Lithium battery capacity meter

[ramonegumpert]

Dear Friends

Is there a good battery capacity measurement device you can recommend?

I need to measure how much capacity a lithium pack really has.

Thanks.

regards
Ramone

 

[MrSparkle]

battery capacity is not that easy to measure accurately. It varies by battery, temperature, and rate of discharge, and all those variables interact with each other.

 

[meBigGuy]

Are you trying to measure a fully charged battery's capacity under certain discharge conditions, or do you want the state of charge, ie how much charge is left in the battery.

The first is easy, so I assume that is not what you are asking. The second is hard.

This addresses state of charge:
http://www.digikey.com/en/articles/...-charge-measurement-for-lithium-ion-batteries

 

[ramonegumpert]

Are you trying to measure a fully charged battery's capacity under certain discharge conditions, or do you want the state of charge, ie how much charge is left in the battery.

The first is easy, so I assume that is not what you are asking. The second is hard.

This addresses state of charge:
http://www.digikey.com/en/articles/...-charge-measurement-for-lithium-ion-batteries

Dear meBigGuy

I am trying to verify a lithium battery pack is as claimed or rated.

Else we are buying with eyes blinded.

 

[ramonegumpert]

I found the answer :
http://www.aliexpress.com/item/Battery-Capacity-Tester-for-Laptop-battery-Power-Bank-18650-Li-ion-Battery-Mobile-Battery-NI-CD/1728618714.html

 

[meBigGuy]

That looks like a good tool to verify battery rating and performance. You want to graph the discharge voltage over time with different discharge rates. Note that batteries are always rated at a specified fixed discharge rate. Commonly it is 0.1C where C is taken from the Ampere-hour rating.

I have always just set up a test system consisting of a programmable load (depends on battery and system requirements) and a gpib dvm and control it with a PC. I can then get a automated family of plots based on discharge rate. But that could cost more than the instrument you purchased. But, I have the istrumentation already available (we also need it to verify our li-ion battery charger hardware and firmware).

 

[NascentOxygen]

I found the answer :
http://www.aliexpress.com/item/Battery-Capacity-Tester-for-Laptop-battery-Power-Bank-18650-Li-ion-Battery-Mobile-Battery-NI-CD/1728618714.html

$300 odd?

Sorry to rain on your parade, but if the answer involves aliexpress or one of the aliexpress sellers, you should prepare to be sorely disappointed. I wouldn't risk even $30 at that site.

Best course is to do your own testing. That way, you can be confident of the testing protocol and of the results.

 

[Borek]

Note that batteries are always rated at a specified fixed discharge rate. Commonly it is 0.1C where C is taken from the Ampere-hour rating.

No idea how to to understand 0.1C, care to elaborate?

 

[Baluncore]

0.1C means charge rate of 10% of capacity, C, in amp-hours.
So a 10 A.hr battery will be charged at 10% of 10 A = 1 amp, for 10 hours.

 

[Borek]

So for 0.1C it is always 10 hours, just with different currents? And for 0.05C it would be 20 hours?

 

[Baluncore]

I believe so.

 

[NascentOxygen]

0.1C means charge rate of 10% of capacity, C, in amp-hours.
So a 10 A.hr battery will be charged at 10% of 10 A = 1 amp, for 10 hours.

Not quite right.

I believe it means (for charging) you would charge at 1A but the duration is not specified. So, in practice, charging would be terminated by sensing the cell's voltage, or if it's a tolerant type and almost flat, simply charge for 15 hrs. In any case, it's still termed a 0.1C charging rate.

This thread is concerned with discharge rate, though the same principle holds.

 

[AlephZero]

So a 10 A.hr battery will be charged at 10% of 10 A = 1 amp, for 10 hours.

Only if (1) it was completely discharged, and (2) the charging was 100% efficient.

When the chemistry was tolerant of overcharging, batteries were often designed so they could be charged at 0.1C for any length of time without damage. Higher charging rates might overheat the battery, or cause chemical reactions too fast for the battery to process, so water was lost as hydrogen and oxygen gas, etc.

Of course some modern battery types (like lithium) are NOT tolerant of overcharging under any circumstances.

 

[Baluncore]

Obviously the state of discharge or degree of charge required needs to be taken into account.

I am surprised that no one has pointed out the obvious dimension error in the guideline.
A scalar, 0.1 or a ratio, multiplied by amp.hour gives amp.hour, not just amp, nor hour.

For a partial charge, the charging rate guideline of 0.1C seems to specify a charge for only one hour.
That is clearly not the case.

