Thread: New Product ESR / IR Meter fo Lipos
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Old Jan 27, 2011, 08:55 PM
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I second that. There are some things where relative accuracy is fine, some where approximate is good enough, and some where good accuracy is essential. And for the last, I am not meaning lab quality, just good enough for toy aeroplane purposes. Measuring cut off voltages for a LiPo charger or rpm is an example of the last one. You want to be as accurate as it is possible to be.

Approximate is fine for some things. kV for ordinary modelers is a reasonable example. Excepting the gurus and motor designers for most of us a kV reading within 100-200 is perfectly close enough for practical purposes (like figuring out what prop might be appropriate). If you are serious, then further measurement with a wattmeter is going to be involved anyway.

Relative accuracy is OK if you wanted (for example) to keep track of your IR over time or compare two similar batteries. Problem here is there are TWO sources of error. The intrinsic measurement accuracy of the meter - which I suspect is probably OK- and the error due to the method of measurement which is not. IMO this meter is WORSE than no meter for measuring IR and I agree with Mark. It is giving you a totally false sense of what you are measuring. You are measuring a small (~ few mOhm) cell resistance in series with a larger (~10's mOhm) lead and contact resistance which will vary from cell to cell and over time. Problem is in this age of digital readouts we all get caught up in the apparent authority of the number on the screen and lose sight of how it got there. The vast majority of modellers would quite reasonably buy one of these, take a reading and expect (say) the difference between 23mOhm and 25mOhm on two cells to mean something. It doesn't. It is much more likely to be the variation in the connection resistance. Any real change in IR of a new good cell will be of the order of 1or2 mOhm and will be swamped by any variation in lead resistance between the balance connections.

IMO, the only realistic way to measure IR for our purposes, which is to assess performance under heavy DC loads, is the way Wayne's meter and some chargers do it. Using 4 wires - one set to carry the fairly heavy current load needed and the other to measure only voltage with negligible current flow and hence the resistance of the leads has no effect on the reading.

Rant mode OFF

John
Quote:
Originally Posted by mrforsyth View Post
In my view, a very inaccurate meter is of less use than putting a battery in a plane, running up the throttle, and checking voltage under load with a wattmeter.

If used as you indicate - to monitor packs over time, I concede that it can have some utility as long as you understand that a portion of the witnessed change in IR could be due to increase in balance connector contact resistance.

Personally, however, it would drive me bonkers as there's really no way of knowing whether the lipoly cell is deteriorating or the balance connector is deteriorating. But then I'm perhaps less tolerant of such things than most...

Mark
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