E-meters, We Love to Hate it! [RE-wrenches]

Tue Nov 21 14:38:31 EST 2000

<x-flowed>Hello Wrenches

These battery ampere-hour meters are essentially electron counters. 
The determination of when a battery is "fully" recharged is very 
difficult. Sometimes these meters go astray.

The efficiency figure delivered by the amp-hour meter is coulombic 
efficiency, not energy efficiency. Since all batteries require 
several volts higher to recharge than they deliver on discharge, the 
energy efficiency is far lower than the coulombic efficiency. Energy 
efficiency in a L-A batt in good condition is between 82 and 88% for 
shallow cycle service such as encountered in a well designed PV 
system..

I often see 98% and 99% efficiency figures from our E-Meters here. 
These high readings occur mostly during the summer when the battery 
is receiving gobs of overcharge and the nighttime discharge cycles 
are shallow, less than 8% DOD.

Peukert's Equation is a mathematical description of an "average" L-A 
batt's ability to store charge (coulombs). This equation doesn't 
directly include factors such as self-discharge, cell impedance, and 
temperature. All factors which radically effect battery efficiency.

In the case of the E-Meter, regular discharge cycles below 10% DOD 
are necessary for efficiency recalcs. The efficiency figure radically 
effects ampere-hour counting as the input amps are scaled based on 
efficiency. It's easy to see why these meters give erratic 
measurements during periods when the battery is not regularly fully 
recharged and also given a fair amount of over charge (about 5 to 10% 
of the capacity).

I think that these meters deliver GREAT service and value for the 
$200 that they cost.

We have both the E-Meter and the Tri-Metric running side by side in 
the same system. Our 24 Volt system also has a Heart/Cruising Link 
2000 two channel amp-hour meter. These three meters almost never 
agree exactly, but they are generally in the same neighborhood. They 
more closely agree when the batteries have been fully charged 
routinely.

I watch the ampere-hour figures in relation to battery volts and 
battery current. We have our 24 Volt system (twenty L-16s) set pretty 
hot with voltage regulation at 29.5 VDC. Since our batts are in a 
heated box, they are always within a few degrees of 77 degrees F (25 
degrees C), so temperature is not a factor here. When the charge 
current drops to about 17 amps (C/100) at 29.5 VDC, I know that the 
battery is within a few percent of being totally recharged. I check 
this against the ampere-hour count on the three meters and reset the 
meters if necessary. The only time I see big differences between the 
meters (and reality) is when the battery is cycled deeply and not 
fully recharged for a week or more.

I totally agree with Spark's comment on noise and the E-Meter. If 
there is RFI or even if there is a PWM charge controller, the E-Meter 
will give bogus ampere reading which will effect the ampere-hour 
count. The Tri-Metrics are much more noise immune.

Hope this helps.

Richard

At 08:55 -0800 11/21/00, R. Sparks Scott wrote:
>	List;
>
>	Recently, on another list I belong to, someone made the (outrageous)
>statement that it required 2Ah of charging to replace 1Ah of consumption
>from a battery bank. I made the retort that my E-meter is displaying a CEF
>of 99%, and that it seems like I see almost exact Ah for Ah equivalents
>when charging, i.e., a 45 Ah discharge being charged at 15 A usually takes
>exactly three hours to full.
>	Got out the E-meter manual and did a quick study on Peukert's Equation,
>and how CEF is calculated. OK, so CEF as displayed isn't actually the
>batteries overall efficiency, but the logrithmic calculation of discharge
>rate applied to charging.
>	So....If I'm seeing Ah consumption being replaced at 99% of 
>the charging
>current (as calculated by the E-meter, and by direct observation), what is
>the actual battery efficiency? Sounds to me like 99% of what goes in as a
>charge comes out and is delivered to the load.
>	About the only thing I can think of that is that efficient is the
>government's ability to waste my tax dollars. I smell a rat. Anybody care
>to comment? Richard, you're the Battery Miester here.....
>
>	-S
>
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------------------------------------
Richard Perez
Home Power
PO Box 520, Ashland, OR 97520  USA
Tele: 530-475-3179
FAX: 530-475-0836
Subscriptions: 800-707-6585
mailto:richard.perez at homepower.com
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 RE-wrenches at topica.com

The archive of previous messages: 
 http://www.topica.com/lists/RE-wrenches/

To unsubscribe send a message to: 
 RE-wrenches-unsubscribe at topica.com

To check out the other RE-Wrench participants:
 www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine: 
 www.homepower.com

For info contact list moderator by email:
 michael.welch at homepower.com

___________________________________________________________
T O P I C A  http://www.topica.com/t/17
Newsletters, Tips and Discussions on Your Favorite Topics

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