# Designing a high capacity 7.2V Li-ion battery pack using 18650 cells



## Footleg (May 15, 2011)

Since my 'DIY battery holder' thread threw up so many questions on the ways to connect up and charge Li-ion cells, and many nights just spent reading other threads on CPF, I decided to start this thread to try to answer this question in detail!

My led circuit requires a power supply in the region of 7.2V and has to handle current between 1.4A and 2.8A with enough run time to allow me to set up and take complex photographs (in caves). So I settled on putting 3 pairs of series connected 18650 cells together in parallel. I was planning to just charge these from the power leads connected to the pack. But then I learned a bit about balance charging and the option of having a center tap. Now that already changes the configuration as it means all three mid points in my 3 pairs would also be wired together. You could view the pack as 3 parallel connected cells put in series with another 3 parallel connected cells.

What I want to determine here is what is the best way to wire up the 6 cells in the pack, and what is the most suitable way to charge this battery pack? To be absolutely clear what we are talking about here, I have gone beyond ASCII graphics and put together the following two diagrams. The top drawing shows the original battery pack as I designed it before setting off on the road to enlightenment in the CPF batteries smoke and fire forum!  
The lower drawing shows what I am thinking I should actually be building now:







Based on cost vs quality recommendations on this forum I have gone for Trustfire 3.7V 2400mAh protected 18650 cells (with the black sleeve featuring a tasteful flame motif oo: ).

My first question is just about the pros and cons of joining all the center points together, without using the tap for balance charging. Is there an advantage or a possible issue between these options if I just use a Li-ion pack charger 8.4V/1800mA to charge this pack? In use, as the cells reach the point where the protection shuts them off, is it better to interconnect the center taps like this so that the pack still gives out power until every cell is spent? Is there any danger of reverse charging of one cell by another here?

Secondly, in order to keep the pack well balanced, I understand I should really use a balance charger. Can I just balance charge the pack using the 3 wires out on the lower drawing? Do balance chargers happily charge cells in parallel like this? It is sort of like balance charging a chain of 2 cells with a capacity of 7200mAh isn't it?

I really do not want to have to dismantle the pack for charging. The entire light containing the battery will be sealed against mud and damp. NOT pressure sealed I hasten to add :huh:, so it will be able to vent if a cell did so. I just want to keep the insides dry when handled with wet muddy hands and be able to charge the entire unit through a socket fitted in the case.

Does the protection in these cells protect from short circuit if a wiring fault developed inside my case? Or do I need to fit a fuse on the +ve lead (and possibly on the tap lead too?) ?

Finally, assuming that balance charging a 6 cell pack is possible, what low cost charger options are there? I'm looking for a light weight charger to go with these lights (I'll be building a few of them for myself) which will be dedicated to just that job. So I do not need a full featured hobby charger unless there is no other option.


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## Mr Happy (May 15, 2011)

Hi Footleg,

Your second configuration with the centre tap is much preferable to the first. Putting three Li-ion cells in parallel just creates one big cell, and Li-ion cells are very happy that way. Your second configuration is therefore "two big cells in series", and this is much less complex than the first configuration.

For protection, I have doubts about using protected cells in this manner. Normally with Li-ion one wants to protect the battery as a whole, not the individual cells inside it. If you could (I am not saying this will be possible) you should try to find a protection circuit for a 3p2s pack and build this into your battery yourself. Failing that, use unprotected cells but be careful with battery management. Use a balance charger, don't over charge, don't over discharge, and don't allow any short circuits. Fuses are your friend.

On balance charging, this aspect is important. You don't need a really expensive charger necessarily; look at chargers like the Bantam BC6. But do use the balancing tap.


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## moderator007 (May 15, 2011)

Footleg said:


> Finally, assuming that balance charging a 6 cell pack is possible, what low cost charger options are there? I'm looking for a light weight charger to go with these lights (I'll be building a few of them for myself) which will be dedicated to just that job. So I do not need a full featured hobby charger unless there is no other option.


 
A hobby charger will probably work best. The time it takes to charge your pack will be considerably less with a hobby charger. 3 batterys in parallel will be as one 7200 mah battery. Your pack will be 7.2 volts with 7200 mah. Charging at 500ma will take you roughly 15 hours to charge. Never charge unattended. The hobby charger will allow you to charge the pack at 7.2v at 4 amps (on the safe side). Charging time will be roughly less than 2 hours. Also you can buy a cigarette lighter plug to power your hobby charger. Charge it while you on your way. You should wire in a balance plug and use while charging to ensure the pack is balanced. If your looking for a small charger try the turnigy accucel 6. This thing will fit in one hand. Try hobbyking they have lots of chargers.

