I need some enlightenment or illumination.

Submitted: Wednesday, Feb 18, 2015 at 21:12
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G'day all, a 12 volt battery will discharge to about 10.6 volts before it is flat. What I would like to know is how many Amps will be useable before the battery is flat. e.g. if I have a 100amp battery, and I run say a 2amp light, will it run for 50 hours before it is flat or will I get less hours of illumination.
thanks in advance for your brightness.
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Reply By: Allan B (Member, SunCoast) - Wednesday, Feb 18, 2015 at 21:36

Wednesday, Feb 18, 2015 at 21:36
Des,

A battery is not quite like a bucket of water. When the level in a bucket has fallen to half the bucket's height it is half full. When a battery has discharged to half its voltage it is empty, or flat. That is a rough explanation.

I will assume that we are talking about a deep-cycle type battery. It should only be discharged to about 50-60% of its rated capacity at which time its voltage will be about 11.8 Volts. If it is rated at 100 Amp hours capacity then you will have used about 40-50 Amp hours.

A 2 Amp light will consume 50 Amp hours in 25 hours.

Discharging a battery below its 50% capacity (11.8v) will cause it harm.
Cheers
Allan

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Follow Up By: Krooznalong - Thursday, Feb 19, 2015 at 14:04

Thursday, Feb 19, 2015 at 14:04
"When the level in a bucket has fallen to half the bucket's height it is half full."
Only holds true if the bucket has straight sides :-)
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Reply By: Roachie.kadina.sa.au - Wednesday, Feb 18, 2015 at 21:42

Wednesday, Feb 18, 2015 at 21:42
G'day Des,

There are a lot of variables, but I would say you wouldn't get 50 hours in the scenario you outlined. The type of battery is important too. AGM batteries are the best for this type of usage as they will best handle being deep discharged.

If the light was an incandescent type, it would simply get progressively dimmer as the battery discharged. By the time it got down to 10.6 volts it would be barely visible as being "on".

If it was a flouro-type light, then it would simply go "OFF" altogether at some point when the available battery charge became too low to run the electronics inside the light; not sure what the actual voltage would get down to for that to occur though.

I like to work on the basis that a reading of 12 volts on a battery is FLAT and I do my damnedest to try to ensure that that never happens. Batteries are too bloody expensive to kill, so I keep them over 12.6 at all times (if possible).

Having said that, we used to often carry around a 7.2 amp/hour mini 12v battery pack and run a 1w flouro lamp off it during the evenings around the camp fire. It would run for several hours (5 maybe??) and then the light would "die".....that was our indication that it was bedtime....time to tell Pesty to shut the F*** up and go to sleep!!! hahaha

That little battery pack finally packed-up after about 7 years of being deep discharged.

Cheers mate

Roachie

PS: The above is just my opinion.....I am not an electro-mechanical engineer (as you know).
AnswerID: 546147

Reply By: Batt's - Thursday, Feb 19, 2015 at 00:48

Thursday, Feb 19, 2015 at 00:48
This may help you as well
http://www.aussiebatteries.com.au/deep-cycle-battery-info/
AnswerID: 546153

Reply By: Sigmund - Thursday, Feb 19, 2015 at 06:12

Thursday, Feb 19, 2015 at 06:12
I gather that as a battery ages as well it has less capacity. It may read 12.8 V eg. after charging but with a drain get much quicker to 12 V. A capacity test is needed to check battery health. Turf it if it has under 70%.
AnswerID: 546156

Follow Up By: OBJ - Thursday, Feb 19, 2015 at 08:36

Thursday, Feb 19, 2015 at 08:36
Is there a way to check a battery "under load" to see how it is holding a charge? I have a couple of older batteries that seem ok for light loads (not the illumination type light) but cannot kick start my diesel car. I'd love to know the real state of these batteries, because one supposedly "dead" one (which I can charge to 13.6v) ran my Engel fridge for a couple of hours a week ago when I was having a minor power crisis on a weekend trip.

Thanks
OBJ
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Follow Up By: Roachie.kadina.sa.au - Thursday, Feb 19, 2015 at 08:50

Thursday, Feb 19, 2015 at 08:50
G'day OBJ,

You should be able to take your suspect battery to a battery retailer and they can put it on a proper load tester and give you an appraisal of its health status.

Roachie
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Follow Up By: Bob Y. - Qld - Thursday, Feb 19, 2015 at 10:30

Thursday, Feb 19, 2015 at 10:30
OBJ,

In a past life, we used to load test batteries with a method that was both crude, and potentially dangerous. So I bought one of these, $80 from Bearing Service in the Isa. At that time the nearest auto electrician was either 400 or 500 clicks away, so it has paid for itself many times over.



