# When charging rechargable battery, should we care about watts and ignore voltage and current?

I know this might be a silly question, but it's from a beginner.

Let's say I want to charge a rechargable lithium-ion battery, so when I charge it should I only check the watts and ignore voltage and current specific values? I mean the current is about how much electrons are flowing, and voltage is how much potential energy is stored in each electrons (this is how I understand voltage and current, correct me if my thoughts are wrong), so I thought that if I charged a battery with 10 watts, 1 amp and 10 volt, then the battery will be charged well, but if I charged the battery with same10 watts, but 2 amps and 5 volt, won't the battery be filled with weaker electrons? I mean this time the battery will be filled with less-energy electrons than the previous example, so should I look at the volts and amps when charging or the watts is enough?

• There are no "weaker" electrons. – JRE Feb 14 at 18:02
• All electrons are equal (and there are no electroncs which are more equal then others.) – Oldfart Feb 14 at 18:04
• If you fill a kettle more slowly from a tap, do you end up with weaker water? – Finbarr Feb 14 at 18:08
• NO! NO! When recharging a lithium-ion battery you must limit current and voltage to safe levels. If you are not sure how to do that then don't try. Fire and explosion can result if you get it wrong. – Elliot Alderson Feb 14 at 18:09
• Beginners should not design lithium battery chargers, period. This is not an appropriate area in which to make learning mistakes. – Chris Stratton Feb 14 at 18:10

All batteries (in contrast with capacitors) exhibit what is called variable charge efficiency. That is, when the voltage is low, forcing 1 coulomb of charge into the battery will allow you to deliver (almost) 1 coulomb of charge when discharging at that voltage. This is a charge efficiency of about 1. When a battery is nearing full charge, the charge efficiency will drop, often to quite low values. Since power is voltage times current, the "unused" current and power near full charge will show up as excess heat, and the battery will start to warm up. So power is not a useful measure of charge effectiveness.

In most chemistries, there exists a maximum voltage, above which all input power is wasted, and this condition usually will damage the battery, as well. In "standard" lead-acid cells, the electrolyte may actually start boiling.

In lithium cells, trying to charge above a certain voltage (nominally 4.2 volts for a 3.7 volt cell), the cell will be irreversibly damaged, and may catch fire.

won't the battery be filled with weaker electrons? I mean this time the battery will be filled with less-energy electrons than the previous example,

shows a really major misunderstanding of how batteries work, to the point that I'm not sure how to respond.

For instance, if you have 3.7 volt lithium cell and you try to charge it with "1 amps and 10 volt", a charger which will supply 10 volts and a maximum of 1 amp will current-limit at 1 amp, and the battery will show a voltage much less than 10 volts (and if it shows more than 4.2, you're hosed). Likewise, a "2 amps and 5 volt" will limit at 2 amps, and less then 4.2 volts - unless you're hosed. Now, it's true that, as a rule of thumb, the 2 amp charge will produce a full charge in (more or less) half the time as a 1 amp unit, but that's not at all what you're asking.

In all cases, the useful effect of charging is to change the composition of one or more molecules. In the case of lithium ions, the most common reaction is of the net form$$LiC_6 +CoC_2 \Leftarrow\Rightarrow C_6 +LiCoC_2$$ and the reactions use the same electron energy regardless of the input current.

• You say "in most chemistries"; I'd like to see one that doesn't! – Hearth Feb 15 at 2:03
• @Hearth - So would I. But I've pulled the "this never happens" pronouncement, and gotten burned by it, often enough that I try to avoid it. – WhatRoughBeast Feb 15 at 2:08

You may not only look at watts when charging a lithium-ion battery and ignore voltage and current limitations.

Charging a lithium-ion battery is very sensitive process. It will explode easily with overcurrent, overvoltage, or overcharging.

You should probably start by understanding the final step of charging "CV" charging. During this step the charger behaves as a voltage regulator, using power to push current into the battery, but as it does all that, it's controlling the voltage across the cell which is why we call it "Constant Voltage"

Constant Voltage charging begins at the end of Constant Current charging where current is controlled as the voltage across the cell rises from 2.7V or so to the Constant voltage limit. At the time the charger changes over from CC to CV, the power into the cell is maximized.

Some chargers cannot achieve this maximum power and have to limit power, running a constant power mode between Constant Current and Constant Voltage.

In each case the charger uses some sort of switch to control the flow of electrons into the cell. But whether it controls voltage or current, the other is a function of the first as determined by the Lithium Ion Cell and a number of other variables like its temperature.

Usually a cell datasheet will have a recomended charging profile with a charge current limit and a charge voltage limit (as well as a recommended termination current). These are followed or violated at the designers discretion and will affect things like the capacity and the lifetime of the cell.

A lithium battery is first charged with a constant current. That current depends on the battery specifications (smaller battery smaller current). When the voltage reach the 4.2V then the voltage is kept on 4.2V. Do not exceed this voltage! When you keep this voltage the current will drop to allmost zero. And then the battery is full.

If you do not use the correct voltage and current to charge a battery you run the risk of having your battery quite literally explode. And if it doesn't, you'll still damage it, reducing its capacity or just making it more likely to explode in the future. Batteries, lithium-ion especially, must be treated with care and you should not try to design your own charging system without some serious research first into how to properly use them.