At what voltage are my batteries at 20% of DoD?

[Olberg]

Here´s a rookie question for you:

I want to program my Victron MultiPlus to start my genset and charge my batteries so my they never go under 20%. The Multi needs to know a “start-voltage”.

I have 640Ah @ 24V of Marathon L6V160 batteries.
Do I have to use the current-discharge tables or is it easier?

Would appreciate it someone could walk me through this.

Thanks in advance!
/O

[Olberg]

Ok, so I found this table:

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12 25,40V
90% 12.5 2.08
80% 12.42 2.07 24,84V
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

Does this aply for AGMs?
Can I use this straight off or do I compensate for loads, solar charging?
Or will I have to do empirical testing with my specific system under some time?

Happy for all input I can get!

/O

[Naptown]

Voltage works if you allow the batteries to rest for at least 4 hours or more.
You really need to create your own empirical chart for your own batteries.

[Beanyboy57]

Ok, so I found this table:

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12 25,40V
90% 12.5 2.08
80% 12.42 2.07 24,84V
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

Does this aply for AGMs?
Can I use this straight off or do I compensate for loads, solar charging?
Or will I have to do empirical testing with my specific system under some time?

Happy for all input I can get!

/O

I think you will have to work this out for yourself over a given period of time. A rough guide would be to use the figures you have given here on this page but remember that AGM's have a recommended float charge of around 13.5v or 2.25v per cell and an absorption charge of somewhere around 14.4v, remembering that battery manufacturers give data based on a set temperature, usually 20c. Therefore 20% DOD would be around 13v at 20 degrees celcius, by my calculations but I am not an expert by any means.
I believe you will have to monitor how your batteries perform at given loads and temperatures over a month or two. You could install a battery monitor such as Victron supply or a commonly used Trimetric type or use a digital multimeter regulary.
Good luck, I for one will be interested with what you come up with after you take some measurements.

[Olberg]

Thanks Beanyboy

My batteries have a float charge at 2,40V per cell but I wonder what their max voltage would be?
(I don’t have them here and can test them)
Pehaps someone has used L6V160 and can tell me?


I found another calculation on the net (see attachment)
Which says that 35% Dod is at 12,6 on AGMs (25,2 @ 24V systems)
Beanyboy says around 13V (20% DoD at 26V @ 24V system)

This seems to be a something I’ll have to test. I’ll start with a value of 25,5V and see how it goes.
Seems reasonable?
Others opinions on this would be helpful!

Cheers
/O

[Sunking]

State of Charge voltages are useless on a working system period. Only true way to get the an accurate SOC is with a hydrometer. I can take a fully charged 12 volt battery and make it read 6 to 16 volts in a blink of an eye.

2.4 volts is way too high of voltage to float at.

[Olberg]

Thanks Sunking for the input.

You are right. 2.40V per cell is my max boost voltage. My wrong.
The operating instructions tell me to float charge the AGMs at 2,27V (20 degrees ).

Would that meen that I can expect the batteries have 6,81V when they are fully charged?

Taking a hydrometer test is difficult with AGMs though. Voltmeter seems like my only option here.

/O

[Beanyboy57]

Thanks Sunking for the input.

You are right. 2.40V per cell is my max boost voltage. My wrong.
The operating instructions tell me to float charge the AGMs at 2,27V (20 degrees ).

Would that meen that I can expect the batteries have 6,81V when they are fully charged?

Taking a hydrometer test is difficult with AGMs though. Voltmeter seems like my only option here.

/O

I thnk the figures are a little flexible and that we all have to operate within an acceptable range of accuracy given that we do not all live in exactly the same environment. When I received my 6v FLA batteries over a year ago they were at 7v initially so I have tried to keep them around that mark or just below, somwhere around 6.8/9v or 2.3v per cell. I called the battery supplier to get his thoughts and soon realised that he knew very little about the actual internal workings of FLA batteries. I found most of my information on this forum and on manufacturers websites. I installed a battery monitor but really the best thing I did was use a multimeter to monitor each cell individually. I like to go to 20% DOD but other people that use my shed (namely my oldest son) don't think about DOD, they use as much power as the want, when they want it! I don't have the luxury of a generator, just the good old sun!