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Old 08-03-2011, 07:46   #1
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What's a Safe Voltage to Discharge to ?

Ok, I now have a great alternator, regulator and AGM battery, but what is considered to be the safe voltage to discharge to before running the motor to recharge? My house is a 190ah AGM
Thanks
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Old 08-03-2011, 07:56   #2
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Re: Whats to safe voltage to discharge to?

Open Circuit Voltage (OCV) is a very rough & unreliable approximation of a battery’s State of Charge (SoC).
Batteries should generally be recharged when discharged to about 60% SoC.
Depending upon battery chemistry, a 60% SoC occurs at an OCV of between 12.4 to 12.6 V.
In your case, I'd recommend recharging at an OCV of 12.5 Volts or higher.

See ➥ OCV vs SOC - Remove the surface charge, measure the Open Circuit Voltage (OCV) with an accurate (.5% or better) digital voltmeter across the terminals, temperature compensate, and... Cruisers & Sailing Photo Gallery
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Old 08-03-2011, 07:57   #3
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Re: Whats to safe voltage to discharge to?

For me it's below 12 volts because you are trying not to discharge more than 50% if you can help it. Things will still work at 11, but below that you will have problems. I think the Xantrex battery monitor flashes "low bat" at 11.5 volts but I'm not sure.
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Old 08-03-2011, 11:30   #4
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Re: Whats to safe voltage to discharge to?

Quote:
Originally Posted by bazzer View Post
Ok, I now have a great alternator, regulator and AGM battery, but what is considered to be the safe voltage to discharge to before running the motor to recharge? My house is a 190ah AGM
Thanks
Is it out of the question to get a proper battery monitor? Now that you have the rest of the good stuff.....

That's certainly what I'd do if I could.

Regards,
Extemp.
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Old 08-03-2011, 11:34   #5
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Originally Posted by Extemporaneous
Is it out of the question to get a proper battery monitor? Now that you have the rest of the good stuff.....

That's certainly what I'd do if I could.

Regards,
Extemp.
Why do need one when I already have a built-in DVM? I'd like to save the bucks and put it towards a solar panel!
Thanks
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Old 08-03-2011, 11:49   #6
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Re: Whats to safe voltage to discharge to?

It all depends on how long you want your batteries to last. Here is a chart of number of discharge/recharge cycles you will get versus state of charge. It is from memory and really rough, but it illustrates the point:

Pct OC V Cycles
80% 12.5 2000
60% 12.2 1000
40% 12.0 500

But also recognize that you get three times the amphours by discharging to 40% versus 80% but your battery life is about 1/4. So it may work out for you to discharge to 40%. You just have to replace your batteries more often but you will need fewer batteries to deliver the same daily amphour usage. So the cost difference isn't that much.

Also note that it is difficult to recharge beyond 85% or so. The current accepted drops dramatically as the battery gets beyond 85%. Solar panels can help. If you recharge your batteries in the morning to 85% with a genset or propulsion engine alternator then your solar panels will top them off during the day.

Personally, I try to operate the batteries between 40% and 85%, with a boost to almost 100% during sunny days using solar panels.

David
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Old 08-03-2011, 11:54   #7
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Quote:
Originally Posted by djmarchand
It all depends on how long you want your batteries to last. Here is a chart of number of discharge/recharge cycles you will get versus state of charge. It is from memory and really rough, but it illustrates the point:

Pct OC V Cycles
80% 12.5 2000
60% 12.2 1000
40% 12.0 500

But also recognize that you get three times the amphours by discharging to 40% versus 80% but your battery life is about 1/4. So it may work out for you to discharge to 40%. You just have to replace your batteries more often but you will need fewer batteries to deliver the same daily amphour usage. So the cost difference isn't that much.

Also note that it is difficult to recharge beyond 85% or so. The current accepted drops dramatically as the battery gets beyond 85%. Solar panels can help. If you recharge your batteries in the morning to 85% with a genset or propulsion engine alternator then your solar panels will top them off during the day.

Personally, I try to operate the batteries between 40% and 85%, with a boost to almost 100% during sunny days using solar panels.

David
David, thanks for your reply, can you tell me what OC V is? Is this table good for AGM's and wet cells?
Baz
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Old 08-03-2011, 12:08   #8
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Re: Whats to safe voltage to discharge to?

Quote:
Originally Posted by bazzer View Post
David, thanks for your reply, can you tell me what OC V is? Is this table good for AGM's and wet cells?
Baz
See post #2.
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Old 08-03-2011, 12:09   #9
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Re: Whats to safe voltage to discharge to?

