# Battery efficiency ?

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Apparently, for every unit of energy extracted from a battery, it must be charged with 1.5 units ?

Thus, battery charging equates to around 75% efficiency ?

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I think its 1/1.5 * 100% = 66,6%

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2 hours ago, chenbeier said:

I think its 1/1.5 * 100% = 66,6%

7 hours ago, Erina said:

Apparently, for every unit of energy extracted from a battery, it must be charged with 1.5 units ?

Thus, battery charging equates to around 75% efficiency ?

Yes chenbier is correct but perhaps you would like a bit more since your battery 'efficiency' is not the same as mechanical or thermodynamic 'efficiency'.

${\rm{efficiency = }}\frac{{{\rm{output}}\;{\rm{energy}}}}{{{\rm{input}}\;{\rm{energy}}}}{\rm{ \times 100}}$

You have an input and a theoretically empty battery.
This battery has a 'capacity' which is the quantity of electricity required to fully charge it or charge it to 100%.

But here your 'output' is the actual amount of energy you need to put into the battery to bring it to 100% charge ie its capacity.
Your 'input' is the amount of energy, an energy meter would read as input to your combined battery and charger.

Some of this input energy is lost in the charger itself as heat and some is lost in the battery as heat.
Both of these losses count as inefficiencies.

so the efficiency is 1000/1500 x 100 = 67%

as chenbier says.

You must have noticed that chargers and batteries get warm during charging ?

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My understanding is that a modern Li ion battery has a charge/discharge efficiency of 80-90%.

75%, let alone 67%,  seems low to me.

Edited by exchemist
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Heat being a factor, I read something on the Tesla Motor Club forum that by controlling this factor the 80% can be achieved:

But in essence, battery efficiency is the base for calculations, and it's anything up from there ?

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1 hour ago, Erina said:

But in essence, battery efficiency is the base for calculations, and it's anything up from there ?

I'm sorry I don't understand this comment.

1 hour ago, Erina said:

Heat being a factor, I read something on the Tesla Motor Club forum that by controlling this factor the 80% can be achieved:

Heat is not necessarily a bad thing.

The capacity of an electric vehicle battery decreases markedly with lowering of temperature, (or raising it above optimum which is in the 20oC to 23oC range)
The car manufacturer will specify the capacity at this temperature.
At 5oC the capacity will be 25% - 30 % lower and it can be as little as 50% lower in freezing conditions.

Also the capacity depends upon the equipment used to charge the battery.

The Kia Nero has a 64kW-hr rated battery.

But it needs a 3 phase supply to achieve this capacity.

Ordinary UK mains can only reach 80% of this value ie 51 kW-hrs,

I recently charged a Kia from completely flat to this level  -  It took 28 hours charging at 10 amps on the UK 240 volt mains.  This equates to 67 kW-hrs.

So the charging efficiency was 51/67 x 100 = 76%

Other types of lithium-ion (there are several types) batteries have different characteristics.

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I meant that the formula was the base number, and then tweaking the process of energy production would allow for greater efficiency, and that's why some quote 80%.

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8 hours ago, Erina said:

I meant that the formula was the base number, and then tweaking the process of energy production would allow for greater efficiency, and that's why some quote 80%.

I'm sorry I still don't understand this.
A formula is not a number.

Perhaps there is some translation problem ?

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People come out with all kinds of numbers (efficiency rates), but I wanted to find the formula to actually calculate it. I have that now.

However, some say that the numbers produced are too low. So my thoughts were that the formula would only yield a base number, of which could be negotiated upwards, otherwise how could folk claim 80% efficiency ?

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14 minutes ago, Erina said:

People come out with all kinds of numbers (efficiency rates), but I wanted to find the formula to actually calculate it. I have that now.

However, some say that the numbers produced are too low. So my thoughts were that the formula would only yield a base number, of which could be negotiated upwards, otherwise how could folk claim 80% efficiency ?

Like @studiot, I'm having some trouble following what you are saying. You can't "negotiate" with electrochemistry or physics. Do you mean "optimising", by reducing  losses or something?

Also there is nothing fundamental about any of the formulae in this thread so far. All we have covered is the arithmetic to work out a % efficiency, given certain measured inputs and outputs.

I think you need to be a bit careful what is meant by efficiency in the context of charging and discharging a battery.

I see that @studiot has used it to mean the efficiency with which a given electricity supply, as input to a charging system, results in actual charge entering the battery. So that's the efficiency of the charging process.

However the figure of 80-90% I was quoting is the efficiency with which the battery itself stores charge, i.e. the amount of charge you get back out for the amount you put in. This is the charge/discharge efficiency.

What is the formula you are referring to?

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Thank you for the clarification: "optimising, by reducing  losses".

Charging and discharging is something that I also didn't take into account.

Charge in:

efficiency = (output energy / input energy) * 100

e.g. 1/1.5 * 100% = 66,6%

Charge release (out):

(charge from discharging / charge consumed in charging) * 100

So basically how much a battery doesn't leak, versus how much Hydrogen is leaked from its stored state, that is the main difference ?

Edited by Erina
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1 hour ago, Erina said:

Thank you for the clarification: "optimising, by reducing  losses".

Charging and discharging is something that I also didn't take into account.

Charge in:

efficiency = (output energy / input energy) * 100

e.g. 1/1.5 * 100% = 66,6%

Charge release (out):

(charge from discharging / charge consumed in charging) * 100

So basically how much a battery doesn't leak, versus how much Hydrogen is leaked from its stored state, that is the main difference ?

I don't understand this at all, I'm afraid.

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This should help.

3 hours ago, exchemist said:

I see that @studiot has used it to mean the efficiency with which a given electricity supply, as input to a charging system, results in actual charge entering the battery. So that's the efficiency of the charging process.

Exactly.

That is what Erina quoted from Tesla

On 8/2/2022 at 9:29 PM, studiot said:

So charging efficiency.

I have attached two diagrams firstly the charging diagram of an ordinary common or garden 12 volt car battery.
As can be seen the charger or alternator produces a (laughingly) constant voltage and the current is initially high for a discharged battery but gradually drops off as the battery voltage rises so the difference between the charging voltage and the battery voltage falls until the charger is just receiving a 'trickle charge'.

This trickle charge can safely go on indefinitely. Processes within the battery dissipate the very small amount of energy safely as heat.
Also since the charger voltage is only just higher than the battery voltage it cannot add further charge to the battery.
Also shown is the dashed constant current line that sophisticated battery 'fast' chargers operate.
It is not safe to try to pump the high initial charging current into the battery so when the finish point is reached this type of charger switches itself of and perhaps disconnects the battery.

Lithium ion batteries can also be charged in this manner but constant voltage charging is slow especially when the amount of charge to be added is considered.
So they use constant current charging and sophisticated controls which stop the process at the point A on the curve.
Now an EV battery is much higher voltage than the old type  - the Kia is over 300 volts - so the safety implications of overcharging are much more serious.
Also there will be statistical variation in the absolute capacity from battery to battery.

So the manufacturer rates the battery below this point and I have shown the statistical safety margin above this on the curve.
But note the curve also turns over compared to a straight line 100% efficient charging, just as the 12 volt battery did.

I have also drawn line BC to show this 100% efficient charging line.
Now the further up the curve you go the further below 100% is the charging efficiency.
So where the manufacturer places their battery rating on the curve determines the charging efficiency.

The Kia is a cheaper car than the Tesla so I expect that they can less afford to offer a generous rating and are pushing their batteries harder.
This would imply that their charging efficiency suffers comapred to the Tesla.

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