Battery capacity question

pheran

pheran

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I've seen it said on here and elsewhere, that a fully charged 12v battery should give 12.8v, at 12.5v its half discharged and at 12v its effectively 'flat'. I am assuming that for my 24v domestic set-up, I can simply double the figures to 25.6/25/24v. Indeed, my inverter shuts off once the available battery voltage falls below 24v showing a 'low input' warning, giving some credence to the bottom figure.
So.....getting to the point, how many Ah will I be able to draw from my domestics for the 1.6v that are nominally available? (domestics supplied from 2 x 235Ah batteries in series to give 24v).
 
Theoretically 117.5, but that assumes that your batteries are in tip top condition. Beyond that it gets complicated.
 
The figures i prefer to use for voltage vs state of charge are <ul type="square">[*]12.7 - 12.8 volts fully charged [*]12.2 volts half charged [*]11.7volts effectively flat [/list] These are it must be stressed figures after the battery has rested for 12 to 24 hours after charging with no load and yes double those for a 24 volt battery.
The figures under load will be appreciably lower and will depend upon the load.

The capacity figures normally relate to a 20 hour discharge rate

So your two 12volt 235 Ah batteries in series gives you 235Ah at 24 volts if discharged at 11.75 amps.

If you are going to cycle your batteries down to 50% of fully charged you will get 117.5 Ah at that discharge rate as login-name says. However the voltage reading will have fallen well below 12.2v but on standing it should recover to about that.
 
The voltage also varies according the temperature if I remember correctly 0.2v / degree? most figures are quoted at 20.00deg C. It's difficult to be precise a hydrometer is the easiest way to check state of charge.
 
Yes it does vary with temperature. I cannot remember a figure but there are so many tables of volts vs SOC about anyway that I don't bother about the temperature correction

Hydrometers are fine if the batteries are not sealed and you have to cater for a thing dripping acid! Even the hydrometer readings should be corrected for temperature.

Personally I find one measurement with a voltmeter a whole lot less bother than taking six readings with a hydrometer . I have hydrometer in the garage but it's not something I would normally take to the boat.

You stick to your hydrometer if that's what you prefer I'll stick with my multimeter.
 
When sizing a batter bank for an inverter you need to decide how much power you want to take out. You can take quite a large current from quite a small battery for 5-10min without problems with the voltage dropping so if you are looking to boil a kettle, run a vac for a few mins or similar jobs then a capacity in a/h of about 1.5x the max DC draw of the inverter should be fine but after a few min the voltage will collapse and it will shut down. If you want power available for more time you really need to go up a lot, at least 3x and preferably 4-5x the highest sustained current you want, ie if you want to be able to draw 100a for 1h you might get away with 300a/h when everything is new but really need 4-500a/h for reliable performance. The reasons for this are to complex to explain here but I have found the rule of thumb above works for me. The only proviso is not to expect cheap leasure batteries to last long with a demanding inverter, you need proper deep cycle jobs and a charge system to match.
 
I was being pedantic /forums/images/graemlins/wink.gif
the question relating to measuring to tenths of a volt and using the answer is only a guide (and a fairly rough guide)

The hydrometer reading however only varies by less than .001 for every degreeC at or around 20.00C, what it does give you is a very good indication of variation between cells.

I havn't used a hydrometer for years!

I use the volt meter method...but 12v can mean 40% or 10% of charge remaining, due to the many other factors involved...
soc_voltage.jpg
 
I have two 85 amh batteries. which are continuously tricle charged from a solar panel. As a result it is never possible to leave the batteries idle for 12 hours to use voltage as a check on their state of charge.
I use a hydrometer which always shows the SG well up in the 'green sector' of the float.
Is a hydrometer a better indicator of the state of charge than a volt meter ?
 
[ QUOTE ]
I feel this page

[/ QUOTE ] Ah thats the one I was looking for!

Pity they dont make the "Voltwatch" now. Rich boat owners who have money to burn prefer the fancy battery monitors I suppose.
 
[ QUOTE ]
Is a hydrometer a better indicator of the state of charge than a volt meter

[/ QUOTE ] It is really provided you have decent one. It is just that IMO a voltmeter is so much more convenient to use.
 
I don't know if it's right but it seems to make sense.
************************************************

It makes perfect sense and please note the comment about shorted cells near the bottom of the page.....It would appear the majority of faulty batteries reported here have suffered this fault. May last forever as 10 volt batteries if you don t mind dim lights /forums/images/graemlins/cool.gif
 
Certainly for submarine batteries SG was the prefered measure of battery state and each battery had reference cells (there were some 200 odd cells) which were checked as required.

Of course with many batteries today such checking is not easy, and perhaps not advisable in a yacht in a sea way, thogh in suitable batteries quite practical in harbour.
 
Many thanks for all the replies. All I have to do now is to figure out why my domestic bank appears to be giving only 30 odd Ah (drawn as mains power via the inverter) before dropping below the critical 24v level when the inverter cuts out. Both batteries were brand new at the end of last season.
 
[ QUOTE ]
only 30 odd Ah (drawn as mains power via the inverter)

[/ QUOTE ]

You don't mean 30ah at 240volts do you?

If so, that's your answer. In crude terms thats equivalent to 300ah at 24 volts.......considerably more in reality
 
100 watts is a very small inverter. You should have no trouble with that size of battery. I'd expect it to run for hours, all day maybe off a suitable battery.

I found the recommend minimum battery size for one 100watt inverter was only 30Ah

Those that I have found any figures for cut out at about 21 volts (and restart again at 25 volts) so I suspect your trouble is that is is cutting out at too high a voltage. If that is what is specified I don't think you'll stand much chance with it.
BUT
You do mean it is cutting out when the volts fall to 24 not they read 24 immediately after it has cut out don't you?
 
A total of around 6-7 hours. Getting down to specifics, the mains devices in question are a TV and a Sky box. The plates on the backs of these give their power requirements as 50W and 32W respectively. Added together 82W so I called it 100W to take account of any inefficiencies in the 24v to 240v conversion.

Just to make it clear, we don't normally watch TV at 6-7 hours at a stretch but did leave it on as a test when we began to suspect a problem.
 
Vic. The inverter itself is a 2000W model. The 100W is the mains current I calaculated I was drawing. Unfortunately, I don't have any documentation for the inverter and the Vetus website doesn't mention this particular model ie a combined inverter/charger. But it does seem to imply that 24V is the cut-out voltage for some of its other models.

You do mean it is cutting out when the volts fall to 24 not they read 24 immediately after it has cut out don't you? My limited knowledge of these matters does not enable me to appreciate the significance of this question but the 23.9V is the reading I get if I stick the multimeter on immediately after the inverter has cut out.
 
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