Best thing since sliced bread! :-)

Bob

Seen it all, Done it all.
Can't remember most of it.

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Follow Up By: Allan B (Member, SunCoast) - Thursday, Feb 19, 2015 at 12:12

Thursday, Feb 19, 2015 at 12:12
That's posh Bob.

Back in the days when car batteries had the individual cells accessible i had one of these............



Applied to each cell in turn, incorporating a 2v meter and a heavy load resistor.
Best thing even before sliced bread! lol
Cheers
Allan

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Follow Up By: pop2jocem - Thursday, Feb 19, 2015 at 12:50

Thursday, Feb 19, 2015 at 12:50
Alan,

You and I must be of similar vintage. I had one of those when I had the garage workshop and I also remember the exposed lead bars between the cells.

Ah, the good old days...LOL

Cheers
Pop
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Follow Up By: Dennis Ellery - Thursday, Feb 19, 2015 at 13:47

Thursday, Feb 19, 2015 at 13:47
I must be older.
I can remember the times when you could take the top off the battery case and rebuild it.
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Follow Up By: Bob Y. - Qld - Thursday, Feb 19, 2015 at 14:02

Thursday, Feb 19, 2015 at 14:02
Allan,

Saw one of those once, on a station I worked on. Should have "borrowed" it, if only for historical aspect........doubt if anyone knew what it was.

Yeah, I can remember the old exposed bars on batteries, bit of a trap for young players, as I recall, if one got a bit careless with the spanners.

You blokes might have a few years on me.......not quite at the septuagenarian vintage yet. :-)

Bob

Seen it all, Done it all.
Can't remember most of it.

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Follow Up By: pop2jocem - Thursday, Feb 19, 2015 at 14:10

Thursday, Feb 19, 2015 at 14:10
Hey Dennis,

I'm not quite to the septuagenarian stage yet but awful bloody close.

I can remember when spark plugs could be dismantled, cleaned and put under air pressure to test. Done a few myself, can't even remember what they came out of. The bloke brought them to me for "reconditioning".
I guess the compression and firing pressures were a hell of a lot lower back then...LOL

Cheers
Pop
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Feb 19, 2015 at 15:30

Thursday, Feb 19, 2015 at 15:30
Actually Bob, the "exposed bars" on a battery could be quite useful as well as a hazard. They were useful for 'tapping' lower voltages in 2v steps.

Back in the 'spotlighting days' we used a 6 volt lamp run from a 12v battery but 'warming up' on the 6v tap then moving to the 8v tap for a superbright light. As the battery discharged we could move to the 10v or even 12v to keep up the lamp brightness.
Sometimes got a bit keen about moving up to a higher voltage and blew the lamp as soon as moving the tap. Bugger!
Cheers
Allan

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Follow Up By: Member - Duncan W (WA) - Thursday, Feb 19, 2015 at 16:30

Thursday, Feb 19, 2015 at 16:30
And to reenergise your battery you gave each cell a BEX powder.

Dunc
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Follow Up By: Sigmund - Thursday, Feb 19, 2015 at 17:03

Thursday, Feb 19, 2015 at 17:03
And a cuppa tea with a good lie down?

Just to be clear, there's a load test for cranking batteries and a different test for deep cycle jobs that involves putting a known draw on it and measuring how long the battery takes to get to 50% charge (c. 12v). See http://www.sidewinder.com.au/page226aaa.html

A battery discharges exponentially over time, so this test will have you running around like a bare etc etc.

Some battery places will do this for you.
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Reply By: Robin Miller - Thursday, Feb 19, 2015 at 07:42

Thursday, Feb 19, 2015 at 07:42
Hi Des

Your 2amp light will run longer , for between 60 & 70 hours.
Robin Miller

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Follow Up By: Allan B (Member, SunCoast) - Thursday, Feb 19, 2015 at 10:40

Thursday, Feb 19, 2015 at 10:40
Longer than what Robin?
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Allan

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Follow Up By: Member - Des Lexic - Thursday, Feb 19, 2015 at 13:41

Thursday, Feb 19, 2015 at 13:41
Allan, I think that Robin is referring to my comment re 2amp's times 50 hours equalling 100 amp total.
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Follow Up By: Robin Miller - Thursday, Feb 19, 2015 at 15:01

Thursday, Feb 19, 2015 at 15:01
I just got back from coffee club and 3 cups of coffee so my reply might be brighter than your lamp Des.

But yep when someone says 100ah it really means that much when discharged at the nominal 20amp rate and as I think we all no if discharged at a lower rate then we get more "amps" out of it.

Please excuse the not very technical terminology , until my caffiene level drops off. (and also the spelling)
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Feb 19, 2015 at 15:52

Thursday, Feb 19, 2015 at 15:52
Actually Robin, the 'nominal' is the '20 hour' rate not the "20 amp rate".

