New headlamp battery design

[spyke]

Hello Cavers!

I'm new here and I'll explain myself as simply as I can:

1. I'm not a caver but I need the advice of cavers.
2. I am a small-time (hobby) manufacturer of high-end headlights. Really I just cover my costs and allow myself to use any profit for hobby-purposes. I've sold about 50 over the last 18 months, mainly to adventure racers and athletes but a few cavers have bought them too. So now I'm trying to include the opinion of cavers into the design.
3. This is not advertising because I'm writing under a false name and not letting you know who I am or how to buy my products. I really just want an opinion from cavers.


I chose this forum to ask because I've read through many threads and you guys seem to be interested in the geekier side of gear and there's clearly a bit of technical understanding out there.


Now the question:

The headlight is the type with a battery pack at the back and a cable going to the front. The single button to operate the light is at the front. I have a few options for how I set up the battery.

The battery is 2 18650 Li Ion cells in parallel.

Option 1: (Existing design)
The two 18650 cells are electrically connected in parallel and placed in a hard, protective and waterproof case with a gland to allow the cable to come out. On the end of the cable is a connector. The user never opens the case, they use the connector to charge the battery and connect it to the lighthead at the front. The case is mounted on the back of the headband / helmet with good quality velcro (simple, but it doesn't bounce when running).

This is like a Stenlight battery except that mine have a hard casing.

Option 2:
The hard battery case can be opened by the user to reveal the 2P 18650 pack which can be removed by disconnecting a connector INSIDE the hard case. The battery pack is charged by disconnecting and removing the pack from the hard case. There is a single unbroken cable running between the hard case and the lighthead.

This is like a Scurion battery pack.

Option 3:
Similar to Option 2, but the 18650 cells are not bundled into a pack. Each cell is placed individually (copper springs at both ends of the case make electrical contact). Charging is done by removing the cells and putting them into a charging cradle.

This is like most lower-end headlights (Petzl, Black Diamond etc) which have a plastic case that is opened up to insert AA or AAA cells.





Pros:

Option 1:
Easy to add an extension cable to put the battery on a belt or in a backpack.
Very quick and easy to disconnect the battery, un-velcro it and swap in a new battery (even with cold and gloved hands in total darkness).
Guarantees that the pack is sealed (doesn't have user-serviceable orings which can be forgotten).

Option 2:
Hard case attaches more securely to helmet / elastic headband.
Simple cable between battery and lighthead (cannot be accidentally disconnected).
Less prone to leaking (the connector for option 1 is good for at least 3m depth but if it's not correctly lubricated then it will not always seal perfectly.

Option 3:
As option 2.
User can buy 18650 cells from many different places. Gives more options. I use good cells (not the very best but up there) and charge accordingly. A user may choose to buy cheap cells or the very best according to their taste.


Cons
Option 1:
Expensive, every battery pack needs the hard, waterproof case.
The user cannot buy batteries from anyone else but me.

The connector is only waterproof when it is connected. A disconnected battery can (very, very slowly) take on water. The contacts in the connector may get dirty if opened and closed in wet or muddy situations.

Velcro mounting is not as good a solid (screwed / riveted) mounting on a helmet

Option 2:

Fiddly to open the case and disconnect the little connector. Especially in total darkness.

Option 3:

Fiddly to open the case. Especially in total darkness.

Slightly more resistance because of the 4 springs making contact with the cell (though I will aim to minimise this).
User may put bad cells in the light and then complain when it doesn't have good runtimes.
Unprotected cells will still be protected by the MCU in the driver circuit, but this is not as good as also having a dedicated protection circuit.




Lots of my thoughts are there, though there's probably a few more I'll add at some point.

Thanks in advance for helping me improve

[Marduke]

Definitely option 3. Being ably to swap out the cells mid-trip gives it virtually unlimited runtime.

[LukeM]

I have to agree. Also, interchangeable li-ion cells is not something that I've seen any caving headlamp producers do, and I've always thought it would be a great option. Buying 2 cells is usually much cheaper than buying a pre-built pack and there are plenty of examples of waterproof battery cases out there.