As mr happy suggested you should use a 7.2v pcb protection circuit wired in to the pack. You can find one here or try ebay.


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## HarryN (May 17, 2011)

Hi,

Mr. Happy, it makes perfect sense to me that the ideal configuration is version 2 with the center tap, and that it should really be built with a pack protection circuit. (which are pretty common BTW).

I am sort of lost on your advice though on using unprotected cells vs protected cells if he cannot manage to add a pack protection circuit. I cannot ever imagine myself using unprotected 18650s, especially if bundled together in a pack with no overall protection circuit setup.

My logic, is that if an individual cell does in fact trip (for example from over dischange voltage) then it has done its job properly and kept the cell from being damaged. The other cells in that section would be soon to follow. Once back on the charger, wouldn't these reset and charge again ?

Please help me understand your concept. 

Thanks

HarryN


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## Footleg (May 17, 2011)

Thanks for the input so far. I also am a little confused as to the merits of using protected vs unprotected cells in packs. Given that I have already purchased individually protected cells, is there any real advantage to using an additional pack protection circuit? Will the individual cell protection circuits upset a pack protection circuit if they start to disconnect individual cells as the voltage drops as the cells become drained?

I can see why for a large scale manufacturer the cost savings of using unprotected cells with a pack protection circuit make sense. But I am only building a few of these packs and I would have thought that the individual protected cells would be safer given that I am building the pack myself and there is always a danger that I could get a short within my pack wiring if something came loose. Is there any fundamental reason why a pack built using individually protected cells cannot be charged and discharged as a pack using the sort of balance charger that has been recommended above?

I don't think the option to use a completely unprotected pack and just be careful to no over discharge would work for my purpose. I need to be able to give these lights to inexperienced assistants to operate while I am setting up and taking photographs. Sometimes the lights have to be positions in hard to reach places and left on until I can get back to recover them at the end of a photo session. So they have to be able to handle being left switched on when the cells start to drain out without damaging the cells. As far as I understand it the cell protection circuit will do that job be disconnecting the cell internally when the voltage drops to a level that would cause damage to the cell.


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## moderator007 (May 17, 2011)

I ask this question once here about protected vs non protected batterys in a pack. You can read the thread here. I did build the pack with protected cells. The same black and red flame trustfire's you are talking about. The pack has tested fine with no problems. Have yet to finnish the light. I have run various discharges and charges to see how things act before it goes into service. I did wire in a pack pcb and a balance plug.

If you have three cells in parallel with a pcb on the bottom of each, like the trustfires do. The cut off current will be the sum of all three battery pcb's together. I know that these trustfires will run 3 amps without the pcb cutting them off. At what current they cut off i dont know. Three connected in parallel will cut off well beyond 9 amps. The pack pcb i linked above cuts off at 5 amps. You can find some even lower if your current draw is lower. 

If you wire in a pcb for the pack it will limit what ever current the pcb of the pack was made to cut off at. A pcb also protects each 3.7v parallel side of the pack. If one side falls behind the pack pcb will cut the whole pack off because of thats side low voltage. It also protects each parallel side from over charging. Balancing the pack is almost a must unless you can charge each cell independtly and check there voltages before use.

Unprotected high quality japan cells is the safest way to go. Which most people here will recommend you use. With a pack pcb wired in with a balancing plug and a charger that will balance the pack. They have proven to be more reliable in packs. Its the same setup in a laptop. This route can be very expensive with a large number of batterys. Not everybody can afford this route. So here we are.:thinking:


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## Footleg (May 18, 2011)

Thanks for that link to your other thread. Seems we are building almost the same type of pack! Very interesting discussion. Also it had not occurred to me that my pack will be able to drive 3 times the max current the protected cells allow because I have 3 in parallel. I think I will be fitting a 5A fuse to the pack as a cheap way to protect from a short. My maximum current draw will be 2.8A for my light (or maybe 4.2A if I decide to put in 6 LEDs instead of the 4 I originally planned) oo:

What might make things safer for you if you want so many cells in one light would be to make 2 battery packs and have a switch that can use either one or the other of them? I considered this for my light. I could make 2S2P packs x 2 and use one to drive half the LEDs and one to drive the other half. Then you have less cells at risk of messing with each other. You also know when the light is half used when you have to switch over from one pack to the other if you are driving just one circuit.


