Thread: Discussion On the topic of parallel charging of Lixx / PB packs. View Single Post
Oct 01, 2008, 10:13 AM
Proud to eat Kraut ;-)
Germany
Joined Dec 2003
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Discussion
On the topic of parallel charging of Lixx / PB packs.

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!

And this is how it's done:

 Parallel Charge Board Demonstration (2 min 40 sec)

Here is an excellent posting about the facts of parallel charging. Each individual cell was measured here:
http://www.rcgroups.com/forums/showt...1#post29362689

1. When does charging in parallel makes sense?
2. What are the advantages of parallel charging?
3. When does charging in series make sense?
4. How do I wire packs in parallel?
5. How do I wire the balancing contacts in parallel?
6. Simple wiring for lazy people and small currents
7. How is the charge current calculated?
8. Mixing old and new packs
9. How come this works with packs of different capacities?
10. What happens if I connect packs with different states of charge?
11. In which order do I connect the various leads and balance contacts?
12. Dangers of parallel charging

1. 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% difference to each other, to be safe).
-When the charger can provide the current needed for parallel charging, but not the voltage needed for series charging.

2. 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.

3. 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.

4. 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?

Meanwhile, a small industry has developed around the process of parallel charging. Now, one can buy parallel boards that make connecting packs really simple:

All the self-made soldering is not neccessary any more.
David has a large variety for all kinds of connectors in his shop.

Here is a very neat setup from Mark:
http://www.rcgroups.com/forums/showt...5#post13743814
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...3#post10828442

5. How do I wire the balancing contacts in parallel?

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. The charger does not "see" each individual cell this way, but this does not matter. When the cells are connected in parallel, they always have the same voltage. They are balanced as a group, and this works perfetly fine.

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

http://www.rcgroups.com/forums/showp...&postcount=122

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

Dieter from KD-Modelltechnik presented the neatest solution I have seen so far in a german forum. The PCBs are not for sale, though. The images should be self-explanatory:

6. Simple wiring for lazy people and small currents

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.

7. 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.
Generally, charge at no higher rate than you would charge your least capable pack when charging singly. (source: mrforsyth)
For example, you have 3 pieces 2.2Ah packs in parallel, and one only can be charged at 1C, while the others accept 2C. This means, that the charge rate for the whole group should not exceed 1C. The whole group has 6.6Ah (3x2.2Ah), so the current at 1C would be 6.6A.

8. Mixing old and new packs

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.
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 roughly equal 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:

9. 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. One can parallel charge packs of any capacity one wants, no matter how big, small, or different they are.

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

10. 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%. If you are not sure, better check the packs before connecting them. There are cheap voltage displays all over ebay for around \$2. While those are not terribly precise, one can still roughly determine the level of depletion.

11. In which order do I connect the various leads and balance contacts?

Warning: There has been some discussion about whether equalisation currents of packs with a different state of charge can be high enough to burn the traces of balance boards, when the balance contacts are connected first.
Until clarity has been achieved in this matter, I recommend checking the voltage of the packs prior to any connecting process. If the difference is too high, bring the packs to an identical voltage first, and connect then.

First: Main contacts and balance contacts of the parallel adapter to the charger.

Second: Balance contacts of the packs to the parallel adapter.

Third: Main contacts of the packs to the parallel adapter.

Reason: http://www.rcgroups.com/forums/showp...postcount=1092

12. Dangers of parallel charging

Generally speaking, parallel charging is perfectly safe as long as all components involved are in good working order, and no mistakes regarding wiring, selecting packs or charger programming are made. This applies to all charging processes in general.
However, packs connected in parallel to a charger make it more difficult, if not impossible, for the charger to detect a single defective pack or cell.
If you connect a single pack to a charger and one cell is defective and has only 2V or so, the charger will rightfully complain about the dangerously low voltage of that particular cell.
But if a handful of packs are already connected to the charger, the voltages of the other cells may mask the unusually low voltage of the presumably defective 2V cell and prevent the charger from detecting it.
To circumnavigate this issue, the safest method is to briefly check each pack prior to connecting it to a parallel harness already populated with other packs. This can be done in different ways:
1) Many chargers have a "display mode", where they show the individual cell voltages of the connected pack without charging. Connect all the packs you want to charge once for a check, and after all individual checks are OK, connect the packs in parallel for charging.
2) Get a checking device like Junsi's CellLog.
A split second after plugging it into the balance cable, it shows all cell voltages with high precision. I use it all the time since it was released.
3) Get a cheap LED cell monitor. Those are generally not as precise, and they only display one cell's voltage at a time, and are difficult to read in direct sunlight, but they are only like \$2.

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Here is a site that explains everything in a nutshell with great diagrams: