How are series connected batteries of different voltages (or even same voltages) charged "equally"?

Question

I am trying to understand conceptually, how a series bank of batteries is charged "uniformly". Let's take the example of of six 6V AGM batteries for a 36V golf cart. The charger is 36V nominal but of course will be somewhere between 36V and maybe 43V. Ok so the electrons from the charger come out of the - (negative) post on the charger, then pass thru the first battery in the series "chain", then pass thru the other 5 batteries, then back to the positive on the charger. How is it that each battery gets charged about "equally"? Shouldn't the first battery the charger "sees" get a stronger charge?

Here is another way to state my question. Suppose instead of six 6V batteries, we had three 8V batteries and two 6V batteries all in series. That is still 36V nominal total. Suppose the batteries are in this order (8V,6V,8V,6V,8V). How is it that while charging, you might see 6.9V across each 6V battery but see 9.2V across the 8V batteries? In other words, how does it "know" to charge each battery the right amount (voltage and current)? Does the order of the "mixed" voltage batteries matter for charging/discharging? Would it help to occasionally reorder the batteries so the "first" battery in the chain is changed every month or so, or is that order irrelevant?

When answering this, you can assume that all batteries are AGMs of similar age and health. Also assume the interconnect wires are thick and of minimal loss and do not get warm.

Answer 1

How is it that each battery gets charged about "equally"? Shouldn't the first battery the charger "sees" get a stronger charge?

In a series circuit, each element receives the same current, so an equal amount of charge flows into each battery. The order of the batteries therefore makes no difference.

How is it that while charging, you might see 6.9V across each 6V battery but see 9.2V across the 8V batteries? In other words, how does it "know" to charge each battery the right amount (voltage and current)?

Each cell has a voltage across it decided by its state of charge. As the current flows through the cell, the cell is charged, which causes its voltage to increase. Nothing needs to "know" that. The voltage increases as a consequence of charging.

Does the order of the "mixed" voltage batteries matter for charging/discharging? Would it help to occasionally reorder the batteries so the "first" battery in the chain is changed every month or so, or is that order irrelevant?

It is irrelevant because each cell sees only the current applied to it, and that is equal for all cells.

Answer 2

so the electrons from the charger come out of the - (negative) post on the charger, then pass thru the first battery

Actually the electrons don't pass through the first battery, they get absorbed by it in the chemical reactions that occur during charging. But an identical amount of other electrons come out from the positive terminal and go into the next battery, charging it by the same amount. The flow of charge inside the battery is via ions (charged molecules) in solution, which move very slowly. That is why a battery cannot be charged or discharged very rapidly.

Only after the battery becomes fully charged do the electrons 'pass through' it. However in a Lead-acid battery they actually contribute to electrolysis of the water into hydrogen and oxygen, which is why an AGM battery must never be overcharged!

If you have several batteries in series and they don't all have the same state of charge then some will become fully charged before others, then they will start to overcharge. In 'flooded' Lead-acid batteries this overcharging causes them to 'gas' and lose water (which can be replaced by 'topping up' the cells with deionized water), until eventually all the batteries get a full charge and become 'balanced'.

Sealed lead-acid batteries are much less tolerant of overcharging than 'flooded' batteries. AGM batteries can absorb a small amount of gas, but too much will cause them to vent and permanently lose electrolyte. Therefore the charger must limit the voltage to prevent gassing. But it doesn't read the voltage on each cell, so while theoretically you could wire several different sized batteries in series in any order and they would all get the same charge, if they do not have identical capacity and state of charge then some will get overcharged and others undercharged.

Suppose instead of six 6V batteries, we had three 8V batteries and two 6V batteries all in series. That is still 36V nominal total. Suppose the batteries are in this order (8V,6V,8V,6V,8V). How is it that while charging, you might see 6.9V across each 6V battery but see 9.2V across the 8V batteries?

All lead-acid batteries are series strings of 2V cells internally anyway, so it makes no difference how they are grouped.

Shouldn't the first battery the charger "sees" get a stronger charge?

Each cell's voltage is determined by its state of charge and the current passing through it, which in a series circuit is the same current for all. The charger only looks at the total voltage and presumes that when it sees eg. 43 V that each cell has 2.4 V across it. If some are higher and some lower due to having different charge states the charger can't tell.