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Old 04-08-2016, 23:26   #1
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12v battery voltage

I have 4 Victron 110ah batteries in my house bank and have had them for 4 years now. Until recently I have been using a NASA BM1 battery monitor and have mostly relied on its % capacity display which I almost never let drop below 65% (correctly configured to my 440ah). It has now occurred to me that I never really paid attention to the voltage or at least I don't remember doing that.

This year because I upgraded my alternator, I upgraded to a Victron battery monitor as I needed a bigger shunt.

Because I like "toys" I added the Bluetooth dongle and now I monitor from my phone.

Now that I have more information on a single screen, I'm paying attention to much more information. What it is highlighting is how quickly the voltage is dropping.

This is the current status after a night at anchor, with the batteries at 95% before sunset (don't remember the voltage). All consumers were off for this reading as I wanted to see if the voltage would go up. The 200w solar are just waking up, hence the positive amps


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12.37v seems very low considering how little amps have been used. The voltage will drop below 12v once the capacity is into the 70's%

Is this a sign that my batteries are on their way out or is this normal and I would know this if I had been paying attention to the voltage more over the years?

I'm hoping it's the latter but I expect it's the former


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Old 04-08-2016, 23:55   #2
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Re: 12v battery voltage

Your batteries are close 45% discharged at 12.37v so you have lost a lot of capacity..

Sorry, Teddy

Ps. Amper counting doesn't work without constantly calibrating..
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Old 04-08-2016, 23:57   #3
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Re: 12v battery voltage

Does seem a bit low, esp when being charged!

Is the reset value set correctly? If it isn't , then the charge state gets out of sync with reality...
Every time the engine has been run ( or any charging source) long enough, the accepted amps gets to a preset (in the batt computer) that will tell it to reset capacity back to 100%. For a 440amp/hr battery bank, that number is normally around 5-7 amps.

The problem is, that as the batteries age, their amp/hr actual capacity falls. One way to tell soc in a flooded batt is measuring specific gravity. Otherwise, when back at the dock, you can switch everything off, and come back the next day, measure the voltage. The batt manufacturer should be able to give you voltage for state of charge. Painfully slow method though ....



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Old 05-08-2016, 00:01   #4
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Re: 12v battery voltage

In reality your bank has maybe 200Ah total capacity left..
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Old 05-08-2016, 00:18   #5
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Re: 12v battery voltage

With my old NASA I only relied on it's % figure which I don't know how it was calculated, but I guess that given that it did not reset the amps used, it was perhaps on the voltage. Now that I think about it, last year the % did drop down fairly fast when at anchor over night (fridge running, led anchor light & instruments & one MFD on for anchor watch). So maybe that was a sign....

The Victron unit resets itself to 100% and 0 amps used at 13.2v. This is the default setting. Not sure how that actually works because it does not rest when the motor is running and charging at 14+ volts

Something I do need to change is a setting that should be set because of the solar. Regardless, even after a long motor session and the voltage is up to 13.2v or so, it does seem to dip down into the 12's v pretty quickly after the sun sets
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Old 05-08-2016, 00:35   #6
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Re: 12v battery voltage

What is "absorption charge"?

Is that the voltage shown when the batteries are full whilst running the motor or shore charger?

Because I have solar, I should be increasing the "charged" voltage to prevent the battery monitor from resetting too early. I don't think there is a problem with the BM showing an optimistic state of charge because of an early synchronise, because of the voltage drop after a long motor, but I probably should set it correctly.

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Old 06-08-2016, 01:26   #7
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Re: 12v battery voltage

There are normally 3 stages to charging efficiently, bulk, absorption and float. Read the manual for your charger, alt regulator, and the battery info for your battery make and model. The battery manufacturer will specify the required charging voltage... Then you set the charging systems to suit your batteries.


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Old 06-08-2016, 04:22   #8
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Re: 12v battery voltage

Quote:
Originally Posted by hoppy View Post

12.37v seems very low considering how little amps have been used. The voltage will drop below 12v once the capacity is into the 70's%

Is this a sign that my batteries are on their way out or is this normal and I would know this if I had been paying attention to the voltage more over the years?
My batteries do the same and have since they were new.
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Old 06-08-2016, 04:29   #9
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Re: 12v battery voltage

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Originally Posted by Neptune's Gear View Post
There are normally 3 stages to charging efficiently, bulk, absorption and float. Read the manual for your charger, alt regulator, and the battery info for your battery make and model. The battery manufacturer will specify the required charging voltage... Then you set the charging systems to suit your batteries.


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I've looked at my Balmar charge controller manual and the default absorption charge is 14.18v for AGM. I will assume that this is still correct, so I have set the charged voltage to 14v on my Victron. I'll see over the next few days to see if it affects the "state of charge" the Victron presents
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Old 06-08-2016, 05:04   #10
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Re: 12v battery voltage

Keeping Your Ah Counter More Accurate

Keeping Your Battery Monitor More Accurate Photo Gallery by Compass Marine How To at pbase.com
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Old 06-08-2016, 05:41   #11
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Re: 12v battery voltage

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Originally Posted by Maine Sail View Post
Wow, a great article, thanks
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