NiMh eMate Battery technical question

[beachycove]

I thought I would put this here rather than in the Newton forum, as it is VERY slow going there.

eMate batteries (which are just 4x1.2v NiMh cells like those found in many old laptop battery packs) have a glass encapsulated thermistor on leads located near the plug that is easily relocated on a replacement battery pack; it literally tapes onto the cells, and you join up the wires a couple of inches from the cells. The original battery pack also has a strap-type safety fuse at the other end, however, as a second line of defence against overcharging. I think this latter is something called a "polymeric positive temperature coefficient (PTC) device." If I have it right, this kicks in if the thermistor fails.

Packs of 4xNiMh cells are readily available without the fuse, but finding ready-made packs with a fuse seems next to impossible. This should not be so much of a problem, you might say, since the fuse can be reclaimed and reused from the old pack. The problem is that it would have to be soldered to the battery terminals, and my experience of soldered batteries suggests that they are severely damaged by heat, so I don't want to have to go there. Waste of time and money, in my opinion, as they just do not last. Reusing the original thermistor, on the other hand, is not a problem.

Thus my question: Will a battery pack without a PTC safety fuse, but with the thermistor re-installed, work safely in an eMate? Has the battery technology advanced so that these secondary fuses are no longer needed?

Anyone here with enough technical background to comment?

 

[TylerEss]

I don't have the safety-engineering chops to speak with authority on this subject. I do have the experience to speak as a total hack, though.

I put a snap-in 4xAA battery tray in my eMate instead of repacking the pack. It works great, seems to charge / discharge fine, and all that. I'm not personally worried that it'll burn my house down, for what that's worth.

 

[beachycove]

Excellent. Did you reuse the thermistor, or just connect positive and negative leads?

 

[directive0]

I've been thinking about doing that battery holder hack.

Did you have to remove a lot of bezel?

 

[TylerEss]

I maintained the thermistor but not the fuse.

I did not have to cut away any of the battery door, but I did have to cut away a fair amount of one wall of the square "bucket" the pack sits in by default.

 

[beachycove]

Meantime, back on the farm, I found an older, failed battery that was undoubtedly in the machine when manufactured in 1996. Popped it in the freezer (Google "freezer trick + NiMh") for a day. Took out and left to warm up and dry off. After a couple of failure notices and subsequent resets (eMates are known for this on replacing batteries), it took a full charge. Huzzah. Gotta love those NiMh cells.

But I still think I am going to order me one or two (I have two eMates) of these. No fuse -- I asked -- but from what I've read here, a fuse is probably not really needed. I couldn't buy a pack of 4 NiMh AA cells at WallMart for as little.

It is worth looking at this guy's listings, btw, as there are other assembled battery configurations available that would be suitable for modest adaptation in 540c/190/Duo/5300 battery packs.

 

[James1095]

Those PTC fuses are commonly referred to as a polyfuse or polyswitch. They act like resettable fuses, going high impedance when a short or overload causes them to heat, then returning to their normal state after they cool down. It's a good idea to have one since NiCd and NiMH cells can deliver *very* high currents for short periods, but there are plenty of battery packs out there that lack them.

Soldering directly to cells is definitely a no-no, it will almost always cause damage if it doesn't kill the cell outright. Battery packs are typically assembled by spot welding nickle ribbon to the cells. I built a capacitive discharge spot welder specifically for this, and there are a number of similar designs floating around the web. You can buy cells with nickle tabs already spot welded to them though which can be soldered to.

 

[TylerEss]

Silver-bearing conductive epoxy is a good choice for homebrew assembly of battery packs, too.

 

[J English Smith]

Question: would it be worth a try re the freezing trick for old Powerbook 1400 batteries, or are those probably too old to be revived? I have several packs now that won't hold a charge at all. Have never pulled the switch on a full recell job, and I don't want to personally try soldering batteries/connectors.

 

[beachycove]

Yes. I revived a Duo battery from 10 seconds to over a couple of hours one time. Battery Recondition or Batter Amnesia help -- the things have to be cycled 3-5 times before you see the full effect of the magic.

This, of course, is assuming they are not actually shorted, etc.

 

[J English Smith]

And there is no risk of explosion with the NiMh while frozen, right?

 

[beachycove]

All I can say is that it is a common practice that has been documented in cyberspace for around a decade.

 

[J English Smith]

I will give it a try and report findings. Pretty sure they are not shorted, just old. I will use one that has a little charge left to start.

 

[beachycove]

... I am going to order me one or two (I have two eMates) of these.

I ordered one for testing, as shipping is free, with a view to ordering some more from the guy if the first worked. It arrived yesterday, after a long wait (cheap postage from China is evidently cheap for a reason). So tonight i soldered it onto the old plug and popped it in an eMate.

It makes an excellent fit and all seems well: the battery is recognized and charging in a way the old one just would not.

Mind you, I think I'll test the smoke detectors before going to bed tonight....

 

[beachycove]

This is just to report one last time that the battery pack mentioned in the previous post is working perfectly, with a long, long runtime, and with the machine's charging circuitry being happy at last. This contrasts with several "solder your own" solutions I have tried over the years, which always ended up as disappointments. You seemingly need specialized welding equipment to make that work, since the cells are fatally damaged by the heat of conventional soldering equipment.

I see that I last posted about this on December 11, when I first charged the battery purchased. After intermittent but regular use (I have turned the machine on more or less every time I have passed it, as well as using it weekly for this and that), I just charged it up for the second time, 78 days later, on Feb 27 — and the battery was not yet completely exhausted. I would expect the second cycle of the NiMh battery pack to give an even better result.

I think I am going to order more sets for other machines, and not just my Newtons, as it would be trivially easy to recell, say, a PowerBook 190cs with a couple of 6-cell NiMh packs. Once that can be shown to work, then maybe a 1xx-series battery would be next.