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Battery datasheet reading

by jumpjack
Tags: battery, datasheet, reading
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jumpjack
#1
Nov12-12, 09:24 AM
P: 201
How do I read such a chart?



It looks to me from this chart that available capacity increases while voltage decreases, but it is not what I see on my batteries: the more I use them, the lower their voltage is, and the less it will be the available capacity.
So...?
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CWatters
#2
Nov12-12, 09:34 AM
P: 3,109
No.

What it's telling you is that as you discharge it the voltage falls.

It suggests that the SCMG battery has a slightly higher voltage when full and maintains it's output voltage for longer. So for example it stays above 3.3V for longer than the other companies cells.
sophiecentaur
#3
Nov12-12, 11:03 AM
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It seems to me that they've used an unfortunate / inaccurate choice of X axis label. I think they mean 'Charge Used (mAh)' and not 'Capacity'. This would tie in with experience of other batteries where the volts drop as they discharge. A slow drop until the end, when they just 'die'. The 'Available Capacity' would be however far you have to go till you reach that 'knee' on the curve. Their model seems to have about 30% more than the rubbish one they use for comparison. (Well, they're not going to compare with a better one, are they?)

It could, alternatively, mean 'Discharge Current (mA)' and that would imply that the internal resistance of 'their' battery was lower because there was less of a voltage drop.


What sort of a battery has a 'capacity' of 10mAh?? Could it be a small Lithium Ion watch or hearing aid battery?

jumpjack
#4
Nov12-12, 11:52 AM
P: 201
Battery datasheet reading

I just picked one of the many similar charts available around: all datasheets uses this V/Ah chart, and I can't understand how it works.
sophiecentaur
#5
Nov12-12, 12:05 PM
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Many of those images are very badly labeled. They all mean to show how the volts drop as the battery is used but many are just labelled very sloppily.
Some of them, correctly, have an X axis label "capacity used' / "charge used" etc. The others are just nonsense - if you take them literally. There is one that suggest when there is zero remaining capacity, the volts are at max and when maximum capacity remains, the volts drop to 10%. Always be careful when reading charts and think what they might actually be trying to tell you.
I assume you understand why the 'correct' ones I quote are correct.?
jumpjack
#6
Nov12-12, 12:28 PM
P: 201
I think I got the reason why these chart are so "weird": they don't just say how V changes during discharge, they also say which is the battery capacity!
Maybe they assume that battery capacity is defined as the X value for which voltage drops below P% of maximum charge voltage (80%), or of the flat-line voltage (I don't know).
Once you know this value, you read the X values corresponding to each Y value as a percentage rather than an absolute value: if it is at half between 0 and max Ah, it means 50%.

I guess so because some charts actually use % in X axis but have same type of curve.
derek10
#7
Nov12-12, 12:40 PM
P: 12
As sophiecentaur said, the X axis represents the battery's capacity discharged relating to its voltage output using a constant current load. Generally when the voltage is < 3.5V in Li-ion, the battery is considered empty. I can see also the same graphs that use a discharge percentage value for the X axis starting with 0% and ending with 100%.
sophiecentaur
#8
Nov12-12, 05:10 PM
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Quote Quote by derek10 View Post
As sophiecentaur said, the X axis represents the battery's capacity discharged relating to its voltage output using a constant current load. Generally when the voltage is < 3.5V in Li-ion, the battery is considered empty. I can see also the same graphs that use a discharge percentage value for the X axis starting with 0% and ending with 100%.
Not necessarily a constant current load - just a value of current that is within the spec. The units of the X axis are mAh, which allow any combination of current and time for a given Ah value. To be fair, they will be chosen at an optimum rate for their advertising purposes.
CWatters
#9
Nov12-12, 05:31 PM
P: 3,109
Quote Quote by jumpjack View Post
I think I got the reason why these chart are so "weird": they don't just say how V changes during discharge, they also say which is the battery capacity!
I don't find them wierd at all.

