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Old Oct 04, 2004, 08:38 PM
Impossible? Hah!
KreAture's Avatar
Oslo Fornebu, Norway
Joined Jun 2003
3,074 Posts
Idea
Building the perfect LiPo pack. <- Post ideas here!

I have been thinking a lot about how to make my LiPo packs.
There are so much to consider, and I find a lot of the comercial packs wanting in many areas. Would be nice to make the pack with every base covered. (So to speak...)


Stuff I found one has to consider:
- tabs prevented from shorting
- "+-+" or "+++" type stacking
- access to center-tabs for balancing
- supporting the exit-edge of the pack
- strain relief on wires
- supporting the cells to prevent bending
- NON permanent gluing/bonding as pack may need service. (Especially for larger packs.)
- some protection from denting ?


Problem is how to cover all bases. the tab-exit edge is prone to leaking at the tabs if you ding it so it really needs protection.

So... How do YOU do it?
Post a picture of how you do it and what benefits you see by doing it like that. Let's have a nice, clean, ON TOPIC discussion!
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Old Oct 04, 2004, 09:36 PM
Registered User
England
Joined Mar 2004
93 Posts
Kreature

It's also good to consider

- minimal wire lengths
- ventilation for cooling
- ability to parallel cells in use and disconnect for cell by cell analysis
- optional cell balancing system that you don't have to carry as payload
- drop-in size and shape to fit models and achieve CG positioning

It's also helpful to have acess to 1000s cells for pre-construction matching, a dedicated QC/QA process and a piece of kit like this:

to help with analysis.

and we end up with this:




Nobody's perfect but we're sure as heck out there trying

Sincerely,

Julian
www.flightpower.co.uk
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Old Oct 04, 2004, 09:43 PM
Impossible? Hah!
KreAture's Avatar
Oslo Fornebu, Norway
Joined Jun 2003
3,074 Posts
Dang! First post and it had to be an advertisement. I am seriously dissapointed right now.
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Old Oct 04, 2004, 09:50 PM
GO THE DOLPHIN!
ndflicks's Avatar
Central North Carolina
Joined Sep 2003
839 Posts
I have to agree.... that's pretty lame to just jump in and hijack someone's thread for 'free advertising'. Kreature didn't title his thread "BUYING the perfect pack". it was titled "BUILDING the perfect pack'

Shameful.... just shameful.
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Old Oct 04, 2004, 10:01 PM
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England
Joined Mar 2004
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Quote:
Originally Posted by KreAture
I find a lot of the comercial packs wanting in many areas.
Not an advert, an answer, and for anyone wishing to DIY some packs, maybe even some tips.

Seriously, no offence intended.

Julian
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Old Oct 04, 2004, 10:10 PM
Straight up is all I ask
Fairfield Ct USA
Joined Apr 2002
2,323 Posts
My Method I can reconfig at will

I have made many packs of Irate 2600
I use PC board to secure the tabs which makes it super unlikley that a tab will ever break due to being worked back and forth . I got the Idea from DNA and added a few touches .
1. Get blank pc board from radio shack and cut into a size that fits in the void at the end of the cell.
2. Peel copper off the back of the pc board and cut a 1/4 inch slot in the center of the top and remove the copper . This divides the + & - sides
3. use 5 min epoxy to glue prepared pc board into the void area of the cell divided side up so that the tabs can be pulled arround and secured with solder .
4. Pull tabs arround the board and trim to length
5. Solder tabs to pc board (pull them tight for no flex)also only handle tabs twice once to trim and once to solder this reduces chance of breakage.
6. I use 1 inch pieces of wire to connect the cells in series soldering to the outer edge of the pc board . You can secure anything you want to pc board without touching the tabs or heating them again. Good luck
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Old Oct 04, 2004, 10:29 PM
Straight up is all I ask
Fairfield Ct USA
Joined Apr 2002
2,323 Posts
A cell

Here it is
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Old Oct 04, 2004, 10:40 PM
Registered User
Singapore Paya Lebar, Singapore
Joined May 2004
142 Posts
jmolwitz,

The pc board certainly protect the tabs, can you draw high current without the copper getting too hot?

Thanks

Triton
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Old Oct 04, 2004, 10:53 PM
Registered User
Singapore Paya Lebar, Singapore
Joined May 2004
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Julian,

Since u wanted to provide some tips, how about telling us- how you match the cells?

Cheers
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Old Oct 05, 2004, 02:16 AM
Impossible? Hah!
KreAture's Avatar
Oslo Fornebu, Norway
Joined Jun 2003
3,074 Posts
One of my monster packs are attached.

I was thinking of using this method again, but I'd like for the pcb to be supported by the cells better. Not just lie against the edges of the cells.
Maby I can add something in the dents to fill the gaps?

Still, strain-relief on the wires going from the pcb would be nice too.

I had to disassemble that pack again because two of the cells in it were leaking. This was not my fault, but rather a production fault. There seemed to be bad sealing at the + poles. Pack was never flown. Sweet smell was what brought me onto the leaky cells so I investigated.
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Old Oct 05, 2004, 02:43 AM
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England
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Triton, I think you are bending my words, I replied to Kreature's post because I was excited that we had addressed his issues and many more besides in a stock product. Nevertheless I will try to live up to your challenge.

To have a pack perform ideally, you need to match cells very finely in respect of capacity, internal resistance and charge state. The latter is the easiest because it can be estimated by voltage. The biggest challenge facing DIY pack building is that you buy bare cells and you get what you get, if you can’t match all that you get then you either have to keep buying more bare cells or accept that you will be making a compromised pack with what you got.

