HP's twin-chamber cartridges are the easiest ones to refill.  This article tries to provide a comprehensive review of methods and troubleshooting tips for the refill.  Because people tend to skip the troubleshooting section, we will start with common troubleshooting first, so we know what do when a problem occurs

Problems

1. Clogging.

Clogging is common among HP printers.  When a printer is clogged, it still prints, but the page is either blank or has stripes.  These printers are extremely easy to unclog.  Many factors can cause clogging:

1) HP printers can use both pigment and dye ink. When we switch ink types, it is not unusual for the printer to clog up first.

2) Refilling the cartridge may introduce air bubbles that clog the printhead temporarily.

3) When the printer is idling for a long time, ink may dry on the printhead and clog it.

4) The printer may be clogged by dirt or debris.

Solution: We will first use the printer's built-in function to clean the printhead twice, then wait a few hours and repeat.  If the clog persists, we will take out the printhead.

• Soak.  We will put about half an inch of water or cleaning solution in a bowl and soak the printhead from thirty minutes to a few hours.  This will soften the clog.  Many clogs can be dissolved this way, and no further cleaning is required.  We can use hot water to wash the bottom of the printhead and put it back.
• Inject water.  If we put the ink intake directly under running water, the water can go through the printhead and clean out the clog.

Or we can do some extensive cleaning:

• Option 1: we can use a house vacuum to suck from the bottom of the printhead
• Option 2: we can use a power steamer to blow steam from the side and blow out the clog.

An example of a printhead cleaning can be seen here: https://youtu.be/wSkB9wy6Ifc

2. Sponge Fatigue.

Symptom: the printer prints the first few pages well, but the ink soon fades.  If we wait a few minutes, the printer prints adequately again.  We will keep adding ink but it may seem the printer can't hold the ink, and the ink dries out very quickly.

There may be a few factors causing this problem.

1. The cartridge is sealed airtight.  When we print, the cartridge needs to be vented.  If the the top is sealed airtight, the cartridge will suffocate.

2. The HP's sponge is not designed to be refilled.  Thus,  it takes less and less ink in each wet-dry cycle.  When a cartridge goes through a wet-dry cycle, goo will be collected in the sponge pores and prevent the sponge from being used again.  When we fill up the cartridge, the excessive ink will flow out of the cartridge.  Also, the cartridge will suffocate itself during the printing process because the sponge pores are closed after multiple refills.

Solution: only print a few pages at a time or switch to a BCH refillable cartridge body that has a sponge designed for multiple refills.  Also, a large tank will keep the sponge wet and prevent wet-dry cycles.

3. Cartridge Missing or Damaged

The printer complains that a cartridge is missing or damaged

Cause: 1) We may have ink on the cartridge chip.  2) The chip has gone bad

Solution: Wipe the chip clean and re-insert.  If this fails, we will try to use a new chip.

4. Used or Counterfeit Cartridge

HP displays a warning message when a cartridge is half full or is a third party cartridge.  The goal of the message is to trick you to buy HP's new "genuine" cartridge.  Although HP is obligated by law to let us refill the cartridge, HP planted some tricks.  When we insert a half-full or a non-HP cartridge, the printer will display the message "Used or Counterfeit Cartridge."  We can click "Continue," "OK"  or "Yes" to dismiss this message.

Another HP's trick is to show a follow-up message: "Did you buy this cartridge as a New and Genuine HP Product?"  The answer should be "No."  If you click "Yes," then HP can stop you from using this cartridge.  This message exists in some older printers.  We put it here for reference.

To summarize, here are the common warning messages:

The cartridge Is Low on Ink.  Click "OK" or "Continue."  If you are using a refilled cartridge, please note that this is normal and keep using the printer.
'Used, Refilled or Counterfeit Ink Cartridge Detected'  Click "OK" or "Continue."
"Ink Cartridge Depleted."  We need to turn off the chip info or "anonymous information" on the printer's LCD panel.
Incompatible Ink Cartridges.  It is either an old version of the cartridge or a wrong model.  You need to change the cartridge.
HP Protected Cartridge Installed. HP can let you "protect" your cartridge, so the cartridge can only be used in your printer.  If you bought a refilled cartridge that is protected, then you won't be able to use it.
Furthermore, if the HP Printer Utility is installed on the computer, we can turn off the  "Display HP Alerts."  

