The Princeton Tec EOS Diet…

SilverFox

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I use a Princeton Tec EOS headlamp all the time at work. I usually set it on low and it keeps me from falling into holes and tripping over things. I use a flashlight to see and inspect with, but for general navigation, the EOS works out very well.

The EOS is regulated and seems to do a good job of sucking Alkaline cells dry. It is advertised to run about 6 hours on a set of 3 AAA cells on high. On low, I believe the advertised run time is something like 60 hours. Not bad for a set of around 900 mAh cells. Many people have been using NiMh cells and have been pleased with their performance as well. I have not tested NiMh AAA cells at 150 mA, but believe that most brands (regardless of what their label says), should come in with similar capacity.

The EOS weighs 105 grams with the strap and with a full set of Alkaline batteries.

While the EOS is fairly lightweight, I decided to put mine on a diet.

I contacted AW and found out that there are Li-Ion cells that are the same size as AAA cells (10440). Their capacity is a bit low at 320 mAh, but if you parallel them you end up with 3 cells at 4.2 volts and 960 mAh. This is pretty much a direct replacement for the 3 AAA Alkaline cells.

I ordered some cells and put them in. The middle cell has to go backwards and the negative end does not contact the metal strip. A small magnet took care of that problem, and the plastic holder of the light will keep the magnet from slipping around out of place. EDIT: You also have to jumper the contacts at the top and at the bottom. The contacts are in two pieces in both places and you need to connect them together. END EDIT

My EOS went from 105 grams to 98 grams and now is more comfortable to wear, should have the same run time, will have better cold weather performance, I am no longer concerned with self discharge, and because the batteries are rechargeable, I am still enjoying guilt free illumination…

Now I am looking for a place to put a "Lithium Powered" sticker.

I must add that all the standard warnings concerning Li-Ion cells should be adhered to:

These cells are unprotected, so charge them often. If you run them all the way down you will destroy them.

You will need a charger than can handle Li-Ion cells and keep in mind that these are 320 mAh cells. The preferred 0.7C charging rate is 22.4 mA. I am charging at 25 mA. You will need some way to hold the batteries for charging. If you charge them in series, you should make provisions to balance them from time to time. If you charge them in parallel, things on the charging side get easier, but how to hold them becomes more of a challenge.

Don't charge Li-Ion cells unattended.

Charge on a fire proof surface.

Tom

As a foot note…

My wife things I am a bit off because I am running around the house with my head lamp on bouncing my head back and forth trying to see how much of a difference 7 grams makes. I have two EOS headlamps, so I don't have to change batteries all the time.

I believe there is a noticeable difference, but I also though that my truck had a little more snap to it when I replace the exhaust system and fitted it with Corvette tips…
 
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cognitivefun

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Good post. Thanks for the idea.

You can also use NiMH which should work okay if you don't mind the self-discharge.
 

SilverFox

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I forgot to add that you have to be careful when replacing the cells. If you don't put them in correctly, you will probably fry the light...

Tom
 

Bogus1

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I like the EOS too and was wondering about this myself. So the trick in converting it to parallel from series is to reverse the middle battery?

Thanks
 

SilverFox

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Hello Bogus1,

You've got it...

At first I was going to call this a mod, but it is actually more of a substitution.

The only difficult part is to get a connection between the base of the middle cell and the contact. Usually the battery nipple makes that connection, so I improvised a nipple on the base of the Li-Ion cell using a magnet.

Tom

EDIT: Please note that this information is not correct. In addition to reversing the middle battery, you also need to join the two piece connectors at both the top and the bottom of the battery compartment.
 
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jar3ds

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i wonder what 1.5v lithiums would reduce the weight.... ? i think i'll be going more that route
 

SilverFox

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I have run into a problem with this project... EDIT: Problem fixed... END EDIT

I have been doing some run times and am getting similar results to what Doug (Quickbeam) got. About 2 hours regulated light on high, then things start to dim.

However, when checking the voltage of the individual cells (total system voltage was around 2.9 volts), I am finding that the two outer cells are pretty close, but the middle cell is over 4.5 volts.

Everyone knows that is not a safe voltage to be at with Li-Ion chemistry.

How does a parallel pack charge its center cell under load? :thinking: :thinking: :thinking:

I am sure there must be a good reason this is happening, but I am at a lost to explain it...

Tom
 
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SilverFox

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Hello Markdi,

Yes, you are absolutely correct. I feel like I left my brains in the ditch...

I did measure the voltage across the whole pack and it was showing 4.134 volts. From that, I thought I had everything in place. Wrong...

OK, let me try again. In order to put all three cells in parallel, you need to add a jumper wire to connect the top two battery connection pieces and another jumper wire to connect the bottom two battery connection pieces. With all of the top contacts connected together, and separately with all of the bottom contacts connected together, you can add the Li-Ion cells (taking care to make sure that you follow the orientation for the cells in slots 1 and 3, but you need to reverse the cell in slot 2) and they will be in parallel.

