Ni-MH to Li-ion conversion

Use recycled Li-ion cells to bring life to an old handheld vacuum cleaner.
Feb 14, 2020 — 8 mins read — Electronics

Ni-MH to Li-ion conversion

In this article and video, we will convert this handheld vacuum cleaner from Ni-MH to Li-ion batteries. 

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This vacuum cleaner is close to 10 years old but in the last 2 years, it was almost never used as it developed an issue with its batteries. Whenever we took it off the charger in order to use it, the vacuum power would almost instantly drop as if it was not charged.

The vacuum will then continue to run for some time but not with the needed power so it was kept in storage for a while gathering dust on it instead of inside it. 

On the back of the vacuum, there is an access port that once removed, we can access the batteries. Its pack is made from 3 Ni-MH cells connected in series to provide 4.5V when fully charged. Once drained, this pack will get down to about 3V making it a perfect candidate to be switched with lithium cells. 

I used my multimeter to check on the old pack once it was out of the vacuum cleaner and it measured 3.8V across the three cells but once I started measuring each cell individually, I noticed that one of them was at 0.6V which is below the voltage it should ever be.

If you are in a similar situation and you don’t want to convert to lithium, you can replace the defective cell and have your appliance fixed. 

The voltage range for a lithium cell is from 4.2 to 2.8V which fits nicely in the range at which the vacuum cleaner already operates so the decision was made. 

You can definitely buy and use new Li-ion cells but I had this laptop battery that I did not use so I decided to salvage some of the cells inside in order to give them a second life. 

The battery case is made from two halves that sandwich the individual battery cells and removing them is quite a challenge. I started by prying open the case from one side with a flat head screwdriver and working my way all around. 

The entire pack is glued together so be careful with the screwdriver as in one moment of carelessness I managed to stab my hand when the screwdriver pierced the outer case. 

Once the case is split open I used my screwdriver to separate the cells from the other half and then with the wire snips I cut out the control board that I won’t need and also separated the three pairs that were in parallel. 

At this point, it is crucial that each of the cell pairs is checked for the voltage it is on, as any that are below 2.5V should not be used as they might have been permanently damaged. I had one such pair so I took one of the good ones and started preparing it to go into the vacuum cleaner. 

To make sure that the batteries are not overcharged, I used one TC4056A module. This module gets 5V on the input and it then charges the lithium cells to 4.2V which is their maximum allowed voltage. Anything beyond that and you risk damaging the cells and causing a fire. 

I’ve first added some solder to all of the module pads, I’ve added two wires on the input pads and I fixed it on the cells with hot glue. I then added two thicker wires from the battery terminals to the battery pads on the module making sure to keep the same polarity as marked.

The pack was now ready to be charged so I proceeded to test the charging with the original charger from the vacuum cleaner that luckily outputs 5V. The output current of this adaptor is really low at 120mA so the charging will take a while but on the other hand, it is much safer that way as the batteries will never get too hot. The cleaner is not used very often and can be easily put to charge overnight. 

In order to identify the polarity, I connected the charger to the wall outlet and used my multimeter to test the voltage on the pins that protrude from the charging deck. Since this is a transformer-based power supply, you can see that the voltage is a bit higher than 5V when measured but this is only because there is no load on the output. 

When measuring for voltage like this, if you get a positive reading, the terminal that you touch with the red probe on the multimeter is the positive connection. In my case, the voltage was reading negative, so I had the probes in reverse. In this case, with a negative voltage reading, the black probe is the positive terminal. 

With the terminal identified, I used two wires with crocodile clips to attach the pack to the charger and leave it to get charged. While charging, the module has a red LED that lights up and when the battery is fully charged, it turns off and a blue LED lights up. 

In its original configuration, with the Ni-MH batteries, the charging terminals are directly connected with the battery. Since we need to add a protection circuit in between them with the new pack, I opened up the vacuum cleaner case and removed the entire motor and batteries assembly.

On its back, we can see that one side of the connection is made through a diode for reverse polarity protection and the other is directly soldered between the charging terminal and the switch.

After giving it a quick clean from the dust that was gathered on it, I used my soldering iron to remove the diode and that broke one of the connections. Using thicker wire, I soldered one end of it to the motor terminal and I then tried to break the solder joint on the other terminal. 

Since there was a lot of solder there and it also seemed that there is extra metal behind it to support it, I used my snips to cut off a small section from that bridge and break the second connection from the terminals. 

The second wire was then soldered to the switch terminal and with that, both of the terminals were now standing free and two wires came out from the inside connections. 

To be sure about the polarity of the terminals, I placed the entire assembly over the charger terminals, while being connected to the outlet and measured the voltage again. I used a red marker to mark a plus sign on both the inside terminal in the assembly and on the outside on the charger terminal. 

I then added some solder to the assembly terminals and soldered both of the module input wires to the terminals according to the polarity marked.

To prevent any unwanted shorts, I used some electrical tape to isolate the battery terminals inside their slot and then placed the new pack in. Luckily for me, the spacing was perfect and the new pack fitted inside without any modifications to the slot. 

As a final step, I’ve cut the wires coming in from the motor to length and I’ve soldered them to the output pads on the module. With everything connected I pressed on the switch to test it out and the motor did move but it immediately stopped. I wasn’t sure what the issue was but I assumed that maybe the batteries were not charged enough and the module is not turning on so I proceeded with installing the entire assembly back to the case. 

The installation was fairly simple as there was plenty of space on the inside to fit all of the new wires. The trickiest part was aligning the switch on the top and after that was done, I returned the three screws to secure the case as one piece.

To see why the vacuum cleaner didn’t work, I used the alligator clips to connect it to the charger and to my surprise the charging LED did not turn on. Thinking that something might have disconnected during assembly, I measured all of the connections and voltages on the charger module only to realize that I managed to fry it. 

By specs, the module is capable of charging the batteries with up to 1A of current but it never came to my mind that that 1A is also the limit that it can provide on the output! When I turned on the vacuum cleaner previously the motor must have pulled a lot more than 1A destroying the module in the process. 

Lesson learned, I replaced the module, and since I did not have any other module that can handle more current I now soldered the output wires directly to the battery wires. This way, the output current does not go through the module and it can’t damage it but also, this way, we lose the over-discharge protection that the module provides. 

In this configuration, the module will only be responsible for charging the batteries and making sure they don’t over-charge and for over-discharge, I’ll try to manually monitor this for now, by listening to the motor speed. Whenever I hear that the motor is starting to run slower than normal, it will be placed on the charger. 

This is not ideal, but I don’t think it will be an issue since the cleaner is only used in relatively short bursts and not continually for a long time. After every few uses, we can put it back to the charger to top it up and never run it dangerously low on the battery voltage. If I see this as an issue in the future, I might add a separate battery monitoring circuit and do a video about it. 

So, with everything working now, I trimmed two of the plastic pieces that pushed on the original battery pack from the cover and closed it all up. After the filter was put back in its place, and the front put back on, I once again had a working handheld vacuum cleaner. 

I hope that this video was educational for you and that you managed to learn something. If that’s true then please hit that like button below, make sure to subscribe and I’ll see you all next time. 

battery reuse tools basic electronics
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