This is more of a note-to-self than a posting for the public. But come along for the ride if you care to.
This is about getting fed up with short-lived rechargeable battery packs for a home device. And deciding to Do Something About It!
I don’t actually make the repair in this post — I need to get my hands on some parts. But I outline my proposed solution.
The problem
Source: Mint 5200 owner’s manual.
I have a Mint 5200. It’s a Roomba for mopping the kitchen floor. (In fact, I think Mint was eventually bought by Roomba). It works, to a degree. It certainly keeps the floor cleaner than it would otherwise be.
It eats batteries. The original battery pack must have lasted about eight years. The first replacement lasted maybe a year. The last one I bought lasted about six months. And, looking on Amazon, it appears that six months is more-or-less the norm.
And so, I can either throw away this $300 device, or throw maybe $35 worth of batteries into this every year. And because these are Nickel-metal-hydride (Ni-mh) batteries, I’m supposed to take them to the Fairfax County Household Hazardous Waste disposal site. Which means, in reality, that they go back in the battery drawer marked “dead”, in the hopes of spontaneous resurrection. Or, more realistically, in the anticipation that I’ll eventually cart them off to the household hazardous waste center.
Here’s what kills me. This “battery pack” is just six AA batteries, connected in series.
The problem is that the only thing I seem to be able to buy is six ultra-cheapo batteries with an exceptionally short battery life. The pack dies after a few tens of full recharge cycles. Worse, that doesn’t mean that every battery in the pack is that lousy. It doesn’t even mean that the typical battery in the pack is that lousy. It means that at least one battery was that bad.
One rotten battery spoils the pack, so to speak.
Meanwhile, I can look on Amazon and see that a top-shelf rechargeable Ni-mh AA batteries (Eneloop brand) advertises up to 2100 full recharge cycles.
How hard could it be to replace that existing “battery pack” with six off-the-shelf rechargeable AAs?
Safety third.
So, inspired by Big Clive on YouTube, I figure, why not take one of these battery packs apart to see how it works? I’ve learned more practical electronics watching Big Clive than I did in a lifetime of trying to figure out electronic circuits on my own.
How complicated could a battery pack be? Plus, as I now have three dead ones to work on, I don’t lack for materials for experimentation.
Here’s the pack, and the same pack with the plastic “skin” off. As I said, it’s just six AAs.
The first bit of oddness is that there are three wires. Back when I was a kid, batteries only had two ends. Plus and minus. Zero ambiguity. Has that changed while I wasn’t paying attention? If not, then what’s the third wire for?
Short answer is that, for whatever reason, the black and white wires are joined within the battery pack. So there’s still just one positive and one negative connection.
Electrically, this is just six Ni-mh AA batteries wired in series, for a total of 7.2 volts. The red wire connects to the positive terminal of the first battery. The black and white wires both connect to the negative terminal of the sixth battery.
So far, so good. I actually understand everything I’m looking at.
But there are two safety devices wired into the circuit. These will interrupt the flow of electricity if there’s either too much current draw, or too high a temperature within the battery pack.
Batteries 3 and 4 are connected through a thermistor. This is a current-limiting device. This prevents a short circuit or a stalled motor (or possibly motor start-up) from drawing enough current to overheat or otherwise damage the batteries. If too much current flows, the resistance of this device rises sharply and limits the current. It then “resets” once the current draw has gone back to normal.
Batteries 4 and 5 are connected through a temperature-limiting device. (You will see similar gizmos in all your kitchen electric appliances). The purpose of this device is to open the circuit if the battery pack gets hot. Some of these are single-use bits of low-temperature solder. One use and they are permanently fried. Some are bimetallic strips that will reset once the batteries cool back down.
The interesting thing is that both of these safety devices appear to be the self-resetting type. In other words, this battery pack is built as if they expected it to last for years. And they didn’t want it to fail permanently in the face of a single over-current or over-temperature episode.
Which is pretty funny, given that the damn things won’t hold up for six months.
Neither of these safety devices failed. (A quick test with a multi-meter shows that both of them are conducting. Neither one is creating an open circuit.) I therefore infer that it’s the batteries themselves that failed. Or, as least, one of the batteries in the pack has failed.
