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Old 20-05-2011, 05:43   #31
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Re: Basic Battery State Monitoring

Phew! Thank heavens. This discussion was getting pretty messy :-)

Here's my take on what's been said/agreed in this and other CF discussions on the topic:

1. Modern battery monitors are pretty good, though not infallible. They measure both voltage and amps in and out, and try to measure capacity remaining. They must be properly set up, calibrated and periodically reset.

2. Voltage monitoring over time can be a useful way to estimate SOC, if done by a sophisticated device (like the SmartGuage) or by someone with knowledge and experience. While on a cruising boat you almost never have a battery bank which has not been charged or discharged for 24 hours, you can still use something short of that, e.g., a battery bank which has received no charge and only a small load for a few hours to take off the surface charge. Not 100% but pretty close (as verified dozens of time by hydrometer readings and, yes, 30 years of experience including conduct of controlled battery tests).

3. State of charge (SOC) is different from battery capacity. A battery with a full SOC may have lost much of its capacity. A voltmeter alone won't show this, nor will a battery monitor (amp counter) unless it's been recalibrated as the battery loses capacity over time.

4. The only reliable ways to measure battery capacity are by using a controlled load over about 20 hours or the use of a sophisticated internal resistance/conductance tester like the Midtronics series.

5. Two engineers in apparent deep disagreement can themselves reach a state of violent agreement -- all without outside help :-)

Bill
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Old 20-05-2011, 06:28   #32
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Re: Basic Battery State Monitoring

There's the right way, the wrong way... and my way

Untill a few months ago I just used a cheap digital multi-meter stuffed into the cigarette lighter socket at the nav station.

Keep the batteries at or above 12.2 and the world still seems to revolve

Now I got one of those expensive, fancy solar thingies that has a big screen and tells me the exact same thing!!!!!
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Old 20-05-2011, 07:21   #33
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Re: Basic Battery State Monitoring

Quote:
2. Voltage monitoring over time can be a useful way to estimate SOC, if done by a sophisticated device (like the SmartGuage) or by someone with knowledge and experience. While on a cruising boat you almost never have a battery bank which has not been charged or discharged for 24 hours, you can still use something short of that, e.g., a battery bank which has received no charge and only a small load for a few hours to take off the surface charge. Not 100% but pretty close (as verified dozens of time by hydrometer readings and, yes, 30 years of experience including conduct of controlled battery tests)
All the rest I can agree with.

SOC estimation as a function of voltage monitoring relies on battery modeling. This is an active area of research and various models have been proposed. The Extended Kalman filter model has the best possibility of being implemented in a microcontroller, but establishing th einitial parameters, especially dynamically is complex and error prone. If SmartGuard does this, great.

But voltage monitor , aka what the ordinary user does, glancing at a voltmeter, is almost useless, in itself as a way of determining SOC. Voltage history, user experience, consistent charge discharge cycles can all help to improve that "uselessness".

An ammeter and some columb counting can equally be as useful. ( or useless as your view maybe)

Dave
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Old 20-05-2011, 07:57   #34
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Re: Basic Battery State Monitoring

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Originally Posted by goboatingnow View Post
All the rest I can agree with.

........

But voltage monitor , aka what the ordinary user does, glancing at a voltmeter, is almost useless, in itself as a way of determining SOC. Voltage history, user experience, consistent charge discharge cycles can all help to improve that "uselessness".

.......

Dave
Well, Dave, we're gonna have to agree to disagree.

I agree and have said repeatedly that what you call the "ordinary user" won't have much luck with only a voltmeter. On this we (almost) agree.

However, I disagree strongly (maybe even violently) with your basic premises that: (1) some sort of computer algorithm is necessary to make SOC estimation with just a voltmeter possible; and (2) that voltage history, user experience and, by implication, user knowledge and competence with battery chemistries and performance are unreliable indicators of SOC.

That's just not so. But, I guess, I just don't have the energy today to debate the issue with you.

Hint #1: Do you think it possible that the experienced human brain may be just as good -- or better -- than a computer algorithm in the case we're talking about?

Hint #2: If you don't, then you likely don't have nearly enough experience either with computers or with real life cruising.

IMHO.

Cheers,

Bill
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Old 20-05-2011, 10:13   #35
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Re: Basic Battery State Monitoring

I'm equipped with panel voltmeter and panel ammeter w/450ah bank. I know that when I drawing 20a and the voltage is north of 12.2v, my batteries are pretty good shape. Drawing the same current with a voltage under 12, I'm looking to start charging.

I use the combination of current draw and voltage and knowledge of how much time has passed since last charge combined with what we've been using to determine when to charge.

Yeah, an ah counter would be nice, but I'll save that $$ towards my next battery replacement.
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Old 20-05-2011, 14:00   #36
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Quote:
Originally Posted by btrayfors

Well, Dave, we're gonna have to agree to disagree.

I agree and have said repeatedly that what you call the "ordinary user" won't have much luck with only a voltmeter. On this we (almost) agree.

However, I disagree strongly (maybe even violently) with your basic premises that: (1) some sort of computer algorithm is necessary to make SOC estimation with just a voltmeter possible; and (2) that voltage history, user experience and, by implication, user knowledge and competence with battery chemistries and performance are unreliable indicators of SOC.

That's just not so. But, I guess, I just don't have the energy today to debate the issue with you.

Hint #1: Do you think it possible that the experienced human brain may be just as good -- or better -- than a computer algorithm in the case we're talking about?

Hint #2: If you don't, then you likely don't have nearly enough experience either with computers or with real life cruising.

IMHO.

Cheers,

Bill
Read my post.

I said that experience etc helps.

Dave 26 years computers 32 years sailing
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Old 20-05-2011, 15:24   #37
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Re: Basic Battery State Monitoring

my battery has more volts in it when fully charged than yours!




and I'm kidding before experts start "explaining" to me why I'm wrong
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Old 20-05-2011, 19:06   #38
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Re: Basic Battery State Monitoring

Great discussion; very informative, especially the highlighting of the two ways in which battery monitors attempt to track state of charge. I am now motivated to try a monitor that uses voltage history/profile to estimate SOC. Good stuff; thanks Pteron, Bill and Dave.
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Old 21-05-2011, 09:36   #39
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Re: Basic Battery State Monitoring

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Originally Posted by btrayfors View Post
3. State of charge (SOC) is different from battery capacity. A battery with a full SOC may have lost much of its capacity. A voltmeter alone won't show this,
Bill
"may have lost much of its capacity" Yes it will
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