Standard Enclosures with 18650 battery access

[Signal Simon]

Hi all,
Last year I designed a pocket-size 10W power supply with Vbatt, 24V and 50V switchable output, which runs off 4 AA Nicd/NiMH/Alkaline cells.
I used a standard enclosure, New Age S1A-404017.

I've had a few people ask me if I could do a version which utilises the common 18650 lithium-ion cells, as they now have a number of sets of these for their torches, plus independent chargers.
I would like to run the circuit on a 2 or 4-cell series arrangement in order to reduce the input current and increase efficiency, possibly also increasing output power capability.
Problem is, I can't seem to find standard enclosures with a 2 or 4-cell 18650 battery compartment.
I've looked at some bare power bank enclosures with a view to changing the front panel but the internal battery contacts are all wired for parallel configurations.
I know there are some decent 18650 PCB battery holders out there such as the Keystone 1048 / 1049, but it is important that the battery can be changed easily without disassembling the unit.

Any suggestions please?

Simon.

[juanfermed]

Hey hello,

Have you looked at this?

http://www.batteryspace.com/battery-holder-2-cell-18650-unlimited-expandable-battery-holder.aspx

Not exactly an easy replaceable holder, but can make the job easier. It does not have contacts though, only "structural" support. The nice thing is that you assemble together as many as you want.

If not, then this must be much better (I hope):

http://www.batteryspace.com/search.aspx?find=holder+18650

Hope it helps!

[Siwastaja]

18650 cells are absolutely not suitable for holders for user access, they have never been meant to be used this way. This is especially true with more than 1 cell. They are only meant to be constructed as a battery pack by the manufacturer (you).

Reasons include:
- Poor insulation on the sleeve, easily causing shorts when handled by users.
- Unlimited short circuit currents, in range of about 100A, enough to cause serious burns
- Protection limited to chemical and cell cap PTC design, not enough when handled separately
- When installing more than 1 cell: (if in series:) cells are grossly unbalanced and the energy cannot be used. (if in parallel:) cells in different SOC are connected in parallel by the user, causing heavy current, cell damage and possibly a risk of fire, or a blown fuse in your device.

If you think your customers can accept a hacky, potentially unsafe device, and they are handling separate 18650 cells in their ecigs anyway, then, why not. Be careful to instruct them to absolutely fully charge each and every series cell before installing them, so that they are in balance, and in this case, a cell-level monitoring BMS is a must.

Be aware that 18650 holders are unsafe to begin with, because the plus side steel spring can scratch and penetrate the poor insulation near the plus terminal, shorting the cell. The holder spring will glow red and the plastic holder melts. These cells are only designed for spot welding.

[sleemanj]

I've had a few people ask me if I could do a version which utilises the common 18650 lithium-ion cells, as they now have a number of sets of these for their torches, plus independent chargers.

Define: "a few"

If 3d printing is in the quantity ball-park, I've been working on a design lately for a parametric box for which the idea is to just pull it into future projects.

Here's the first prototype I printed sized for 2 AA cells, but it's designed to be expanded in both cell number and size, can be wired in whatever configuration required.



I'm still trying to resolve a problem with the strength of the lid's latch which works but tends to break after a couple of cycles, I think printing the latch separately in a better orientation and gluing to the lid will work, but realistically a better solution is probably to dispense with the latch and use a threaded m3 insert and screw.

Anyway if it's any use to you it's in OnShape: https://cad.onshape.com/documents/be6d5f907bc0e83693f00d8c/w/96e1eeaa962fd97433430fbb/e/82ec095f7bb481b306ae66ea

[mikeselectricstuff]

Might be worth looking at off-the-shelf packs for cameras, camcorders, phones etc.

[Signal Simon]

Thanks for the replies.

Siwastaja, the main reasons I've stayed away from the 18650s are the inherent risk and the necessity for comprehensive charging and discharging protection and balancing.
I've had experience of bad 18650 holders. The ones I mentioned are a more sensible design but require something above to keep the cells in place .

The end users are Test Engineers, so they have an understanding of some design limitations.

Because very low volumes are involved (10-20), I did not want to be using a custom case.
Thanks for your idea Sleemanj.

Looks like I will be better off making a few optimisations on the original 4AA design (why does TI Webench come up with a completely different solution than it did last year for the same design parameters and controller??) and for a higher power / longer runtime version maybe do an 8AA or other nominal 12V design and use an off the shelf wide input range converter brick.

Mike, I like where you're going with the battery pack idea, but we get back into the need for an item-specific pack and  charger, rather than using something they already have.
Now if I was making this for say the construction industry, I would adopt this idea like the cordless tool manufacturers have with their site boombox radios.

Simon.

[tszaboo]

The end users are Test Engineers, so they have an understanding of some design limitations.

I would just avoid their request and make it with 4xAAA or 4xAA  or something similar. There are plenty of benefits to do that. You can buy close-able holders with built in switch, for example. Or you can take your electronics on an airplane (what service technicians might do).
And the best is: This is probably for your company, internally. For AAA, you can buy everything at Farnell, Mouser, etc, with proper paperwork. But you already know this. You designed it correctly. So let them cry for the features, that they dont really need.

[nctnico]

Thanks for the replies.

Siwastaja, the main reasons I've stayed away from the 18650s are the inherent risk and the necessity for comprehensive charging and discharging protection and balancing.
I've had experience of bad 18650 holders. The ones I mentioned are a more sensible design but require something above to keep the cells in place .

The end users are Test Engineers, so they have an understanding of some design limitations.

Nevertheless go for readily made battery packs with a UN38 certification with internal protection circuitry. There is too much which can go wrong with 18650 Li-ion cells and there is a good reason you can't buy them from LG, Samsung, et al directly.

[voltsandjolts]

I agree, giving the average joe access 18650 bare cells is a bad idea but its already too late...the world is awash with torches, power banks, nicotine vapes etc. with accessible 18650's.

You could re-purpose a cheap power bank like this one:

http://www.ebay.co.uk/itm/4-Bays-TOMO-V8-4-power-bank18650-battery-charger-18650-Battery-Charge-DIY-Power-/311660609817

[Edit: Ahh, sorry, re-read first post here, you already thought of this. This power bank is probably parallel wired too]