RE: RX voltage display

RE: RX voltage display
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RE: RX voltage display - 10/29/2004 12:01:44 AM

Hangtime

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Hi Bill!!

quote:

Consider two parallel 1000mah packs compared to one 2000mah pack (same total capacity). NiCad’s parallel packs giveback a higher maximum current. The same is true with NiMH.

Is the above statement accurate?

Assuming the 1000mah single pack impedance is the same or close to the 2000mah single packs impedance, the above statement is accurate, yes!

(in reply to BillS)

Post #: 101

RE: RX voltage display - 10/29/2004 12:46:05 AM

BillS

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OK. Pardon my ignorance. Is it also logical to assume that smaller packs would have smaller impedance?

Bill

(in reply to Hangtime)

Post #: 102

Impedance - 10/29/2004 1:12:40 AM

JPMacG

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Anyone care to speculate on why the battery manufacturers specify impedance rather than resistance? Seems to me impedance is meaningless information unless you know at what frequency it was measured. Even if you did, what possible use would it be? Do some elaborate calaculation involving the rise time distortion of the servo current waveform?

(in reply to BillS)

Post #: 103

RE: RX voltage display - 10/29/2004 4:15:29 AM

Hangtime

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Hiya Bill!

Still makin' me work fer my supper, sir?

BTW, you have nothin to apologize for.. yah got good guestions, I just hope my answers make sense. If I err here, hopefully Red or one of the electrical engineers here will step in and help fill in the areas where I missed the obvious.

quote:

Is it also logical to assume that smaller packs would have smaller impedance?

Usually, it works the other way... the smaller the cell dimension; the higher the impedance. In other words 'fat' cells generally (but not always) offer lower impedance than the skinny ones compared in the same technology types. Then generally, a nicad cell of a 'given dimension' will have lower impedance than a nimh cell of the same dimension. There are some exceptions... below is a smattering of cells we commonly use in the hobby and thier impedance specs. Data from Sanyo. (one from GP)

Cell Type/Designation ----Technology -- Dimension ---- Impedance ------------ Notes

Sanyo CP2400SCR --------- Nicad --- Standard Sub-C --- 4.5mOhm-----Big Bird Rx packs, popular in electrics
GP 3300 SCH----------------- NiMH --- Standard Sub-C --- 5.0mOhm----- same as above, ignition
Sanyo CP1700SCR ---------- Nicad --- 4/5 Sub-C --------- 5.5mOhm----- same as above, ignition
Sanyo KR1400AE ------------ Nicad --- Standard 'A' ------- 10.0mOhm---- Common IMAC/Pattern Rx pack
Sanyo HR-AU (2700) ---------NiMH---- Standard 'A' ------- 20.0mOhm---- Common IMAC/Pattern Rx pack
Sanyo KR1500AUL------------ Nicad --- 4/5 Height 'A' ----- 16.0mOhm---- Lighter Pattern Rx, .60 size
Sanyo HR-4/5FAUP (1900) -- NiMH----- 4/5 height 'fat' A - 5.0mOhm------ IMAC & Big Bird Rx, Ignition, electrics*
Sanyo KR-1100AAU ---------- Nicad --- Standard 'AA' ---- 19.0mOhm---- Upgrade Tx, .40/.60 sized Rx
Sanyo KR-800AAE ------------ Nicad --- Standard 'AA'---- 12.0mOhm---- Upgrade Tx, .40/.60 size Rx
Sanyo HR-AAU (1650) -------- NiMH --- Standard 'AA' ---- 25.0mOhm--- Upgrade Tx, smaller pattern/aerobatic
Sanyo HR3U (2100/2300)----- NiMH --- Standard 'AA' ---- 25.0mOhm--- Upgrade Tx, Not reccomended airborne**

* new development cell.. shockingly good performance from a NiMH 'A' cell.
** poor fast charge performance, easy to damage, thin wall case, poor temp resilience.

Ok, as you run down the list, on the left hand column, number's are mah rates. You'll see with only a few exceptions, as the cells get dimensionaly smaller, the impedance trends upward. The 'Notes' are relevant to the way I see them commonly used.

You can also garner (I think) why the guys that run the high capacity NiMH setups and the 'A' sized nicad setups generally run 'em in parallel; cuts that high impedance number down to a lil more comfortable level while bumping up total system capacity & adding a nice redundant saftey factor on switches and connectors. Down tick is it's a bit heavier and field support with out a dual port charger can be problematic.

