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Old Sep 30, 2010, 07:20 PM
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Parallel battery charging current test

I see many comments on the parallel charging thread where people seem to not be aware of what is actually going on when you connect batteries in parallel. So what happens when you connect batteries in parallel is that current flows between them with little interference until they equalize. This means high current if the initial voltage difference is big. There is no magic, it’s current and time to equalize packs. Packs are not equalized when you connect them, it takes a few minutes.

Examples (two 3S 1300mah packs)

1. Pack one 12.0V, pack two 11.5V (0.5V difference)
Initial current 4A, after one minute 1.6A


2. Pack one charged (12.6V) pack two discharged (11V)
Code:
Time  Current (Amp)
00:00 15
00:10 7.5
00:20 6.3
00:40 7.5
01:00 4.8
I stopped here because the packs got warm

3. Pack one charged (12.6V) pack two storage (11.4V)
Code:
Time  Current (Amp)
00:00 10
00:10 6
00:20 5.4
00:30 5
00:40 4.7
01:00 4.3
01:30 3.7
02:00 3.3
02:30 2.9
03:00 2.6
03:30 2.3
04:00 2
04:30 1.8
05:00 1.6
4. Pack one storage (11.4V) pack two empty (10.5V)
Code:
Time  Current (Amp)
00:00 5.6
00:10 3.6
00:20 2.7
00:30 2.3
00:40 2.0
01:00 1.6
01:30 1.2
02:00 1.0
02:30 0.9
03:00 0.8
03:30 0.7
04:00 0.7
04:30 0.6
05:00 0.6
Notice that in all experiments initial current is over 2C (battery spec), and the batteries did warm up a little bit in all experiments, where in experiment two they warmed up too quickly for my taste. There is also relatively high current for a few minutes. This means that if you connect the packs to the charger before they equalize, one pack will see even more current.
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Old Sep 30, 2010, 07:33 PM
Southern Pride
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But recomended SOP is to connect only LiPolys of the approx. same state of discharge and any LiPoly made in the past year that is 20C or greater rated can handle 2C charge rates with no problems.

My flown packs are between 3.6 and 3.8 volts per cell. I also charge 2200 LiPolys at between 10 and 22 amps. and yes they are rated for these charge rates.

Charles
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Old Sep 30, 2010, 07:42 PM
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Some people say its perfectly fine to connect charged and discharged packs
Parallel charging 3x 3s packs with a kink (6 min 28 sec)
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Old Sep 30, 2010, 07:54 PM
TJin(Guy + Tech)
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Joined Jun 2008
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Here is something to consider. When people parallel charge several packs, say for a 450, they often charge more than 2 together. Some charge as many as 6. I usually charge 3 or 4. So if one of the 3 or 4 packs is mistakenly connected when fully charged, the current will be split up between the other 2-3 packs. The current being drawn from the charged pack will actually be higher in this situation but the division of it means that it is much less of a concern.

Also here are my numbers from my experiment. I used a pair of Hyperion VX 6s 2600 packs. One pack was charged and the other was just under 4V per cell. The initial current was 18.7A. After 1sec it was 12A, 5sec was 8A and 20sec was 5A.

Next I plan to connect the same 2 packs together, but one charged and one discharged pack. I just have to find the time to do it.
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Old Sep 30, 2010, 08:02 PM
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Ignoring whether it's perfectly fine, why would someone want to connect a charged battery to a charger?

Aren't they just making an extreme statement along the lines of: even if you do something this goofy - it still doesn't blow up?

By the way, some chargers like FMA's that have a "C" rate, will also charge the batteries for significantly higher than that rate while they home in on what the battery really wants. They claim that a few minutes above the spec'd C rate is not a problem.

All that said, I never connect any in parallel with more than .1v/cell difference. That's just my comfort level.
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Old Sep 30, 2010, 08:03 PM
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How is what he did in the video any different than your test? You both connected a fully charged pack to a fully discharged pack to prove something. Yes, your test was more informative, but in the end you were both trying to prove something and just happen to come up with different conclusions, primarily because he used no scientific analysis to reach his conclusion. I don't think you, he, or anyone would knowingly connect these two packs together in the real world, what would be the point? Ok, so maybe to equalize the packs for storage, but without a resistor to limit amp flow that would be a pretty poor way to accomplish that task.

Don't get me wrong, I appreciate your findings, as I've often wondered what the amp flow would be as the voltage difference increases. But I fear it's only going to show the amp draw for those specific packs, as I would guess the flow will be dependent upon the IR and C rating of each pack that is connected to the parallel connection. In other words, your test shows that yes, there is the potential for significant amp flow for some period of time, how much that will be and for how long is totally dependent on the specific packs.
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Old Sep 30, 2010, 09:02 PM
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True, there is no reason to connect a full pack to an empty one.
I still however end with packs that are 0.5V+ different than each other after a session. Normally the lower C packs have to stop earlier (higher voltage) since they can’t provide enough power, while the bigger/higher C packs can provide power all the way to 3.6V rest voltage (and even less).

