Alright, I should probably know this, but I don't. For a school project, I need to be able to charge a 12v battery from an alternator. What I am stuck on is the method of how batteries (generally speaking) are charged. Is it just plain old voltage going into the battery? Or is it something more?

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    \$\begingroup\$ Here's a good place to start: batteryuniversity.com/learn \$\endgroup\$
    – m.Alin
    Commented Apr 23, 2012 at 14:07
  • \$\begingroup\$ Wow, that is one heck of the start, to say the least. :) thanks \$\endgroup\$ Commented Apr 23, 2012 at 14:11

3 Answers 3


The method of charging a battery while being effective but without doing damage varies with battery chemistry.

Since you have a "12V" battery, I'm guessing it is a lead-acid type like you find in cars. Lead acid are fairly forgiving in how they can be charged, unlike other chemistries, especially certain types of lithium. Basically charge a lead acid with power that is both current and voltage limited. The voltage limit is usually 13.6 V for car batteries. The maximum current depends on the size of the battery. A ordinary car battery can take several amps easily. For example, a power supply that is limited to 5 A and 13.6 V will work just fine to charge ordinary car batteries, although that's not pushing the maximum allowed current. That means it will drop the voltage to not exceed 5 A or drop the current to not exceed 13.6 V, whichever is lower. Hardware and automotive stores sell chargers for car batteries that have all this built in. The only gotcha is a charger advertized as "fast" may abuse the battery. A full charge should take a few hours, although most of the time the battery should not get low enough to require that.

If your battery is smaller, then you have to dig up its datasheet or somehow get specs for it and make sure the charger doesn't produce too much current.

If your battery is not lead-acid, then things could be quite different. In that case you really have to get the specs, which should include the required charging profile. Getting this wrong, particularly with some types of lithium, could result in pyrotechnics.

  • \$\begingroup\$ Alrighty, that makes sense. So is there any easy way I can regulate the voltage coming out of an alternator? \$\endgroup\$ Commented Apr 23, 2012 at 14:10
  • \$\begingroup\$ Rule-of-thumb: limit the current to 1/10 of the capacity in Ah. So for a 40Ah battery limit to 4A. \$\endgroup\$ Commented Apr 23, 2012 at 14:15
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    \$\begingroup\$ You could use a smps(switched-mode power supply) to regulate voltage. I would suggest you to buy one rather than make as they can be pretty hard to fine tune \$\endgroup\$
    – Shungun
    Commented Apr 23, 2012 at 14:18
  • \$\begingroup\$ @Shungun: switcher ICs are very easy to use these days. They often only need 4 external components. The only thing when you don't choose the most optimal parts is that the efficiency will be somewhat lower. \$\endgroup\$ Commented Apr 23, 2012 at 14:25
  • \$\begingroup\$ Okay, the "original" focus of this question was to see what was required to charge a battery in the terms of like if anything more than pure power is required to charge a battery... \$\endgroup\$ Commented Apr 23, 2012 at 14:52

batteries are source of electrical power and this power is characterized by two things Voltage(volts) and Current(amperes) now when you give a close look at the battery (in this case 12V) then what it means is that the battery can Provide a maximum Potential Difference of 12V between its Contacts.. there is also a Power rating available with the same.. a Cellphone battery may read 1300mAh@ 3.2V which means that the battery can supply 1300mA current for 1 hour at 3.2V. The chemicals inside the battery undergo some reaction and Produce a potential difference, the reaction stops at a particular Potential Difference, a voltage above which the reaction cannot proceed (in your case it is 12V) although it does not imply that the battery is weak! as you draw power from the battery you provide a way for these electrons to move out from one end and reach the other terminal thus providing a way for the reaction to occur, the reaction continues till the time any power consuming device is connected to the terminals and the chemicals are being used up in the reaction. when the battery is depleted it is identified by falling terminal voltage in your case the voltage levels shall fall below 12V.

now comes charging: during charging we provide a voltage (12V or more in your case) in opposite direction, that is the +ve of your charging source to -ve of battery and -ve of your charging source to +ve of battery this creates an electron path in opposite direction for the Battery being charged, this reverses the chemical reaction (this is the difference between Rechargeable batteries and non-rechargeable batteries, the later has no reversible reaction) and takes the chemicals inside to a state when they were charged and produced a potential difference of 12V.

in a way we are reverting the chemicals inside to an earlier state, we are not storing any electrons inside...also based on the drift velocity of the electrons it is fairly impossible for the electrons from the socket to reach your battery before it is fully charged.

  • \$\begingroup\$ Rare Good Explanation \$\endgroup\$ Commented Sep 26, 2018 at 15:59

For practical charging, a standard power supply can destroy your battery (constant-voltage supply.)

Instead, the simplest method is trickle-charging with a constant small current. The exact current would depend on the type of battery and its capacity rating. (ALso don't mess with DIY chargers for explody lithium, use NiCd or lead-acid. Or just buy a charging device for Li rechargables.)

I've charged batteries by using a lab-supply with an overcurrent knob.