Maybe they assume that battery capacity is defined as the X value for which voltage drops below P% of maximum charge voltage (80%), or of the flat-line voltage (I don't know).
Normally that voltage is defined by the cell technology rather than as a percentage of max capacity. For example a 1.2V Nicad or NiMH cell might be considered flat at about 0.9 or 1.0V. Some types of cell can be damaged by deep discharge and the manufacturer sets a minimium safe voltage below which the life of the cell is affected.


Once you know this value, you read the X values corresponding to each Y value as a percentage rather than an absolute value: if it is at half between 0 and max Ah, it means 50%.

I guess so because some charts actually use % in X axis but have same type of curve.
A percentage chart is less useful. If you were to plot Voltage against % capacity used you would not be able to compare different makes of battery so easily. They would all be flat at 100% discharge but you would have no idea how much energy each could deliver before that point was reached.
rcgldr
#10
Nov12-12, 06:38 PM
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One bit of information missing from that chart is the discharge load (current). Usually this is stated as as some multiple (or fraction) of C, where C is the capacity rating divided by 1 hour. For a 2700 mah battery, C would be 2.7 amps. Usually the higher the load, the lower the capacity of a battery.
sophiecentaur
#11
Nov13-12, 05:26 AM
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Quote Quote by rcgldr View Post
One bit of information missing from that chart is the discharge load (current). Usually this is stated as as some multiple (or fraction) of C, where C is the capacity rating divided by 1 hour. For a 2700 mah battery, C would be 2.7 amps. Usually the higher the load, the lower the capacity of a battery.
But the capacity follows the same curve as long as the load isn't outside the 'reciprocity' range. Batteries are not necessarily on constant continuous load . That is as long as self discharge can be neglected. It would be difficult to describe behaviour under a wide range of conditions with just one curve. The Ah is a pretty damn good measure aamof.

But don't get me started on the way that battery capacity is specified. Apparently, 'ordinary' 12V lead acid batteries are knackered once they are taken down beyond half 'capacity' so why don't they sell a 100Ah car battery as a 50Ah battery????? Bastids.
CWatters
#12
Nov13-12, 06:16 AM
P: 3,109
I'm not sure about all countries but it annoys me that here in the UK there is very little data provided with non-rechargable batteries (eg AA primary cells). We have to take it on trust that brand X, costing 3 times as much as brand Y, will deliver 3 times the energy.

Last I heard the makers claimed that a common standard for specifying capacity can't be agreed because usage patterns vary such a lot. Well I don't buy it because they can do it for rechargable cells.
sophiecentaur
#13
Nov13-12, 12:52 PM
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I think there is a small excuse for the makers not specifying their mAh figures. You would need to give much more info about the capacity , such as the load for the given capacity. People would not make any sense of the figures and would, I'm sure, go for the maximum capacity - even if the cell were designed for a totally different load. That would cause loadsa problems for them. They at least divide their battery types broadly according to load. If a particular device is using batteries at a high rate then you can experiment to find the best buy - it will cost you either way. But you have to admit that we all have many battery powered devices that we can just fit and forget about for ages. Modern batteries are total magic in many respects. Long shelf life and they don't usually bleed.
rcgldr
#14
Nov13-12, 04:25 PM
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Quote Quote by sophiecentaur View Post
I think there is a small excuse for the makers not specifying their mAh figures.
I think most of Europe requires capacity be stated for at least automotive type batteries. I'm not sure about other type of batteries.

Batteries used for radio control models specify a capacity (usually for a 1 C load), a maximum continous load (20 C to 30 C for the higher current battery types), and sometimes a maximum burst load (higher still). The capacity at maximum continous load usually isn't stated, but it's somewhat (15% or so) less than the capacity at 1 C load. (C is the capacity rating divided by 1 hour). At higher loads, the shape of the curve will be similar, but the horizontal component of length will be shortened due to the reduced effective capacity. This web page has a graph showing the effect of load versus capacity. Scroll down to the discharge rates section.

http://www.mpoweruk.com/performance.htm
sophiecentaur
#15
Nov13-12, 04:53 PM
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Quote Quote by rcgldr View Post
I think most of Europe requires capacity be stated for at least automotive type batteries. I'm not sure about other type of batteries.
It never seems to appear on primary cells.


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