# Low cost DIY 18650 battery holder



## Footleg (May 9, 2011)

It seems that battery holders for 18650 Li cells are thin on the ground, and not cheap for the few that are available commercially. This was a problem for me as I am building a set of LED lights for underground photography. Needing to build several means keeping the unit cost as low as possible. Searching CPF there are very few threads about how to make holders for these cells, so I thought my own solution might be of interest to people.

For my particular design I required double the voltage of one cell, but a high capacity. So I decided to have three parallel pairs of cells (each pair being 2 cells connected in serial). I used protected cells to give some safety in charging and to protect the cells from damage if they become less well matched over time. I will be easy to see how this design of battery pack can be adapted to suit different numbers of cells and different arrangements.

So I needed two end plates with contacts for the cells. For one end I used some strip board/circuit board with the strips horizontal so that the end contacts of each row of three cells would all be connected together. For the other end I wanted some sort of spring contacts connecting vertically the top cell in each row to the corresponding cell in the bottom row. The solution I came up with for the spring contacts was a fuse holder, bent out flat. I cut a piece of stiff plastic from an old box and drilled pairs of holes in it so that the flattened fuse holder pushed in and held itself in place.

Here are the parts described so far (the ruler is metric, so the numbers are cm):






This is the plastic end plate with two of the clips in place, plus one shown as purchased (costing (UK) 22 pence each).






The +ve end of the 18650 cells contacts the copper strips on the circuit board fine, but the –ve ends are flat and slightly recessed, so I added blobs of solder across three strips to make a better contact for the batteries. I also soldered wires onto the board. The +ve lead bridges two copper strips as the cells contact both. The –ve contacts three strips which the solder blobs bridge so the current is shared across all three. You can also see the six Li cells held in formation with a large cable tie.






The two end plates are deliberately made slightly smaller than the dimensions of the six cell pack so that the cable ties used to hold it all together sit in the grooves between the cells. This helps stop everything slipping around. Two more cable ties are used around the outside to hold it all together. This is what the finished battery pack looks like.






It needs something to hold the cells flat or the middle row of cells attempt to pop up. In my case the pack fits snugly in the case I am using so this holds everything solidly in place. If your case was larger then a couple of rigid plastic panels on top and under the pack held in place by the outside wrapping cable ties should sort it. Mine are held tight by the case itself so nothing more is needed.






And finally a top view:





I plan to charge the pack in the case via a socket so I will not have to remove them. But as nothing is permanently attached to the cells you could remove them by cutting the cable ties if any developed faults or you wanted to reuse them in another project later. The total cost of the battery pack apart from the cells themselves is only few pennies really.


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## Norm (May 9, 2011)

This post may interest you: What are the Differences in Charging in Parallel or Series 
Norm


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## Nate (May 9, 2011)

Even lower cost & easier to use:
http://www.rigidmount.com/battery_case.html

Just replace the molded plastic bits i have with circuit board or paddlepop sticks under the contacts.
& use "proper" battery contacts.
The cheapie case was <$1 and waterproof

Covered here:
http://www.candlepowerforums.com/vb/showthread.php?314221-2-x-18650-Battery-Holders


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## Footleg (May 10, 2011)

Norm said:


> This post may interest you: What are the Differences in Charging in Parallel or Series
> Norm


 
Thanks Norm, that was most interesting. I'm chasing more info on that thread now. My research on CPF is never ending!


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## Footleg (May 10, 2011)

Thanks Nate. I had read your battery holder post with interest during my researches before posting this thread. I agree your design is excellent for a separate battery pack. I did wonder where you got the battery clips from as I failed to find any before starting on my build. 

Your posting came just too late for me. It seems there were no new threads on battery holder building for 18650 cells for around 2 years, and then like buses 2 come along together! :laughing:

However for my build I require a battery pack that is really no larger than the cells to fit in the case I am building the rest of the light into. So I would not have room for the plastic box your holder is built from, and certainly not room for 3 of them! It was pure luck that my battery pack is such a snug fit in my chosen case. Otherwise I would have had to pad it to hold the cells square and keep them firmly contacting the contacts.


