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Old 20-06-2018, 00:46   #1
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Batteries, Margin an Recharging

I have 2 Problems:


1.) Margin
1 have a house bank of 3x225Ah = 675Ah


so I tried to calculate the margin


A: 675 = 12,7V therefore 675/12,7 x 12,3 ...
675Ah 12,7
654Ah 12,3 50% 21Ah
643Ah 12,1 20% 32Ah


which, I believe, is far too less



B: 675 = 100% 675/2 = 337,5..
675Ah 100%
337Ah 50% Margin 337Ah
135Ah 20% Margin 540Ah


which is far to much!


SO HOW TO CALCULATE??


Charging:
the easier part: 675Ah+ Starter Battery 225Ah = 900Ah
Charging shall be not more than 20% = 180 Ah (My electrician said 10%??)
so alternator shall be 200A (with 10% calculated loss) and chargers 2x80A


Is this correct?


I think it would be of great value for the community if someone gives a professional short answer for dummies like me
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Old 20-06-2018, 01:26   #2
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Re: Batteries, Margin an Recharging

I have no idea what your first question is. Are you trying to ask at what voltage you've drained how many amp hours? That has a lot of factors.

For a base line. 12.8v is full battery. 12.0 is 50% battery. Half your ah used.

Size means nothing.

At -340ah you will be at 50% battery. Should be 12.0v. No load

Flooded batteries should not go below 50%

Get a battery monitor like a victron 702

2nd depends on battery type. Agms will take much more then flooded.

Charger depends on shore power and gen size. Two 80a charger would be good. But will close to max out a 3.5k gen or 30a 120v shore plug with nothing else can run.
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Old 20-06-2018, 05:07   #3
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Re: Batteries, Margin an Recharging

For a base line. 12.8v is full battery. 12.0 is 50% battery. Half your ah used.


Thank you this is basics to calculate with. 337Ah 50% Margin 337Ah


2x 15A 230V fuses are actially not the problem


And 10% loss with the alternator is ök?
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Old 20-06-2018, 05:16   #4
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Re: Batteries, Margin an Recharging

moseriw, you could find your battery manufacturer's spec sheet, which should list percent of charge remaining at various voltage readings. 12.0V is not universally a 50% charge; depends on your maker's model, etc.

For example, for one of our banks. 12.18V is 50% state of charge (SOC), 12.06 is 40% SOC, 11.94V is 30% SOC, etc. Info direct from manufacturer.

-Chris
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Old 20-06-2018, 05:46   #5
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Re: Batteries, Margin an Recharging

Where are you using 15 amp 230 volt fuses?
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Old 20-06-2018, 07:29   #6
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Re: Batteries, Margin an Recharging

Quote:
Originally Posted by ranger42c View Post
For example, for one of our banks. 12.18V is 50% state of charge (SOC), 12.06 is 40% SOC, 11.94V is 30% SOC, etc. Info direct from manufacturer.

I should mention the maker says that's open circuit voltage, after resting for a minimum of four hours.

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Old 20-06-2018, 11:21   #7
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Re: Batteries, Margin an Recharging

Well look at the vendors datasheet - nowadays a thing to get angry:


http://productsheets.varta-automotiv...337.1529515073
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Old 20-06-2018, 12:40   #8
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Re: Batteries, Margin an Recharging

Quote:
Originally Posted by Zil View Post
Where are you using 15 amp 230 volt fuses?

It is in fact 16Amps for 230V and this is standard in the EU.
My 230V Network consists of Shore/Generator 7,5Kw/Inverter 350W/1500W
+ galvanic isolator for the 230V Shore-Earth connection
+ Kw counter

+ ground circuit fault interrupter

+ 7 16(12)A fused circuits for Washer/Charger/Boiler/Workshop Plugs/Bow Plugs/Strn Plugs


And I think this is a good setup.
But this is off topic here.
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Old 20-06-2018, 12:49   #9
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Re: Batteries, Margin an Recharging

btw. I have a victron 702 but I want to understand the basics. Yes I can read some params but I simply want to know how it works.


