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Old Nov 07, 2003, 10:02 PM
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what is safer series or parallel charging of multi lithium packs?

I just got an upgrade on my orbit to v6.3.

I'm thinking, now I can charge three 3S1P packs in series or just charge X amount of 3S1P packs in parallel.

Which is safer? Or is this not safe at all?

thanks, JT
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Old Nov 07, 2003, 10:40 PM
RIP Ric
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Buy more chargers, and charge them individually!
..a
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Old Nov 07, 2003, 11:41 PM
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Like Andy said, buy more chargers... What you want to do isn't safe at all.

WJ
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Old Nov 08, 2003, 02:12 AM
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I didnt realize that.

I always thought that if I discharged the packs with my discharger, then I could charge them in series up to as much as my charger would go (as long as they were of the same type and properly discharged). In this case three 3S packs.

In parallel, all the talk about lithiums self-balancing each other in a parallel charging circuit, I would have thought that I could charge all my packs (same type and discharged) as much as I wanted.

Where did I go wrong in my assumptions?

JT
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Old Nov 08, 2003, 06:57 AM
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Bing!,

You might be able to safely charge your packs in parallel if you used one of Kokam's safety circuits for each pack, but I'm not sure. Ask JJ Hong.

Series charging the packs would definitely be unsafe. You would have the imbalance problem multiplied - not only imbalance within each pack, but imbalance from pack to pack.

- RD
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Old Nov 08, 2003, 07:56 AM
RIP Ric
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The only way it would be safe would be if you matched the packs from the beginning, and flew them as one - in other words, made up a 3s2p pack..
..a
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Old Nov 08, 2003, 08:44 AM
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"In parallel, all the talk about lithiums self-balancing each other in a parallel charging circuit, I would have thought that I could charge all my packs (same type and discharged) ..."

"Where did I go wrong in my assumption?"

I'm not sure you did. I can't think of any mode of failure within each or all of the several packs, caused by parallel charging per se.

Let's ask JJ Hong.
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Old Nov 09, 2003, 04:51 PM
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Parallel charging of series-connected packs

Bing,

If I correctly understand JJ Hong's response at

http://www.rcgroups.com/forums/showt...58#post1476858

you can safely charge your 3s packs in parallel.

If you go without safety modules, your risk is the same PER PACK, whether charging one-at-a-time, or in parallel.

If a safety module disconnected one pack, the entire current set on the charger for all the packs would go through the remaining packs. While overamperage charging is not so critical as overvoltage, it would still be wise to monitor the safety circuit's status closely during charging, if the charging current is set as high as no. of packs times C.

- RD
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Old Nov 10, 2003, 09:41 AM
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Thanks RD!

Looking at JJ's very reserved answered, it looks like its OK to parallel charge multiple lithium packs. The danger would be no different from charging a regular parallel pack. NOTE: There is always danger in charging R/C Lithium batteries due to no. of variables associated to their use.

I believe that my plan to discharge the packs before hooking them for parallel MASS charging adds a margin of safety.

I will be doing careful testing next weekend and will report results.

Thanks to all!

JT
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Old Nov 10, 2003, 11:26 AM
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You're welcome, JT.

Discharging the packs isn't neccessary. Just turn the charger on and connect a first pack to the charger. Then connect each successive pack in parallel with the earlier-connected pack(s) as you go.

In other words, don't connect the packs together in parallel before you connect them to the charger. That's because a substantial residual voltage difference between packs would cause current to flow from the higher voltage pack(s) to the lower voltage pack(s), but connecting each to the higher voltage charger will assure current flow from the charger to each pack. The current flow to each pack will be substantially proportional to it's voltage and the packs will equalize to the voltage set on the charger.

Also, the danger IS greater than charging a regular parallel pack, if you mean a pack with no cells in series, e.g., what's sometimes called a 0s3p (or 3p) pack. But the danger doesn't lie in the parallel charging settup; it lies in each of the series-connected packs within the parallel settup. Therefore, it would be wise to use a safety module, such as Kokam's, for each pack.

- RD
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Old Nov 10, 2003, 11:37 AM
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If the voltages of the various packs aren't real close to the same, you will have the same problem regardless of whether the charger is plugged in or not. Voltage will flow from highest to lowest. Adding packs while the charger is operating can also be dangerous. A sudden jump or drop in voltage could cause many of these "autodetect" chargers to change their mind about how many cells are in the pack(s).

Here's the best rule of thumb, IMHO: Charge packs the same way they were discharged. That is, if you used two separate 3S1P packs to make two separate flights, charge them separately. You should only charge in parallel if the packs were discharged in parallel.

If you're going to connect two packs in parallel for the first time, the best way is to first charge each pack separately, then connect them. This way, they start out almost perfectly balanced. From there, as long as you keep them connected for flying and charging, they will be fine.
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Old Nov 10, 2003, 04:45 PM
The blade numbers go up to 11
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How about if you have 3x3S1P packs which you connect in series for 9S1P to fly with - could you connect them in parallel to charge if they are discharged all in series together? What would happen to cell balancing then? How would you ensure that each pack received the same charging current? 3 separate chargers, perhaps? Thanks

Stu Maxwell
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Old Nov 10, 2003, 06:41 PM
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Stu,

Three 3s packs which have been flown in a 3-pack series can be disconnected and charged in parallel on a single charger.

If they are of identical manufacture, e.g., three Kokam HD1500 3s packs, each will draw very close to the same current: Nominally, the current set on the charger, divided by three.

Moderate differences in current draw when charging in parallel are not so critical as voltage differences resulting from cell mismatches when charging in series. In other words, charging a cell at 110% C is not ruinous. Charging it to 110% of 4.23 volts IS ruinous.

- RD
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Old Nov 10, 2003, 08:04 PM
The blade numbers go up to 11
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Cool, thanks RD, I thought it might be OK. The good thing is that it means I don't need a booster from 12V for my charger.

Stu Maxwell
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