Bill, Here's a link to a pretty reasonable decription of battery impedance and it's effects from the 'Battery Handbook for Non-Engineers'..

http://www.buchmann.ca/Chap6-page3.asp

quote:

Anyone care to speculate on why the battery manufacturers specify impedance rather than resistance? Seems to me impedance is meaningless information unless you know at what frequency it was measured. Even if you did, what possible use would it be? Do some elaborate calaculation involving the rise time distortion of the servo current waveform?

Hiya JPMacG!

Sanyo's ratings are done at 1000Hz, but as to why they use an impedance rate instead of a resistance rate.. I dunno. Never thought to ask.. possibly Red can shed some light on it. For our purposes as a 'comparative' number, we can at least evaluate the cells against each other; and on that level the numbers do us some good.

Hope all this helps..
<
edit: messed with the columns to get 'em to display better>

< Message edited by Hangtime -- 10/29/2004 4:19:46 AM >

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(in reply to BillS)

Post #: 104

RE: Impedance - 10/29/2004 4:17:31 AM

JohnMuchow

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quote:

ORIGINAL: JPMacG
Anyone care to speculate on why the battery manufacturers specify impedance rather than resistance? Seems to me impedance is meaningless information unless you know at what frequency it was measured. Even if you did, what possible use would it be? Do some elaborate calaculation involving the rise time distortion of the servo current waveform?

Internal resistance is often measured with a 1KHz AC discharge current waveform so "impedance" works. There are DC-current internal resistance tests but, I believe, the AC numbers look better as there is less influence from variances in electrode voltage due to the current flowing through it (not just a simple DC voltage drop). There is a lag time before these variances affect the voltage level so testing with AC current lets the total internal resistance/impedance numbers stay lower....and look better for a manufacturer.

DC current tests, depending on the application, may be a better indicator of the cell's true voltage-under-load. This can be source of confusion when one user's experience with the cells doesn't agree with the published internal resistance numbers. It all depends on whether it was measured using AC or DC current and whether the application is pure-DC or AC. Check the data sheets for your cells carefully before buying if the internal resistance numbers are important to you (they should be).

Sanyo's CADNICA Engineering Handbook has a good description of the different cell resistances involved in determining the total internal resistance or impedance and why DC and AC tests are used (Section 3-2, page 11): http://www.sanyo.com/batteries/lit.cfm

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Post #: 105

Impedance. - 10/29/2004 4:50:43 AM

JPMacG

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Thank you John and Steve.

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Post #: 106

RE: Impedance - 10/29/2004 6:43:37 PM

backfire

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I just ran a few quick resistance tests on some common sub-C NiCd cells ranging from 1500 to 2400 mAhr using the direct current method shown on page 12 of the Sanyo engineering manual for NiCads. My results indicate that the DC resistance (most important for our purposes) is at least five to ten times greater than the published impedance specification (that's measured at 1000 Hz) for a given cell. The impedance specification is only useful for calculating the effect of short, low-duty-cycle load pulses. There might be some useful correlation between the published impedance value and the DC resistance but I haven't seen it in the literature so far.

(in reply to JohnMuchow)

Post #: 107

RE: Impedance - 10/29/2004 7:18:40 PM

JohnMuchow

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quote:

ORIGINAL: backfire
[snip]...The impedance specification is only useful for calculating the effect of short, low-duty-cycle load pulses. There might be some useful correlation between the published impedance value and the DC resistance but I haven't seen it in the literature so far.

This is where I'm still a bit confused as to how to apply the impedance numbers. The typical R/C load is pulsating DC, a varying duty-cycle waveform from 0 to full current draw for the motors (same current as 100% throttle), at up to a several KHz rate depending on the ESC. It's not AC, but it is a series of short pulses and the cell has a bit of time to recover between each pulse (when the current is zero). If you're running full out, then it's pure DC current being drawn from the cells.