Set the supply voltage above the battery's working voltage (so, 14V for a 12V batt.) Then set the constant-current knob to a high current for fast-charge of batteries having too low voltage. As the battery voltage quickly rises, the high current drops back to a small value for long-term trickle-charge.


A battery is a charge-pump. It pulls electric charge in through one terminal, pumps it through itself, then spits it out through the other terminal. No charge ever builds up inside. This makes sense, because the battery electrolyte is a good conductor, and all batteries act like "short circuits," with very low internal resistance. Batteries are made from conductive materials, and the path for current is through the battery, through the electrolyte between the plates, then back out again. No charge ever builds up inside.

And with water pumps, no water builds up inside: the path for the current is through and back out again, same as for charge-pumps.

So, when we "charge" a battery, we're not storing up any charge? Yep, that's right. The total electric charge inside a battery never changes.

But something does change. Batteries are charge-pumps, chemically-fueled charge pumps. They can only "run" until their chemical fuel is all used up. When it's gone, then the pumping-action halts. That means your brand new flashlight battery is full of chemical fuel. And a "dead" battery has lost its fuel, and only contains waste products, so we send it out to be ground up and recycled.

What then is "recharging" of batteries?

Ah, now we've uncovered a problem with words. Batteries are never "charged" with electric charge. They're only "charged" with energy, energy in the form of chemical fuel. The word "charge" has more than one meaning. (And, cannons are given a charge of gunpowder. A fully "charged" cannon, it doesn't involve voltage or amps, or even coulombs!)

Charging and discharging a battery involves the movement of "charges" of energy measured in joules, or watt-hours, etc. Not coulombs. Whenever some energy flows into a battery or out of a battery, the coulombs just flow through.

Rechargeable batteries do something very weird. If we run their "electricity pump" backwards, for example by connecting the battery to a generator ...then the waste products are converted back into chemical fuel again! The zinc chloride in flashlight batteries gets converted back to zinc metal. Or, the cadmium hydroxide in your NiCd battery gets converted back to cadmium metal. This is the reverse of normal battery operation, where the metal plates provide energy as they dissolve away while the "electricity pump" is operating. A metal plate can provide energy by corroding. And if we want to "uncorrode" a metal plate, this takes energy supplied from outside the battery.

So, during battery discharge, the metal plates themselves are the "chemical fuel" which drives the current-pumping operation. The plates corrode as the battery runs, and the metal turns into dissolved chemical-waste products. To "recharge" a battery, we just force the current in the opposite direction. The metal plates get electroplated. They thicken up, ideally becoming the same as when new. And ideally, if the metal plate supplies a certain amount of energy, then the same energy must be injected into the battery when we un-dissolve the metal plate. "Charging" a battery is undissolving its metal plate. "Discharging" a battery is corroding its metal plate in order to power an external device.

Batteries are small metal-burning electrical generators, no steam turbines needed! But with normal power plants, if we run the turbines backwards while pushing the smoke back into to boiler, it doesn't create any new coal or oil!

When analyzing batteries mathematically, everything becomes quite easy to calculate because battery voltage is almost constant.

That means, if the battery produces a variable electric current in an external circuit, it also sends variable energy into that circuit, and the rate of energy-flow will be proportional to the amperes. And, the total energy contained inside a battery will be proportional to the electric charge pumped through it. One coulomb of charge equals one ampere-second. (One amp flowing for one second means one coulomb of charge has passed through the battery.)

This means we can (temporarily) ignore voltage and then estimate the energy inside batteries in terms of ampere-seconds, amp-hours, etc. (Note that it's amps-times-seconds, not amps-per-second.)

But ...were any amps ever stored inside the battery? Or any amp-hours stored? Nope. Amp-hours (if multipled by the constant voltage) are just an oversimplified shorthand for energy, and electrical energy is always based on voltage and coulombs. Since we don't want to work with coulombs of electric charge, and prefer amperes instead, ...and since the voltage stays constant during charge or discharge, ...then AH ampere-hours becomes our main energy rating. Yes, it's quite twisted up and hard to understand.

Energy is actually volt-coulombs, which is the same as volt-amp-seconds, which is the same as volts times AH times 3600. But if volts stays the same, and 3600 stays the same, then all the changes happen only in the Amp-Hours ratings. In the end, we rate batteries in Amp-hours. Yet the true ratings behind this are: volts, times the total coulombs able to be pumped by the battery, through the battery.

To calculate the actual energy stored, multiply amp-seconds times volts. Or, use amp-hours times 3600sec/hr, times volts. That gives us the total joules of chemical energy stored inside a battery.

But unfortunately the use of amp-hours convinces everyone that ampere-hours are a form of energy, or that AH gets stored inside the battery. Or that batteries are charged with electric charge, when actually they're only charged with joules of electric energy. The electric charge inside a battery never gets bigger or smaller.

  • \$\begingroup\$ what float charge voltage do you recommend? \$\endgroup\$
    – user2497
    Commented Sep 25, 2017 at 10:31

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