Tec-40 battery pressure (spring mod?)

I just got a PT Tec-40 and really like it. It also puts out great light with NiMH batteries and KPR-103 which is a big plus for me. I've read on the forums that the Tec-40 puts a lot of pressure on the batteries and that has caused NiMH batteries to deform for some users. So I'm interested in preventing that and I've read on the forum that some users have put foil in the bottom of the battery compartment to reduce the battery pressure. [edit: correction - the foil is to improve contact on some batteries, not reduce pressure. Thanks for the clarifications]

I think the spring under the bulb is the key to how much pressure is exerted on the batteries. If a softer spring of the same dimentions could be installed, or the original spring cut shorter, the pressure could be tuned down to a lower level. I'd rather get a softer spring, as that would be a reversible modification.

Has anybody tried to change or modify the Tec-40 spring? Does anybody know how to get an appropriate spring? I looked through my other flashlights for a donor, but found none. I'm going to try Ace Hardware today, but the size is going to have to be very close to fit around the PR bulb base and also inside the bulb holder.

Looking for any advice. Thanks.

Lurker,

I think what's really needed is a battery compartment like the Rage, with springs at the bottom. I haven't tried to mod one, but I would like to see PT do this themselves. I'm sure it's a common problem. They seem to have given up all R&D on the Tec40/20, though.

Yes, I am sure the problem could be designed out with little difficulty by the PT engineering department. It's a shame they left this one flaw in a light that has so much going for it. Still, I am hoping that someone has come up with a real fix for it. I've read about some pretty amazing modifications in here.

Never mind! Helps to read the origional posting first!

My reason for putting foil in the bottom had nothing to do with reducing pressure. It had everything to do with making an effective electrical contact with the single cathode end that had to sit on the flat connector strip on the bottom of the light.

I guess that PT figures that this problem is worth the tradeoff of the additional length that springs would require in the bottom of the body. I'd vote for springs also.

When I use rechargeable batteries in my Tec 40, I have to put aluminum foil in the bottom or it won't make contact with the metal in the bottom.

It would be my guess that PT designed the Tec 20 and Tec 40 for alkaline use. Their concerns about using rechargeables is likely not very significant.

I'm using my PT Tec 40 with NiMh's. I haven't found a problem with it yet, but this thread is making me nervous. I think I'll stop using it until I can be sure it will not result in any issues from using such batteries.

The negative end of my rechargeable batteries does not stick out as much as on alkalines. That is why I have to usse aluminum foil.

Thanks for the feedback. I tinkered with the light a little more and came up with what I think is a big improvement for use with NiMH batteries. It is based on trying to solve these 2 problems:

(1) The spring under the bulb is stiffer and longer than necessary and fully compressing it to turn on the light puts a lot of pressure on the batteries.

(2) Once the spring is fully compressed and the light turns on, any extra turning of the bezel just crushes down on the batteries like a vice. This is because there is virtually nothing else compressible inside the flashlight to relieve the additional pressure (therefore, be sure not to overtighten the bezel when turning the light on). Actually, the contact strip in the bottom of the flashlight has a thin piece of compressible material (rubber?) under it, but this is not sufficient to provide the necessary spring action.

So I made the following changes:

(a) To solve problem 1, I got a different spring from the hardware store, stretched it so it would fit around the bulb and cut it about 3mm shorter than the original. The spring is also just a little softer. You could cut the original spring as well, but I wanted the mod to be reversible. This took a lot of pressure off of the batteries but didn't address problem number 2.

(b) To address problem 2, I got a piece of foam rubber about 2mm thick (a piece of weather stripping that happened to be the correct density), wrapped it in aluminum foil and dropped it in the bottom of the light to act as a sort of spring under the batteries. There isn't much room to add anything here, so this is about as much "spring" as you can fit. In addition to allowing a bit of overtightening of the bezel, this should also improve contact on rechargeable batteries with recessed negative terminals.

These two changes seemed to float the batteries pretty well and should be much gentler on them. Time will tell for sure. The bezel doesn't screw down on the flashlight quite as far as before, about 3mm less before the light comes on, but this doesn't seem to be a problem. You could continue to tighten the bezel further, but again, I think it is best to stop here.

(c) One advantage of the high original stock spring pressure is that it held the batteries pretty securely in place. My changes left them rattling against the sides of the battery compartment a little bit, which I don't like. I wrapped a small sheet of paper around the inside of the battery compartment to eliminate the rattle. The whole thing feels very solid again.

The flashlight can be returned to stock condition very easily. This wasn't too much trouble to do, but you might wonder why bother when you can get other 4AA flashlights that do not have the problem in the first place. It is worth it because the Tec-40 has what I think is a better beam than the 4AA alternatives, and I prefer using rechargeable batteries when possible. Just my personal preferences. I hope this helps someone.


P.S. Now to make it fool-proof I need to add an o-ring around the body of the flashlight where it will act as a stop to keep the bezel from being screwed down too far. The light already has a stop there, but the O-ring will raise the stop position a little bit. That will keep me (or more probably another user) from accidentally overtightening the bezel while trying to turn the light off in the wrong direction.

Lurker,

You know what else you could do to address problem 2, is to replace the 2 solid contacts with springs. These are the two metal nipples that make contact with the batteries at the front of the light. They screw in and out with a pair of needle nose pliers. I'm not sure exactly how you would put springs in, but it should work.

Great suggestion, Lux. It would have never occurred to me that those would unscrew. That could work really well with the right springs. The main thing is that the springs would have to be strong enough not to fully compress before the light comes on. I'll have to work on that idea.

Meanwhile, my foam rubber spring didn't hold up too well. I have been trying to find a metal spring to work in the bottom of the light and am currently using a bent pocket clip from a pen. I also found something called a push nut that might work.

Also open to any other ideas.

I took a close look at my PT Blast flashlight, and as was suggested earlier in the thread, PT completely fixed the problem in that flashlight, even though the overall design is very similar to the Tec 40. They could easily make that slight change to the Tec 40 and solve the problem. All it takes is a spring in the bottom of the battery compartment and 2 plastic ridges in the body of the flashlight for the bulb-holder to bottom out against instead of bottoming out on the batteries. But that would make the light slightly longer and require retooling the production equipment, so I guess PT isn't going to rush out and change a successful product.