IMHO, DC current tests are probably the best way to rate the cells. It stresses them the hardest and is the easiest test to reproduce so just about anyone can do the tests. But, it might represent a worse-than-worst-case scenario and may not accurately represent how a cell will perform when supplying power to an ESC (pulse-width modulated current). IMHO, a lot will depend on how a NiCd/NiMH/LiPo cell responds to, and recovers from, these short pulses. <sigh> Time for more research. :-)

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CamLight Systems

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Post #: 108

RE: Impedance - 10/29/2004 8:24:27 PM

backfire

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Here's something else interesting to think about. Although, while digressing from the Rx application a bit, consider an 8-cell pack powering an electric plane at say 25 Amps. Using a typical impedance value of 5 milliohms for each cell, the power dissipation for the pack would be 25 Watts. Given the temperature these packs get to in five minutes, this seems about right. If on the other hand, a value of 50 milliohms is used, the dissipation would be 250 watts. In this case, the pack would probably catch fire in five minutes. It seems apparent that for the purposes of calculating voltage drop for longer duration loads (maybe over 1/4 second), the resistance derived from the DC method should be used. For the purposes of battery power dissipation, use the published impedance value. According to Sanyo, the major 'resistance' component of the DC method is due to cell polarization. It looks like cell polarization is just a change of cell voltage and not resistive power dissipation.

(in reply to JohnMuchow)

Post #: 109

RE: Impedance - 10/29/2004 9:15:44 PM

JohnMuchow

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Here'a a data point to throw into the mix...

Eveready's method of calculating DC internal resistance of NiCd cells (copied from their on-line guide):
Internal resistance (Re) is calculated using the voltage drop method as described in ANSI C18.2, which states that a fully charged cell rated at less than 5Ah shall be discharged at 10.0C1A(capacity rating at 1 hour rate in terms of amps) for 2 minutes then and switched to 1.0C1A. The voltage shall be recorded just prior to switching and again upon reaching its maximum value after switching. The effective internal resistance, Re shall be calculated as indicated below:

Re = DV/DI where DV = VL - VH and DI = IH - IL

Notations:
Re = Internal Resistance
DV = Delta-V, Voltage Change
DI = Delta-I, Current Change
VL = Voltage recorded after switching
VH = Voltage recorded prior to switching
IL = Current recorded after switching
IH = Current recorded prior to switching

For 50% discharged cells, multiply Re by 1.2 factor.

Their guide to NiMH cells doesn't list a DC resistance measuring method so, IMHO, the method can be used for both (perhaps all) cell chemistries.

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John Muchow
CamLight Systems

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Post #: 110

RE: Impedance - 10/29/2004 9:21:43 PM

JohnMuchow

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Another bit of info from Eveready's NiMH guide (with an interesting difference between NiCd and NiMH cells pointed out):

For purposes of electrical analysis of the battery cell, the Thevenin equivalent discharge circuit shown in Figure 7 is often used. This models the circuit as a series combination of a voltage source (Eo), a series resistance (Rh = the effective instantaneous resistance), and the parallel combination of a capacitor (Cp = the effective parallel capacitance) and a resistor (Rd = the effective delayed resistance).

See Figure 7 below: Equivalent Discharge Circuit for a Nickel-Metal Hydride Cell

Eo = effective cell no-load voltage
Re = (Rh + Rd) = total effective internal resistance
Rh = effective instantaneous resistance
Rd = effective delayed resistance
Cp = effective parallel capacitance
E = cell termination voltage

For steady state purposes, the cell voltage at a given current is Eo - iRe, where Re, the effective internal resistance, is the sum of Rh and Rd. The transient response is shown in Figure 8 where the initial voltage drops immediately to Eo - iReh and then transfers exponentially (with a time constant = Cp *Rd) to the steady-state voltage. Obviously the process reverses when the load is reduced or removed. For many application the steady-state voltage is adequate for describing cell performance since the time constant for most cells is small: usually less than 3 percent of the discharge time. When compared to a nickel-cadmium cell, the steady-state voltage for the nickel-metal hydride cell will be reduced since, although the instantaneous resistance is comparable, the delayed resistance will be on the order of 10 percent higher.

See Figure 8 below: Example of Transient Voltage Profile for a Nickel-Metal Hydride Cell

[Edit] The left-hand graphic is the equivalent circuit (Figure 7) and the right-hand graphic is the discharge curve (Figure 8).

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John Muchow
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Post #: 111

RE: Impedance - 10/30/2004 3:53:23 AM

BillS

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Gosh from an engineering perspective the information is great.

The original post was in search of greater field safety. How can the recent discussions be translated into greater field safety for everyone?

Bill

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Post #: 112

RE: RX voltage display - 10/30/2004 4:30:30 AM

Hangtime

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From: Babylon, NY, USA