 

[AlephZero]

I am surprised that no one has pointed out the obvious dimension error in the guideline.
A scalar, 0.1 or a ratio, multiplied by amp.hour gives amp.hour, not just amp, nor hour.

The "C" in 0.1C is a unit of current. For any given battery, 1C it is the current that will fully charge or discharge the battery in one hour.

There is no dimension error.

 

[Baluncore]

The "C" in 0.1C is a unit of current. For any given battery, 1C it is the current that will fully charge or discharge the battery in one hour.

There is no dimension error.

That may be your assumption. I disagree.
The C is always taken from the data sheet “capacity”, specified in “amp.hours”.
That has a singular unit amp, but plural hours.

 

[AlephZero]

That may be your assumption. I disagree.

It's not just "my assumption". http://web.mit.edu/evt/summary_battery_specifications.pdf

But if you want to invent a different interpretation and then complain that it doesn't make sense, that's not my problem.

 

[Baluncore]

C- and E- rates – In describing batteries, discharge current is often expressed as a C-rate
in order to normalize against battery capacity, which is often very different between
batteries. A C-rate is a measure of the rate at which a battery is discharged relative to its
maximum capacity. A 1C rate means that the discharge current will discharge the entire
battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge
current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would
be 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge
power to discharge the entire battery in 1 hour.

[pedantic] I note that even MIT measure capacity in Amp-hrs. I believe that unit of capacity should be the singular A.h, the amp.h or the amp.hour, never the plural hrs, or capitalised Amp. [/pedantic]

It can get confusing since the symbol C represents the SI unit of charge, the coulomb.
1 amp.hour = 1 coulomb per second, for one hour = 3600 coulomb = 3600C.

[/PLAIN]
In the late 1700s, Charles-Augustin de Coulomb ruled that a battery that receives a charge current of one ampere (1A) passes one coulomb (1C) of charge every second. In 10 seconds, 10 coulombs pass into the battery, and so on. On discharge, the process reverses. Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.

Most portable batteries are rated at 1C, meaning that a 1,000mAh battery that is discharged at 1C rate should under ideal conditions provide a current of 1,000mA for one hour. The same battery discharging at 0.5C would provide 500mA for two hours, and at 2C, the 1,000mAh battery would deliver 2,000mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is a half-hour discharge.

I see, 1C = “The current to discharge in one hour” has now been defined somewhere, by the battery industry, which makes it easy to calculate from the capacity in A.h and so eliminates the time dimension problem. But I still wonder where it was ever defined?

I note that the term “battery”, from “Battery of Cells”, can now also apply to a single cell.
I wonder if the term “battery” from “Battery of Guns” will ever refer to a single gun?

 

[nsaspook]

I see, 1C = “The current to discharge in one hour” has now been defined somewhere, by the battery industry, which makes it easy to calculate from the capacity in A.h and so eliminates the time dimension problem. But I still wonder where it was ever defined?

It's even more confusing as most people don't use the fractional notation but write something like C20 or C/20 when talking about battery charge/discharge rates in the solar power industry. For instance C20 is the standard discharge rate to qualify the battery Ah capacity for RE and traction types.

http://www.trojanbatteryre.com/About_Us/Press_Releases/Industrial_Line2VPR.html

So if you are designing a battery monitor that accounts for the Peukert factor in some types of cells, C20 is the baseline for increasing or decreasing the Ah capacity at the current discharge rate.

 

[NascentOxygen]

The C is always taken from the data sheet “capacity”, specified in “amp.hours”.
That has a singular unit amp, but plural hours.

Throughout 3 years of my studies we had an Indian lecturer who is memorable for his fastidiousness about only one thing, viz., the term "amps". For some reason, he disliked hearing us in the lab giving a current reading in amps. He'd interject with, "Is not 'yamps'! Is 'yamperes'! We grew tired of hearing this, so cured him of this peccadillo by taking on board his exhortation: in the machines lab we began carefully enunciating the units of current as "yamperes".

Regarding the battery charging convention, the C has no units. The battery has a capacity of C ampere-hours, and the charging current (in amperes) is related as a fraction of C, e.g., 0.1C amperes.

In the workshop, engineering vernacular can become a bit sloppy, but when correctly spelt out, there is no conflict in units.

 

[meBigGuy]

Either:
C is the current that "theoretically" discharges the battery in 1 hour(s) or
C is the scaler value that indicates the capacity of the battery in A-H

I happen to like the first, since then the sentence "discharge at 0.1C" makes sense. No one (presumptuous of me) says "discharge at 0.1C (y)amperes"

Note that the capacity in AH (total energy delivered) at 0.1C and at 1C discharge currents will not be the same (I assume all know that) because the chemistry is discharge rate dependent.