Prismatic Cells How to Determine Amp Hours

DMZ

New Member
I apologize if this sounds elementary

If the Lifepo4 prismatics cells have no markings
How does one determine the actually amp hours.
If a volt meter is used and a reading is taken...Say 3.2 volts...
what then?
my question arises as a first time purchaser of cells from someone on ebay locally... And the cells look new, no stickers...
how do I know they are 280ah
how do I know how much life is left.
thank you as I intend on going to purchase these in a few days
 

gnubie

Photon Sorcerer
The way would be to do a discharge test. Bring the cell up to fully charged, then use a known load to fully discharge the cell. The manufacturer's spec sheet will tell you the voltages involved. If you don't have a known load you can use a watt meter in series between the cell and the load. During the test you check the cell voltage to determine where you are in terms of state of charge. lifepo4 cells have a fairly flat voltage change graph most of the time with rapid change in voltage at fully charged and full discharged.

If you are testing individual cells you'd need a 'lab' power supply or the like. I think I've seen a single cell charger discussed on the forum too. As you can guess there is a fair bit to testing the capacity of a single cell.

Example of lab power supply (not a recommendation to buy this item)
Example of watt meter (not a recommendation to buy this item)
 

DMZ

New Member
Thanks very much.
So what does this really tell me...
If I were to use a volt meter getting the initial reading of 3.6 volts...
1 Can I figure amp hours from here given volts?
2 I'm aware of doing the load test... Which I will not be able to do standing in their driveway...

As I am going to meet them and I guess all I get to do is check volts and internal resistance using their volt meter?

So I think I only will be able to tell if the volts are close enough to full capacity.
 

gnubie

Photon Sorcerer
It's going to be pretty rough using just internal resistance as there's a fair bit of variance between manufacturers, in as much as you can call 0.1 milliohms a fair bit. Without a reliable known 'new' internal resistance for your cells it's a bit of a guess. I'm sure someone far more schooled in this than me will pipe up in the next 24 hours. :)

Here's some CALB data to give you an idea.

1601056374040.png
 
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DMZ

New Member
It's going to be pretty rough using just internal resistance as there's a fair bit of variance between manufacturers, in as much as you can call 0.1 milliohms a fair bit. Without a reliable known 'new' internal resistance for your cells it's a bit of a guess. I'm sure someone far more schooled in this than me will pipe up in the next 24 hours. :)

Here's some CALB data to give you an idea.

View attachment 23685
Thanks for the chart it's informative!
 

Craig

Watts are Watts!
Staff member
Moderator
Going to bottom line it...

In the driveway you can measure voltage, internal resistance and LxWxH dimensions of the cells and compare to known volumes. Anything else takes hours.

You are taking a chance. Period.
THIS!! ^^^^^^^

But use that as a bargaining chip.
 
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snoobler

Solar Honey Badger
On second thought...

A resistive 12V/100A battery load tester will probably put about a 25A load on a 3.2V cell. It should be able to handle that with flying colors. You'd need bolts to adequately grip the terminals.

This isn't definitive, but it's another quick check that the cells are in comparable states of charge with low IR.

Also, most voltmeters don't measure IR. At least I've never seen one.

Still taking a chance!
 

DMZ

New Member
That's good to know ,
I've been reading lots on all aspects of solar and Batteries for awhile now and still even the more I learn the more I realize I Kno nothing!

Thanks.
How helpful is knowing the actual weight...
To differentiate between 100 ah vs 280 ah
 

snoobler

Solar Honey Badger
That's good to know ,
I've been reading lots on all aspects of solar and Batteries for awhile now and still even the more I learn the more I realize I Kno nothing!

Thanks.
How helpful is knowing the actual weight...
To differentiate between 100 ah vs 280 ah

It's another data point. If they all line up, you can get more confidence, but you won't know until you do an actual load test.
 

GSXR1000

Solar Addict
voltage will be about the same on different AH same type battery at the same charge so it really wont tell you anything
 
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