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Roomba 521
Battery Error 3 Repair


In this project, the dreaded “Battery Error 3” charging error was tackled on an old Roomba 521 vacuum robot.


This error can be found in older models of Roomba robots and is often referred to as the “Red Ring of Death” for robot vacuums.

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In this project, the dreaded “Battery Error 3” charging error was tackled on an old Roomba 521 vacuum robot.


This error can be found in older models of Roomba robots and is often referred to as the “Red Ring of Death” for robot vacuums.


  • The vocal message “Charging Error 3” when attempting to charge the robot through the docking station or by direct connection.

  • A red ring appears when attempting to charge the device.

  • The battery does not recharge regardless of how long plugged in.

  • The battery re-learn feature cancels, reporting the error.


Quick Fix’s

There are many reasons why an error may occur when charging your robot, from a bad battery, or a loose connection, all the way to a bad motherboard. Below are some initial tips to attempt to remedy the issue.


  • Wipe clean the connections on the charging dock

  • Plug the charger directly into the robot

  • Clean the connections to the battery itself

  • Try another battery (if one is spare)

  • Try a battery reset (Hold down “Dock” and “Spot” buttons for 10-20 seconds and let go - the robot should make a tone) which will let the robot re-learn the battery's charging and running characteristics.

The Investigation
The Investigation
1 - Overview

To initiate the repair, we first need to strip down the robot to expose the error, as the basic checks show the fault lies within the robot. Below is a picture of the top face of the robot in question.


This robot is a standard Roomba 521 model designed for home use, compatible with most floor types, including tiles, carpet and hardwood.


To start the repair, we need to turn the robot over, exposing the bottom side of the robot in order to access the case screws.

Taking a closer look, the exact model number can be revealed.
2. Backplate Teardown


The backplate contains the battery and battery connections, which are the first point of internal inspection for this error.


The first step is to remove the side brush, as it will prevent the separation of the backplate. This brush rotates in normal use allowing for debris to be collected within reach of the main vacuum rollers beneath. This component can be undone by removing the single screw in the centre. To keep things safe, I often refit the screw into the motor so that it will not become lost.


With that task completed, the highlighted screws can now be undone. These screws are typically captive screws, which do not fully remove from the outside cover when loose. This is beneficial for often removed access panels, as it reduces the risk of a vital case screw becoming lost.

bottom case.png

Once undone, the backplate can be lifted off the body to expose the battery compartment and other components.

3. Battery Inspection


As this is an error related to charging, the next step is to investigate the connections to the battery, which for this fault, may have corrosion. To remove the battery, simply pull on the two green tabs to lift it evenly from the compartment.


With the battery out, the area surrounding the connections can be examined for any potential damage.


It can be seen that the left topmost terminal is exhibiting signs of corrosion (green furr), which may very likely be the cause of such an issue. Below is an image taking a closer look.


Examining the battery itself, there is no sign of leakage or corrosion. With a fault such as this, it is typically due to the motherboard being exposed to moister such as a spilt drink on the robot, damp storage conditions or cleaning with too wet of a cloth.


Corrosion can cause damaged, shorted, or broken connections which will prevent the robot from accurately measuring the charging of the battery, leading the robot to shut down the charging circuit to prevent further damage. This means to correct the issue properly, we will have to closely inspect the motherboard, remove all the corrosion, and repair any damage present.

Top-Case Teardown
Top-Case Teardown
4. Top-Case Teardown


The motherboard is on the top side of the robot, under the face plate. But first, we have to remove the body panels that restrict access to the screws and the components that block the removal of these panels.

The first component to remove is the front bumper plastic. A thin plastic strip helps retain the outer bumper shell to the robot body. To remove the strip, un-do the following highlighted screws. BEWARE! These screws are not captive and will have to be stored safely (such as in a baggie or a part sorter compartment).

bumper screws.png

Tip: Make sure to keep note of what screws went where as you progress through the teardown, as not all screws are guaranteed to be of the same dimensions. Additionally, recording what sizes the screws are will help you find replacements later on if some were already missing.

mini carmalan robot.png

Below is an image of the screw size removed at this step. For this model of robot, a quantity of 10 screws is expected.


With the screws undone, the plastic strip should easily lift away.


Carefully turning the robot on the other side, the bumper cover should slide off upwards.

BEWARE! The top IR sensor for docking and virtual wall detection will still be connected to the motherboard by a thin cable. Take care when removing the bumper, not to damage the cable or connector, as the cable plug may not be easily accessible at this time to unplug.


The back bumper is far easier to remove, with only needing to press the button at the rear. This will allow the wastebasket, with an internal suction fan, to effortlessly slide out.