For a detailed analysis of HP alerts, we can visit: https://www.bchtechnologies.com/blog/stop-annoying-status-messages-windows-from-hp

5. Ink Levels.  

HP's printers do not have devices to measure how much ink is left in a cartridge.  Instead, they count how many pages have been printed since a cartridge was installed.  Because we are going to use a cartridge and refill it over and over, HP will not be happy about it. HP will stop us from printing and claim "Ink Depleted."  We can disable the "cartridge chip info" or "Anonymous Usage Info" on the printer, so the printer will keep working.  Consequently, the printer will not display the ink level after the ink depletion because it will print too many pages by HP's standard.  We need to keep an eye on the print quality and add ink when needed.  For a detailed explanation of HP's ink level chip, visit this page: https://www.bchtechnologies.com/blog/how-to-reset-hp-cartridge-chip

How to turn off "Cartridge Chip Info"

Printer without LCD Panel:

Press and holds the (Wireless) and (Resume) buttons for 3 seconds

Printer with LCD Panel:

Not a Touch Screen

Touch Screen, Before 2016

Go to printer's product control panel Setup->Preferences->Chip Info and turn it off.  

Touch Screen, After 2016

 After 2016, it is called "Anonymous Usage Info"

Background Information

Anatomy of the Cartridge

HP's design is similar to Canon's twin-chamber system.  The back chamber is the ink reservoir which holds ink.  On the top of the ink chamber, there is a fill-hole which allows HP to fill up the ink chamber quickly.  The fill-hole is sealed with a plastic ball.  The back chamber connects to the front chamber through a small opening on the bottom.  For regular (non-XL) cartridges, HP only fills the front chamber and blocks the opening.  As a result, if we add ink to the back chamber, the ink will not be able to be used by the printer.

The front chamber is filled with a sponge which regulates ink pressure.  When the sponge is wet, the ink will be drawn to the bottom outlet by gravity.  As a counterforce, the ink will also be held by the sponge's surface tension.  On the top, there is an air hole which may be connected to the outside by a series of channels.  A cartridge needs air when it prints.

Ink outlet.  Because HP's cartridge doesn't have much ink, it can sit on a black grommet without leaking.  However, people tend to fill up the cartridge as much as possible, so we recommend having an extra washer installed on top of the existing grommet.

Picture Legend

Back Chamber: ink reservoir. It is not used by non-XL cartridges.

Front Chamber: sponge pressure regulator

The Opening: XL= open; non-XL=closed

Refill Hole: sealed by a plastic ball.  It is not used by non-XL cartridges.

Air Hole: open to the air

Washer (recommended): underneath the ink outlet.   

Chip

The chip doesn't know how much ink is in the cartridge.
The chip counts how many pages are printed.
The chip can be turned off.

All chips are the same: the original, third-party chip without ink level, third-party chip with ink level.  The chip will show a full tank of ink (except ones without ink level), and work until the ink is depleted after the first use, and then it will become a chip without ink level.  You can get any chip, as long as it is the correct model.  Also, you can convert the existing HP chip and make it work with refillable cartridges.  

Update: HP made some design changes for their new lines of startup cartridges:

Refill Methods

Our goal is simple: adding ink to the cartridge.  For the non-XL, we will add ink to the front chamber.  For the XL cartridge, we will add ink to both front and back chambers.

Front Chamber Refills: All Types: XL, Regular, Startup

Drip

This method is a fool-proof-no-brainer: We will flip the cartridge over and drip ink on the felt pad of the ink outlet.  We will keep dripping ink until the pad is saturated.  There is no tool required, and we can add more ink than stated by the non-XL cartridge specification.  This method works on both XL and non-XL cartridges.  

