# DIY battery pack q's: Protection, balancing, lipo vs li-ion



## langen (May 21, 2005)

So I'm thinking about changing out at least one of my two batterypacks, and I think I want to go the DIY route.

I've read this thread
http://forums.mtbr.com/showthread.php?t=571398
but still have some questions, that I hope someone could answer 

*Protection - single cell vs for the whole pack:*
I think I'm going to buy protected 18650's, and just connect them in series using e.g. SDNatives holders.
I can't see any other disadvantage of single-cell protection vs pack protection other than if one of the cells reach cutoff voltage long before the others, the cell (and your whole pack) shuts down.
On the other hand - if this happens, the cell in question probably is faulty, and the shutdown of the pack prevents a too deep discharge of this cell. This would not have happened with pack protection only.

*Balancing:*
Is this really necessary for bike light batteries that don't see very heavy use? 
I understand that when batteries are used heavily as in RC, this becomes a more or less requirement.
But - if I'm gonna build a pack from scratch, I might as well build in wires for balancing. 1-6 cell balancing chargers are also quite cheap:
http://www.hobbyking.com/hobbycity/...ame=Turnigy_Accucel-6_50W_5A_Balancer/Charger

*Li-Po vs Li-Ion:*
On the site linked to above, it seems like you get more Wh's / $ with LiPo's than with Li-Ions. But do these LiPo packs have built-in protection circuits??
If I understand correctly, LiPo can take higher charge- and discharge current than Li-Ions, but that's not really needed for my use.
And the safety aspect: Charged and discharged correctly, I guess neither of these are much safer than the other?

So - a battery solution (4 x 18650's) for a 3 x XP-G light would look something like this:
Balancing charger:
[See above link]
+ suitable balancing wire harness
+ suitable power supply (probably have one lying around the house)

Batteries (2500-2600mAh):
-From DX:
http://www.dealextreme.com/details.dx/sku.5790
or
-From AW on cpf (highly regarded, but about 4 x (!!) the price of the DX ones.)

Holder:
http://www.luminousdiy.com/Light Kits and Parts.htm#holders

Are there other factors I should take into consideration?

..and thanks for reading!

(I would have posted this on CPF too, but the site is down...)


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## lsocoee (Oct 29, 2009)

I don't think the Lipo batteries from Hobbycity are protected. I've destroyed many of them by discharging them too much. 

I think that RC plane guys would rather risk destroying a battery and getting their planes down safely.


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## vroom9 (Feb 24, 2009)

RC lipo's = no protection

Using a battery holder like SDNative's allows you to remove the cells for charging. Charging them individually is balancing and is by far the safest way to charge. If you build a pack it's not that hard to add the balancing connector and a "real" charger like the one you linked to will work well.

Don't rely on the protection circuitry to turn your light off. The voltage on most of them is too low for long life.


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## bikerjay (Sep 16, 2007)

There have been several threads on this exact question before....

I am starting to this we need a big sticky battery thread.

this thread links to some previous questions.
http://forums.mtbr.com/showthread.php?t=571398

Vroom is totally correct. It would be most cost effective to either use unprotected individual cells soldered together and a balance charger to charge, or use protected cells and charge individually. If you are going to use a balance charger you may as well peel off the shrink wrap and remove the protection circuit and use the tabs to solder the cells into a pack, eliminating the need for a cell holder.

The big question in your set up is what are the electronics in the light(s) you will be powering. The reason I say this is because it should dictate your whole pack solution, from the number of cells to the protection scheme. If you are not using electronics to drive the LED that cut out a set voltage programed according the number and type of cells you are using you absolutely need some sort of protection.Vroom eluded to this point. The protection in many cheap single cell schemes doesn't work like it should and will often allow over drain any way, and vrooms point about protection kicking in with the cut off too late is also quite true. A taskled.com flex driver handles low voltage cut off at a programmable point and also does fuel gauging.

Another solution is to use the balance plug to use a cell monitor / voltage indicator/ alarm. These are cheap from hobbycity as well.


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## langen (May 21, 2005)

Thanks guys!

So LiPo's are out.

I'm going to use a bFlex - both for temp and voltage sensing. 
It will either be a 3 x XP-G or an MC-E (2S2P) + a single XP-G. Either way about 10W on max power.

Since I don't feel comfortable soldering batteries together, I think a solution with a holder seems like a good one. Even though it may be overkill, I'll balance charge the batteries.

