Combine 2x2s LiPo to increase Capacity and Discharge Capability?

[monkeyracer]

I wanted to check to make sure that I was thinking correctly about this...

I know that if you combine 2 identical 2s packs in parallel, the combination effectively becomes a 2s2p pack. So if these packs are 1500 mah, the new combined pack is 3000 mah... Correct? (Series increases voltage, parallel increases capacity)

If that's true, then is the following true?

If they are 15C packs, that would mean the new combined pack could be discharged at up to 45A? (3Ah x 15)

Also, since they would be used together, I could (should?) charge and balance them as if it were one pack, so if the new capacity is 3000 mah, charging at 1C would mean 3A charge rate right? (I would make a 2s to 4s balance adapter and a parallel deans y-cable.)

Now what if I had 3 identical packs?

I think this is right, I just want to make sure before I go starting lipo fires and everything.

Thanks!

[TattooKenny]

Ask John at Holmes Hobbies, he is your guy to help ya here

[clockworks]

Your thinking on usage and discharge is fine.

For charging, I think that you'd need a 2s Y adaptor for the balance cables, rather than a 2s to 4s.
if it was me, I'd just charge the packs separately.

[monkeyracer]

OK that helps. I acquired a couple receiver packs from a friend, and they are only 15C, but I thought for my scaler that I could combine at least 2, and get a longer run time, and get the better current capability.

Glad to know that was sound reasoning.

As far as the balance connector is concerned, I'll probably end up charging them seperate until I can make the balance adapter. I saw a tutorial on another forum that showed how to do it, it's just a matter of getting the right plugs. With it combined, the charge time will be less overall.

BTW, how can I find out what type of plugs they are to be able to order the right plugs to make the adapter?

So just to make sure the math is right, if I use the 3 batteries in parallel to make a 2s3p pack, I'd have a 4500 mah 2s pack capable of up to 67.5A discharge?

Thanks for the help with this.

[JohnRobHolmes]

Yes, your reasoning is sound.


I would generally charge up the packs seperate, but if you checked each individual cell voltage first you could plug the cell taps in parallel and charge them as one big pack.


Add another pack in parallel, add another 1.5 amp hours of runtime and 1.5x15c of discharge rate.

[monkeyracer]

Originally Posted by JohnRobHolmes

Yes, your reasoning is sound.


I would generally charge up the packs seperate, but if you checked each individual cell voltage first you could plug the cell taps in parallel and charge them as one big pack.


Add another pack in parallel, add another 1.5 amp hours of runtime and 1.5x15c of discharge rate.

I guess I was looking at this new pack as a 2s3p lipo, rather than 3 seperate lipos. I saw a thread on another forum about it, can't find it now, but found this:

Originally Posted by traxxas forum

Everything you need in terms of a "how-to" can be found at BrianG's website;
http://www.scriptasylum.com/rc_speed/_lipo.html

This is the graph you need.



as for the connectors you need;
you can get the male ones here;
http://www.bidproduct.com/part/Produ...ECTOR_263.html

and the female ones here;
http://www.bidproduct.com/part/Produ...ECTOR_264.html

remember a two cell lipo needs a 3 pin connector, and a four cell lipo needs a 5 pin connector.

Other then that, get some 22g wire and solder them the way you need.

I buy most of my wire, connectors, shrink wrap etc from hobbycity.

The other thread basically said that the only reason they should be charged like this is if they will be used and drained together (e-revo, etc.) and that's what I plan on for these batteries.

I think I've got the info I need to get started, I'll charge and balance them seperately, at least for now.

Thanks for the help guys!

[JohnRobHolmes]

That pic is for series use too, not parallel.


If you measure the cells and wire them together you would be fine keeping it as one pack. Just make sure the cells are the same age such. Weak cells will pull down the parallel buddy.

[monkeyracer]

Originally Posted by JohnRobHolmes

That pic is for series use too, not parallel.


If you measure the cells and wire them together you would be fine keeping it as one pack. Just make sure the cells are the same age such. Weak cells will pull down the parallel buddy.

Hmm, must have skipped over the part that it was a 4s connection in the end...

Ok, charge/balance seperately.

The batteries have all been through pretty much the same scenario. He got them thinking it would work for his crawler since they are so small, not knowing the C rating, so he's used them maybe 5 times a piece. I'll check the cells to see where they're at, and they'll each get a good balance charge to make sure they're all equal.

[clockworks]

There's nothing to stop you using them in parallel (2s2p) and charging them in series (4s1p).

[Spiftacu1ar]

YES, charing lipos in parralell is compeltely safe, as long as they are at similar charge states. Lipos have a very flat power curve, so if you connect packs in parrallel that are not in identical charge states, it is ok. They will balance each other at a relatively low rate. The voltage difference is not large enough to induce dangerous currents (unless one is near full charge, and one is dead, lol). anywhere in the middle of the charge state, they are realtively trhe same voltage, but at the beginning and end they taper off. It's best to charge them to storage votlage (3.7v/cell) before combining them into parralell packs. To be safe, I'd say any battereies within about .01 - .02v as far as charge state are safe to charge in parralell. All you have to do is math up the wires going to blaance conncetor.

Lets call 1 pack pac A, and the other pack B, so for 2s, the cells are 1a, 2a, 1b, and 2b.
The balance leads for 1a and 1b should be shorted together, as should 2a and 2b. Also short the main connector toghther (so in the end you have 1 main connector, and 1 balance plug).