The last step before removing the top case cover is taking off the face plate. The face plate is a replaceable cosmetic panel that shields various access ports and case screws. This facia is held in place by clips. 


BEWARE! These clips can become brittle with age, so take great care when prying the panel loose. Cold plastic clips can also be more likely to break. Robots previously stored in a cold garage, left to match the temperature of a normally warmed room, may be less likely to result in broken housing clips.


To release the clips, gently pull the plastic around each clip one at a time, unclipping them from the body. Repeat this process around the face plate until the place becomes free. On this robot, there is no indented tab that can be pushed to release the clips, as common on some other types of devices.


With the face plate detached, the remainder of the case screws are revealed, along with the communication port on the right-hand side.


The next step is to remove the following highlighted screws. These are the first ring of screws that hold the top section of the case to the bottom section.

top plate screws 1.png

Below is an image of the screws removed from this section, of which 6 should be expected.


Next, the inner ring of screws need to be undone.

top plate screws 2.png

Below is an image of the screws removed from this section, of which 4 should be expected.


Below this ring is an additional single screw of a different size, as highlighted below.

top plate screws 3.png

The following image shows the size of the sole screw expected in this step


The final screws of this stage are the two below the handle. These screws do not pass through to the body underneath and are not required to be removed. However, they are shown below for future reference.

top plate screws 4.png

The following image shows the size of these two screws.


With all the mentioned screws extracted, the top cover should now be withdrawable from the robot's base, exposing the main circuit board (motherboard) below.


The wastebasket can be re-fit if desired, as it may help keep the robot level when performing repairs.

Motherboard Removal
Motherboard Removal
5. Motherboard removal

BEWARE! With the motherboard now exposed, it may be at risk of Electro Static Discharge damage. Wearing an ESD strap, as shown below, is recommended, so static energy from your body can be dissipated to ground and not damage your robot.

With the top case detached, the plug for the bumper sensor is now accessible. Gently pull on the connector (ideally not on the wires themselves) to disconnect the plug from the socket.


Tip: The placement of sockets and the colours of wires can differentiate between different models of devices. It is recommended to take pictures of each plug before disconnecting to ensure that the correct sockets are reconnected later. Additionally, if a wire became loose on a plug, it may help refit that pin in the correct location later.

mini carmalan robot.png

With the bumper fully separated, the front bumper sensors can now be seen.


Tip: If the robot's bumper has lost its bounce, or the robot appears to occasionally “bump” into objects that do not exist, slightly bending the large metal tabs shown above forwards may help. These tabs act as springs on this version of the bumper and can become slightly compressed over time from continuous bumping. Pulling the spring tabs slightly further out will put increased pressure on the bumper plastic, allowing the bumper to be properly extended and not misread.

mini carmalan robot.png

Tip: Cleaning the IR LED sensors on the inner bumper will aid in properly detecting objects. The front IR sensors are used to detect upcoming objects so the robot can slow down before hitting them. If these sensors are covered in dust, they may not properly work in operation and cause a symptom of the robot heavily hitting objects. The IR sensors below can also be cleaned to ensure that “cliffs” or other large drops can be properly detected.

mini carmalan robot.png

A plastic sheet may be present to protect the top of the motherboard. To remove this sheet, undo the screw internal to the small plastic pillar, as shown below.


Below shows the length of this pillar and the corresponding screw.


Now you should be left with a fully revealed motherboard.


Before the motherboard can be withdrawn, a few remaining steps need to be followed.


First, unplug the robot's speaker cable. This is the speaker that produces the speech and tones.


Secondly, remove the charging cable (left) and suction fan cable (right), as shown below.


Next, there are 5 retaining screws for the motherboard to be undone, as highlighted below.

motherboard screws.png

The size of the screw for this step is shown below.


The motherboard should now be loose and can be lifted from the body. 


BEWARE! The cables below are still connected, so take care not to damage the wires. Additionally, the drive motors have inserted sockets on either side, so lift gently and evenly up to not break these connections.


The last step is to unplug the remainder of the plugs from the underside of the motherboard. These plugs are primarily for sensors. Connections to the drive motors should be as small PCB boards inserted into the motherboard sockets, and the connection to the side brush may also be springs, similar to the battery connection. 


Additionally, if any of the motors appear to work intermittently, carefully wipe clean the connectors to ensure no dirt is preventing a reliable connection.


Cautiously unplug each cable one at a time. On this model, each plug should be of a different size, but taking images of each and their corresponding wire colours will prevent any confusion when reconnecting later.


With this done, the motherboard should be removed, leaving only another plastic sheet and a metal tray underneath.

More Coming Soon
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