I guess my remaining questions are:
-Are there any other (dis)advantages to using single protected cells in series rather than one protection circuit for the whole pack? I haven't read anything that I think answers this one 100%. 
-Are AW cells *that* much safer than DX cells?


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## yetibetty (Dec 24, 2007)

> I guess my remaining questions are:
> -Are there any other (dis)advantages to using single protected cells in series rather than one protection circuit for the whole pack? I haven't read anything that I think answers this one 100%.


 Only if the cells are solderd together as a pack. this is due to the fact that the single cell PCB is only designed to take an input voltage of 4,2v from a charger.

So if for example you were to build a 14.8v pack with single protected cells and try to charge it as a pack the 16 odd volts from the charger would pop each PCB, that's when you require a pack PCB.... does the same job but can handle the chargers output.


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## BlackDawn (Nov 19, 2007)

You can always use a pcm from battery space to protect, charge & balance the 4 cell pack from hobby city (my plan  )


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## mtroy (Jun 10, 2005)

BlackDawn said:


> You can always use a pcm from battery space to protect, charge & balance the 4 cell pack from hobby city (my plan  )


Very interesting. Cool link. So what charger would you use for this...just a 14.8V DC supply or like this?


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## langen (May 21, 2005)

yetibetty said:


> Only if the cells are solderd together as a pack. this is due to the fact that the single cell PCB is only designed to take an input voltage of 4,2v from a charger.
> 
> So if for example you were to build a 14.8v pack with single protected cells and try to charge it as a pack the 16 odd volts from the charger would pop each PCB, that's when you require a pack PCB.... does the same job but can handle the chargers output.


This must be wrong.

It's been some years since the electrics classes at university, but I'm pretty sure that voltage in a circuit divides itself among the components in the circuit, depending on the resistance of the component. Assuming this applies for a battery charging circuit as well.
If - as you say - each cell sees 16 odd volts, then the charger would have to put out 64 odd volts for 4 cells. And it doesn't.
(Series connections are voltage additive)

Besides - a balancing charger measures each cell to charge it up to 4,2V.

Edit:
The Turnigy Accucel6 manual says: 
"max.charge voltage:4.2V/cell"
which I think proves my point


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## OverTheHill (Dec 3, 2004)

langen said:


> This must be wrong.
> 
> It's been some years since the electrics classes at university, but I'm pretty sure that voltage in a circuit divides itself among the components in the circuit, depending on the resistance of the component. Assuming this applies for a battery charging circuit as well.
> If - as you say - each cell sees 16 odd volts, then the charger would have to put out 64 odd volts for 4 cells. And it doesn't.
> ...


This post would seem to empirically back up your argument:

http://forums.mtbr.com/showpost.php?p=6243025&postcount=8

Does anybody else have experience of this one way or another?

Regards,

OTH


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## yetibetty (Dec 24, 2007)

Yes your right a balance charger charges each cell @ 4.2v just like charging each cell with it's own charger, I use a balance charger with packs with no protection apart from a polyswitch.

I was thinking more along the lines of a "normal" 14.8v charger with single PCB's on each cell .
I could be wrong on this but if 2 or 3 cells have charged and their PCB's have cut the charge to them then won't the remaining cells and their PCB's have to take the full juice of the charger.


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## langen (May 21, 2005)

yetibetty said:


> Yes your right a balance charger charges each cell @ 4.2v just like charging each cell with it's own charger, I use a balance charger with packs with no protection apart from a polyswitch.
> 
> I was thinking more along the lines of a "normal" 14.8v charger with single PCB's on each cell .
> I could be wrong on this but if 2 or 3 cells have charged and their PCB's have cut the charge to them then won't the remaining cells and their PCB's have to take the full juice of the charger.


This is how I *think *it is:

Using an unbalanced charger: 
When one of the cells reaches cutoff @ 4,2V, and the PCB for that cell kicks in, the current stops going through that cell - i.e. open circuit. This is in practice the same as disconnecting the battery pack from the charger. If your cells are balanced - fine. But if there is a large voltage variation, it is the cell with the highest voltage that dictates when the charging stops, and the cell(s) with the lowest voltage will never reach 4,2V.

Using a balanced charger: 
Same as above - when the first cell cuts off @ 4,2V, the main charge circuit will be open circuit. BUT now you still have the balancing circuits to top off the remaining cells. 
*Unless *the whole charging (both main and balancing circuits) shuts down when the main circuit detects open circuit. Anyone know what is the case here?


