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There are numerous products that enable lipo batteries to be charged in parallel.

Most sources are clear on voltage and capacity requirements, which follow pretty directly from basic circuit calculations:

  • batteries must be same number of cells
  • mAh capacity of batteries can be mixed

One area of confusion is how much the batteries can differ in their voltage; a commonly cited value is 0.3V, but many different answers have been provided. Some of them are from quite old forum postings, so the answers may have been correct circa 2005-2006 but superseded by newer technology.

So, how much can voltage differ between two lipo batteries before it is inadvisable to charge them in parallel?

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  • \$\begingroup\$ Are you sure you don't mean "mAh capacity of batteries can't be mixed"? \$\endgroup\$ – pjc50 Jul 11 '13 at 12:54
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Parallel charging LiPo packs has become very common in the RC hobby. Granted, there is not a lot of empirical evidence about how good or bad this is. Only the fact is that a lot of people do this on a daily basis.

Personally I have been parallel charging 6s LiPo packs for 2 years now with good results. I have some budget packs that are over 100 cycles, so in that regard, I am happy with the life I got out of the pack.

My parallel charging routine was not very stringent. I likely never charged packs with over a 0.25V/cell difference during this time. I think in the future I will be more careful about the voltage of the packs. My recommendation would be to stay under 0.1V/cell difference.

The problem with parallel charging is that it is quiet easy to make a mistake, and connect packs of dissimilar voltages. So if you are going to do this, I would always double check your pack voltages before connecting.

A very good resource on parallel charging can be found on the Tjin Tech site. This is a very thorough examination IMO. If you scroll to the bottom it also addresses the potential surge currents when connecting the packs, and also includes experimental measurement of current that is released at the initial connection.

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Charging multiple lithium batteries in parallel is a bad idea. Some people may have a rule of thumb where it's sortof OK if everything is just right, but that doesn't make it a good idea. Read the battery datasheet and see how it needs to be charged. Unless you can see clearly how to stay within the proper charging regime by tying both cells in parallel, I wouldn't do it.

This applies to pretty much any battery that must be charged with current, not voltage. I might be tempted to do it if necessary with lead acid batteries if they were full enough to allow fixed voltage charging. But, lithium cells generally require careful current charging and monitoring of the voltage.

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Summary:

  • It's bad idea but may often not be fatal.
    YMMV.

  • Rough BOTE calculation suggests that it would be eays enough to get say doubling of the maximum allowed charge rate of a cell if cells with say 0.3V imbalance are hard interconnected at the cells and then charged immediately after connection.

  • If batteries are not hard connected at the battery but instead have leads to a common power supply point then not charging them for about 10 minutes after interconnection *should* allow safe enough [tm] self balancing. Adding a very small resistor in each battery lead or ensuring leads of a minimum resistance would aid this process. See text.

  • The existing rule of thumb is probably an empirical one based on the practical reasons behind the guideline in the paragraph above - see text.

    • Mutual balancing "on the bench" befor installation with a resistor or a purpose built bidirectional current limiter would be a good idea.

NOT an authorative statement. I've never hard paralleled LiIon cells.
But I do have lots of general battery experience and have thought about this specific issue before now.

Hard paralleling should be avoided if possible. With modern electronics it is extremely easy to make a switch which allows independent current paths when charging and discharging.

The "rule of thumb" MAY be based on experience and this in turn may be based on the happenstance of battery connection resistances - see below.

If you have cells which area rated at say 1C max rate and you charge two together at 2C, the charge may distribute unevenly and in addition you can get considerable intercell currents. The net result is that (it seems to me) that you could easily enough double a single cells rate charge rate.

Even simpler, if you can tolerate a small amount of voltage drop from the battery then adding a small amount of resistance in each lead such that it drops say 0.1V at full charge will allow quite substantial differences with minimal effect. If max charge is say 1C (common for many LiIon, some manufacturers allow up to 2C) the R ~= 0.1/C (C = Ah capacity in amps). So eg an 18650 cell (not a LiPo but same principle) may have a 2Ah capacity so R = 0.1/2 = 0.05 ohms. You can achieve something like that just by using two battery leads to wherever the cells connect to rather than hard connecting between cells and using a single lead. If 1C (2A of charge flows between imbalanced batteries the drop will be 0.2V - so 0.2V of imbalance at initial connection will be accommodated within spec if you have only battery-to-battery balancing to considerAs a very very rough rule of thumb LiIon capacity increases by about 6% per 0.1V across the constant current charge region. (That's based on a quick mental calculation of Vmin = 3.0V, Vmax = 1.2V, capacity at constant voltahge pedestal ~~ 80%, linear capacity change with voltage change). Capacity is NOT linear with voltage change but it gives us some idea. So a say 0.2V differential ~~~= 2 x 6% = 12% of C.If mac intercell balancing current = 1C then this will take ~~ 12% x 1 hour =~ 7 minutes. So if you parallel connect two cells with >= ( R/0.1C) lead resistance in each cell's leads and don't charge them for say 10 minutes after connection you'll "probably be OK" [tm]. Operation from battery immediately after interconnection is OK.

Effect on charge and discharge: As the above allows for about 2C of intercell transfer and as cells are not normally discharged at a 1C rate (laptop users usually value having more than one hour of battery operation) then enough resistance to provide Heath-Robinson interconnection protection would have minimal effect on cell discharge voltage. If charging at maximum capacity via these resistors te cell voltage will be reduced accordingly but as the system comes off constant current into constant voltage mode current will drop and battery potential will be made up. So the net effect is to slightly increase charge times.

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The only concern with parallel charging LiPo cells is that when connecting a mostly discharged cell in parallel with a mostly charged cell (the worst case scenario), the charged cell may dump power into the discharged cell faster than its maximum charge rate allows safely. A small (0.5 Ohm or less) resistor inline with each cell will mitigate this without significant reduction in charge rates. Use less resistance with bigger cells...

This can be repeated for any number of parallel LiPo's, and each cell will charge to the correct stopping voltage with little downside to having mismatched or significantly different charge states of the cells.

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Parallel charging LiPo batteries is a good idea, for a knowledgeable user. However, do NOT parallel charge NiCad or NiMH cells. When parallel charging LiPos, my personal experience tells me that LIPo batteries put in parallel need to be within ~25% State of Charge of each other, or else the higher-charged LiPo will drive a current into the lower-charged liPo which is too great, for too long a time, for the lower-charged LiPo to remain within its recommended safe-charge limits. I prefer to make sure LiPos are within ~15% State of charge of each other, however, before plugging them in parallel with each other. Also, I give the batteries time to equalize their voltages (and thus have time to share electrons) before starting the charger. For my 15% rule above, a 15% State of Charge difference in a LiPo between ~3.7V/cell and 4.0 V/cell corresponds to a Voltage difference of ~ 0.08V/cell. For my 25% rule above, the voltage difference in that 3.7V/cell to 4.0V/cell range is more like 0.12V/cell. I determined that these were State of Charge range values I liked by plugging in a power meter between two batteries and watching the current exchange between them.

For much more detailed information, including a plot of State of Charge versus cell voltage, and a plot to give you a rough idea of how long to let the batteries' voltages equalize before beginning the charge, as well as for some more background behind how parallel charging works, see my much more detailed article I wrote here: http://electricrcaircraftguy.com/2013/01/parallel-charging-your-lipo-batteries_22.html

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    \$\begingroup\$ Answers that are just a link are usually discouraged on stackexchange sites. Linking to more detail is great, but it's better if you can include enough detail your answer for it to stand on its own. \$\endgroup\$ – Phil Frost Jul 11 '13 at 3:43

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