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## Justin Case (May 10, 2011)

You might consider wiring the pack for a balance tap and charge the pack using a hobby charger. Basically, you have three parallel strings of 2S 18650 cells. So run a common wire connection across all three of your flattened out fuse holder pieces. Now you have three wires coming out -- Batt+, Batt-, and the center tap. You would then consult the wiring diagram for whatever hobby charger you get and connect those three wires appropriately to whatever connector your hobby charger (or hobby charger balance board) uses.


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## find_bruce (May 10, 2011)

Looks good - I like the way the heads of the zip ties keeps it away from the aluminium body. With a balance tap as described by Justin Case you will have a nice robust case. It might just be my eyes, but it looks like you still have a bit of room at the end of the case, can you shorten it or add some foam so the batteries don't rattle

Is there any way of making it waterproof?

Cheers

Bruce


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## Footleg (May 10, 2011)

I'm actually going to fill the remaining space in the case with a block of aluminum to mount the LEDs on to transfer heat out to the case. So there will not be any room at the ends of the battery pack. It will need a bit of foam alongside the battery pack to stop it rattling around though. I'm still building it so I've not got pictures of that yet.

These Hammond extruded aluminum cases do come in waterproof versions, but these are a bit larger and cost over twice as much. They don't have the slide out side panel. I'm going to make this splash proof with some silicon rubber sealant mainly to allow me to wipe off the mud after trips underground.


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## Footleg (May 10, 2011)

Justin Case said:


> You might consider wiring the pack for a balance tap and charge the pack using a hobby charger.


 
This is new info for me. Can you point me to an explanation of a balance tap or explain what I gain from this charging method over just charging the pack as constructed via the power leads?


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## Norm (May 10, 2011)

Balanced charging
Use the google search box at the top of the page to limit your search to CPF.
Norm


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## jasonck08 (May 11, 2011)

Footleg said:


> This is new info for me. Can you point me to an explanation of a balance tap or explain what I gain from this charging method over just charging the pack as constructed via the power leads?



You'll have big problems with the pack if you don't balance charge, especially with lower quality cells.


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## Footleg (May 11, 2011)

OK, to summarize what I think I have learned so far on the charging front:

1) I can probably safely charge my 2S3P cell pack with an 8.4V charger without balancing because I am using quality cells with built in protection. All cells being the same brand and age. All individually charged before building the pack initially.
2) Balance charging is the safer option because it will handle differences which might develop in the cells over time, and would be required if I was using unprotected cells?

I have found on eBay a 'Mystery CX450 battery balance charger' which quotes:
- Input voltage: DC 11.5-13.5v 
- Output current: 800mA (constant current) 
- Cutoff voltage: 8.4V, 12.6V 
- Battery pack : 2-cell (7.4V), 3-cell (11.1V)

Would this be suitable for charging my 2S3P configuration?
The 2-cell connector is labelled 7.4V and has 3 pins. I would connect these to the ends of my pack plus one to the middle terminals which would all need to be wired together?

This link should turn up current listings for these chargers:
http://shop.ebay.co.uk/i.html?_from...ery+Balance+Charger&_sacat=See-All-Categories


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## Footleg (May 11, 2011)

Or would one of these B6 balance chargers allow me to safely charge my pack faster?
http://shop.ebay.co.uk/i.html?_from=R40&_trksid=m570&_nkw=B6+balance+charger

I guess my main question is can these chargers cope with having 3 pairs of serial cells being charged in parallel?

Using my best ASCII drawing skills, where one cell is represented as +===

My pack would be wired as follows to the balance charger:

1 2 3
! ! !
 ! ! !
[+===|+===]
[+===|+===]
[+===|+===]

With the 3 [ characters all connected together and to the 1st wire from the balance charger.
The 3 | characters all connected together and to the 2nd wire from the balance charger.
And the 3 ] characters all connected together and to the 3rd wire from the balance charger.
The wires being represented by the ! characters.


