A123 battery pack questions

[1967geezer]

Hello,

I have some general questions about A123 battery (& similar) configurations and wiring.

I'm thinking about hooking two 6s2p packs in series to give me 39.6v, 4500-4600mAh. Apparently turnigy make the individual packs, see here:

HobbyKing Online R/C Hobby Store : Turnigy 4500mAh 6S2P 30C LiFePo4 Pack $69.99

Or I could get the ebay seller that jerrysimon bought his a123 cells from to make me a custom pack:

6S1P 16.5V2300mah A123 NEW Battery Pack for Models-2 on eBay (end time 04-Jan-10 11:50:13 GMT)

(the link is to a 6s1p, but they offer a custom build service)

My first question is about balancing the 6s2p packs. What kind of balancing plug would the pack have? In the photo of the turnigy 6s2p pack in the first link above it looks like there is only one 6-way balance plug, but there are 12 cells in the pack. How can you balance 12 cells with only 6 pins on the plug? Or is there another balance plug hidden under the visible one? If there is one plug, are they balanced in pairs? If so, surely one of the pair could be higher than the other? Or isn't that how they work?

If each pack actually needs two balancing plugs then that will mean four plugs in total, which would make the charging and balancing procedure rather cumbersome.

Second question, can 2 such packs actually be connected in series? And if they can, is there a way to do all the balancing on all the cells via one balancing plug?

If there is no obstacle (such as "it can't be done" ) I would be keen on getting two custom packs made up by the ebay seller. One 6s2p pack will just about fit inside a standard drink bottle, having two drink bottle batteries is appealing because of the unobtrusive aesthetics. Otherwise it would have to be a ping 36v 10Ah, which I'm not keen on, admittedly it is small but it is still quite blocky. I have already found out that ping are quite restricted in configuration due to the fact that their v2.5 packs are made up of prismatic cells.

Thanks in advance,

Chris.

 

[jerrysimon]

Chris,

The 6s2p are in parallel so one can be balanced with one plug i.e. in pairs as your say. This shows how

R/C Calculations

Click 5s then 2p and show balance wire hookup.

Technically I think you can connect up 2 * 6s2p in series. I need to think about about it a little though

EDIT *** Ok I have it, of course you would just connect these up using the series lead I show in my A123 build thread here.

You would have seperate them to charge/balance them with the Turnigy and if you used my cheap ebay charger then it would take about 3hrs to charge them in one 39.6v, 2 * 6s2p as its only charging at 1.6amp and you have a 4.6Ah pack.

I think I prefer the option of having the two packs of 2 * 6s1p in series using one or two packs paralleled depending on how far you are travelling. That way you can adjust weight accordingly, rather than having to take one 2kg pack out all the time. I am probably going to make up two more 6s1p packs so I can have a second pack for just that.

Finally it may be that you hardly ever need to balance them but I am yet to prove that.

PS Concensus is that the first link you showed is in fact not the cylindrical A123 cells but the soft pouch rectangular ones. There was some discussion on the other forums that they do not perform as well as the cylindrical A123 cells and they certainly wont take any physical knocks of the packs as well as the cylindrical ones.


Regards

Jerry

 

[NRG]

To truly balance 6s2p a second JST connection would be required connected to the 'other' 6 cells, however, as the pack is 2x 6s paralleled and sealed there's no way of disconnecting the two packs and breaking out a second balance connector. So the pack has to be balanced with cells in pairs as Jerry's link shows.

Also, as Jerry indicates two separate packs may be better but this will make the pack wiring a bit more complex.

Yes its possible to connect two packs in series to give 39.6v but don't forget that is your nominal voltage. The full charge voltage will be 3.6*12 = 43.2v, may not be a problem but check your controller and motor ratings.

For balancing in this configuration its probably best to balance each pack individually otherwise you would need to make up a Y cable to link the balance wires into a single JST connector and then use a 12s balancer/charger, however, I don't think any off the shelf device exists for 12s... I don't think you will need to balance very often as these cells are very forgiving and stay balanced without intervention for a long time.

 

[jerrysimon]

NRG, much better put and you just taught me something about pair balance/charging too

Yes I was quoting nominal voltage.

There is a setup where you can use two 6s balancers together with this Hyperon charger, but it gets much more expensive than the cheap 6s Turnigy.

Air Craft - Electric R/C Aircraft World Shop

Regards

Jerry

 

[1967geezer]

Thanks for the replies, I think I understand

I understand that each 6s2p pack (if I go for 2) would need balancing individually, which leads to another question, is there any kind of standalone balancer that I could leave wired up to each 6s2p pack permanently? I've seen these, would they suit being permanently connected inside the battery housing?

Xtrema Balancer - TME - Tejera Microsystems Engineering

The reason for two 6s2p packs together is to increase the Ah/range. I only have a 5 mile round trip, and I would hope to get 2 or 2.5 round trips out of a 2300mAh battery. Doubling this would be more convenient, and would hopefully mean 1 charge a week.

Chris.

 

[NRG]

I don't think theres much point in connecting a balancer permanently. After a few battery conditioning cycles the cells if good will remain very close in voltage. The only device I would connect when a pack is in use is some form of low voltage warning or cutoff device like these:

HobbyKing Online R/C Hobby Store : BM-6 Cell Voltage Monitor 2-6S Lipo (New Version) $12.95

The LVW is settable between 2 and 4v and the rolling display gives total pack voltage and individual cell voltage. The device will alarm if a cell falls below the set warning voltage. It also gives the maximum voltage difference / delta between cells and I use this to determine if I need to balance my Lipo 7s3p pack. Also theres load of others to choose from on the HobbyCity site but I prefer this version.

 

[jerrysimon]

NRG, IMHO I really don't think these cells need those/or any on board BMS. Most controllers have an LVC, mine cuts out at at 31.5V which equates to 2.6V/cell. Don't forget these are not the same chemistry as Lithium Polymer cells but are closer/identical to Life04 chemistry.

These cells can survive at much lower voltages. Take a look at the table at the bottom of this link, 2v officially but almost empty at 2.5V. Can go down to 1.0V!

'Fly Electric' - A123 Lithium cells

I don't even think they are going to need balancing that often.

Not only this but I have been told and I am also convinced they don't need Diodes for battery combining.

But we shall see I may be proved wrong and have to eat my words. I have purchased them new, so I can monitor them over the six months or so to see what happens.

Regards

Jerry

 

[NRG]

Yes, thats true, I was going by my experience of the Lipos which need more careful handling! Still, I like to know what the voltages are and these little units are useful for that. Plus its still not recommended to take A123 down to 1v and 2v is a safer minimum voltage....

Edit: Not sure when that Fly Electric page was last updated but some of the statements for Lipos don't hold true with the latest packs that are now available

 

[1967geezer]

In browsing a link posted by daniel.weck in another thread (http://www.pedelecs.co.uk/forum/electric-bicycles/5098-building-small-a123-battery-pack-6.html#post67002) I came across these balance plug adapters, which allow you to parallel the balance plugs from several packs.

Products

They look like they will give me all options.

If, as people say, these packs don't need much balancing then maybe paralleling the balancing on two or four 6s packs will suffice. If they turn out to need individual balancing, I will still have that option.

My project moves forward incrementally

Chris.