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I'm having a long discussion on an RC board regarding LiPo parallel charging.

The basic crux is that my understanding of how charging works is something like this:

Charger Line voltage of x(8.4v in this case) @ y amps (5.0) with 4 batteries connected. The battery capacities are listed as 2x1000mah and 2x1200mah.

Each battery internal voltage will rise as current is soaked into the battery. As each battery approaches 8.4v it will naturally soak less current up until the whole battery bank is approaching 8.4v at which point the peak current will drop low enough that the charger shuts off.

My assumption is that each battery will increase in voltage at more or less the same speed given that the battery bank is wired in parallel... That means the 1200mah batteries are soaking proportionally more current than the 1000mah batteries...

Or am I drinking too much moonshine?

The second part of the question regards balancing, my understanding is that the individual cells (2) are wired in series, this means that those cells have the potential to charge at different rates. Therefore balancing is recommended, but probably not mandatory all the time?

Thank you for your time.

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  • \$\begingroup\$ Key question: What are the max allowable charge current rates for the 1000 and 1200 mAh cells. Without this information the question become svery hard to give a good answer to. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 2, 2012 at 8:32
  • \$\begingroup\$ After seeing your comments, on the answer, I'm not sure that you understand how lithium ion, lithium polymer and lithium ion polymer batteries are charged. When you have ~40 minutes available, take a look at this video. This guy explains how process works from engineering point of view. You don't have to understand everything (unless you want to go into electronics and make your own charger), but it should give you an idea how charging process works and what are major components of a charger. \$\endgroup\$
    – AndrejaKo
    Commented Jan 2, 2012 at 17:14
  • \$\begingroup\$ Thanks AndrejaKo, it's appreciated. I watched the video, what I got out of it was that a lipo charger runs at escalating voltage @ set current until 4.2v and then runs constant voltage and cuts off when the current drops below a threshold. Given all that, besides the question of balancing the cells I'm not sure where I would run into problems... This isn't to say I'm sure there aren't problems, just that possibly there aren't problems or I don't know enough about batteries and charging to understand the problems. \$\endgroup\$
    – Gary
    Commented Jan 3, 2012 at 1:39

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What colour is the magic smoke when it vents with flame ? :-)

Rushing immensely, more later, but ...

What you describe seems to run a severe risk of doing damage.

To be ure, first you need to specify the allowed MAX charge rate for each LiPo.

You have 1.0 + 1.2 = 2.2 Ah in parallel so 5A = 5/2.2 = 2.27C.
This MAY be OK if cells are specd as 2C or more and are balanced in draw.
If specd at say 10C then it all may survive.
If specd at 1C it is very very bad.

BUT when a cell pair plateaus at 8.4V it's current will start to drop and if the charger is able to make 5A the extra current WILL flow into the still in current more other battery pair.

If max charge rates are not >> 2C then what you describe is at best an extremely poor compromise and at worst a disaster either in magic smoking or in cell lifetimes.

If max charge rates are around 2C then what you describe is at best a beating of some of the cells regularly and at worst a journey towards magic smokedom.

In an arrangement like this with different capacity pairs wired in parallel you need to carefully monitor individual cell pair or even cell voltages to prevent discharge-damage. This is going to make balancing more important, although I have been impressed with how well cells from the same batch seem to track when I have checked it (not often).

Operating cells of different capacities in parallel is an immednsely bad idea usually unless you manage and turn off each pair individually.

Apart from one pair endpointing before the other and throwing more charge or discharge onto the other there is a lack of certainty re how cells load share.
eg Say you have a 1000 and 1200 mAh cell and load both with 1000 mA. The large cell will see this as less of a percentage load so it's natural terminal voltage will be larger and it will "happily" supply the extra current. but there is no guarantee that it will o this in the ratio of the tywo capacities. The large cell may prove very "sacrificial and provide most of the load for most of its capacity. BUT when it finally falters the small cell will then take up most of the load and may now be overloaded. And there is no certainty that the LARGE cells will not now expire and be driven into a damaging mode. Probably not, but. too many uncertainties.

Why run cells of different sizes and in this 2 x 2 pattern?

Key question: What are the max allowable charge current rates for the 1000 and 1200 mAh cells.
Without this information the question become svery hard to give a good answer to.

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  • \$\begingroup\$ Actually these cells are all at the 2c+ max allowable charge... But that's not part of the question. I'm not asking if it's a good idea, I'm asking about the mechanism of charging. In addition we are talking about a charger that tapers it's charge, so we may start at 5amps, but my understanding is that as 8.4v approaches in the 'bank' than there will naturally be less current on the line... In this case @ 8.4v the amps will go down substantially until they hit 3% of the max charge rate, then the charger stops entirely. \$\endgroup\$
    – Gary
    Commented Jan 2, 2012 at 13:14
  • \$\begingroup\$ Also, again I'm not asking this as part of the question... But the charger itself will taper charge down. Full 5amps will only be until the bank measures 8.4v at which point it tapers down substantially... IE it's a smart(ish) charger. \$\endgroup\$
    – Gary
    Commented Jan 2, 2012 at 13:56
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    \$\begingroup\$ 2c+ is not good enough - there should be an actual spec - and it matters. And the taper charging will not save you. In a LiIon / LiPo system it is NOT the charger that tapers - the charger simply goes into constant voltage mode. It is the CELL that tapers. So if one pair of series cells starts to taper and the other set do not (as still not charged enough) then all the current that one set stop accepting will be taken by the other set instead. Actual max I and better still actual cell brand and type needed, but bad things are possible here. | \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 2, 2012 at 15:12
  • \$\begingroup\$ Re "soaking less current as the cells approach 8.4V - NO. Cells will accept whatever current you offer them all the way to 4.2V cell (8.4 V for 2 in series.) It is the charger going into constant voltage mode and not therefore giving the cell enough drive to drop voltage across its internal resistance that start it slowing down.| I'd guestimate that an imbalance of 0.1V of where a cell pair think they are on their curve wrt another pair may start to cause problems and a 0.2V mismatch may cause real problems. This is an out of spec way to charge cells and Murphy may or may not catch you out. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 2, 2012 at 15:14
  • \$\begingroup\$ Ok, fair enough. What about charging like capacities with a balancing board? How about like and unlike capacities with a balancing board? I'm under the impression that This will prevent imbalances from happening. \$\endgroup\$
    – Gary
    Commented Jan 2, 2012 at 17:25

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