verald_stevens

My Feedback: (11)

I bought a couple of 3 cell 700 mAh lipos at Toledo two years ago. They do not have any markings on them and I need to know what the C rating is for each battery so I can know how much amps they can put out. Is there anyway to figure this out without destroying them?

Matt Kirsch

My Feedback: (21)

The only way to figure that out is to remember who you bought them from.

draganfly.com

My Feedback: (40)

Run them on the West Mountain Radio CBA.

Matt Kirsch

My Feedback: (21)

What's that?

Matt Kirsch

My Feedback: (21)

Oh! Computerized Battery Analyzer, CBA.

I would dare say that what verald paid for those packs would certainly not justify the expense of a tool like that for a one-time test. They don't rent them, do they?

Still, that doesn't change the fact that he would have to test at least one pack to destruction to determine its true capabilities. Between the battery analyzer and the loss of at least one pack, verald would be further ahead just tossing the packs in the trash and starting over with a known quantity

Two years ago, I'm assuming Toledo '04 (that was "two years ago" in Toledo show terms). As I recall, 10C technology was just starting to poke its nose out most places. FMA had some 20C stuff, with bigger 15C and 20C packs on the way. The bargain basement stuff was 5-6C at that time, so that's where I'd assume these packs fall.

Fred Marks

There is a way if you want to take the time and trouble. You need either to have a ammeter and a DVM or a temperature sensor. The discharge curve for a 1st Gen cell, the KOK 835, is attached. As you increase current drain two things happen that can be measured. When the max allowable discharge current (C rating) is exceeded, output voltage drops sharply. For the example, this occurs as current begins to exceed about 2C= 1.67 amps. If you try to increase current to, say 5C=4+ amps, the drop is very sharp. You don't have to apply this load very long as the drop is almost instantaneous and the cell recovers to the topmost curve instantly when the overload is removed. A short pulse will not harm the cell.

The alternative is to increase currrent gradually and measure cell temperature. When the cell is being overloaded, temperature will rise. Arbitrarily, when cell temp reaches over 130 Deg F, the cell is near the max rating. This is not the preferred method as the cell will suffer some small amount of damage if that temperature is exceeded.

Worth it? Only you can tell us.

verald_stevens

My Feedback: (11)

This will be very helpful and I think I can follow it. Now, all I need is an ammeter. Is that the same as an Astro Flight Whatt Meter?

Also, can you cxplan the curves in your graph. I'm assuming that the vertical axis is voltage (and this is one cell). What is the horizontal axis?

Fred Marks

1. A Whatmeter will do what you need.
2. The vertical axis is voltage of a single cell.
3. The horizontal axis in the example is delivered capacity at constant current.
4. That axis may be converted to Time in hours by dividing capacity delivered at any point on the curve by the constant discharge rate, i.e., AH/Amperes=hours. Hours X 60=mimutes.
5. A lot of discharge curves are published based on use of the handy and reasonably accurate West Mountain Computerized Battery Analyzer (CBA). The above is typical of curves that can be plotted with that excellent little device. However, the discharge curve means little unless you know how to use it to compare cells or to select the pack needed. Imagine trying to stack up a half dozen separate sets of curves for comparison- almost impossible. So, most get published with no real explanation of what they mean.
6. There are some things on the curve itself that can be interpreted directly:
a. The amount that the average voltage drops as load is increased is called voltage depression. It is the difference in average voltage from, e.g., the red to the yellow curve to the green, etc. A better way of displaying will be described later. Most casual students of Li Pos have a sense of the importance of voltage depression.
b. The “flatness” of a given curve is an indicator that pack voltage will not keep falling rapidly so that throttle adjustments are needed continually. By the same token, when voltage does drop, it is rapid and cut off can occur abruptly.
c. Capacity depression is reflected in the spread between end points at the right end as current load increases The KOK 3200 illustrated is a particularly “stiff” battery, i.e., it loses little voltage or capacity as load is increased.
d. Note that casual observation of the curves above as a standalone bring little sense of how good the cell is. Comparison with other cells in its family below will illustrate the importance of being more thorough.
4.The four key data points listed form the basis for a great deal more analysis that can be done if one is interested in knowing how to do a true evaluation. The key data may be entered in the spreadsheet below for analysis.