[jharman2]

I'm leaning toward option 3 also. I assume the regulation and protection circuitry are built into the lamp. The losses associated with the spring contacts will likely be minimal especially at lower drive currents.

[potholer]

If it's not a daft question, in the case of option 3, what stops someone putting two cells in wildly different states of charge in the battery pack, and what stops that causing problems if it happens, apart from relying on protection circuits on the cells?

[spyke]

Thanks for your help guys.

So far option 3 seems to be the clear leader. This does not surprise me because it's the 'expert-user' option. Being able to put your own cells in gives much more freedom.

If it's not a daft question, in the case of option 3, what stops someone putting two cells in wildly different states of charge in the battery pack, and what stops that causing problems if it happens, apart from relying on protection circuits on the cells?

..and this is the obvious problem, that with too much freedom people can do stupid things. I think in the above case it would be ok since one cell could charge the other though the rate might get a bit high. I can think of worse cases if unprotected cells are used:

1. The cells get put in the wrong way around - So the circuit shuts down and the user complains of no light.
2. One cell is put in wrongly - Again the circuit shuts down but inside the battery case you've got up to 8.4V being shorted.
3. The user gets 'clever' and tries to make it brighter by putting in 2 or 4 CR123 cells which is at the very limit of my driver's comfortable range (6V).

I think I'll have to put in some mechanical orientation protection (an insulating ring at the +ve battery end) to prevent contact being made if a cell is put in the wrong way. The disadvantage here is that you'll have to use cells with button tops.

[spyke]

Definitely option 3. Being ably to swap out the cells mid-trip gives it virtually unlimited runtime.

Just to clarify; all the options can have the battery swapped mid-trip. They just swap in different ways.

[Tenzin Beck]

All flashlights and headlamps are vulnerable to not having batteries inserted properly. Clearly instruct on what's acceptable, and trust users not to be idiots.

[Marduke]

With cells in a parallel arrangement, dissimilar charge states are not disastrous. It's only series arrangements that cause reverse charging and overdischarge.

Using Cree XM-L's, a series configuration is not warranted anyhow, as the Vf is well below that of the minimum discharge for li-ion cells, including circuit losses. You can still use a buck-only circuit. You can use a parallel arrangement that can run on any number of cells installed, even with some slots left empty. You could even use 2xCr123 in each slot for backup if desired.

Reverse polarity protection can easily be done electronically. Using a mechanical reverse polarity protection will mean people can't use the majority of 18650 cells, which are flat tops.

[potholer]

Reverse polarity protection can easily be done electronically. Using a mechanical reverse polarity protection will mean people can't use the majority of 18650 cells, which are flat tops.

The neat/simple way of doing it with little wasted power by using a reversed FET doesn't seem to translate directly to protecting individual cells in a 2P arrangement.
There may still be a neat way of doing it, but not one that leapt out at me.

[spyke]

Ok, new question:

Would anybody not prefer a four cell holder? They're all parallel so you could use one or two or three or four.

It would weigh a little more and be a bit bigger. Does that bother you guys?

[rlboyce]

Wouldn't bother me.

[LukeM]

Personally I'd probably prefer whatever helps balance out the headlamp best, as long as the capacity is enough for most trips. Cavers don't tend to run their headlamps on high most of the time like an adventure racer might. Plus, if all I have to do is pop in a few more cells in once during a long trip I'd rather save a little strain on my neck.

[bif]

I'd rather like to see these lamps. Come on Spyke, chin up.

[Cody JW]

Spyke, Not sure who your target market is but I think one aspect that is important to cavers (or at least me) is the ability to easily screw the light and case to a helmet. I used the duo for years and was spoiled on the way it has holes in the pack to screw the battery case to a helmet. I temporarily switched to the apex and Fennix and found that it was not as easy to screw the light to the helmet. Also I got spoiled with the ease to reach the switch on the duo, even with a gloved hand.I found myself fumbling to reach the switch on both the Apex and Fennix. Thankfully both Potholer and Bif came out with current generation bulbs so I can go back to the duo. I also got spoiled by the ease of getting into the battery case of the duo and was dismayed by the difficulty of getting into the case of the Apex, Fennix was not bad. As far as the vote on battery case goes #3 for sure. Let us know what you come up with.