Discussion About parallel charging of Lixx / PB packs.

[Julez]

Hi!

I did notice that some people do not know about the advantages of charging multiple packs at the same time on only one charger, wired in parallel.
Instead, many even bother with charging in series.
Therefore, I want to make this thread.
Attention: Do not charge Nixx packs in parallel!



When does charging in parallel makes sense?

-When I have multiple packs with the same cell count.
-When I want to charge them at the same time, thus reducing overall charge time, or avoiding the hassle of starting multiple charging processes.
-When the packs have slightly different states of charge (<30%, to be safe).
-When the charger can provide the current needed for parallel charging, but not the voltage needed for series charging.

What are the advantages of parallel charging?

-No need to care about different capacities or slightly different states of charge.
-Reduced balancing times.
-Simple wiring, fits for a multitude of packs.

When does charging in series make sense?

-When I have multiple packs of the same capacity constantly wired together in series, making a bigger pack
-When the pack's state of charge is nearly identical.
-When the charger can provide the voltage needed for series charging, but not the current needed for parallel charging.

How do I wire packs in parallel?

That is very simple: Just connect every contact of one pack with the corresponding contact of the next pack.
The positive contact is connected to the positive contact, the negative contact is connected to the negative contact.
Simple, isn't it?


And the balancing contacts?
The first balancing contact is connected to the first balancing contact, the second balancing contact is connected to the second balancing contact, and so on.


One can see, that this adapter board has enough contacts to also work with 6s packs. So one board fits all pack cell counts from 1-6s. No need to build different adapters for different packs.

The board in red shrink wrap is a little difficult to solder, but there is a simple alternative:



One can easily see how the contacts are connected.
The pin row headers take the balancing plugs of the packs, and the female header takes the balancing cable coming from the charger or balancer.

Here in another nice image how it can be done:
http://www.rcgroups.com/forums/showp...postcount=1895
One can see clearly which contact goes where.
This is another neat example:
http://www.rcgroups.com/forums/showt...9#post10883849
Note: Long cables mean no harm, but they can in some cases mean a longer charging and balancing time.
Here one can see a harness in detail:
http://www.rcgroups.com/forums/showp...&postcount=274

Another neat example, using real balance plugs instead of row headers:
http://www.rcgroups.com/forums/showp...&postcount=122

http://www.rcgroups.com/forums/attac...mentid=2254168
http://www.rcgroups.com/forums/showt...5#post11255155

But this is not all: When one only charges with small currents <2A, it is not even necessary to connect the packs' main leads. The balancing cables can cope with this current, so just one pack's main lead has to be connected to the charger.

I have charged up to 5 slowflyer packs 2s 900mAh this way.
One can even use the provided standard adapter for parallel charging. It must only be ensured, that the contacts of all balance ports are connected in parallel. Better check with a multimeter first.



It is very convenient: Just connect all packs at once, and start charging. No need to bother with starting 5 separate charges.

One can see, that just one pack is connected to the actual charge cable. The charge current spreads out to all other packs via the balancing cables.

Here is a very neat setup from Mark:
http://www.rcgroups.com/forums/showp...50&postcount=5
He can charge 11 packs at a time.
Note how he connected the main leads to the outer balancing contacts.
This is a very good solution, provided one always charges packs with the same cell number on a given adapter/harness.

If you want to charge with more current, wire the packs like this:
http://www.rcgroups.com/forums/showt...7#post10909267

How is the charge current calculated?
Simply add the currents of the single packs.
Say, one wants to charge a 1Ah, a 2Ah, and a 3Ah pack together, each with 1C.
1A+2A+3A=6A.
So these packs are charged with 6A.

There may, however, be the issue, that the packs connected in parallel have a different inner resistance (IR). The older a pack, the higher the IR. This means, that such a pack will not accept the charge current as willingly as a brand-new pack with a very low IR.
Now in the hypothetical situation that one has a lazy old 5000mAh pack connected in parallel with a brand spanking new 500mAh pack, and decides to charge at 1C with 5.5Ah, the chances are good that the small pack will see more than its share of 500mA.
My advice is this:
Know your packs and their health. If you are pushing for the maximum rated charge current of the packs, be sure to connect only those in parallel that are not way off concerning age, capacity, and general health.
If you, however, slowly charge with a small current over night, I do not see a problem.
More on this subject.

Generally, when one has the time, I advise to charge LiPos as slow as possible. Let's say, you want to charge your lipos over night. In this case, there is no need to charge than at 1C or even 2C.
If you have 12h time, use it. Set the current to 1/10C.
I do this all the time. Less current means less problems, less thermal stress for the chargers and power supplys, less risk of anything malfunctioning.

Questions that might arise:

- How come this works with packs of different capacities?
This is because they are connected in parallel. This way, they will always have the same voltage. It is not possible to overcharge a single pack.

Although all pipes have different shapes and volumes, they are always filled to the same level. The same applies to the packs' voltage levels.

- What happens if I connect packs with different states of charge?
A current will flow between the packs. However, this current is not very high, even if one pack is full and the other depleted. Yet, it should be avoided to connect packs with a state of charge differing over 50%

Please note: A troll named "cstratton" frequently claims in this thread, that parallel charging is dangerous.
Although he has no data to back up his claims, he does neither accept the data collected by users, which suggest parallel charging is perfectly safe, nor numerous experiences by credible users, which suggest the same.
It is up to the reader to decide whom to believe.