There were lots of questions about equalization, some people assume that once you connect you can treat the packs as one for charging/discharging purpose, while the reality is that it is not. The packs are still separate. An example is a question about why the pack voltage is different after a charge if you parallel charge two different packs (like two sizes). The reason is that every equalization takes time, a few minute normally. I hope this example makes it clear that equalization is a long process, and packs need quite a bit of time to equalize.

My use case is simple: I have all my packs at storage voltage always. When I decide to go fly I put them all parallel and charge them all in half hour. The different voltages are normally after a day when I want to get all packs to storage level.
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Old Sep 30, 2010, 09:27 PM
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Quote:
Originally Posted by 4Dim View Post
My use case is simple: I have all my packs at storage voltage always. When I decide to go fly I put them all parallel and charge them all in half hour. The different voltages are normally after a day when I want to get all packs to storage level.
I do the same thing, however, if I do end with a couple of different voltage ranges I connect them in steps. For example, 1 is 3.7 per cell, another is 3.8 per cell, and yet another is 4.0 per cell. I connect the 3.8 and the 4.0, give them some time to equalize, somewhere around 3.9. Once they are stable, I then connect the 3.7 pack. At no time is anything connected with a huge voltage difference, even though they may have started with one. I do something similar at the field when charging, charge the lowest packs first, once they reach close the the voltage of packs that didn't get used as deep, I add them to the charge, of course stopping the charge in the process. I don't want anyone to think I just add them during the course of a charge.

Personally, I've never noticed a significant difference after the charge is complete, but then again I don't race over to take them off the charger. So it's possible there is, they just equalize in the time it takes for me to remove them.
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Old Sep 30, 2010, 10:20 PM
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I also do something like that if the voltages are different.
When you connect your packs together and they have different voltages, put a watt meter between them. Always interesting to watch. This gives a good idea of the current in different volt levels.

Another thing to play with
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Old Sep 30, 2010, 11:37 PM
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I am calling BS on the video above. He stated "are the leads getting warm? No, not at all. [pause] none of the leads are getting warm, none of the balance connectors are getting warm..." LOL ---> none of the balance connectors are, um, connected?

Rick
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Old Oct 01, 2010, 05:24 AM
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First off I must say well done to 4Dim on the video. I also frequently parallel charge packs, and they are not always at the exact same potential when connected together. The only difference in my system is that I made up a balance board using JSTXH sockets all connected in parallel, so that each balance plug is inserted in "pin parallel" configuration which allows charging
( only at low rate ) without needing to connect all the batteries main plugs together.
This low charge rate is simply fed via the outside edge balance leads ( pack -/+ ve )

It was interesting to see the comments earlier as to how long all the batteries took to equalize with each other, I would have expected equalization to be quicker.
As for the end result, well I still consider that once paralleled and equalized, the resultant large battery is then simply "seen" by the charger as one larger capacity battery.
After all, electrical node theory states "All leads connected to a single node (i.e. point) have to be at the same potential (i.e. voltage). All cells (packs) in parallel are connected to the same plus and minus nodes, ergo, no cell can "get ahead" or "fall behind" the voltage of any other".
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Old Oct 01, 2010, 07:07 AM
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[QUOTE=4Dim;16182659]Some people say its perfectly fine to connect charged and discharged packs
Excellent post and video.

Just for fun put a watt meter in the mix as source when you connect a fully charged pack with an empty pack and record it.
The current draw is very small and very safe.

Oops, just saw that someone else suggested that idea.
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Old Oct 01, 2010, 08:44 AM
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I wouldn't call it safe. The tables posted by 4Dim clearly show that it is fairly easy to exceed the charge C rating of many batteries.
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Old Oct 01, 2010, 09:02 AM
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Quote:
Originally Posted by Turner2 View Post
I wouldn't call it safe. The tables posted by 4Dim clearly show that it is fairly easy to exceed the charge C rating of many batteries.
But then that 1C charge rate is the minimum at best.
Most newer batteries can take far more than one c and remain safe in doing so.

Do some test on your old batts and be enlightened as to their ability to drink from the well So to speak...
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Old Oct 01, 2010, 09:35 AM
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Not sure what you are getting at. In the first table above the current flow starts at 15 amps. The second at 10 amps. Both these figures far exceed the charge C rating of any of my batteries.
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