, and BLN-Ex-2 U.

  • THR9 platform compatibility: The THR9, HT9588AA, and HR8509AA share the same battery bay geometry, contact layout, and BMS handshake protocol. All three accept this pack without modification to the door latch or contact rail.
  • Bench tested on actual hardware: We cycled this pack through the THR9 charger dock and monitored BMS response through transmit current spikes. The protection circuit handled the sustained RF output load without tripping, and the dock progressed through its charge stages correctly.
  • First-insertion contact cycle on the THR9 dock: If the dock shows a fault LED on first insertion, remove the battery and wipe the gold contact strip with a dry cloth before reseating. The THR9 charger requires a clean contact cycle to complete the BMS handshake before it will begin charging.

Why the THR9 cuts out mid-transmission on a new BLN-5i

The THR9 draws a sharp current spike the moment PTT is pressed — transmit load is significantly higher than standby draw. If the BMS reads that spike as an overcurrent event, it trips the protection circuit and kills output mid-call. This usually happens when the cell is at storage voltage, not fully charged, and internal resistance is temporarily elevated. Charge the pack fully in the dock before first field use and the BMS will handle PTT spikes without tripping.

Bar indicator showing one fewer bar than expected on a fresh BLN-5i pack

The THR9 reads voltage thresholds to display its bar indicator — it does not use a fuel gauge chip. A new cell ships at storage voltage, typically around 3.6–3.7V, which the radio reads as a partial charge state. This is normal and not a fault with the pack. Run one full charge cycle in the dock and the indicator will reflect the actual 5700mAh capacity correctly.

Compatible Models

CASSIDIAN THR9 HT9588AA HR8509AA

Replaces Part Numbers

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

Technical Specifications

Voltage3.7V
Amp Hours5700mAh
Capacity5700mAh
Rate21.09Wh
Net Weight126g /4.44 oz
Gross Weight196g /6.91 oz
Approximate Weight196g /6.91 oz
Dimension 98.00 x 53.50 x 23.90mm

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 dock is blinking a fault LED and won't start charging the new BLN-5i — what's happening?

The dock uses a voltage-threshold handshake to accept a new pack. If the cell arrives below the dock's acceptance floor — common after extended shipping or storage — the charger flags it as a fault rather than starting the charge cycle. Remove the pack, wipe the gold contacts with a dry cloth, and reseat firmly. If the fault persists, connect the dock to power for 30 seconds with no battery, then reinsert the pack to force a fresh handshake attempt.

The THR9 drops to reduced TX power partway through a shift even though the battery bar still shows two bars — is the BLN-5i pack failing?

This is voltage sag under sustained RF output load, not a failed cell. During extended transmit periods, the cell voltage dips under load even if resting voltage is acceptable — the radio's power management steps down TX output to protect the link. Two bars on the indicator reflects resting voltage between transmissions, not loaded voltage. Swap to a fully charged pack mid-shift and confirm the second pack holds TX power consistently; if it does, the original pack needs a full charge cycle, not replacement.

The BLN-5i won't wake up after sitting unused in a drawer for several months — is it dead?

Prolonged storage can push the cell below the BMS recovery threshold, causing the protection circuit to lock the pack out entirely. Place the battery in the THR9 dock and leave it connected for at least 60 minutes — some chargers will trickle current into a deeply discharged cell before beginning the main charge cycle. If the dock shows no response at all after 60 minutes, check that the dock contacts are clean and try a different power source. Recovery is possible if the cell voltage is still above approximately 2.5V.

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