[steve1814]

Julez,

First - thanks for the thread -very informative

Now, I have the 1010B charger that I mainly charge 5S1P A123 packs on. currently two of the three packs fly in the same plane as parallel packs (doubling my mAh) and the third flies alone in another plane. I use the balancing board that came with the charger and connect the two balancing tabs to the "b" and "b1" ports and then connect a homemade serial deans adapter to the main discharge leads which then plugs into to the main output leads on the charger. I set the Charger up for 10S 2300 mAh and start charging....

Would connecting them up parallel and setting the charger up for 5S 4600 mAh be any faster/better?

Cheers,
Steve

[BlueAngel2000]

Great explanation Julez.
This thread will help many people to don't burn their lipos or chargers because of false connecting lipos in series.

[Rodney]

Just remember, this only works for LiPo and Pb batteries. It will not work for NiCad and Nimh unless you like to ruin a lot of cells. In fact, it can be downright dangerous with some as the ends will blow out hard enough to cause serious injury or damage.

[Julez]

Steve:

currently two of the three packs fly in the same plane as parallel packs (doubling my mAh) and the third flies alone in another plane
[]Would connecting them up parallel and setting the charger up for 5S 4600 mAh be any faster/better?

It would be more simple, but not as fast, unless you do not charge with more than 5A currently.

When does charging in series make sense?

-When the charger can provide the voltage needed for series charging, but not the current needed for parallel charging.

Does the balancing phase need a lot of time? If yes, charging in parallel might be beneficial.
Charging 10s, the charger can output 200W. Charging 5s, the maximum power would be 180W, 10% less. So your charging time might increase a little, but the balancing time will decrease. Hard to predict time gains in this case, I would check each method. If they turn out to need the same time, I'd still prefer parallel, as the charge current will be lower throughout the whole charge, more friendly for the cells.
Maybe its like this:
Series: 20min total time
15min charge with high current, 5min with low current and heavily balancing

Parallel: 20min total time
19min charge with not so high current, 1min with low current and a little balancing.

So when lower currents do not translate into longer charge time, I'd always charge with lower currents.

Angel: Thanks!

Rodney: Thanks, you are right. I will update my posting.

[steve1814]

Originally Posted by Julez

Steve:


No. As long as you discharge these 2 packs always in series, you can also charge them in series, as I wrote:

gotcha. I misread that to mean "hardwired". -yeah that would just be a bigger pack

Thanks,
Steve

[Julez]

No, I missed that you discharge your packs in parallel
I have updated my last post.

Cheers,

Julez

[steve1814]

Originally Posted by Julez

No, I missed that you discharge your packs in parallel
I have updated my last post.

Cheers,

Julez

Julez,

The balancing does take a while as the packs only have about 5-6 flights on them and haven't really "sync'd" yet. It wouldn't be hard to build a 2-to-1 balance tap adapter and just plug it into the "b" slot -I already have the parallel deans from the plane. I'll have to time the difference and see.

Thanks again,
Steve

[Julez]

It wouldn't be hard to build a 2-to-1 balance tap adapter

Not necessary!
I just updated my post, you can use the provided adapter board:
http://www.rcgroups.com/forums/attac...mentid=2095955

Maybe you need to file of one notch of the 5s plug to fit it into the 6s jack though.

[steve1814]

Originally Posted by Julez

Not necessary!
I just updated my post, you can use the provided adapter board:
http://www.rcgroups.com/forums/attac...mentid=2095955

Maybe you need to file of one notch of the 5s plug to fit it into the 6s jack though.

Great Idea!

[ww8s]

Originally Posted by Julez

- What happens if I connect packs with different states of charge?
A current will flow between the packs. However, this current is not very high, even if one pack is full and the other depleted. Yet, it should be avoided to connect packs with a state of charge differing over 50%

How can this be? If I connect two batteries at different charge levels I would expect large currents to flow until the batteries equalized.
Dave

[rpage53]

Originally Posted by ww8s

How can this be? If I connect two batteries at different charge levels I would expect large currents to flow until the batteries equalized.
Dave

No, the discharge voltage is fairly flat from 90-40% discharge. A large voltage difference is necessary for a large current. The remaining capacity has nothing to do with it.

Rick.

[kjetilmh]

Originally Posted by rpage53

No, the discharge voltage is fairly flat from 90-40% discharge. A large voltage difference is necessary for a large current. The remaining capacity has nothing to do with it.

Rick.

It would be nice to have some numbers about how high ( or low ) the current
would be. Measurements anyone ?

[Julez]

Well, just compare a charge and a discharge graph.
While the resting voltages might have a considerable difference, as soon as a pack is charged, its voltage rises rapidly, and the voltage of a discharged pack decreases rapidly, this making the actual voltage difference of 2 connected packs fairly small.

I once connected a fully charged and a fully discharged pack (100% charge difference), and the current was about 2C initially. It became much less after 30s, and after 2 minutes, it was only a couple of hundred mA.
With a max of 50% charge difference, the current should be even lower.

Cheers,

Julian

[kjetilmh]

Originally Posted by Julez

I once connected a fully charged and a fully discharged pack (100% charge difference), and the current was about 2C initially. It became much less after 30s, and after 2 minutes, it was only a couple of hundred mA.
With a max of 50% charge difference, the current should be even lower.

Cheers,

Julian

Thanks !