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## moderator007 (May 18, 2011)

Footleg said:


> Also it had not occurred to me that my pack will be able to drive 3 times the max current the protected cells allow because I have 3 in parallel.


 
Not only is it three times the current but six times the power of a single cell because its 7.2v.

I did consider 2 packs with a switch. But my friend wanted the charging to be external same as it was. Plus trying to have plugs external for charging would have been quite challenging with balancing plug for each.

I used a hobby charger and a 5 pin mini din plug to make the connection from charger to light. Wired up a custom 5 pin mini din plug on the hobby charger so its simple plug and charge. I have the balancing wires on three pins and power on the other two.


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## Footleg (Jun 9, 2011)

moderator007 said:


> A hobby charger will probably work best. The time it takes to charge your pack will be considerably less with a hobby charger. 3 batterys in parallel will be as one 7200 mah battery. Your pack will be 7.2 volts with 7200 mah. Charging at 500ma will take you roughly 15 hours to charge. Never charge unattended. The hobby charger will allow you to charge the pack at 7.2v at 4 amps (on the safe side). Charging time will be roughly less than 2 hours. Also you can buy a cigarette lighter plug to power your hobby charger. Charge it while you on your way. You should wire in a balance plug and use while charging to ensure the pack is balanced. If your looking for a small charger try the turnigy accucel 6. This thing will fit in one hand. Try hobbyking they have lots of chargers.


 

Thanks for the advice. I just received my first hobby charger in the post. After researching a few websites of possible suppliers I ended up getting the Turnigy Accucel 6 from Hobby King so your suggestion was spot on.


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## RepProdigious (Jun 9, 2011)

Well, you've already got a charger that can balance so why not go with a pre-made pack? Something like this;

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14072

Hooks up to your accucel straight from the box if im not mistaken.....


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## RepProdigious (Jun 9, 2011)

Or this one, way cheaper 
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9172


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## Footleg (Jun 10, 2011)

I did consider pre-built packs. But both of these are too large for the case I am building my light in. I need the 7.2V so the 6.6V is too low for my requirements. Then there is the capacity. My 2S3P pack will give me 7.4V at 7200mAh. Cost around $29 to make too. That second cheaper pack looks like great value. But is just too large for my needs.


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## Footleg (Jun 13, 2011)

So over the weekend I dismantled my prototype battery pack in order to fit a balance tap. On measuring the cells, I discovered that while 4 had discharged in use, 2 had not. I realized that one of my cells must not have been making contact with the holder, so the pair it was in were not being used at all. Now with a balance tap as shown in my second diagram at the top of this thread, a bad contact on one cell would result in just that one cell being left out. So I would in effect have 2 cells in parallel, connected in series with 3 further cells in parallel. Or to put it another way, a 4800mAh cell in series with a 7200mAh cell. Given that all my cells are individually protected, is there an inherent danger here if one cell failed to contact the cell holder properly and and was to charge/discharge this pack with a balance charger?


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## CKOD (Jun 13, 2011)

Footleg said:


> So over the weekend I dismantled my prototype battery pack in order to fit a balance tap. On measuring the cells, I discovered that while 4 had discharged in use, 2 had not. I realized that one of my cells must not have been making contact with the holder, so the pair it was in were not being used at all. Now with a balance tap as shown in my second diagram at the top of this thread, a bad contact on one cell would result in just that one cell being left out. So I would in effect have 2 cells in parallel, connected in series with 3 further cells in parallel. Or to put it another way, a 4800mAh cell in series with a 7200mAh cell. Given that all my cells are individually protected, is there an inherent danger here if one cell failed to contact the cell holder properly and and was to charge/discharge this pack with a balance charger?



With the individual protection, you should be fine, obviously the 4.8AH bank of cells would discharge first. If you were unprotected, you could run into overdischarging (damaging to the cell, not dangerous until you start to chage them again) or reverse charing(definitely dangerous) 

The 2 cells that discharge should go into protection and the pack would cut out, however if your load cuts out as the first bank drops off in voltage, you should be fine. I.E. if they 3x bank is at 4.0v and the 2x bank drops off to 2.5V, and 6.5v will make your light cut out, then you wont overdischarge the bank missing the cell too severly, though hopefully by 2.5v the protection circuit should have turned on already. 

Would you consider something like http://www.luminousdiy.com/Light%20Kits%20and%20Parts.htm one of their 6X holders? The individual springs would make it so you dont run into the issue as easily (they can be wired however you like, not just 6S or 6P ) Contact them to find out the dimensions of the holder, and if they are still available. Not sure just how tight it is in your light.


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