House battery question

[Jerry Liebler]

Dallas,
     It's really the same equation but put in a form that doesn't require what is called the 'Peukert capacity' but instead uses what's usually given by most manufacturers amp hours achieved in a timed discharge.
if you have  two sets of  amps and time values to discharge a single battery you set up the first equation
twice then subtract one copy from the other dropping out the Cp(Peukert capacity) and solve the resulting equation for the exponent.
Regards
Jerry 4107 1120

[Sean]

Dallas -- sorry I beat you to the punch.  As Jerry noted, the page I posted cautions against the "simple" version of Peukert's equation -- the only "capacity" that works in that equation is the "Peukert Capacity" -- the amp-hours available at a 1-amp draw rate.

Jerry -- Peukert refers only to drawing energy out of the batteries.  Putting energy back in also has a "fudge factor" that must be applied, often called the "Charge Efficiency Factor (CEF)."  This measures how much of the input energy actually gets translated to chemically stored energy in the battery.  (To answer the obvious question, "where does the rest of the energy go?" -- it is heat, as can be easily demonstrated by measuring the temperature of the batteries as they are charging.)

The meter I use (which I highly recommend), the Xantrex Link-10 (not to be confused with the very similar but lower-quality Xantrex Battery Monitor), has settings for both.  The Peukert Exponent must be entered by the user -- the factory default of 1.25 (not 1.15 as I guessed earlier) is sufficient for most users if the exponent is not known.

The CEF may be manually entered, but the recommended setting is to allow the meter to calculate it on a rolling basis.  It does this by using a set of user-entered parameters to know when the charging process if finished, including final voltage of the system and amount of current flowing from the charger.  If you set these correctly, then every time the batteries charge up fully, the meter resets the amp-hours "used" to zero, and re-figures the CEF (since it knows how depleted they were, and how much energy was put back in).

The Link-10 also keeps a running total of the number of cycles your batteries have been through, what the average Depth-of-Discharge (DOD) is, and what the deepest-ever DOD was.  Very handy information.  I think I paid around $135 for mine on eBay -- a worthwhile investment.

-Sean
http://ourodyssey.blogspot.com

[Jerry Liebler]

Sean,
     Thank you!  Was/is the Link 10 originally the 'Emeter' from a Seattle area company named 'Cruising Equipment that was bought by Xantrex?  I'm quite sure it is/was.  I've had many lunches with the 'Emeter's designer as it uses a microprocessor made by my former employer.  The reason I kinda want to roll my own is  I don't want a resistor to sense current.  I've got some neat current sensors that measure the magnetic field around a wire, essentially lossless (it takes some power for the hall effect sensors and electronics).   
Regards
Jerry 4107 1120

[Sean]

Jerry,

You seem to have answered your own question... Yes, indeed, they are one and the same.  Only the screen-printed decal on the meter's face has changed.

As for substituting Hall-effect sensors for the shunt, you can probably still use the Link-10 (E-Meter) if you can translate your Hall-effect signal to a 0-50mV output (WRT ground) -- the Link shouldn't know the difference.  Just remember that you need to measure the current flow at the interface between battery negative and ground, rather than at the positive end of things.

FYI, installation and operating instructions for the Link-10 (in .PDF) are here:
http://www.xantrex.com/web/id/72/docserve.asp

-Sean
http://ourodyssey.blogspot.com