And I will contact Varta for correct data.
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Old 20-06-2018, 12:49   #10
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Re: Batteries, Margin an Recharging

Quote:
Originally Posted by moseriw View Post
Well look at the vendors datasheet - nowadays a thing to get angry:


http://productsheets.varta-automotiv...337.1529515073

I'd guess that's probably just a basic sales brochure. They probably publish more tech specs somewhere... or maybe will supply if you ask them...

While you're asking, you might look for projection of longevity, perhaps expressed in number of (laboratory) cycles to a particular depth of discharge (DoD, often 50%).

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Old 20-06-2018, 13:31   #11
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Re: Batteries, Margin an Recharging

mose, when you say "margin" I believe you are looking for the word "capacity" as it refers to "state of charge".

With conventional wet lead acid batteries, you will get the best life out of them (the most amps over the longest number of cycles) if you draw the batteries down to about 30% discharge. You will still get good life by drawing them down to 50% discharge. Which number to use if a personal choice; if you must go to 50% or deeper, then you do. If you can get by with recharging at 30% discharge (70% remaining) then all the better.

With a 675Ah bank, the math is simple. You can draw 30-50% of that, depending on your decision, so you can use some 202-367 Ah before you need to recharge. Forget complicated math and voltage for this, all you need to know is the percent of discharge capacity.

As for the voltage of a "full" battery, 12.8 is unlikely unless you have perhaps pure lead battery or other special premium type. 12.7 is more typical.

If you are relating battery voltage (which should only be taken for a 'rested' battery, another definition that folks argue over) then the long standing numbers are that for every tenth of a volt, your battery has lost 10% useful capacity. So from 12.7 volts "FULL" you are effectively dead at 11.7 volts. Halfway discharged is 12.2 volts, no less, for a rested battery. Yes, you can still start an engine at 11.7 volts, but there's no useful ong-term power in it, it is effectively discharged by then.
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Old 20-06-2018, 15:55   #12
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Re: Batteries, Margin an Recharging

SO HOW TO CALCULATE??


Charging:
the easier part: 675Ah+ Starter Battery 225Ah = 900Ah
Charging shall be not more than 20% = 180 Ah (My electrician said 10%??)
so alternator shall be 200A (with 10% calculated loss) and chargers 2x80A


Is this correct?


I think it would be of great value for the community if someone gives a professional short answer for dummies like me [/QUOTE]



On the discharge question - the question has been adequately answered. Buy a decent charge/discharge monitor.

On the charge question you don't have to worry about the size of the alternator or the % rate of charge since with a good regulator the internal resistance of the battery will dictate the charge rate.
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Old 20-06-2018, 17:40   #13
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Re: Batteries, Margin an Recharging

If your combined capacity is 900AH, then most wet cell makers would say to recharge them at not more than a 20% C rate, which would be 180A charging power. That's 180A out of the charger or alternator, whatever is putting out the final power.
You'd need that device rated for 180A continuous duty, which might mean a device with a 250A or higher rating.
And for an alternator, that might mean you'd have to settle for what you can, bearing in mind that around 100A you need to go to ribbed belts not v-belts.
Your electrician might have said 10% because that's better for battery longevity, the plates reform better and you'll boil off less electrolyte so you'll spend less time topping it up.
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Old 21-06-2018, 04:13   #14
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Re: Batteries, Margin an Recharging

WOW - thank you very much for the professional posts.



I think this thread is highly valuable for the community too. As soon as I have the vendors reply I will post it here for reference.
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Old 21-06-2018, 04:39   #15
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Re: Batteries, Margin an Recharging

Charging: the Alternator


you ca buy 2 sorts of Alternator:


1.) car/truck alternator for around 100 USD
2.) marine alternator for around 900 USD


What is the difference?


common automobile alternators are designed to recharge the discharging of the starter for a very short period about 10 to 20 minutes. Then the altrnator will swtich to conservation charging.


marine alternators are designed to run at max (-10% mostly) without getting to hot.


A possible solution maybe to buy an automobile alternator that has a defined all time power without getting hot and to reduce charging with an adopted charge controller.
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