In any case, you are referring to 'Peukerts Effect' which is related to the nominal discharge rate and introduces significant variation if discharged at a higher current rate but very little change if discharged at a lower current rate than the nominal 20hr as is the case in Des' original expression.

But once again chaps, a simple battery question of imprecise detail is being treated as a though we are dealing with Rocket Science. There are so many variables not entertained in the OP that the answer can only be an approximation and I imagine that is all Des expected and has received. What IS IT about 'battery threads'??? Geez!
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Follow Up By: Robin Miller - Thursday, Feb 19, 2015 at 17:42

Thursday, Feb 19, 2015 at 17:42
Yep Allan , thats why I gave a simplistic 1 line answer showing the trend , not the exact.
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Follow Up By: Member - Des Lexic - Thursday, Feb 19, 2015 at 18:19

Thursday, Feb 19, 2015 at 18:19
LOL Guys. Simple answers suit my simple mind. Thanks for your luminosity.
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Follow Up By: Happy Frank - Thursday, Feb 19, 2015 at 21:05

Thursday, Feb 19, 2015 at 21:05
Have you seen the light yet?
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Reply By: Tomdej - Thursday, Feb 19, 2015 at 09:29

Thursday, Feb 19, 2015 at 09:29
Manufacturers of batteries publish the capacity of their batteries.

For example, here are some of the published specifications for a 100Ah GEL battery. (Jaycar SB1695)

Capacity (25 degrees C)
20 hour rate (5.1A) 102AH
10 hour rate (10A) 100AH
5 hour rate (16.2A) 81AH
1 hour rate (58.5A) 58.5AH

The 10 hour rate is what is manufacturers should publish so comparisons can be made.

If you divide the total AH by 10 you get the load that the battery should be able to supply for 10 hours.

If the load is higher than 1/10 the capacity of the battery is lower than expected.
If the load is lower than 1/10 the capacity of the battery is higher than expected.

In your case with a 2A load you should get a great deal more than 50 hours.

The link to the specification page is:
Jaycar SB1695 Spec sheet

I have used this Jaycar battery as an example only. I have no connection with Jaycar and do not endorse any type of battery.
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Reply By: Frank P (NSW) - Thursday, Feb 19, 2015 at 10:23

Thursday, Feb 19, 2015 at 10:23
Hi Des Lexic,

Most manufacturers publish a 20 hour capacity - C20. Some provide others as well or instead of, but the most commonly advertised capacity is C20.

If the C20 capacity is 100 amp-hours, then a new fully charged battery will provide 5 amps for 20 hours before being fully discharged (under test conditions).

If you put a 2 amp load on the battery, then because of a thing called Peukerts effect it will last longer than 50 hours (100 divided by 2). The extra time is determined by a complicated formula called, unsurprisingly, Peukert's constant.

These figures apply to a new battery. As a battery ages and is cycled its capacity is reduced and the Peukert's constant changes, so any testing you do with a used battery may not give the results you expect.

Allan B's advice re avoiding discharge below 50% is sound. Even if a battery is specified as "deep cycle", deep discharges reduce its life. The commonly accepted "sweet spot" between practical useage and battery life is to discharge only to 50%. Yes, with a deep cycle battery you can go further and it will recover, and you can do that quite a number of times, but you will reduce its life.

Cheers
FrankP

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Reply By: pop2jocem - Thursday, Feb 19, 2015 at 12:47

Thursday, Feb 19, 2015 at 12:47
With your typical caravan installation using 2x (or whatever number) of 105 AH AGM DC (which seem to be a pretty common arrangement) batteries charged by the vehicle through an Anderson plug while on the move and plugged into CP power usually when parked for the night(s), what do you do when at home for 4, 6 or whatever months?

Leave the batteries on charge (float)?

Or just fully charge, disconnect or turn off the charger, remove any loads (disconnect) and monitor the voltage over time. Recharging as required?

The charger I have is a smart charger which has the usual bulk, absorbsion and float stages. The selector switches provide selection of wet (flooded) or GEL/AGM and float voltages of 13.2, 13.5 and 13.8 which seem to be pretty typical.

I have heard various opinions about which float voltage is best and which practice is best. Charge and then disconnect or leave connected with appropriate float selected.

The batteries are about 4 years old and seem to do what I need but maybe there is a better way??

Cheers
Pop
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Reply By: Krooznalong - Thursday, Feb 19, 2015 at 14:15

Thursday, Feb 19, 2015 at 14:15
So Mr Lexic - is there light at the end of the tunnel?
Batteries in essence are a simple device but crickey why are they so damn difficult to understand?
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