when is a battery flat?

Submitted: Tuesday, Mar 24, 2009 at 21:37
ThreadID: 67163 Views:10427 Replies:11 FollowUps:7
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I have a 100ah fullriver agm battery. When away we camp unpowered until the battery runs down then go to a powered site to charge it up.

At what voltage do you consider that it is time to charge up a battery in this case?

Michael
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Reply By: Member - Graham H (QLD) - Tuesday, Mar 24, 2009 at 22:17

Tuesday, Mar 24, 2009 at 22:17
Probably at about 10 volts if you want it to last.






AnswerID: 355976

Reply By: Member - Roachie (SA) - Tuesday, Mar 24, 2009 at 22:31

Tuesday, Mar 24, 2009 at 22:31
Personally I consider 12 volts to be low enough for my Fullriver 120a/h and my other batteries too. I know you can run them lower but the lower you go and the more often that you do it, the more the life of the battery will be adversely affected (IMHO).

Roachie
AnswerID: 355979

Follow Up By: Patrol22 - Tuesday, Mar 24, 2009 at 22:51

Tuesday, Mar 24, 2009 at 22:51
I'm with you on this one Roachie...I consider the battery flat at 12 volts.
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Follow Up By: Maîneÿ [wa] - Wednesday, Mar 25, 2009 at 06:19

Wednesday, Mar 25, 2009 at 06:19
Yes,
will second that too
(technically it's just ½ flat, but that's as far as it should go anyway)

Mainey . . .
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Reply By: Member - 1/2A - Tuesday, Mar 24, 2009 at 22:32

Tuesday, Mar 24, 2009 at 22:32
Hi Mintrax,
600 cycles @ 80% discharge, the more the dischared the less the cycles.
Arthur
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Reply By: Motherhen - Tuesday, Mar 24, 2009 at 22:47

Tuesday, Mar 24, 2009 at 22:47
If i recall correctly, our Prostar controller turns of the load at 11.8 v.

Motherhen
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AnswerID: 355991

Reply By: Patrol22 - Tuesday, Mar 24, 2009 at 22:53

Tuesday, Mar 24, 2009 at 22:53
mintrax - don't you have some means of charging the battery from the vehicle when travelling? Even if you don't have an isolator it is pretty easy to hook the vehicle to the extra battery through an Anderson Plug to charge it while travelling...just have to remember to unplug when you stop:-)
AnswerID: 355993

Follow Up By: mintrax - Wednesday, Mar 25, 2009 at 07:01

Wednesday, Mar 25, 2009 at 07:01
Hi Patrol,

Yes I do have a charging situation when travelling. But it's not the worlds best.






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Reply By: Olsen's 4WD Tours and Training - Tuesday, Mar 24, 2009 at 23:19

Tuesday, Mar 24, 2009 at 23:19
I am not a battery expert, but I've been told , as above 80% of static charge (12.6V) ie. 10 V min. Though I would recommend 11+V if you want a longer life
AnswerID: 355997

Reply By: pop2jocem - Tuesday, Mar 24, 2009 at 23:44

Tuesday, Mar 24, 2009 at 23:44
mintrax

As others have said the more often and more deeply you draw your battery down the more you shorten it's life. I have two 105 amphour AGM 's that will run our Engel for three days without recharge at which point the volts are down to about 12.2. I have been told that this is about 50% and then I recharge.

Hope this helps some

Cheers Pop
AnswerID: 356002

Reply By: mintrax - Wednesday, Mar 25, 2009 at 07:04

Wednesday, Mar 25, 2009 at 07:04
Thanks for all the opinions. I guess 11.8 to 12.2 is where I will call it quits.


Michael
AnswerID: 356016

Reply By: ABR - SIDEWINDER - Wednesday, Mar 25, 2009 at 09:01

Wednesday, Mar 25, 2009 at 09:01
We are often asked how do you know what power is left in a battery, condition or state of charge. (SOC)

Here is a simple graph that shows SOC vs. voltage.

Please note this is an average and there are slight variables that are effected by temperature and battery type.

Best is to keep your battery above 30%.

AnswerID: 356037

Follow Up By: Member - Mike DID - Wednesday, Mar 25, 2009 at 09:48

Wednesday, Mar 25, 2009 at 09:48
. . . and if you want this graph to be meaningful, you have to read the not-so-fine print at the top "battery voltage at rest - no load" - and that means waiting after charging too.