The front-chamber refill is used by Costco and many refill stores.  They will add ink to the front chamber and then glue a one-time XL chip on it.  Although the back chamber is not filled, the ink volume will be somewhere between the level of XL and non-XL.  We would be happy if the printer is back working, and won't notice that the cartridge prints 30% fewer pages than a true XL.  For example, the black OEM XL has 14 ml of ink, and non-XL has 7.5 ml of ink.  The front-chamber refill can add 11 ml of ink.  We won't notice if it prints a little less than the OEM XL cartridge.  For the colors and photo black cartridges, the XL has 7.5-8.0 ml of ink, and the non-XL has 4 ml of ink while the front-chamber refill can add 6 ml of ink.

Drill in Front of Air Hole

This is the method used by big refill stores, for example, Costco.  First, we will find the air hole on the cartridge.  Secondly, we drill a hole on top of the air hole.  After that, we Inject ink until ink comes out.  We will use a piece of tape over the hole.  It is for preventing dust, so doesn't need to be airtight.  Put the cartridge upright and let the excessive ink flow out.  After few minutes, the cartridge should be able to stand upright without any leaking.  Place the cartridge back into your printer. Run a cleaning cycle 1-3 times. Do not run a cleaning cycle more than 3 times.

Pedestal

The "refill pedestal" is a glorious version of 1).  It has a U-shaped tube which allows us to refill the front chamber without flipping over the cartridge.  We put a cartridge on the pedestal and push ink from an injection opening.  

Poor-Man’s Clip Refill 

There is a cheaper version of pedestal refill.  It uses a set of cartridge clips.  Before making a hole, the clips can be used to store cartridges.  We will use scissors to drill a hole in the orange clip and then make another hole on the silicone pad.  Next, we can use a syringe to inject ink from the bottom of the cartridge.

Back Chamber Refill: XL and BCH Blank Body

Drill at Refill Hole: XL OEM Cartridges

For XL cartridges, we can open up the fill-hole and add ink to the back chamber.  We can drill through the plastic ball, add ink, and then seal it with a hot glue gun or a piece of packing tape.  Also, for our convenience, we can use plugs.  BCH has two kinds of plugs: 4mm and 4L, and they both work very well to re-seal the hole.

As an alternative, we can heat up a paper clip with a cigarette lighter.  Insert the paper clip halfway into the ball and allow the paper clip to cool down.  After a cool-down period, we will have a good handle on the ball and can poke it out of the refill hole.  We can re-insert the ball after a refill.

If we do not install extra washers, we will add 14 ml for regular black and 7 ml for photo black and color cartridges.  If we install washers, we will add ink until the cartridge is full.

BCH Refillable body (blanks)

HP 564, 920, 902 cartridge casings all have the same design.  Therefore, BCH makes cartridges casings with transparent walls.  We can see ink levels through the cartridge and leave no room for guessing.  Adding ink when needed will reduce the chance of drying and thus reduce sponge fatigue and air bubbles.The BCH sponge is different from HP's OEM sponge.  The BCH sponge is designed for long-term refill usage and is less prone to fatigue.

The air hole is in the front of the cartridge, covered by yellow tape.  For the first usage, we will need to tear off the yellow tape and expose the air hole.  We will make sure the air hole is fully exposed.  If left-over glue covers the hole, we will use a needle or tag to poke open the hole.

Option 1: If we have a chip, third party or OEM, we will remove the orange clip and glue the chip at the bottom of the cartridge.  We will find a marked box which the chip should be lining up with at the edge of the box.  After installing the chip, we will put the orange clip back to prevent ink spill during refill.

After exposing the air hole, we will remove the color refill plug and add ink to the back chamber.  We will fill it all the way to the top, only leaving room for the plug.  Then, we will put the plug back and leave the cartridge in its orange clip.

To prevent ink leaking due to increased volumes, BCH has provided a silicone washer for the printhead.  The washer has a shorter end and a longer end.  We will point the shorter end towards us and the longer end away, and gently lay it on the top of the ink intake.

Option 2: If we do not have a chip, we can make a chip holder from the used cartridge by following this tutorial [Link].  We will lay the chip holder on the cartridge box and make sure the chip will have good contact with the brass pin. We will next remove the orange clip and install the cartridge.

Refillable Cartridge with Chip

We make some cartridges with a chip installed on them.  Follow 5) and skip two options for installing chips.

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