Finally, the device itself (the Mint 5200) has been designed with this battery pack in mind, so there is no spare room for (e.g.) inserting a standard plastic battery holder (with six AAs clipped in) in place of this battery pack. So if I build one from scratch, and put it back in the cavity within the Mint, I’m going to have to build it as the existing pack is built, by soldering tabs onto AA batteries (or buying specialized cells with the tabs already in place).
The replacement concept
In theory, I could replicate this exact battery pack, just using quality AA Ni-mh batteries, so that it will last longer. I can solder tabs onto battery terminals without killing the batteries. In practice, a) I’ve done that exactly once before, b) I recall that it’s no fun, c) it’s particularly no fun with a low-powered soldering gun, and d) did I mention that it’s not a lot of fun? Lot of failures for every one that sticks. At least in my experience.
If I replicate this battery pack, I replicate all of its weaknesses. If one cell out of six fries, the pack is toast. And when it dies, I’ll have to build another one.
With that as context, the top-shelf solution of replicating the battery pack probably isn’t the smartest solution. It’ll be the best-looking one, for sure. But that’s a lot of effort to go through to save a dozen dead Ni-mh AA’s per year.
Perhaps a somewhat lower-effort (a.k.a. redneck) approach is called for. At the end of the day, this whoop-de-doo battery pack is just six AAs. Plus some safety features. It’s hardly rocket science to cobble together something that will work.
My wife informs me that I cannot mention the traditional Virginia redneck name for the tool pictured above. That’s because her grandmother lived in West Virginia. But those of you from Virginia, of my generation, will recognize that this is just about all the tool kit you need to complete this task.
Putting on my best redneck thinking cap, it seems like the obvious least-cost solution is to convert this to device with an exterior battery pack.
Here’s the plan:
- Buy a standard six-AA battery holder,
- wire in the safety devices as appropriate,
- connect it to the white plug that interfaces with the Mint
- fill it with six high-quality adequate-capacity Nimh AAs.
Arguably, I’m actually going to be able to salvage six usable AAs out of the three dead battery packs in my possession.
After that, I’ll use a stylish bit of black duct tape to attach this to the top of the Mint. Run the wire from the battery pack into the case of the device, plug it in, and everything should work just fine.
That’s the plan, at any rate. All I have to do is order the parts and put it together.
The more general problem of battery-powered tools.
As I wait for the parts to arrive, I’d offer one final comment.
The issue here is that a manufacturer took an absolutely bog-standard piece of hardware — the ubiquitous AA battery — and needlessly built that into a customized battery pack. So that, when a cell dies, all six must be tossed, replaced with a new custom pack.
I’m sure that makes the original engineering a bit easier. But in the end, it doesn’t even add to the manufacturer’s profits, as anybody can make up these simple battery packs. Aftermarket profits are undercut by the fact that there’s really nothing to these customized battery packs, so there are plenty of cheap no-name aftermarket packs to choose from.
You see this all the time. This is the reason I’ve quit buying battery-powered tools of any sort. They are convenient as all get-out. But I get tired of tossing battery packs in the trash, and of paying through the nose for replacements. Particularly when all of those packs are built from industry-standard batteries. If not AA, then some other common size.
So this time, for this device, I’m fighting back.
If I were clever enough, and had access to the right equipment, I bet I could engineer a battery holder that would literally fit into the space allotted for that custom battery pack. So that you could replace the batteries in your Mint just the same way that you’d replace the batteries in your flashlight. A permanent, slightly greener replacement for the existing battery pack.
For some devices, such as those with extremely high current draws, or designed for harsh environments, this might not be feasible. Those are going to require some sort of specialized batteries or hardware or both. But for an application like this — a six-pack of AAs with only modest peak current draw — there’s no good excuse for building this around a custom battery pack.
Maybe I ought to be thankful that the battery is replaceable, period. It never ceases to amaze me that we have entire generations of people in the U.S. who think it’s normal to toss out an electronic device once the rechargeable battery dies. That may or may not be economic, it may or may not make good sense from an engineering standpoint. But that’s never going to seem normal to me.