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Why do need one when I already have a built-in DVM? I'd like to save the bucks and put it towards a solar panel!
Thanks
"need"?
You may not need one but it is a much better more accurate way to evaluate the SOC of your batteries. Considering your using AGM batteries you cannot even check your SG with a hydrometer.
I guess the way I see it is that electricity production, transportation, storage and use is so important (not to mention, expensive) on a boat that I would chose to do better if I could.

Warm Regards,
Extemp.
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Old 08-03-2011, 12:49   #10
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Re: Whats to safe voltage to discharge to?

Quote:
Originally Posted by djmarchand View Post
It all depends on how long you want your batteries to last. Here is a chart of number of discharge/recharge cycles you will get versus state of charge. It is from memory and really rough, but it illustrates the point:

Pct OC V Cycles
80% 12.5 2000
60% 12.2 1000
40% 12.0 500

But also recognize that you get three times the amphours by discharging to 40% versus 80% but your battery life is about 1/4. So it may work out for you to discharge to 40%. You just have to replace your batteries more often but you will need fewer batteries to deliver the same daily amphour usage. So the cost difference isn't that much.

Also note that it is difficult to recharge beyond 85% or so. The current accepted drops dramatically as the battery gets beyond 85%. Solar panels can help. If you recharge your batteries in the morning to 85% with a genset or propulsion engine alternator then your solar panels will top them off during the day.

Personally, I try to operate the batteries between 40% and 85%, with a boost to almost 100% during sunny days using solar panels.

David
These are valid points. Does one want "battery reserve and lifespan", or alternator powered charging efficiency...

I was going for 100% solar charging during the 90% of the time we are at anchor. During that other 10% of the time spent at sea, our consumption has doubled, so we run the engine about an hour, at 6:00 AM, when the batteries are lowest and can accept the most amps. Then the solar does the rest, to 100%, over the day.

We normally (at anchor) cycle our batteries down to no more than about 88 or 90% charged, and top them off every day with the solar panels, (usually fully charged by noon). This gives us a battery lifespan of over 10 years (on our 6v X2 @ 340 a/h, "wet" Trojans), and also gives us huge cushion for cloudy days.

On normally "medium dark" and overcast days, we can still top off our batteries, as we have enough solar for our normal a/h needs... X 2.

On "totally dark" overcast days, so dark that you can hardly read, we start accumulating an energy deficit, (like 10 a/h per day), but are still good for a week of this much overcast sky, before we have to crank the engine.

Also, since we have batteries that are mostly full, the v is high and things run better. This has been our magic combination for 15 years, but we are a super energy efficient trimaran, and have room for that much solar, relative to the need. (285 w of solar)

Admittedly, our formula doesn't work out or make sense for a lot of folks, but we have found that the money saved on diesel fuel and repairs over the years, more than paid for the slightly more expensive, (lower energy consuming) accouterments, as well as the solar panels.

It is also more pleasant.
Mark

PS... You need a smart monitor like a "link 10 or Link 20". Voltage tells you very little about a batteries state of charge.
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Old 08-03-2011, 14:12   #11
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Re: Whats to safe voltage to discharge to?

Quote:
Originally Posted by Mark Johnson View Post

(...)

PS... You need a smart monitor like a "link 10 or Link 20". Voltage tells you very little about a batteries state of charge.
What you need even more is understanding of what is going on, how to use the batteries and how to measure the battery's condition.

My observation is that many, many owners who have battery monitors onboard do not know how to calibrate, reset them, nor do they know how to interpret the readings. On the other hand, many long time cruisers will do very well with the simplest of tools.

Not to say battery monitors are useless. I think they are great. But they do need to be set up properly and then reset temporarily, which too many users tend never to do! It also takes some reading thru the manual and some exercising to get the swing of the tool.

b.

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Old 08-03-2011, 14:19   #12
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Re: Whats to safe voltage to discharge to?

The Percent OCV you speak of is Percent of Charge (first column), Voltage in the second column.
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Old 08-03-2011, 14:20   #13
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All I want to know is a rough And ready method of assessing the state of charge of my batteries without going into Phd and spending a lot of cash, preferably none. I do have voltmeters and the knowledge on how to use them.
Thanks
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Old 08-03-2011, 14:31   #14
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Quote:
Originally Posted by bazzer
All I want to know is a rough And ready method of assessing the state of charge of my batteries without going into Phd and spending a lot of cash, preferably none. I do have voltmeters and the knowledge on how to use them.
Thanks
There isn't one measuring voltage with the battery in circuit is allmost useless as a method of determining SOC amp counting is perhaps the only reasonable method

Dave
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Old 08-03-2011, 14:38   #15
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Re: Whats to safe voltage to discharge to?

Top them up whenever you can. The difference in savings or longevity is not worth splitting hairs over. Going under 12 volts is when I start getting nervous.
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