Battery voltage conversion to % charged while lightly loaded at say 5a

[sailorboy1]

Originally Posted by hellosailor

Don, do you think the folks at Deka / East Penn are intentionally lying, or simply mislead, when they say:

How can you tell if an
VRLA battery is fully charged?
By using a voltmeter.

Open Circuit Voltage vs. State of Charge Comparison*
% Open Circuit Voltage Charge - Flooded Gel AGM
100 - 12.60 or higher 12.85 or higher 12.80 or higher
75 - 12.40 12.65 12.60
50 - 12.20 12.35 12.30
25 - 12.00 12.00 12.00
0 - 11.80 11.80 11.80
NOTE: Divide values in half for 6-volt batteries.

I fully believe this. But we haven't been talking about a rested open circuit voltage.

But if if you want, you 'win"

[sailorboy1]

Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine at pbase.com

[btrayfors]

Originally Posted by Dreaming Yachtsman

All this battery theory is great and ideally a Xantrax or equivalent monitor would be convenient for keeping track of battery SOC. However, my boat came with a digital voltmeter (VM), digital ammeter (AM), and a selector switch to choose port or starboard battery and I have never seen a strong need to add more instrumentation. When I want to check the SOC of the house bank, I either wait until the refrigerator and freezer have both cycled off on their own or open their breakers on the DC panel immediately below the VM and AM, wait 30 seconds for the voltage to stabilize (usually rises ~ 0.1-0.2 VDC), then read the voltage. If less than 12.3 V, I start the generator.

In response to the OP, when I can't unload the battery (e.g. underway with navigation instruments energized), I adjust the meter reading by adding 0.2 VDC for each 5A of load to compensate for (I*I)*R voltage loss in the battery cables. This correction factor is based on empirical observations with fridge and freezer, each of which draws about 5 amps.

This is one of those cases where perfect is the enemy of good enough.

John,

A perfectly intelligent way to do things. Here's a case of an owner who knows his boat and is able to do mental interpretation of SOC, and recognizes the trigger point(s) where recharging is indicated.

I've done exactly the same on my boat for over 20 years. I don't have a battery monitor and don't feel I need one. A digital voltmeter and an analog ammeter on the alternator, plus portable high-quality calibrated multimeters and AC/DC true RMS clamp ammeters are more than sufficient to monitor the SOC of my six Trojan T-105's in the house bank.

OK, I'm not your usual boatowner, since my professional focus these days is power systems and electronics on boats. Still, IMHO any owner who approaches the problem armed with a good understanding of electrical circuits and batteries can be well served by a good voltmeter and AC/DC clamp ammeter.

Bill

[hellosailor]

Don, I don't disagree with you. I'm just saying that a voltmeter, used wisely with some knowledge of the issues, is "good enough for government work".

Like the gas gauge, I don't really need to know when it is down to the last half liter. All I need to know is that when I'm down to a quarter tank, I'd really better think about refueling.

[Adelie]

Originally Posted by Don Lucas

That $120 would almost get you a monitor!

Far as using voltage as an indicator under small loads as an indication of charge: on the boat this weekend I was logging SOC, amp ahours out, voltage, amps; and with less than 1 amp the voltage would indicate the battery was less than 50% charge when it was in the 80s. So to me unless the load has been off the voltage is not worth much as 1 amp was enough to effect the voltage reading.

$120 will get you about 1/2 of a Link Lite monitor ($225-300 on-line)

The affect the amperage draw has is why there should be a family of curves rather than just a single curve.
See following link, page 3 for said family of curves. http://www.magrev.org.au/wp-content/...ing_graphs.pdf

For some reason this link didn't embed into my previous post.

How did you know the SOC was in the 80's, was that off a battery monitor or did you check the specific gravity of cells to determine that?

[sailorboy1]

I spent $158 last year for my Vitron monitor. If I wanted to spend the trouble to put up the sole, open the battery boxes and measure the SG why would I have gotten a monitor?

I have NO questions open or asked on this thread. All I said was that the voltage on the monitor or panel meter would have you belive SOC is 50% where it is 80%+ when operating loads!

[Adelie]

Here is the graph from http://www.magrev.org.au/wp-content/...ing_graphs.pdf :

[GeoPowers]

Originally Posted by Don Lucas

When did I say my voltage was 12.1? Didn't just say I didn't record voltage?

I like how you start making qualification statements while knowing nothing about my qualifications. Nothing is wrong with my system!

Sorry to hear your system is all Jacked up. 50% state of charge after 1 amp for an hour- ouch! You really should look into getting that fixed, could ruin your house bank.... .

[sailorboy1]

Originally Posted by GeoPowers

Sorry to hear your system is all Jacked up. 50% state of charge after 1 amp for an hour- ouch! You really should look into getting that fixed, could ruin your house bank.... .

next week I'm going all wild and shutting one of those other breakers and see what happens