Help me Understand Batteries

Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
I'm having issues with understanding battery specs and discharge rates, SOC and DOD, specifically AGMs (Lifeline). I have a single Lifeline 8D (255ah). I understand, under most situations, I should not go below 12.3 volts. Can I assume that this while there is no load and the battery is at rest? I can draw it down with my inverter to the 11's drawing 50+ amps but when it is turned off it jumps right back up to 12.7+. Additionally, the manual states to test the condition of the battery to apply a 25 amp draw on it until the volts go to 10.5 volts. I'm assuming this is also under.

Additionally I do not understand the chart provided in the manual
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I'm really not a idiot, really! But I feel like I'm missing something regarding this.
 
Jan 7, 2011
4,724
Oday 322 East Chicago, IN
Hi Mike, I know way more knowledgeable people will chime in here, but I can comment on one issue…the volts dropping while the batteries are in use, and then “rebounding”.

I think that is perfectly normal…and I noticed it on my new bank of 2) 6-volt batteries I installed this week…

I left dock and the materiel were at13. Something bolts, but as I left the dock, turned off the motor and started sailing, the voltage dropped (slowly) down to around 12.5 volts…I had the fridge running, stereo,VHF, AP, Chartplotter, and other instraments…

Voltage dropped a bit more over time…I turned off the fridge and the batteries stabilized around 12.4 volts I guess.

The point is that when under load (or when being charged), the battery voltage will be lower (or higher) than the standing voltage, which is what you really want to monitor….and I think it takes about 24 hours to “equalize”…

A battery monitor can, I think, calculate these things without the long wait for the battery to stabilize…or some people use 2 banks and alternate so 1 is always “resting” and they can check the resting voltage.

I am looking forward to all of the other responses…hopefully not correcting me…but I will learn something I am sure.


Greg
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
I understand, under most situations, I should not go below 12.3 volts. Can I assume that this while there is no load and the battery is at rest?
Yes, the 12.3 threshold sounds like the “open circuit voltage”. The threshold under load will be lower, as reflected by the chart you posted. Basically, the chart is showing that depending on the load, the battery will read a certain voltage at a certain rate of charge. You’ll first want to convert the “x hour rate” loads across the top of the table into amperage at your battery size. For example, 1 hour rate for your 255 aH battery would mean 255 amps. So, if you are pulling 225 amps and you read 12.23 volts you are at 10% depth of discharge, which is 90% state of charge. In the right hand column, the 120 hour discharge would be pulling only 2.1 amps. So if you’re pulling 2.1 amps and read, for example 12.18 V then you’re at 50% depth of discharge. The idea is to stay above that 50% mark.


some people use 2 banks and alternate so 1 is always “resting” and they can check the resting voltage.
Agree with everything else you said, and you might be right about people doing this too, but it’s not a great practice. Pulling from one battery is less efficient than both together. So if someone is really concerned about keeping the best state of charge possible they’d be better off keeping the bank combined.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
I don't quite understand the chart you are referencing, it seems to be related to doing a capacity test. However, the next chart on the page is clearer and perhaps more helpful. It lists the Open Circuit Voltage and the approximate State of Charge.

For example if a 12v battery has an OCV of 12.18v it is at a 50% SOC (or 50% Depth of Discharge, DOD, if you are the glass is half empty kind of guy).

The manual can be found here:

 
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Nov 13, 2013
723
Catalina 34 Tacoma
I think it's saying for example, to get to a DoD of 50% in 1hr, a fully charged battery under load would read 11.83 volts.
 
Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
On a side note. I installed a Balmar Sc200. It seems to be the best opportunity to remove some of the confusion and guessing. It is showing SOH now but it is still in the learning phase.
 
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Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
After a good night's sleep the chart you reference makes more sense. It is used when load testing the battery.

The most common load test is the 20 hour load test. To do this, divide the ah capacity by 20 and apply that load. For a 100 ah battery the load would be 5 amps, thus after 20 hours at 5 amps 100 ah would be used and the battery would be at 100% DOD or 0% SOC. The chart shows the estimated voltage readings along the way.

Using the 20 hour rate again, when the voltage reading is 12.65v the battery is at 90% SOC or 10% DOD, when it is at 12.15 volts it is at 50%, at 11.83v the SOC is 30% and DOD is at 70%. Finally when the voltage reaches 10.5v it is completely discharged.

The other columns are for different loads. The 8 hour rate for 100ah battery would require a load of 12.5 amps applied for 8 hours. The voltages listed are for the intermediary stages, 10% DOD, 20% DOD, etc.

The manual suggestion of a 25 amp load is based on a 10 hour test, the results of which are not shown on the chart.

The second chart on page 32 of the manual, gives the information that @Davidasailor26 was referring to. On that chart if the OCV is 12.3 the battery is at 60% SOC. This is a resting voltage with no loads or charging for at least 4 hours. David is correct in advising not to go below a 50% SOC charge and to return to 100% as soon as possible to prolong battery life.

A key idea is the difference between resting voltage (Open Current Voltage) and voltage under load. All battery voltages will drop when under load, the greater the load, the greater the drop. Once the load is removed the voltage will rise and it is that voltage (resting) that indicates the SOC. In your scenario, drawing 50 amps for 1 hour from a fresh fully charged 250 ah battery, 20% of the capacity will have been removed, according to Chart 2, the battery should read about 12.54v.

The first chart, will give you an idea of how long the 50 amps can be drawn from the battery. (although it would require extrapolating between the 8 and 20 hour rate). Making the math easier, a 250 ah battery with a 30 amp load will last about 8 hours before it reaches 10.5 volts. Decrease the load to 12.5 amps (250/20) and the battery will go 20 hours before it reaches 10.5 v.
 
Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
So by the chart and if my battery health is 100% and SOC starts at 100%, I could put a 255 amp (its a 255amp/hour) load on it and once the loaded volts reading reachs 11.83 it is considered 50% DOD?
 
Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
is there a maximum amp draw these battery's should be limited too? I can't seem to find anything on that. I turned on my 1000watt inverter and ran my shop vac and heat gun at about 156 amps (reading on the SC200) The battery dropped to about 11.8 volts and stayed there for about 20 minutes which when shut off for about 5 minutes the volts would rise up to 12.4 and then slowly rise from there. Would this be ok to do as long as the resting volts did not drop below 12.3'ish?
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
So by the chart and if my battery health is 100% and SOC starts at 100%, I could put a 255 amp (its a 255amp/hour) load on it and once the loaded volts reading reachs 11.83 it is considered 50% DOD?
That would be my understanding of the table.

is there a maximum amp draw these battery's should be limited too? I can't seem to find anything on that. I turned on my 1000watt inverter and ran my shop vac and heat gun at about 156 amps (reading on the SC200) The battery dropped to about 11.8 volts and stayed there for about 20 minutes which when shut off for about 5 minutes the volts would rise up to 12.4 and then slowly rise from there. Would this be ok to do as long as the resting volts did not drop below 12.3'ish?
That seems to be consistent with the table. 156 amps for 20 minutes is about 50 ah or about 20% DOD. At 20% DOD the battery should read 12.54v but it may take several hours to reach that level.

Would it be OK to do this? Yes, with a caveat as I understand AGM batteries. The caveat is the time the battery remained at less than a 100% SOC. I really don't have an idea of what that time limit would be, however, the shorter the time the better.
 
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