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## OverTheHill (Dec 3, 2004)

I may be wrong here but logically to me it seems like it is not the protection PCB which is cutting out on charging with an unbalanced charger. Isn't it the charger itself which is sensing the overall voltage of all the cells and stopping charge? Isn't that the case with 1 cell protected on a 4.2V charger or 4 on a 16.8V?

Edit: Also I just checked on All-Battery.com. Their individual cell protection boards are rated at an overcharge protection voltage of 4.325V. Surely this would indicate it is the charger which is cutting out at 4.2V not the protection circuit?

Reading ktronic's posts on the whole thread I linked to also indicates to me that he is not balance charging his 14.8V 4S individually protected battery pack because he advises it is possible to charge unbalanced for 10 charges and then balance charge after that.

It would be nice if we could get a definitive answer on this question from someone.

Regards,

OTH


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## langen (May 21, 2005)

OverTheHill said:


> I may be wrong here but logically to me it seems like it is not the protection PCB which is cutting out on charging with an unbalanced charger. Isn't it the charger itself which is sensing the overall voltage of all the cells and stopping charge? Isn't that the case with 1 cell protected on a 4.2V charger or 4 on a 16.8V?


I agree - *if *the pack voltage reaches 16,8V, the charger will stop charging. But with individual protection, one of the protection PCBs will have to open the circuit before the whole pack reaches 16,8. 
When one cell reaches 4,2V the 4 cells may look like this (a balanced pack):
4,2V (prot PCB opens the circuit and stops charging. Pack voltage = 16,77V)
4,19V
4,19V
4,19V

or like this (an unbalanced pack)
4,2V (prot PCB opens the circuit and stops charging. Pack voltage = 16,2V)
4V
4V
4V

For the latter example your 3 remaining cells are not nearly done.



OverTheHill said:


> Reading ktronic's posts on the whole thread I linked to also indicates to me that he is not balance charging his 14.8V 4S individually protected battery pack because he advises it is possible to charge unbalanced for 10 charges and then balance charge after that.
> 
> It would be nice if we could get a definitive answer on this question from someone.
> 
> ...


This seems very logical. If you balance charge every 10th charge, your cells will more or less always be pretty balanced (like my 1st example), and you almost get full charge every time.

Hope this makes sense to more people than me...


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## bikerjay (Sep 16, 2007)

Black Dawn the board you linked to does not handle charging, you still need a LiOn charger. The PCM you linked to just evens out the cells voltage after the pack is charged.

With one of these all you need is a constant voltage power supply to charge: 
http://www.batteryspace.com/cmbfor1...alimitwithsmartchargingandfullprotection.aspx


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## MtbMacgyver (Jan 23, 2007)

langen said:


> I agree - *if *the pack voltage reaches 16,8V, the charger will stop charging. But with individual protection, one of the protection PCBs will have to open the circuit before the whole pack reaches 16,8.
> When one cell reaches 4,2V the 4 cells may look like this (a balanced pack):
> 4,2V (prot PCB opens the circuit and stops charging. Pack voltage = 16,77V)
> 4,19V
> ...


There isn't as much difference in a whole pack protection PCB and per cell protection as you're making out. The whole pack PCB also monitors the voltage of each cell individually and shuts down the charge or discharge current if any cell voltage goes out of the protection range.

The reason your example above doesn't happen in the charge cycle for a reasonably balanced pack is that the upper protection voltage is always in the 4.3 - 4.4 volt range. So the cells have to be unbalanced by a larger margin than your example for the protection to kick in. In practice, that really never happens when things are working normally. The charge is always ended by reaching the 4.2V per cell limit imposed by the charger in the normal case. The protection PCBs really almost never kick in unless a cell has really started to fail or someone has connected the pack to a broken or incorrect charger. But, that will happen at some point near the end of life of the pack and that's when it saves you from a catastrophic event.

In the case of individually protected cells, once one of the cell protection PCBs opens it does break the series connection through the whole pack. That's the effectively the same as disconnecting the pack. The higher charger voltage should not therefore damage any of the other PCBs on the other packs.

There are two reasons I prefer built up packs over cell holders.

The first is reliability of the power connection. This isn't much of an issue if your light doesn't contain electronics, but it can cause a problem when there are electronics. Vibration and jolts can cause the metal to metal contacts between the cells and holder to disconnect or change resistance for very brief periods of time. This can cause noisy power input to the electronics. There are ways to eliminate most of this risk such as a sturdy cell holder with good spring pressure on the clips and good filter capacitors on the input to the electronics. But, if you want ultimate reliability why not eliminate the source of the potential issue.