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## find_bruce (May 15, 2011)

Footleg said:


> I'm actually going to fill the remaining space in the case with a block of aluminum to mount the LEDs on to transfer heat out to the case. So there will not be any room at the ends of the battery pack. It will need a bit of foam alongside the battery pack to stop it rattling around though. I'm still building it so I've not got pictures of that yet.


 
Your project is sounding neater all the time !! It would also appear your ASCII drawing skills are complete.


While I am familiar with safety systems and NiMH batteries, like you I am new to lithium batteries. I have found that thinking about your questions has helped me to understand them a bit better so I thought I might set them out , but I don't claim any expertise & others may disagree.


In my view using a protection circuit, either on the pack or individual cells should be considered the last line of defence, rather than the first. I am not saying don't have it, but rather that I would prefer to avoid needing to use the protection.

It helped me to think about your pack as being 2 groups of 3 cells. A pack that is out of balance has a problem. It might be a very minor problem that the cells are not identical & drift apart over time. But it might also be a major problem such as a dead or dying cell. What happens if one of the cells dies, or the protection is activated on that cell ?


You now have a group of 2 cells and a group of 3 cells and a bit of a problem. If you now try to charge the pack to 8.4 volts, the same amount of electrical energy will go into the group of 2 as the group of 3. Lets say the group of 2 cells get to 4.2 volts, the group of 3 cells will be under that voltage. Alternatively the 3 cells get to 4.2 volts & the group of 2 will be over 4.2 volts. Every time you charge the pack and use it, the problem will get worse until it makes itself known in dramatic style. Hopefully the protection in the other cells will kick in to save you, but like I said I would prefer to keep that in reserve.


Using a balance charger works fine to fix minor problem, but it won’t fix the dead or dying cell. Sure balance charge will make sure both the group of 2 and the group of 3 will be charged to 4.2 volts, but when you use them the group of 2 is going to work a lot harder than the group of 3 & so the voltage in the group of 2 will be lower. The pack may be at say 6 volts, but the group of 2 could be something like 2.4 volts while the group of 3 is still 3.6 volts.


I plan on using a simple multi-meter to measure the voltage of each group before every use and after every charge – ie the voltage between the positive wire & the centre tap should be the same as the voltage between the centre tap and the negative wire. I haven’t been able to find figures on how close they should be, but I would expect them to be within about 0.1 of a volt. If the pack it is slightly out of balance, use a balance charger to fix it. If it gets out of balance again within 5 or 6 cycles I would be starting to look for a problem with one or more of the cells.


I was thinking of getting a Turnigy Accucel-6 because apart from doing balance charging, it will also tell me the capacity of the cells, which I intend to use to balance the capacity of the pack before putting it together.

Given that you are building several lights & packs, if I was in your position, I would be considering a charger that also would charge all of them at once. If I understand it correctly, the Accucel-6 will allow you to charge 3 packs of 2s in parallel, but it will cut off when the first pack hits the maximum voltage & you will need to finish off the other packs separately. The IMAX B6 will also do this, or you may want to look at a charger specifically designed for multiple packs


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## Footleg (May 15, 2011)

find_bruce said:


> Your project is sounding neater all the time !! It would also appear your ASCII drawing skills are complete.


 
Thanks. Glad you like the plans. The feedback I've had on batteries and charging have led me to spend hours reading the CPF battery related threads. I've just started a new thread to try and get to the bottom of the battery pack and charging questions as this thread was intended to be about the physical design of the cell holder, and I now have so many questions about the issues of Li-ion cell packs and chargers. This is the new thread: http://www.candlepowerforums.com/vb...ty-7.2V-Li-ion-battery-pack-using-18650-cells


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