Batt voltage when discharged, under load?

[Skipper]

It's said that one should not allow a battery to discharge past 12.1 volts.
However, how do you measure voltage under load?
At rest, batt 1 reads 12.6 volts. With instruments and nav gear then activated, says 12.3 volts. Add LED nav lights, down to 12.2. Engage glow plugs, down to 11.8.
Similar result with batt 2, but slightly less.
Do I have to unload the battery to test voltage/state of charge?
Both are your average group 27 deep cycle wet cell. 5 and 6 years old. Maintained. No anomalies.
I'd like to be able to monitor state of charge from the helm, using the chart plotter which has a readout available. Have verified voltage reading is accurate, by comparing to a multimeter attached directly to selected batt.

 

[Bill Roosa]

You have to know what the amp draw is and the bank capacity. Use the graphs attached. "C" is the battery capacity so you will probably want to lable each graph line with what that current flow is for your bank. Note that one chart is for charging and one for discharging the batteries.

 

[Skipper]

You have to know what the amp draw is and the bank capacity. Use the graphs attached. "C" is the battery capacity so you will probably want to lable each graph line with what that current flow is for your bank. Note that one chart is for charging and one for discharging the batteries.

Thanks, Bill. Just what I was looking for.
Is the number after the C the capacity in amp hours?

 

[Skipper]

Thanks, Bill. Just what I was looking for.
Is the number after the C the capacity in amp hours?

Whoops. I just noticed, it still does not show the load. IE if I'm drawing 5 amps, what would be the coresponding voltage at given percentage of charge?

 

[Davidasailor26]

The number after the C is the fraction of the total capacity that you're discharging at. For example, the C/5 curve is what you would use if you were drawing current at 1/5 of your AH capacity (20 amps for a 100AH bank).

You may also want to pay attention to other advice given by Maine Sail on similar matters. For example http://forums.sailboatowners.com/in...ltage-vs-state-of-charge.176854/#post-1266663

 

[Maine Sail]

Thanks, Bill. Just what I was looking for.
Is the number after the C the capacity in amp hours?

No the "C" is Ah "C"apacity and the number is your hourly discharge rate when compared to your batteries Ah capacity.

A discharge of C/20 on a 100Ah battery would be a 5A discharge or Ah capacity divided by 20. A C/100 discharge would be Ah capacity divided by 100 or a 1A load on a 100Ah battery. In a perfect battery that meets is Ah capacity rating a C/20 load should allow the battery to deliver C/20 for 20 hours at 77F. A C/100 rate for 100 hours at 77F. The rate of discharge affects the voltage sag under load.

Keep in mind that chart is quite conservative for many batteries and that testing was done on an L-16 battery, which is nowhere near the battery your automotive type batteries are.

Bottom line: Charge when you hit 12.2V under your "typical average" house load. High amp short duration loads you can ignore on the voltage dips but take your average discharge load and don't let terminal voltage dip below 12.2V and you'll be doing okay. If your discharge rate is really low, in relation to Ah capacity, like C/100, then move your recharge voltage point up to 12.3V+..

 

[Bill Roosa]

MS is correct. the formula is C/xx = amps flow. ie if your banks Capacity in Ah is 400 Ah then the C/100 curve is actually 400/100= 4 amps flowing and C/5 would be 400/5= 90 amps flowing. I also agree with MS that this chart is pretty conservative and you can ignore high amp short duration (starter, winches, windlasses etc) loads. Best to use something that cycles so you get two data points. My reefer draws about 5 amps and the rest of the 'stuff" draws 2-3 amps so my system cycles between 3 and 8 amps. I read both and make sure they agree or choose the one that shows the LOWER SOC.

 

[heritage]

Bill,,, looking at your chart that shows battery state while discharging, I see that a battery that is 50% discharged and has a C/100 load should be at 12.5 volts. That seems high,,, or am I misreading the chart.

 

[Skipper]

No the "C" is Ah "C"apacity and the number is your hourly discharge rate when compared to your batteries Ah capacity.

A discharge of C/20 on a 100Ah battery would be a 5A discharge or Ah capacity divided by 20. A C/100 discharge would be Ah capacity divided by 100 or a 1A load on a 100Ah battery. In a perfect battery that meets is Ah capacity rating a C/20 load should allow the battery to deliver C/20 for 20 hours at 77F. A C/100 rate for 100 hours at 77F. The rate of discharge affects the voltage sag under load.

Keep in mind that chart is quite conservative for many batteries and that testing was done on an L-16 battery, which is nowhere near the battery your automotive type batteries are.

Bottom line: Charge when you hit 12.2V under your "typical average" house load. High amp short duration loads you can ignore on the voltage dips but take your average discharge load and don't let terminal voltage dip below 12.2V and you'll be doing okay. If your discharge rate is really low, in relation to Ah capacity, like C/100, then move your recharge voltage point up to 12.3V+..

Thank you sir. And others.
Making sense of the math brings ease of monitoring.

 

[Bill Roosa]

heritage
You are reading it correctly. and yes it is conservative. Experience is the best tool as this is a black art. Lets hope MS can give us some real data and we can mod the charts for a more realistic tool.