In mid-2011 I purchased a Shark SV75SP 14 cordless hand-held vacuum cleaner for small messes. The unit is shown in the picture below.

The batteries on the Shark lasted for about 10 minutes before they were drained and I was constantly recharging the thing. Worse yet, the instructions said to charge it for 16 hours (that’s a long time!).

My Shark vacuum spent more time on the charger than it did cleaning the messes it was designed to tackle. Enough was enough, something had to happen.

 

Figure 1: my 15.6 V Shark handheld vacuum

The Original Shark Cordless Vacuum Hack

There is a Batteries + Bulbs store in my area, so I took the vacuum there and told them I wanted to upgrade the batteries to ones with a higher amp-hour capacity. Constructing battery packs with a regular soldering iron can ruin the batteries, and I lacked the spot welding device they used at the store, so I had them build the pack and install it.

The new batteries were 4000mAh NiMH cells. Since the capacity was greater, the charge time was even longer than the original 16 hours. This was unacceptable.

To charge them quicker, I got my hands on a generic laptop charger shortly after I got the vacuum back.

The charger produces 24 V at 1 amp. The connector didn’t fit the unit, so I simply cut the connectors off both chargers and soldered the old one onto the new charger. I then wrapped everything up in electrical tape.

Now, I was able to charge the unit in about 4-5 hours and use it longer between charges than I could before.

This worked fine for about four years.

Something is Wrong…

Then, in early 2017 I went to use the vacuum to clean a mess. Nothing. Completely dead.

I threw it on the charger figuring the batteries were just dead.

Still nothing.

No green LED (which lights up during charging), no motor humming when I flipped the switch, no suction. Now I had a big green paper weight instead of a handheld vacuum.

I’m a pretty busy guy who runs several businesses (and this blog), so the vacuum sat for a month or two before I got a chance to take it apart and look at it.

Finally, I grabbed my screw driver and went to work. Upon opening it and removing the guts, I was face to face with the somewhat simple design shown below in the picture.

 

Figure 2: the guts of my Shark

 

The design is pretty simple, but sometimes simple things elude us. Instead of connecting my power supply directly to the motor (after disconnecting the batteries) to see if it works I went full throttle and started testing components on the small circuit board.

A close-up of that board is shown below (with one terminal on one of two parallel resistors and a diode unsoldered).

 

Figure 3: a close-up the PCB

 

On the board, we can see a SPST switch, an LED with its current limiting resistor, two bigger 68Ω resistors in parallel, and two diodes, likely there to prevent inductive kick-back from the motor.

My ohm meter told me the switch was good, as were the two resistors. After unsoldering one of the leads on each of the two diodes, I found that the diode continuity test on my meter did not seem to work right.

But if the diodes are there just to prevent kickback, the unit should still theoretically function, right? Was the thought that ran through my mind. My next thought was perhaps I should buy a new meter…

No time for this now though, I had to spend some time actually doing work that makes me money. And then there’s that thing about writing new blog posts, too.

A week or so later, I finally decided to disconnect the battery pack and insert my benchtop power supply in its place. See the figure below.

 

Figure 4: You can’t tell from the pic, but the motor’s running! The left display is current, the one next to it is voltage.

 

I dialed in 15.6 V, hit the switch on the unit, and voila! The motor kicked on right away (even with both large resistors and diodes removed from the circuit!).

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I now knew that the problem was the battery pack.

As a side note, it’s interesting to note the voltage drop from 15.6 V to 15 V and the current draw of 4.77 A.

Would the voltage drop and current draw differ if I soldered the components back into the circuit? I thought to myself.

To find out, I fired up the soldering iron and repeated the set up. We can see the results were slightly different in the picture below.

 

Figure 5: repeat of figure 4, only now the 2 resistors and 2 diodes are back in the circuit.

 

As we can see, the voltage drop and current draw is even greater. At almost 5 amps, a 4000 mAh battery pack would theoretically drain after about 48 minutes of use. This is neglecting start-up transients and ignoring other losses. The actual run time is likely shorter. No wonder the original batteries would drain so quickly!

Next, it was time to see exactly what kind of batteries the unit came with and compare them with the new batteries.

After peeling back the label on one of the original batteries and doing a search on the characters printed on the metal case underneath the label (which read WT170311 (RDC-001) (T)), I was able to find no information on the cells.

The vacuum is a 15.6 V model and there are 13 cells, so each cell is 1.2 V. I’m sure they’re either NiCd or NiMH batteries but am not sure on the capacity of the cells.

A picture of the old battery pack can be seen below. The cell that stands alone was wired in series with the otherwise rectangular pack yielding a total of 15.6 V.

 

Figure 6: the original Shark batteries

 

At this point their capacity doesn’t matter much.

What matters now is figuring out why the new batteries are dead and not taking a charge.

Was it simply their time to go?

Did I force too much current to quickly into them with the new charger? If so, why did they last so long before dying?

The New Shark Cordless Vacuum Hack Plan

I’ll admit that I’m no expert on battery charger design, so I’m hesitant to simply buy another battery pack and repeat the same saga.

The motor has what appears to be a part number on it. The higher the voltage you apply to an electric motor (up to a limit), the faster it spins. This could create more suction and a more powerful vacuum.

The plan is to look up the specs on the motor. Can I turn my 15.6 V Shark into an 18 V Shark? How about a 20.4 V Shark?

Maybe.

The next order of business will be to investigate the batteries.

How should they charge?

How much current is safe?

Will they even fit inside the case of the unit if I increase the total voltage and/or capacity?

Should I build some sort of battery charge control circuit and implant it into the unit, or are the smarts already inside the charger?

Perhaps a tear-down and reverse engineering of the original charger coupled with some research can answer these questions.

But that’s all going to be in part 2 of this series.

When will I get a chance to do these things?

I’m not sure, but I sure do miss the convenience of having a small battery powered vacuum!

And the concept of hacking this thing to make it a better model is motivating.

Right now, I have a Shark handheld vacuum that would be functional if the batteries weren’t shot. I also have a good project to work on.

I’ll keep you updated on the Shark cordless vacuum hack.

Do you have any ideas or suggestions on battery charging or making the vacuum better? Feel free to comment and share your thoughts!

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