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Suppose I have two 12v AGM batteries and I connect them in a series to up the total voltage to 24v.

My question is: how come this doesn't overload the batteries if individually they can only be charged at 12v~? How come they can collectively now be charged with a 24v input when connected in a series? I thought excessive voltage damages electronics. Also, what's the max number of batteries that can be chained together in series and how can I determine what kinds of voltage would damage a battery?

(I tried searching for a long time to understand why batteries might allow this but the only information I could find on this was explaining how to do it instead of why it works. Apologize if this is a basic question.)

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    \$\begingroup\$ How could a 12V battery ever be charged, when it's actually 6 2V cells in series? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 28 '16 at 9:57
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    \$\begingroup\$ I really don't know. This isn't my field but if you know the answer an explanation is helpful. \$\endgroup\$ – user125022 Sep 28 '16 at 10:06
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Let us keep it simple for the moment. One 12V AGM battery has 6 cell of each 2V in series making a total of 12V. So if you connect a second 12 V battery in series then you end up with 2 x 6 cells in series making 12 times 2 V or 24V.

Theoretically there is no limit of connecting batteries in series. Systems can be realised at higer voltages. However than there are also measure concerning extra insulation and protection against electrocution.

To charge a battery (combination of cells in series) you need a higher voltage then the battery voltage to make charging possible. That is why you need a 24V charger in your situation. This charger supplies with a little higher then the battery voltage the required current for charging. During charging the cell voltage goes up a litte bit and ends at 2.4 V per cell. For your two batteries in series being 28.8 V

Charging a 24V battery with a 12 V charger is not possible. In that case you need to charge each 12 V battery separately and when charged connect them in series before you can use them.

A proper charger also limits the charging current. The charger stops when the cell voltage reaches 2.4 V.

Note: The above is valid for AGM batteries. Not for Lipo systems. Lipo systems require a complete different charging system.

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    \$\begingroup\$ I believe the point of the question was to ask e.g. how come one of the cells doesn't get charged with 3V and another with 1V. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 28 '16 at 10:22
  • \$\begingroup\$ What is the voltage at each cell? \$\endgroup\$ – user125022 Sep 28 '16 at 10:25
  • \$\begingroup\$ Thanks a lot. It just clicked and I get it now. A+++ explanation. \$\endgroup\$ – user125022 Sep 28 '16 at 10:40
  • \$\begingroup\$ @Ignacio: I did not recognise and still dont that in the question. \$\endgroup\$ – Decapod Sep 28 '16 at 10:55
  • \$\begingroup\$ @Ignacio: Looking again at the question I understand now your remark. I took the why not further than the series connection of cells. You expected an explanation why the individual cells remain at 2 to 2,4 V. I think this would lead to far in this situation. \$\endgroup\$ – Decapod Sep 28 '16 at 11:32
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How you charge a battery of cells in series is determined by how they behave with overcharge. If they can tolerate overcharge, then it's easy. Limit the over-charging conditions to the tolerated range, and the battery will self balance, as fully charged cells wait for the others to catch up. If they fail with overcharge, then you have to take steps to control unbalance.

Nickel chemistries. Terminal voltage is below that of water dissociation, so excess charge current turns into heat. The ones to reach full charge first just get hot, and the battery self-balances as the other cells finish charging. Limit the current to control the temperature rise, and you're OK.

Wet cell lead. Terminal voltage is above that of water dissociation, so excess charge current turns into hydrogen and oxygen venting. The ones to reach full charge first just vent, and the battery self-balances. Top up the low cells before they get too low, and you're OK. Fail to top them up, you eventually damage the battery.

Sealed lead. You can't top up the cells, so the cells have a modified chemistry and construction so that at low enough overcharge rates, the evolved oxygen has time to diffuse to the other electrode and recombine with the hydrogen back to water, generating heat. Limit the charge voltage to limit the overcharge current, and you're OK. Allow high current to pass when overcharging, gases are generated faster than they can recombine, you damage the battery.

Lithium. You cannot overcharge the cells without causing damage, period. Every cell in a battery has to be monitored for terminal voltage, and charging stopped on that cell while the others finish their charge.

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