Best series/parellel set up for lipo pack

[ProEV]

Hi,

The series I am racing in allows LiPo packs with a maxium rated capacity of 1,000 Watt hours. I have purchased 9 of these 108 Watt hours packs that I need to wire together to make my racing pack : https://hobbyking.com/en_us/rhino-4900mah-6s-22-2v-25c-lipoly-pack.html

This will give me a total of 982 Watt hours. For the correct voltage for my system, I need 18 cells in series.

So my set up will be 3P18S. The problem is the nicely sealed packs are 6S.

Are there any advantages or disadvantages to:

A. Running three seperate strings of 16S and only connecting them at total pack negative and total pack positive.

B. Connecting three of the purchased packs in parellel, so 6S connected 3P to make a bundle and then connecting each bundle in series.

C. Disassembling the packs, bundle each cell as 3P and then connected in series 18S. (which would require removing shrink material, disconnecting cells and regen sealing in shrink material).

The cells are rated for 25C but I will be discharging at max of 5C and an average of 1C. My goal is to get maximum energy capacity from the pack rather than maximum power.

 

[amberwolf]

I have purchased 9 of these 108 Watt hours packs
<snip>
I need 18 cells in series.
<snip>
So my set up will be 3P18S. The problem is the nicely sealed packs are 6S.

I don't see what the problem is. 18s / 6s is 3. Just wire 3 sets in series, then parallel 3 of those sets. Connect up the balance wires between the 3 paralleled sets, so you can monitor all the cells with whatever charger or BMS or other device you wish to use.

All done.

A. Running three seperate strings of 16S and only connecting them at total pack negative and total pack positive.

Why go to all the trouble to make 16s strings out of them, when they are already 18s if you use 3 packs in series? Then I assume you'd be using the leftover cells to make 2s packs to put in series with it? Lots of work for no reason?

C. Disassembling the packs, bundle each cell as 3P and then connected in series 18S. (which would require removing shrink material, disconnecting cells and regen sealing in shrink material).

that's a lot of work for no benefit I can imagine.

 

[ProEV]

Amberwolf,

I guess my question is not clear. Let me try and re-phrase it.

The simplist set up is put 3 packs in series and use three parellel strings (which is choice A, 18 total cells in 3 strings).

Do I gain anything by cross connecting the parelleled strings (choice B)? Would this balance the load in some way?

The only things I can see is that it might lessen the total wire resistance or it might allow the three strings to share load more evenly.

If I was building the pack from scratch, I would parellel each set of 3 cells and then put them in series so I only had to monitor 18 voltages. If I was really getting fancy, I would capacity test each cell and choose my cells so each set of three cells combined capacity was as close to equal as each other sets combined capacity. Weak with strong cells, average with average etc.

 

[amberwolf]

As I noted in my reply, I would parallel the balance leads (I also meant to say to parallel the main leads of each pack too, but forgot that part).

THen, as you say, you only have 18 cell(group)s to monitor.

Load sharing? unless there's something wrong with a cell or cells, that won't happen anyway--all the current runs thru the series connections if all cells are identical.

Only when there are problems in one string or another will there ever be any current thru parallel connections.

 

[dogman dan]

Flip a coin. It will be quicker and easier to series to 18s, then connect the three strings in parallel.

You could also connect 3p first, but if you do, make the three way connections permanent, not a bunch of three way plugs. In a race, you do not want to add more plugs, each one with a voltage drop however small.

Overcharge for the actual race. to 4.3v per cell. Unless they actually check that.

 

[ProEV]

Thanks for your replies,

Amberwolf

I missed your idea of connecting the balancing leads so that each cell is connected in parellel without having to undo the pack. That is clever.

Dogman Dan

The electrathon rules state battery capacity is determined by manufacture specs so finding a clever way to get a little more capacity like charging to a higher voltage is legal. Whether I am willing to pay the toll in cycle life will depend on how close my competitors are.

 

[amberwolf]

I missed your idea of connecting the balancing leads so that each cell is connected in parellel without having to undo the pack. That is clever.

It is very very common, and an obvious solution used for many many years.

If you look around and read the many many battery build threads for RC packs you'd see that is how they are often put together. A number of such threads even have titles similar to yours.

You'd also get lots of other tips and tricks for pack building.

 

[ScooterMan101]

ProEV,



What kind of speeds and range are you getting ?

Using how many Watt Hours total pack ?

In Regards to batteries
The 3 6s packs in Series I understand since I am now starting to use 2x 6s packs together to make a 12s pack.

However

Connecting the Balance Plugs in Parallel ? When you find out
how to do that please send pictures , and link.
I have spend days before on E.S. trying to find something that someone said was on previous threads only to get way off course with the hundreds/thousands of threads with many pages each.

 

[dogman dan]

Cut off both balance plugs,, then put one plug back on, splicing the wires and matching up colors.

Or, get Jst extensions, and may Y cables with a similar splice, but with plugs on both ends.

 

[ProEV]

Hi Scooterman101,

You wrote

"I am now starting to use 2x 6s packs together to make a 12s pack. "

So think of think of taking 3 of your 12s packs side by side instead of adding them lengthwise. Same voltage but more amp hours.


The pack capacity is 980 Watt hours.

Range is dependent on the type of track we are racing on. Most of our tracks are tight and twisty and we manage around 25 miles in an hour. The record, set on an oval is 62.1 miles in an hour. More information here: http://www.electrathonamerica.org/Welcome_to_Electrathon_America.html