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I know current through series elements is always the same. I was wondering if the same is true for series supplies (batteries) or if it is possible for one supply to provide more or less current than another series supply.

I ask because a project for a BMS system states a specific requirement for per-cell current measurement and this got me confused that maybe there is something I don't know.

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    \$\begingroup\$ "It is possible for one supply to provide more or less current than another series supply" but since the current through them is the same, I think it would be determined (limited) by the "weakest" supply. \$\endgroup\$
    – Circuit fantasist
    Commented Sep 26, 2020 at 21:13
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    \$\begingroup\$ No, if the batteries are in series then the currents through them will be the same (unless you have some leakage around them for some reason.) The universe pretty much insists that nodes between batteries do not collect charges for any useful length of time. The forces are very much larger than you can imagine. Small charge differences could quite literally be used to move the moon away from the Earth. They are unimaginably powerful. \$\endgroup\$
    – jonk
    Commented Sep 26, 2020 at 21:14
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    \$\begingroup\$ @N.Berg I just did a quick calculation to illustrate. If you string up a silver wire from the moon to the earth, whose cross-section totaled just 1 cm^2, and you could arrange for a constant voltage difference between the ends of 1 GV (easily doable), then in one century's time you'd have enough charge difference between the earth and moon to completely cancel out their entire mutual gravitational attraction. \$\endgroup\$
    – jonk
    Commented Sep 26, 2020 at 22:03
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    \$\begingroup\$ BMS systems state per-cell current because cells are sometimes connected in parallel or in mixed configurations. \$\endgroup\$
    – fraxinus
    Commented Sep 27, 2020 at 8:28
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    \$\begingroup\$ @PeteBecker I'm not one to argue words when we both know the same situation equally well and don't need words. But things can be both in series and in parallel. In this case, it would be a series set of batteries with parallel leakage. So I'm not sure why you care or even want to bother with "they aren't in series" with me. Was there some purpose? \$\endgroup\$
    – jonk
    Commented Sep 28, 2020 at 16:19

6 Answers 6

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In a series circuit, the current in each component of the series will be the same. This is true no matter what the component may be: resistor, capacitor, inductor, diode, battery, etc.

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    \$\begingroup\$ Is this true even when the batteries are of different voltage ratings and may be of different physical sizes? \$\endgroup\$
    – schizoid_man
    Commented Sep 27, 2020 at 8:09
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    \$\begingroup\$ @schizoid_man yes. Batteries themselves may not be happy about the fact. \$\endgroup\$
    – fraxinus
    Commented Sep 27, 2020 at 8:26
  • \$\begingroup\$ This answer is not as useful to the OP as it might seem. Whilst it is factually correct (series components see the same current), in BMS configuration it is very likely that some cells will be connected in parallel. BMS typically deployed for modern battery chemistry setups where a mix of serial/parallel cells are used to get an optimal voltage/capacity combination. As such, the per-cell voltages do vary and require balancing by the BMS to optimise charging and overall battery life. \$\endgroup\$
    – simon coleman
    Commented Feb 16, 2021 at 14:40
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I think the key part is the BMS system you mentioned. I believe it refers to charging control circuits that have circuitry to monitor individual cells or sets of cells, and circuitry capable of shunting charging current 'around' cells that are reaching full charge before others. As such, this arrangement is NOT strictly in series, as the additional circuitry provides parallel current paths on each cell (or set of cells). I see this typically when I open Li-ion battery packs.

I don't believe they do any shunting during the discharge cycle (but I could be wrong).

Technically, everyone else stating that if just the batteries alone are connected in series (no balancing circuitry) that the current is the same in all, are also correct.

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    \$\begingroup\$ A simple solution of this idea is to shunt each cell of the battery by some kind of diode - LED, two Si diodes in series, etc. The LED solution will also indicate the cell state. \$\endgroup\$
    – Circuit fantasist
    Commented Sep 27, 2020 at 8:40
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    \$\begingroup\$ @Circuit fantasist I don't know if that would work well; have you seen it done? I think it would be difficult to find the exact combinations to equal exactly the 'full' voltage. The voltage of a low cell and full cell can be very close to the same, depending on battery type. (A selected zener might be about the only practical approach.) I'm not saying this won't work, I'm just skeptical... \$\endgroup\$
    – troubleshooter
    Commented Sep 28, 2020 at 5:46
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    \$\begingroup\$ troubleshooter, I saw this idea in 80's, in an old German book photos.app.goo.gl/AaapkH2ouMkDgCKN8. I agree with you... but the simplicity of this idea is attractive. I use it as another example of "current steering" (besides the well-known differential pair). \$\endgroup\$
    – Circuit fantasist
    Commented Sep 28, 2020 at 6:16
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    \$\begingroup\$ @Circuit fantasist Nice link! I love it. I was contemplating if it was intended for lead acid cells. I was debating if 'back leakage' at close to conducting voltage would drain small cells excessively. Also, would not work on Lithium, as they DECREASE voltage as they finish charging (I assume you know that too). -Could be a triggered melt-down! --- "Ah, my battery's done charging, the zeners just melted!" LOL EDIT: OOPS! guess it would simply just not charge past that point; or if not just right, would overcharge and ruin the cell. \$\endgroup\$
    – troubleshooter
    Commented Sep 28, 2020 at 17:09
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Assuming no other connection between the batteries, no. This would be a violation of Kirchoff's Current Law. Imagine a node between the batteries. All the current entering the node must leave the node.

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I was wondering... if the same is possible for one supply to provide more or less current than another series supply

No, the current through them would be the same... and it would be less than the current that can be provided by the "weakest" supply when working alone.

The reason for this is that each power supply has some internal resistance... and when they are connected in series, their internal resistances are summed.

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The current (charge/discharge) for true series cells will be the same. The BMS is not truly series under charging, since it has to deal with how effective each cell is converting the charge back into the chemical structure storing it. Small variations in how the reactions (wanted & unwanted) are propagating through the cell structure leads to variation in charge storage rate. Since overcharge can lead to deterioration of the cell capacity, the BMS divides the battery to cells and charges each independently, hence the need to separately monito each one.

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The current is the same when charging or discharging. The problem is if one cell is discharged faster its polarity reverses and it gets damaged. If in charge one gets charges faster it can overheat or explode when it's lithium.

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