Things are working much better now... I am about 2.5 hours into a run time test and am still in full regulation.

While I was checking things out, I swapped the optic for a Fraen optic that I got from Mr Bulk a while back. It makes the hot spot wider at the expense of some spill. It measures less on my light meter, but the beam shape seems more useful. I will have to give it a try and see how I like it.

Tom
 

Krit

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As I know, It's rather dangerous if we parallel li ion cell together. Because we don't know exactly capacity each. The high capacity cell might charge another low cell and affect to low cell. So it's rather risk when use paralell mode without control circuit for paralell propose.
 

SilverFox

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Hello Krit,

I have matched these cells for capacity and there is very little danger in using them this way. The problem came from me not hooking things up properly.

Another problem is that these are bare cells and I have to be on top of the discharge. When I see the light fall out of regulation, they are at about 3.0 volts per cell, and it is time to charge them back up again.

My usual habit is to recharge frequently. I am checking run times right now. After 3 hours on high, I am still at 3.65 volts and still in full regulation. This is better performance that I have observed on Alkaline cells or NiMh cells. I wonder if I can get 4 hours...

I usually use the light on low and can expect a weeks use from it between charges.

Charging in parallel also will eliminate any imbalance between the cells. It is a little tricky to set up, but is quite easily done.

Tom
 

SilverFox

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Well, I didn't get 4 hours...

At 3.5 hours (around 3.06 volts), the EOS dropped out of regulation. At 3.75 hours I was down to 3.0 volts and shut things down.

Doug (Quickbeam) reported 2.25 hours until the light dropped to 50% and with the Li-Ion cells in parallel, I am getting 3.5 hours. That is like a 50% increase in run time in regulation.

Not bad.

The down side is that when the light starts to go dim, it dims down very rapidly with the Li-Ion cells.

To lighten things up even more, you could put the jumpers in and run with one cell. I will have to try that and see what the run time is like.

Tom
 

SilverFox

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This is getting very interesting...

My original plan was to power this light with a single Li-Ion cell, but after realizing that the capacity for an AAA Li-Ion cell is very low, I went on to put 3 cells in parallel.

I just finished running it on one cell. 75 minutes on high until it dropped out of regulation, and quickly dove to the 3.0 volt mark in the next 5 minutes.

I have now gone from the original 105 grams to just slightly over 80 grams. That is a significant drop in weight. Although there is some loss in run time, it is not as drastic as I first thought it would be. Since I usually run the light on low, this may be the perfect way to go for me.

I have some other headlamps around here... I may have to see what I can do with them.

Tom
 

jar3ds

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woudl it be possible to get a pic of exactly how your are running these in parraell?
 

SilverFox

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In order to complete this project, I did a run with 2 Li-Ion cells. The weight with 2 cells is 85 grams and the light ran 2.5 hours on high before dropping out of regulation.

This gives me a wide variety of ways to manage my energy needs.

I will probably start out with 2 cells in the light and carry a third for a back up.

Tom
 

SilverFox

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Hello Jar3ds,

Here is a picture of how things are set up. The picture shows all three cells installed, but it will run on 1, 2, or 3 cells.

EOSDietcopy.jpg


Tom
 

SilverFox

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Update:

I ran the light with one cell on low.

At 16 hours and 39 minutes, I noticed it getting very dim. I was below my goal of shutting things down at 3.0 volts and the voltage was at 2.5 volts. At this low current draw, it did not spring back up to over 3.0 volts, so I quickly put it on my charger. My charger complained that it was over discharged at first, but then the voltage came up and everything is fine.

I think it would be best to limit the run on one cell on low to 16 hours. That means that with 3 cells you would get around 48 hours of run time.

I now have run times on high and low. I guess I should do a test on medium...

Tom
 

SilverFox

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Update:

I finished the test on medium. Here is a summary of run times with this set up:

On high
1 cell gives 1 hour and 15 minutes,
2 cells give 2 hours and 30 minutes, and
3 cells give 3 hours and 45 minutes of run time.

On medium
1 cell gives 4 hours,
2 cells give 8 hours, and
3 cells give 12 hours of run time.

On low
1 cell gives 16 hours,
2 cells give 32 hours, and
3 cells give 48 hours of run time.

Once again these run times are with the unprotected 10440 Li-Ion 320 mAh capacity cells, running in parallel.

Tom
 

jar3ds

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awesome! how would you think a protected 10440 would work?

I wonder if we could get you to do a energizer lithium 1.5v runtime test in all three modes just to compare runtimes... maybe even a NiMH? The weights of those two battery chemistries by the weights of your mod with the runtime data woudl give a ton of excellent feedback on the EOS...

i can't wait to get mine and figure out how to make it flood as much as possible... =)

the eos is such an awesome light

thanks for the thread silverfox! great mod!
 
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