# Understanding current through batteries and Kirchoff's Law

i've just started studying electricity a while ago , but there is something that i've been trying to understand and it's making a huge gap inside my head to understand electricity. Now in kirchoff circuits where you haven't got just one battery but more , how does current flow through the battery ? If it's through the electrolyte then if electrons can do that why batteries die then ? I'am just starting to understand it , thanks in advance and i owe whoever make me understand it

• Batteries store charge like huge electrolytic caps in xxx Farads, but with a nominal cell chemical potential voltage. By convention we say current drains out of a battery going from its voltage to a lower one thru resistance and might accept charges visa versa. Charge, Q=CV and current is dQ/dt – Tony Stewart Sunnyskyguy EE75 Sep 29 '17 at 19:30
• @TonyStewart.EEsince'75 those formulae for charge are for a capacitor, not for a battery – Claudio Avi Chami Sep 29 '17 at 19:41
• @Claudio, yes they apply but you use between initial and near final voltage rather than initial voltage and zero. since there are secondary charge hysteresis effects 90 to 10% SoC gives best results. But I digress... – Tony Stewart Sunnyskyguy EE75 Sep 29 '17 at 20:10
• You should edit your post. It's hard to follow, and doesn't need all that "I don't know what the hell I'm doing"-fluff you inserted. – Iam Pyre Sep 29 '17 at 23:04
• Youtube has lots of good videos on batteries, cations, anions, electric potential of half-reactions (relative to hydrogen) and a host of other details. Lots of good videos to take you from, quite literally, a sing-song follow-along approach with music and animation, to serious stuff. You should watch some. – jonk Sep 30 '17 at 0:11

Current flows through batteries, electrons do not...

Batteries are sort of like bottomless capacitors, continually refilled with charge by a chemical reaction.

When in circuit, electrons leave the negative terminal and are immediately replaced by an electron entering the positive terminal. The arrival of this electron causes the chemical reaction to advance which in turn creates a new electron ready to leave at the negative electrode.

The whole process repeats until all the chemicals involved in the chemical reaction are expended and the battery is dead.

However, while still alive, since the number of electrons leaving the negative terminal equals the number arriving at the positive terminal, we can say current flows through the battery.

You guessed right. The path of current is through the electrolyte.

When batteries are powering a circuit, the charges are flowing into one battery terminal, through the battery, then back out through the other terminal. No charge is ever lost during discharge. A battery forms a complete circuit, like an ideal zero-ohm resistor. And whenever we "recharge" a battery, no charge builds up inside. In this way batteries are no different than resistors or wires. Or different than DC generators with all those thick coils inside which short everything out.

In batteries and in wires, the amount of charge doesn't change, and the flowing charges are provided by the conductors. All conductors are always full of vast quantities of movable charges. (The word "conductor" really means "charge-filled matter, where the charges are mobile.)

Why do batteries die?

They die when they run out of chemical fuel, when one of the plates has entirely corroded away. All batteries are charge-pumps: chemically-powered charge-pumps. As with any pump, the "fluid" is not provided by the pump, and also it's pumped through the device and back out again. Batteries are closed circuits, same as resistors. Think of a battery as a constant-pressure, variable-speed water pump, where the "pump" turns off as soon as the constant pressure is reached.

The battery electrolyte is a good conductor, a short circuit. Why doesn't it short out the battery? Simple: because all batteries are actually composed of two "half-cells." One half-cell appears at the metal surface where the liquid electrolyte touches the positive plate. The other one is found where the electrolyte touches the negative plate. All the weird and interesting stuff is happening at these two contact-areas. The actual battery part is actually quite thin: it's a molecule-thin surface layer where a liquid conductor touches a solid conductor. ANd there are two of them, not one.

So, the battery-electrolyte is actually a sort of "shorting bar," like a "heavy wire" which connects between the two internal half-cells.

When grade-school science books tell us that batteries "store electricity," they're just wrong. Batteries store chemistry! Batteries provide energy via the corrosion of metals. During "discharge," one battery plate is dissolving into the electrolyte. (Your zinc plate becomes zinc chloride solution.)

What about recharge? No charge is ever injected into a battery. Instead, we "charge" them with energy, with joules not coulombs. In rechargeable batteries we can reverse the current and un-corrode the metal plates; electroplating new metal out of the dissolved compounds in the electrolyte. Since the path for charges is through the battery and back out, "recharging" means filling the battery with new chemical fuel.

Batteries are very weird, since no electrons ever flow through the electrolyte. In lead-acid batteries, the electric current is made of protons. Conductivity in acids is caused by movable protons, not movable electrons. (This is easy to miss in chemistry classes, since protons are called "+H hydrogen ions.")

While inside metals we have electrons jumping between atoms, in acids we have protons jumping between water molecules. Metals and acids are both conductors. (Double-Es should all be familiar with Drude and Grotthuss, but we usually miss it, since they're both fairly concealed in university physics and chem classes.) In addition to these two, we also have salt-based electrolytes where the moving charges are copper ions and chloride ions for example. Also there are alkaline solutions, where positive sodium and negative OH ions are moving along as the "electric current."

So, whenever you're starting your car, the few hundred amps in the battery acid is a proton-flow mostly, while the same amperes turn into electron-flows as the current in the acid enters the battery plates. And since batteries aren't charged with electric charge, it makes sense that one way to "recharge" a lead-acid battery is to take out the plates and coat them with chemical fuel, oxide/sulfide paste etc.