The second reason is the logistics around keeping a set of equivalent cells together. There are a lot of advantages for keeping a set of cells together such as more naturally staying balanced. I always build packs from a set of cells of the same age. Once built into a pack, I don't have to worry about keeping the set together. Once a cell starts to go bad, typically the rest of the cells aren't far behind so I always dispose of the pack at that point. With a holders and individual cells, I can label cells or use other methods to keep track of the history of cells, but frankly that ends up being more work than I want to deal with. Once you start mixing and matching cells issues with imbalance and the potential for the pack cutting out un-expectantly go up tremendously.


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## langen (May 21, 2005)

MtbMacgyver - thanks for a very informative answer! :thumbsup: 
It's nice with someone with real experience - I was just making assumptions..



> In the case of individually protected cells, once one of the cell protection PCBs opens it does break the series connection through the whole pack. That's the effectively the same as disconnecting the pack. The higher charger voltage should not therefore damage any of the other PCBs on the other packs.


Just one question remains:
Are you talking about normal or balance charging in the quoted text?


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## bikerjay (Sep 16, 2007)

When using a sophisticated balance charger like the turnigy accucell 6 the charger monitors the main leads as well as the balance connector when the input is something unexpected its going to cut off the charge. If you disconnect the balance lead while its charging you get an error message, same for the main leads. 

The only real problems with protected cells from DX is the quality of the protection circuit. This is only a big problem if you rely on the protection circuit to cut off the pack at the end of its discharge. Not a problem for a taskled flex powered light with a properly programed cut off. A real problem for a light using a simple constant current regulator like a buckpuck. 

With A DX protection PCB the cell voltage is only checked on the connection of discharge. What this means is that if you hook up a load to a cell that is above the cut off voltage it will happily discharge all the way to zero volts. If however you disconnect the load after the cut off voltage has been reached and try to connect it again the circuit wont close.


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## vroom9 (Feb 24, 2009)

Lots of good info here, but I'll add a bit more.

Lipo's -- The big issue with them is physical protection. They are fragile compared to the metal canned 18650's. Hit them hard enough to deform them and they are pretty much done. After damage they can even catch fire right away or when charged. The low impedance of Lipo's also makes shorting a pack an exciting experience. Hundreds of amps can flow which results in melting wires and often fire since the components of the battery are flammable. Fuses are good.

That said I'll be using them in one lighting application because they will fit o much better.

Balancing chargers -- Perhaps some high end models do charge each cell individually, but the less expensive ones charge the whole pack from the "ends." The balancing connector is for a set of precision resistors that even out the cells. In order for this to work the cells can't be way out of whack.


Protection circuits -- These things are there to prevent fires. If they cutoff at 4.35 volts, that is way too high. The battery will still be destroyed if that happens. Even a single cycle of that will result in measurable loss. Same thing for a 2.5 volt low cutoff. Make those limits 4.1 volts and 3.1 volts and the cell will last longer.

Individual protection circuits will work, it's just that one may trip resulting in an open pack. It will still do the job of preventing catastrophic failure.


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## MtbMacgyver (Jan 23, 2007)

langen said:


> MtbMacgyver - thanks for a very informative answer! :thumbsup:
> It's nice with someone with real experience - I was just making assumptions..
> 
> Just one question remains:
> Are you talking about normal or balance charging in the quoted text?


Well, it could happen in either case, but would be far more likely in the unbalanced case. In the balanced case, it would typically only happen when the charger voltage wasn't properly limited and all the cells started getting too high. One cell's protection circuit will be the first to pop and then charging will stop.

I really wish there were chargers and protection circuits that had an indicators telling the end user if a protection circuit tripped. I wouldn't use a pack where a protection event occurred unless I understood specifically why. They really shouldn't happen in normal usage.


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## MtbMacgyver (Jan 23, 2007)

bikerjay said:


> With A DX protection PCB the cell voltage is only checked on the connection of discharge. What this means is that if you hook up a load to a cell that is above the cut off voltage it will happily discharge all the way to zero volts. If however you disconnect the load after the cut off voltage has been reached and try to connect it again the circuit wont close.


I haven't played with any individual cell protection circuits extensively so I don't have any first hand experience, but if that's how the DX circuit works I wouldn't consider it acceptable. I do have a few protected DX cell in some flashlights, so now you have me worried. I may have to do a little testing.


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## lidarman (Jan 12, 2004)

Good webinar on building li-ion packs if you are inclined.

http://edn.resourcecenteronline.com/resource_center/asset/1408

Unfortunately, you do have to register.


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