crlock

Hi guys, i'm looking to do some experimenting with an old battery i have. It's a saddle pack, a turnigy 5100mah 2s 3p.
I already took it apart and wired it to leave it permanently connected, so now I have a single pack. but i was thinking
Can I wire this thing to make it a 3400mah 3s 2p? i believe each cell in the pack is 1700mah, so 6 cells at 1700mah can be wired to make a 3400 3s2p pack right? or im just about to destroy this pack?

AndyArdmore

That seems right. Just be careful. Make sure the cells are balanced, and be careful with the soldering iron's heat. It might be safer to just go and buy a 3s pack for your vehicle of choice, in the desired form factor.

killer rock

It can be reconfigure that way but you do run the risk of destroying the batteries and catching them on fire.

I wouldn't do that. But it can be done.

nbTMM

What you have is two 1S3P saddle packs - in each saddle pack there are thee cells connected in parallel, and then the two 1S saddle packs are connected in series to make a 2S battery. You could change the wiring in each saddle pack from parallel to series configuration which would make two 3S1P 1700mAh packs, each would need a 3S balance cable added and be balance charged separately. Technically you can connect the packs in parallel to get 3S with 3400mAh capacity however this is somewhat dodgy as if they aren't both fully balanced charged to exactly the same voltage when you connect them together, a huge current will flow from more charged 3S pack to the less 3S charged pack, damage and fire may occur. Using each pack independently as a small 3S 1700mAh pack or connecting them in series as a 6S1P 1700mAh can be done without issue however.

The least dodgy 3S solution would be to make a permanent 3S2P pack which would require extra permanent connections between saddle packs since 1 cell in a saddle needs to be wired in parallel with one cell from the other. Would make more sense to make a new regular size casing or permanently bolt the two saddle casings together. Any time you connect 2 or more cells in parallel you need to make sure they are balanced before connecting them together with high current wiring. This can be accomplished with a balance charger or connecting them in parallel with a power resistor for some time before adding a direct connection.

If you have any doubt about what you're doing (which it sounds like you do) I'd recommend not attempting it and buying a premade 3S pack.

billdelong

iTrader: (33)

I would encourage you to read the following article so you understand the risks of mis-matched cells:

LVC Adjustments and Packs in Series

nbTMM

This is the biggest load of crap I have ever read, and if that is representative of SMC's understanding of electrical engineering it is no wonder why people have so many problems with the batteries they sell. It beggars belief that a battery 'manufacturer' would have such a misinformed understanding of how their own products can be used. If this is not the case, I can only assume that they would publish misinformation like "the second pack sees double the resistance" to try to sell more higher voltage packs and discourage customers from connecting two existing lower voltage packs in series.

There are no issues with running two identical packs in series to create a higher voltage pack, and this is exactly what is happening inside a single physical pack with multiple series cells. The only difference is that by having two physical packs it puts the onus on the owner to:
1. Ensure that both packs are charged to the same level (read: fully charged) before use as a series pack. For two physical packs I would always recommend charging them individually with the same charger, or making a balance cable adapter so they can be balanced charged together in the same configuration that they are used in the car.
2. Ensure appropriate cutoff voltage is set such that no cell is over-discharged in use - the same has to be done for a single physical pack if there is a capacity mismatch between the cells. And yes you CAN use two packs which are not perfectly matched in capacity, just that the cutoff has to be set higher to prevent over-discharging the lower capacity pack. If they are the same brand and model pack, you should be golden. Just check your cell voltages every few minutes the first time you run it and when any cell is getting too low (<3.4V), your safe cutoff voltage becomes whatever the total voltage is at that moment.

Of course you want to do #2 again periodically to keep an eye on how things are going and that the packs are wearing equally. This can be as simple as looking at all the cell voltages each time you hit low voltage cutoff. You should be doing this already with single packs, because cells inside the same casing can wear unevenly too. Theoretically, over time you'll have to set your cutoff voltage higher and higher as cells become more and more mismatched. Eventually the cutoff becomes too high to be practical but by then the capacity must have dropped to something pretty terrible anyway.

billdelong

iTrader: (33)

I can speak of first hand experience with mis-matched 2S packs that I ran in 4S series many years ago... both packs were identical in age but were used individually in my 2S powered cars, and to save money I ran the same packs in my first 1/8 buggy. What I found was that my buggy would hit LVC prematurely and 1 pack would always drain quicker, often the cells of the pack that drained first would go below 3.0V... after damaging that pack I replaced it with a new pack and this only made problems worse... because the resistance was too high on the old pack, I would repeat the exact same problem and killed my brand new pack within 2 race days

I would then buy 2 new packs as a "matched pair", but I didn't rotate them (because I hadn't read the SMC article yet) and within 5-6 race days I killed another pack.... at this point I stopped running packs in series and have since ran single packs with true matched cells.

nbTMM

One pack draining quicker doesn't have to be a problem, just means that you don't use the full capacity of the higher capacity pack. Where people go wrong is that they have the voltage cutoff set too low and consistently over-discharge one of the packs, which damages it causing it's capacity to become worse every time it is cycled, becoming more severely over-discharged with every cycle until it is so degraded in capacity that it becomes noticeable..

