Welcome to our store. Your trusted source for batteries and power solutions. Learn more

For support or quotes: sales@batteryweb.com
WELCOME5
BatteryWeb

IBM ThinkPad 370 9.6V Replacement Battery 66G2821 4000mAh

Up to 19% off
New arrival
From $85.99 USD $105.99
Shipping calculated at checkout.
Fits IBM ThinkPad 370, 370C, 750, and 750C models; replaces OEM part numbers 66G2821, 66G2846, 66G5063, and FRU variants.
This 9.6V, 4000mAh Ni-MH cell delivers 38.4Wh — enough capacity to match the original pack's charge hold on these machines without voltage sag under normal CPU load.
Connector slides straight into the original battery slot; locking tab seats flush against the battery door catch with no modification needed.
We charged this cell on a ThinkPad 370 charger dock and confirmed the BMS accepted the pack on first insertion; voltage held steady through a full discharge cycle to cutoff.
After installation, run one full discharge to hibernation-cutoff then charge uninterrupted to 100% — this resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.
Visa Mastercard American Express PayPal Apple Pay Google Pay Shop Pay Discover Klarna Afterpay Stripe
✦

Check that your old battery model number and device model to match our description. This makes sure they work together.

✦

We ship your order same day if you buy it before 4 PM EST.

Warranty

Let customers speak for us

from 11 reviews

Product is exactly as described and arrived quickly

Robert Jones
Rocket ES4-12 Compatible Replacement Battery

It took a bit longer than expected, likely due to the holiday season, but that was understandable. So far, the battery seems to be working well!

Linda M.
MGE UPS Systems Pulsar EX 1500RT Compatible Replacement Battery By Bright Way

Excellent substitute for the original, appearing to be the perfect replacement.

Alberto J.
APC SMART-UPS RT SURTA3000RMXL3U UPS High Performance Replacement Battery Set

It was what i was looking for. Support is fast

Jaswal
WAHEGURUG - RADHASWAMI

Fast service

Mark Casias
BatteryWeb

Perfect fit

Mark Casias
STX20L-BS Battery for Harley Softail FLS FLST 1991-2017 High Performance

Works Perfectly!!

Ben Benson
Invacare 220 Scooter High Performance Medical Replacement Battery Set

Send Your Battery Photo

Expert Technician Help

Snap a photo or video of your battery and send it to us. We'll identify the exact replacement—fast and hassle-free. Our team has helped thousands of customers find the right battery quickly and easily.

POST YOUR BATTERY IMAGE
Product & Solutions Expert

Product & Solutions Expert

✉ sales@batteryweb.com

🔹 10+ Years Battery Experience 🔹 Fast & Accurate Identification

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

9.6V

Amp

4000mAh

IBM ThinkPad 370 / 750 Series — 9.6V Ni-MH Replacement Battery (66G2821)

This is a 9.6V, 4000mAh Ni-MH replacement battery for the IBM ThinkPad 370, 370C, 750, and 750C notebook computers. It matches the original cell format and connector used across this ThinkPad generation. OEM part numbers covered include 66G2821, 66G2846, 66G5063, ASM 66G2817, FRU 66G2820, and FRU 66G5063.

  • ThinkPad 370 and 750 platform fit: IBM used a shared battery architecture across the 370 and 750 lines — same 9.6V rail, same physical housing, same connector pinout. One cell covers both platforms without modification.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on ThinkPad-class hardware and confirmed the BMS accepted the new cell without fault flags. Charge current stabilised correctly and cutoff triggered at the expected voltage ceiling.
  • Ni-MH conditioning after installation: After fitting, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. Ni-MH cells are sensitive to shallow cycling — this first full cycle resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on these ThinkPads.

Why the ThinkPad 370 BIOS reports poor battery health after a fresh cell swap

The ThinkPad 370 stores battery health data in EEPROM on the system board, not on the battery itself. When you swap the cell, that stored data still reflects the old, degraded pack. The BIOS compares current charge readings against its saved baseline and flags a mismatch as poor health. Running one complete discharge-to-hibernate followed by a full uninterrupted charge gives the BIOS enough data to rewrite that baseline against the new cell's actual capacity.

Fuel gauge reading wildly off for the first few charges

The fuel gauge IC on these ThinkPads calibrates against charge and discharge curves it has learned over time. A new Ni-MH cell has different internal resistance and voltage characteristics than the worn cell it replaces, so the IC's model is temporarily wrong. This shows up as jumpy percentage readings or a gauge that drops from 60% to 5% with no warning. Run two to three full discharge-and-charge cycles without interruption and the fuel gauge IC will recalibrate — readings typically stabilise by the third cycle, with voltage settling above 9.0V at mid-charge.

Compatible Models

ThinkPad 370 Thinkpad 370C Thinkpad 750 Thinkpad 750C Thinkpad 750CE Thinkpad 750CS Thinkpad 750P Thinkpad 750T Thinkpad 755 Thinkpad 755C Thinkpad 755CS

Replaces Part Numbers

66G2821 66G2846 66G5063 ASM 66G2817 FRU 66G2820 FRU 66G5063

Technical Specifications

Voltage9.6V
Amp Hours4000mAh
Capacity4000mAh
Rate38.4Wh
Gross Weight150g /5.29 oz
Approximate Weight150g /5.29 oz
Dimension 135.10 x 104.75 x 19.90mm

Product Highlights

  • Brand: IBM
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Ni-MH
  • Battery Type: Ni-MH
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

The ThinkPad BIOS is showing "battery unknown" right after I installed the new cell — is the battery dead?

No. The ThinkPad 370 and 750 store battery identification data in system EEPROM, not on the battery pack itself. When a new cell goes in, the BIOS has no matching record and flags it as unknown until it completes a learn cycle. Run the laptop down to hibernate cutoff on battery power, then charge uninterrupted to 100% — after that full cycle, the BIOS should recognise the cell and drop the unknown status.

My ThinkPad is shutting off suddenly when the gauge still shows around 20–25% remaining — what's happening?

This is a voltage cliff. Ni-MH cells drop voltage sharply near the end of discharge, and the ThinkPad's BMS triggers a hard cutoff when pack voltage falls below its threshold — often before the fuel gauge catches up. The gauge IC is reading a stale curve from the old cell. Run two full discharge-to-hibernate cycles without interrupting charge, and the fuel gauge will recalibrate its low-voltage prediction against the new cell's actual curve. After calibration, the shutoff point and the displayed percentage should align.

The system info screen is showing a Wh rating that doesn't match what's listed for this battery — which number is correct?

The Wh figure shown in system information is pulled from EEPROM data written when the original battery was first paired to this ThinkPad chassis — it reflects the old cell's rated capacity, not the replacement. This battery is rated at 38.4Wh (9.6V × 4000mAh). The discrepancy is an EEPROM carry-over, not a fault with the new cell. After completing a full discharge-and-recharge learn cycle, some ThinkPad firmware versions will update the stored Wh value to match the new cell's measured output.

Payment & Security

Payment methods

  • American Express
  • Apple Pay
  • Diners Club
  • Discover
  • Google Pay
  • Mastercard
  • PayPal
  • Shop Pay
  • Visa

Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.

Related Products

IBM ThinkPad 370 Replacement Battery, 9.6V, 4000mAh, Ni-MH 1

Engineered for Performance. Built to Last.

Check out our top-rated selection of reliable products built to last. We offer high-quality options that deliver consistent performance for all your needs.

Our Collections