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determining batteries state of charge?

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Toby

Jan 1, 1970
0
Hello all,

I'm wondering if taking a voltage reading on a battery under no load will
give me an accurate state of charge (discharge). I have a basic voltage/ohm
meter that I'm using and the batteries seem to be consistantly around 12.5
volts. I currently have 2 automotive batteries ( I know this is a bad
choice, it's temporary) linked in paralell to power an inverter. These
batteries are charged by the vans alternator (several hours a day of
driving). So I guess what I'm asking is how do I go about getting an
accurate reading, and what should that magic number ideally be?

thanks
Toby
 
D

Damien

Jan 1, 1970
0
Toby said:
Hello all,

I'm wondering if taking a voltage reading on a battery under no load will
give me an accurate state of charge (discharge). I have a basic voltage/ohm
meter that I'm using and the batteries seem to be consistantly around 12.5
volts. I currently have 2 automotive batteries ( I know this is a bad
choice, it's temporary) linked in paralell to power an inverter. These
batteries are charged by the vans alternator (several hours a day of
driving). So I guess what I'm asking is how do I go about getting an
accurate reading, and what should that magic number ideally be?

A healthy 12V automotive battery should actually be around 13V or so (6 cells @ 2.3V gives a max of 13.6V). It should
also not drop much more than around 1V under load, so if it's below about 12.5V under load you may want to keep an eye
on it. Below 12V, and it is getting flat and could do with a charge. And if it ever drops below 10V, it is stuffed and
should be replaced.

As for getting an accurate reading? Easy! Just put your voltmeter across the battery terminals with the load connected.

Damien
 
T

Toby

Jan 1, 1970
0
Thanks for the info guys, and I know the batteries won't last. They were
free and allow me to get things up and running on the cheap side for the
time being.

Toby


Damien said:
A healthy 12V automotive battery should actually be around 13V or so (6
cells @ 2.3V gives a max of 13.6V). It should
also not drop much more than around 1V under load, so if it's below about
12.5V under load you may want to keep an eye
on it. Below 12V, and it is getting flat and could do with a charge. And
if it ever drops below 10V, it is stuffed and
should be replaced.

As for getting an accurate reading? Easy! Just put your voltmeter across
the battery terminals with the load connected.
 
D

daestrom

Jan 1, 1970
0
Ron Rosenfeld said:
How about 48 :)) ?

Seriously, if you get it, I'd like to know how it works out for use in
testing multiple cells.

One 'trick' we used on submarine batteries, was to pick two 'pilot' cells.
Chosen to be representative of the 'average' cell in the battery, we took SG
and individual cell voltage (ICV) readings on these daily. When they showed
a need for a charge, we did one. Once a month, after an equalizer charge,
we took all cell SG & ICV's and compared them with the 'pilot' cells to
confirm the 'pilot's still represented the overall average.

Might do something similar with a rather expensive SG-electrode. Buy one or
two and install them in 'pilot' cells. Then confirm the pilots are still
valid by comparing with monthly or quarterly readings of the entire battery.

daestrom
 
B

Bob Adkins

Jan 1, 1970
0
I'm wondering if taking a voltage reading on a battery under no load will
give me an accurate state of charge (discharge). I have a basic voltage/ohm
meter that I'm using and the batteries seem to be consistantly around 12.5
volts. I currently have 2 automotive batteries ( I know this is a bad
choice, it's temporary) linked in paralell to power an inverter. These
batteries are charged by the vans alternator (several hours a day of
driving). So I guess what I'm asking is how do I go about getting an
accurate reading, and what should that magic number ideally be?

IF you use your batteries often and don't abuse them
IF you know your batteries are in good condition
IF you periodically test the batteries under load
If you periodically test specific gravity&volts

Then voltage is an accurate state of charge indicator.

Mine runs 13.8-14.1v after 2-3 days of solar charging. If this voltage
should change, I would know something is amiss.

I'm glad you brought this up. It's time for me to do a load test. :)

Bob
 
V

Vaughn

Jan 1, 1970
0
Matt said:
Bob,

What do you use to do your load test?

A quarter of a century past, the shop that I worked in had a great Sun load
tester we used to determine if a battery had any might remaining (or should we
simply replace it). These days it seems that folks just replace (car) batteries
when their current one repeatedly fails. That seems to miss the larger picture of
what the "system" is doing. Automotive suppliers still sell some pretty hefty
units (e.g. Snap-On SUN VAT 45 Diagnostic Volt-Amp Tester). I'm wondering what
the recommended set of tools for home power folks is.

Can anybody point references (vendors & books) to me regarding good tools and
best practices for battery management? And I'd love to find a decent/cheap load
tester if anybody has a source for them as well......

You can get a servicable unit at virtually any auto parts store for
about $30.00 or less. They typically have an analog meter, a switchable
load, and some heavy leads with terminal clamps. At work, we modified one
of these units by installing a digital meter module so we can log a more
accurate voltage with and without the load, but you can do the same thing
with your digital VOM if you don't mind the fuss with the extra test leads.

Vaughn
 
N

Nick Hull

Jan 1, 1970
0
What I want to know is how to determine the state of charge of a sealed
'no maintainance' battery with one bad cell.

This is very practical, I use these batteries all the time to power my
6vdc electric fence chargers. I borrow 'dead' batteries that won't
start a car and use them for a year to provide power out in the field.
I only need 6 volts so a '10 volt' battery is fine and the load is low
and I can easily convert 6-12vdc into 6vdc. I do need to recharge them
every couple of months, but am never sure when they are properly charged.
 
T

Toby

Jan 1, 1970
0
BINGO!!!!!
Thanks Bob, that's the type of gizmo I was looking for. There's a Harbor
Freight tools store in town. They sell crappy quality tools, but at their
prices, gee, it's hard to beat. I abuse my tools and equipment on a regular
basis, so I plan on having to replace them. Case in point, their 2 1/4 ton
floor jacks on sale for $34. I buy one a year and don't sweat it when it
gets beat up. Some of their hand tools and pullers I actually carry a spare
on my truck.

Toby
 
B

Bob Adkins

Jan 1, 1970
0
BINGO!!!!!
Thanks Bob, that's the type of gizmo I was looking for. There's a Harbor
Freight tools store in town. They sell crappy quality tools, but at their
prices, gee, it's hard to beat. I abuse my tools and equipment on a regular
basis, so I plan on having to replace them. Case in point, their 2 1/4 ton
floor jacks on sale for $34. I buy one a year and don't sweat it when it
gets beat up. Some of their hand tools and pullers I actually carry a spare
on my truck.

I like their air tools. Can't beat an impact wrench for $20, an air hammer
for $8, and a brad nailer for $15. Typically, you get only a little less
service than top quality stuff. Even if you get only 25 uses out of them,
you're doing great at those throwaway prices. I got a big ole honkin' 187
pound, 16 speed drill press for $1 per pound. It's already paid for itself
from doing projects for the wife, which are priceless. :)

Bob
 
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