If you are using a wetcell deepcycle, you'll have to wait many hours to get an accurate voltage.

If you're measuring voltage while connected to a load and you disconnect at 11.6 volts, you might find the battery still has 90% of its charge - it depends on the discharge current.
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Follow Up By: Gronk - Wednesday, Mar 25, 2009 at 11:22

Wednesday, Mar 25, 2009 at 11:22
And it really comes down to the situation you are in at the time .....

How much value you place on your batts versus the priority of cold beer ???

I have 6 AGM's in the CT and don't like to see them drop below 12V...........but if they were 12V and I was going home the next day and there was beer to be drunk, then they would have to suffer a bit..

But getting to know how long your batt(s) lasts, usually prevents them going flat..
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Reply By: Mandrake - Wednesday, Mar 25, 2009 at 12:15

Wednesday, Mar 25, 2009 at 12:15
Just as a matter of interest how do you know when a battery reaches 12.2 volts -- reason I'm asking this is I was playing the other day with a 12volt electric car heater on my battery / solar system and the voltmeter on the projecta regulator dropped from 13.00 volts to around 10.9 volts when I switched on the heater ( it sucks 11 amps out )and when I switched it off again it promptly bounced back to 12.6 ish ... Is there a voltmeter that can tell you accurately and immediately state of charge voltage ? Everyone here tends to agree that letting the battery lie idle for a couple of hours gets an accurate voltage figure - but if its in use how accurate is it ? Shouldn't you be working out amp hours used rather than voltage ? in example above I run the heater for 1 hour at 11 amps and technically my battery drops from 85 to 74 ah and I should start re-charging ?? Is that the way it should work ?

Cheers

Mandrake
AnswerID: 356082

Follow Up By: Gronk - Wednesday, Mar 25, 2009 at 13:40

Wednesday, Mar 25, 2009 at 13:40
Measuring with a volt meter should be done when under no load....and although the battery needs time to fully settle after being used...it will give you a pretty good indication as to SOC..

If it shows 12.6V then its good....if it shows under 12V then charging at the earliest opportunity is the go ..

For most people , it doesn't have to be a super accurate science....just a general guide as to when the battery is getting a bit low and may need a charge..

I have a cheap LCD voltmeter ( off e bay ) that I know is between .1 and .2V out ( reads a bit low...which isn't a bad thing ) that lets me ( when I can be bothered to look ) keep an eye on SOC...

You can buy an A/H capacity meter that lets you know how many A/H's are left, etc,etc,...but they are approx $200 plus..
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Follow Up By: Robin Miller - Wednesday, Mar 25, 2009 at 14:15

Wednesday, Mar 25, 2009 at 14:15
Thats a good question Mandrake because the answer isn't readily apparent and is a base reason for lots of wrong impressions.
This isn't helped by the compromises with using an AH estimator.

I like Gonks explanation to.

First, a battery is actually flat at about 10.5 volts as this is when the electrolyte seperates out leaving no acid left just water (using standard wet cell for ease of explanation).

But I'm with the group here in that 12v is a good and safe figure to use.
I take mine to the limits and use 11.7v as it does vary a bit with temperature and battery type.

The practical way to measure the battery is with a good multimeter but mostly importantly the battery should not have been charged or discharged heavily within roughly 1/2 hour
of when you measure it, and not subject to excessive heat/cold.

I.E. if your battery is running a fridge and drawing up to 6 amps, it may measure 11.8v but if you turn the fridge off and wait 1/2 hour the battery voltage will read closer to 12v , its true value, and this gives you a better state of charge reading which can be used with Sidewinders graph above.

You can use a bit of commonsense hear to get you closer though. E.G. if you read 12v with the fridge running then
you can assume its actually a bit higher and still ok to discharge a bit more.
Robin Miller

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Reply By: Rolly - Thursday, Mar 26, 2009 at 15:57

Thursday, Mar 26, 2009 at 15:57
Very intelligent and informed replies all round, but the original query, "when is a battery flat" , is probably better explained by the ratios of the height to the width and the depth.
Any battery which has one physical dimension significantly smaller than either or both of the others can be properly described as "flat".
Like the little one in my mobile phone.
8-D
AnswerID: 356351

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