Assuming you have bought sufficient cells to be able to make choices, you can get pretty close to a matched pack in a home set-up: Firstly by running individual cells full cycle and get an idea of capacity for each cell. You will be selecting cells that match closely for capacity (so they dump together). You also need to test for Internal Resistance. You can either purchase an expensive IR meter or estimate IR by doing a voltage-under-load test using a static load like a big resistor (big coil of heavy gauge wire can work). Tight control on IR is required so the cells don't suffer a thermal runaway imbalance under load. Once satisfied that you have a group of 2 – 10 cells for your pack, you should also ensure that they are brought to an equal state of charge before putting them together – full charge is probably the easiest and this is fine. For an even higher precision result, do all the measurements twice separated by time - say a week or a month depending on your level of patience and check for drift in any of the stats for weaknesses that will show up over time.

It is also useful to be able to deploy repeated cell balancing by voltage at the end of the charge cycle using the principle of centre taps and differential charging. This will have the effect of repeatedly rescuing the pack from mild charge imbalance so long as over discharge or thermal imbalance has not impaired one or more cells in the pack beyond use. Every issue with cell balancing becomes increasingly critical with larger packs.

FlightPower has a proprietary cell matching protocol on our overseas production line that takes all of the above into consideration. The process can deliver entire 10-cell packs to a matching standard that falls within the resolution of quality measuring tools. It is fair to say that replicating this process for DIY packs would be extremely expensive in terms of wastage. For this reason alone I would venture to suggest that an organisation such as FlightPower has a useful role to play for the majority of modellers.


Hope this helps,

Julian

EDIT: Kreature, your pack looks nice, but you have heavy soldering low down on the tabs - the heat involved in this procedure is almost bound to de-nature the seals where the tabs pass through the pouch - most likely the cause of the lekage. Hence the use of ultrasonic welding. Try using smooth metal snub-nose pliers on the tab under the PCB as a heatsink while you do solder joints like this.
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Old Oct 05, 2004, 03:40 AM
Impossible? Hah!
KreAture's Avatar
Oslo Fornebu, Norway
Joined Jun 2003
3,074 Posts
FlightPower
I had 18 cells. I only used 8. From the 18, 2 of the used ones have leaked, in addition to 1 confirmed but possibly 2 of the other, unused cells.

It is a fabrication flaw, not a heat flaw. I know as I carefully watched the temp on the inner part of solder-tabs. (clamp-on temp sensor)
I also used a semi-high temp iron and low temp solder for a quick bond. No joint was heated for more than 2 seconds and temp of the joint between tab and cell was fine.
Also, I used a thick PCB to distance the soldering a bit more. (2mm thick)

I do agree though, that it can be a source of leaking.

Oh, and this is exactly what we need to discuss in this thread. How to solder the packs safely, at home. Not comercial solutions.
Cell-matching is relevant, but not when it comes to the physical building of the packs.

Edit:
All the extra solder on the board was put on seperately btw.
(For more conductivity.) When I soldered the actual joints, the extra solder helped quickly lead heat away from the bond just soldered to cause minimal heating down through the tab. I do agree on another thing though: LiPo's are not too friendly when it comes to assembling.
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Last edited by KreAture; Oct 05, 2004 at 03:48 AM.
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Old Oct 05, 2004, 04:55 AM
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Singapore Paya Lebar, Singapore
Joined May 2004
142 Posts
Flightpower,

Thanks for the reply, ok ok, I bent your words a little. But you are a good sport.
Cell selection is tough for DIY. Having said that, with limited samples, the lesser cells are identified and sorted for the best compromise.

KreAture,

Your pack looks very well assembled. Do you have any air gaps between the cells? I incorporate air gaps in my packs after experiencing intermediate cell failure.

I am about to built a 10s5p pack for a power boat, and I am thinking of clamping the tabs onto the pcb instead of soldering. Have u had any experience in clamping the tabs?

Triton
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Old Oct 05, 2004, 04:56 AM
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UK
Joined Jun 2003
452 Posts
Well, nice thread and not a hint of bashing other manufacturers and some good tips!


Nick T
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Old Oct 05, 2004, 05:30 AM
Impossible? Hah!
KreAture's Avatar
Oslo Fornebu, Norway
Joined Jun 2003
3,074 Posts
Triton
I use 2mm foam tabs between each set of paralell cells, and 1mm foam tabs between each paralell cell. (That gives you an idea of the physical size of the back LOL)

My only beef with that pack is what happens if the pcb get's a beating... It would knock on the edge of the pack with tabs, where it is the most vaulnerable.

I thought maby I could make some balsa spacers under the pcb to support it softly against the actual edge of each cell. Then I could actually increase the distance a bit and maby even use heatshrink around the base of each tab to protect agains serious crashes where the balsa seperating the tabs would be turned to mush...
(Not that such a thing should ever happen.)
Also, as FlightPower points out, I'd get some more distance between the soldering and the pack for less danger in heating.

Another thing... The PCB should maby be designed such a way that the tab penetrates the pcb, and then is folded to the side where it is soldered, leaving a slight distance to the hole so that any pushing on the pcb would not cause strain directly down into seam of pack. Instead it would flex the tab in a nice manner. (yes a lot of flexing will breake the tab, but a lot less flexing is needed to breake the seal on a LiPo. Try taking a LiPO tab with a pair of pliers and pull/push carefully a dozen times or so, and the pack may start to leak. It's as easy as that.
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