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I've been doing some discharge tests in order to figure out the Peukert exponents of two sealed lead acid batteries with capacities 7 and 9 Ah, in addition to an initial current of 2.1 and 2.7 A, respectively.

I've started noticing a strange hissing noise coming from these batteries whenever I discharge them at very high currents (never beyond their 1C rate though, I always make sure to stay 1 A below that). I heard that it is common for batteries to make that noise when they are being overcharged but does that also hold for discharge?

winny
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A.H.Z
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  • that's hydrogen gas leaking. you are overheating the battery. internal resistance times current^2 is the amount of power you are dumping in there. over-charging also overheats the battery. – dandavis Jun 09 '22 at 01:36
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    @dandavis - hydrogen gas release during charging is not due to heating - it is due to electrolysis. Hydrogen does not normally get released during discharge. It is also unlikely that the cell overheats during discharge to cause gas release. – Kevin White Jun 09 '22 at 03:00
  • @KevinWhite: i didn't say it was causal. Just about any battery gets hot if discharged too fast. What is causing the hissing if not for temp going up while volume stays the same? It doesn't sound like a battery (SLA) that would have a cheap SMPS built-in with some audible coil whine, so what is making the noise? I don't see many options aside from over-heating... – dandavis Jun 09 '22 at 03:08
  • Exactly how are you drawing the current out of the battery? – Bruce Abbott Jun 09 '22 at 04:38
  • @BruceAbbott using an electronic load I designed ( linear mosfet) – A.H.Z Jun 09 '22 at 08:17
  • @A.H.Z - What is the battery voltage when the hissing occurs? If a cell has reversed voltage it could cause gas production and "hissing". – Kevin White Jun 09 '22 at 14:24
  • @KevinWhite The battery's terminal voltage can be anywhere from 11V to 2V during the discharge tests. the hissing sound occurs throughout the test ( it's always there) – A.H.Z Jun 09 '22 at 15:09
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    If the voltage gets below ~10.5V it often indicates that one or more cells have reversed polarity and are actually charging in the reverse direction - that can cause electrolysis and hydrogen generation in the same way as charging. – Kevin White Jun 09 '22 at 15:43
  • @KevinWhite but isn't that drop in voltage a normal part of discharging? the battery terminal voltage ( not the battery's internal voltage) is supposed to decrease as we continue discharging, especially at high currents. That is because the internal resistance of the battery is increasing as we discharge the battery further. So how does that relate to cells have reversed polarity? – A.H.Z Jun 09 '22 at 18:28
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    2V at less than 1C is very bad. Why are you taking it down that low? – Bruce Abbott Jun 09 '22 at 19:38
  • @BruceAbbott I'm doing that in order to plot the discharge curve of the battery at that specific current. it is necessary for me to fully discharge it. In the case of a previous discharge test I quit the discharge at 4V ( 4A discharge rate) because the battery could no longer provide 4 A of current ( i had a 1 ohm resistor in the drain of the mosfet in case anything went wrong). – A.H.Z Jun 09 '22 at 20:20
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    @A.H.Z - I provided an answer explaining cell reversal. – Kevin White Jun 09 '22 at 20:24

1 Answers1

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This problem could be caused by over discharging the battery causing a reversed voltage on one or more of the cells.

A 12V lead-acid battery will consist of 6 cells in series. Ideally they would all have the same characteristics but in practice they will all have different capacities and the differences tend to increase with the age of the battery.

During discharge the same current flows through each cell and the voltage across each cell will gradually reduce as the cell discharges. The weaker cells with less capacity will discharge (possibly to zero volts) before the stronger ones.

If discharge continues beyond the point where a cell has zero volts the weaker cells will continue their voltage decline with the stronger cells providing the current until the voltage across them goes negative and they start charging in the reverse direction. This voltage reversal can cause permanent changes to the cell and also cause the same effects as with charging. If the voltage increases enough electrolysis can occur that results in gas generation. (In Li-Ion cells it can cause plating of the copper resulting in dendrites that short the cell).

Lead-acid batteries normally do not have individual cell monitoring and protection that are required with Li-Ion batteries so this cell reversal can occur if the total voltage across the battery drops to less than about 10.5V. This value is enough to guarantee that the worst case is a single cell at zero volt and five cells at 2.1V.

The situation is more complicated when under load as the terminal voltage will also be reduced because of the cell internal resistance, since the internal resistance of a cell increases at a low state of charge the weak cell is impacted even more and will drop its individual terminal voltage more than the others.

Here is an example 6V battery where the load has reduced the terminal voltage to 3V. In this case top and bottom "strong" cells still have 2V on their terminals but the centre "weak" cell has discharged to the point that the cell voltage has reversed and it has negative 1V at its terminal. This can cause effects that you have noticed or damage the cell permanently with some chemistries, particularly with Li-Ion.

schematic

simulate this circuit – Schematic created using CircuitLab

Kevin White
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  • Understood, so now it's best if I avoid discharging these batteries at high currents right? – A.H.Z Jun 09 '22 at 20:31
  • @A.H.Z - If the load causes the terminal voltage to go below ~10V or so there is a risk that cell reversal can occur. It will get worse as the battery ages. – Kevin White Jun 09 '22 at 20:36
  • And I suppose that this is killing the useful life span of my batteries correct? – A.H.Z Jun 09 '22 at 20:38
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    @A.H.Z - almost certainly. In general lead-acid batteries don't like being in a discharged state for very long and there are additional complications with the starved electrolyte sealed batteries. – Kevin White Jun 09 '22 at 20:40
  • So if I understand you correctly, you should not discharge 12v lead-acid batteries below 10.5V no matter the discharge current/ internal resistance, if you want to avoid ruining them prematurely that is. – A.H.Z Jun 09 '22 at 20:44
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    @A.H.Z - yes, ideally. – Kevin White Jun 09 '22 at 21:34