It is also safe, believe it or not, to use 2 lipos of different capacities together (as long as they have the same # of cells, so you have to use 2s with 2s, not a 2s and a 3s). The reason for this is that the battereis constantl;y balance eachother as based on voltage, not current. That means when being charged, they will stay at the same PERCENT charge (since votlage of a lipo is based on in its charge state as a ratio / percent, not how much mah is in it). The same goes for discharge. Even if one is 1000mah, and the other pack is 5000mah, when used in parrallel, they will both discahrge in the same amount of time. The 1000mah pack will NOT discharge 5 times faster than the 5000mah pack.

If you need more explanation (or a drawing for wiring), let me know.

EDIT: forogt to address one issue: WEAK CELLS will NOT pull down the parralell buddy. This is for the same reason that working a 1000 mah and 5000mah will work properly. Suppose you are using 2 3s 5000mah packs. One has a weak cell that holds only 4000mah. This means that the combined cell is 9000mah. ALl this means is that one of the cells is efectively 9000mah, and the other 2 are 10000mah. Ifyou thikn abnout it, this is no different than where you started, which is that before you had 1 cell at 4000mah, and 2 at 5000mah.

Having the weak cell jsut means you MUST balance on after each run (which you shoudl do anyways, especially with 2 5000mah, and 1 4000mah)

EDIT2: I also forgot to mention some stuff about c rating. The current drawn from each pack is realtive to its capacity, so it's best to use the same c packs. if you use packs with different c ratings, on;y the lowest one counts. If you have 2 packs of equal capacity in parrallel, and try to draw 50 amps from them, it will distribute the load as 25amps on each. if you have one 1000mah,a nd one 4000mah pack, and try to draw 50amps, it will take 10 amps from the small pack, and 40 amps from the large pack. If the 1000mah pack is 30c, and the 4000mah pack is 10c, the C rating of the set is 10c. ifyou try and draw 50amps from the set, it will pull 10 form the first, and 40 from the other, as mentioned earlier. 40amps is 10c for the 4000mah, and is therefore the limit. 50 amps is also 10c of 5000mah, indicating that 50amps is the limit of the pair. In that sense, a weaker cell does limit the pair.
However, ussually a weakened cell maintains c rating, but loses its capacity, so its a moot piont.

[monkeyracer]

ok, then I am going to order the balance connectors to make the splitter cable.How can I find out what type of connectors they all are? Or, should I convert them to the same connectors?I've got robitronics batteries (the 3 rx packs) excaliber and losi batteries with a B6 charger.Where is a good place to order the connectors?

[Spiftacu1ar]

OMG, i jsut realized i madea huge typo (dangerous) in my post. I edited it now so its correct. Before, i wrote they should be within .1 and .2v of eachother. I MEANT to write .01 and .02v. (i edited the actual post to reflect this as well).

[monkeyracer]

so, I should probably balance them individually first, then once they are all with the voltage you corrected, they can be charged together?

[Spiftacu1ar]

Originally Posted by monkeyracer

so, I should probably balance them individually first, then once they are all with the voltage you corrected, they can be charged together?

Ya, that would be a good idea. Sorry for the typo up there.

If you don't want to balance them, you COULD put a resitor between the packs to limit current, BUT, I am not going to reccomend that, only becasue if you screw up doing it, it could result in a fire. Therefore, I am not even going to give instruction for that

[monkeyracer]

so where's a good place to get balance connectors? And how do I find out what type of connector the batteries use?

[monkeyracer]

OK, here's what I found about the balance plugs:

I have the following batteries:

3 x Robitronic R05201 - 1500mah 7.4v 2s1p 15C
2 x X-Caliber Black Knight - 1000mah 7.4v 2s1p 20C
1 x Losi LOSB1207 - 1000mah 7.4v 2s1p unsure of C rating

Each brand has a different type of balance plug, and from what I found on rcaccessory.com, the x-caliber has the more common JST-XH type connector.

The robitronics have the wires laid out as such:
Pin 1 - Black
Pin 2 - Black
Pin 3 - Red

The Losi has:
Pin 1 - Black
Pin 2 - Blue
Pin 3 - Red

The X-calibers make it confusing though because the colors aren't the same... One of them has:
Pin 1 - Black
Pin 2 - White
Pin 3 - Red

But the other is:
Pin 1 - White
Pin 2 - Black
Pin 3 - Red

For the robitronics batteries, I was going to charge and balance all three as if it were one 6s pack. The deans connector is easy, for the balance plug I think it's supposed to be the following:

Pin 1 - A1 - First black wire in series
Pin 2 - A2
Pin 3 - A3 and B1
Pin 4 - B2
Pin 5 - B3 and C1
Pin 6 - C2
Pin 7 - C3 - Last red wire in series

The losi battery will be charged/balanced seperately, but the 2 x-caliber batteries will be charged as if it were one 4s pack. The first battery from above, we'll call battery A, and the second as B again, deans connector, no problem, balance connector, I am assuming is:

Pin 1 - A1 - First black wire in series
Pin 2 - A2
Pin 3 - A3 and B1
Pin 4 - B2
Pin 5 - B3 - Last red wire in series

What seems strange to me is that the connector isn't numbered like the other ones, there's an arrow on the red wire. Also the wires aren't arranged the same, so I am worried this might cause issues when balancing them.

For all the batteries, I plan on just converting the plug to the same type so I can standardize my own packs. Since my charger has a JST-XH plug, and the X-Caliber batteries have the JST-XH plug (don't have the charger yet, in the mail...)

So, is this right?

[JohnRobHolmes]

Don't worry about the colors, just worry about the placement and order of wires. The Red is generally the most positive on any pack, so go from that.