Voltage vs discharge curve is non linear so what may see like an alarming voltage mismatch may not be a significant mismatch in capacity. For example, 3.6V/cell is ~90% discharged, 3.4V is ~95% discharged, 3.2V is ~97% and 3.0V is ~100%. So you could have one pack at 3.6V/cell and the other at 3.2V/cell and you have only 'missed out' on about 7% of your overall capacity. In that case you might find that a safe cut off setting is around 3.5 or 3.6V/cell. LiPos only become damaged below 3.0V, so as long as you aren't discharging any cells that far, no harm done, they will maintain good capacity and give you plenty of use. On the contrary, if one pack is say 3.8V/cell when the other hits 3.2V/cell, that is probably an unacceptable mismatch but this implies that the packs are wildly mismatched in capacity to begin with (one has about double the capacity of the other) - you're only going to get the capacity of the 'lower' pack, which tempts the user to set an aggressive LVC which operates the lower pack at dangerously low voltages, reducing it's life and causing it's capacity to drop even further, making it run into low voltages faster and faster every time until it is badly degraded and unusable. I.e. 3.8 and 3.2V quickly becomes 4.1V and 2.9V... the pack is being damaged but the LVC is blind to it because the higher capacity pack is maintaining voltage. If you note the cell voltages after every run, you should be able to catch this before it happens and worst case you just have to start using the packs separately or increase your cutoff voltage and accept a slight reduction in overall usable capacity.

Internal resistance does not have much effect unless you are drawing stupid currents because LiPos have extremely low resistance to begin with. The main concern is the overall capacity of the packs under normal use and as long as it is within a reasonable tolerance (say, 20% of each other), and you keep and eye on your cell voltages, series packs should be no issue.

As mentioned above, the exact same potential problems can occur within single packs, just that you're keeping all the cells together so they tend to age similarly (as the user is basically forced to only use them together) and they might be a little better matched out of the box. Most packs would be lucky to have their cells matched better than 5% out of the box anyway, so all it takes is an end user to set a too aggressive LVC and the pack will see premature death the same way that too aggressive LVC will kill series packs.

billdelong

iTrader: (33)

What you are failing to understand is that the pack with higher resistance won't let the battery drain equal capacity along with the pack with lower resistance, you end up killing the better performing pack with lower resistance as that pack will dump it's capacity quicker... you won't get the full capacity of both packs combined when each pack has different IR, but yes in a 5 min qual it's probably no big deal, but trying to get mix matched packs to last a 10 min main was nothing short of a disaster in my experience.

Everything stated by SMC is 100% legit, please re-read the article I linked, they know what they are talking about.

I will never run separate packs in series ever again

crlock

Great discussion, thanks everyone

Finally i'm going with two 1700mah 3S1P packs. it'll be just a matter of rewiring the already matched cells on each pack.
For the discussion above.
Connecting two packs it's the equivalent of connecting two identical hoses and opening the tap, the electrons flows at the same rate trough both packs. However if a cell on one pack is damaged, the electrons will encounter some resistance due to the crystalization of said cell, this can cause trouble. Thats why when you have an old pack and you put it together with a new one, you may end up damaging the new one. It has nothing to do with the act of connecting two packs, just make sure both packs are properly balanced and healthy.
Now i'm out to do some soldering, hope nothing explodes in my face xD

killer rock

​​​​​​
smc web site is a prime example of someone paraphrasing someone else's research not understanding it and then posting it on their own. That is the biggest load of hogwash and bull I've ever read. The person doesn't understand current flow resistance or voltage. You see the same current through a Series Battery both A and B. A resistance is the same for the most part whether it's a single pack 6s or two 3s packs wired together in series. im not going thro that this and picking it apart. but understand its full of lies and miss information and a general lack of understanding one the writers part mainly to sell batteries.

as a side note
there miss information will not stop me from buying there batteries as there cheap. and good quality batteries.

and there reaility in there is only a few factories that actually make batteries. i for get something like 3-5 factory smc is just like all those other batteries seller buying from them branding and selling.



Edit: I just readed finded reading smc article for my amusingment. I think the person that wrote the article English is a second... No third language. My punctuation and grammar is bad but there's is on another level.

trf211

What nbTMM said is correct from my experience. Having a healthy battery and a worn battery wired together will cause quicker discharge of the weak battery, not the healthy one. The old battery has LESS capacity, therefore drains quicker. Resistance does affect how much power can leave the battery but the amount of resistance in these batteries is very low to begin with, making it less of a factor.

billdelong

iTrader: (33)

This is possible, but only for an aged pack that has lost capacitance, if you have 2 packs that are close to age with identical capacity but different IR's, then the pack with least resistance will drain first... I have killed enough brand new packs to prove this to myself
100% false... not only does IR of each pack make a difference, but also the wiring of each connector, you are flirting with miss-matched resistance when too many different connectors are in the circuit, nothing good will come with miss-matched packs

Keep in mind that my experience with running packs in series was about 6+ years ago with 35C packs, I didn't have an IR meter at the time but was able to confirm the 6 month old pack still had the same capacity as the new pack. Both packs came from the same brand and same specs... the new one ballooned on me.

I switched to a single 4S pack and all my LVC issues went away, I also continued to use the 6 month old pack that was in my series circuit in my 2WD SCT for another 6 more months before any signs of swelling appeared.

trf211

There's many factors at play and I'm not going to break down what happened in your scenario right now.

What nbtMM said is true. If you use a high current setup and the LVC too low, it can quickly lower the capacity in whichever cell goes low first. High amp draw makes your battery voltage dip and whicever cell has the highest resistance will have the LOWEST voltage during high amp draw(if all cells have equal voltage). Think about this when you are full throttle and LVC kicks in, which cell is taking the abuse? Your healthier cells will be above 3.2v while the weaker cells will be going below 3.2v. It has to do with capacity AND resistance.

For this reason the weakest cell always gets damaged further and further, not the strong cells, and why we always try to use the most balanced battery packs possible. And why I wouldn't recommend mismatching battery packs/cells if you plan to run series.

the rc guy

Yes you cam do as you want build 2 ..3 cells packs. I would not .I have made 4 cell packs 3 ..Good luck with soldering tabs to make your battery. Id pass......