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Old 12-14-2019, 11:17 PM   #1
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Help me understand this chart

Hey everyone, i wanted to pick the collective brain of the hive to see if i can better understand this discharge rate chart.
I currently have two Lifeline batteries, GPL-4CT, and im trying to understand how low i can safely discharge them.
Everywhere i read that you shoukd not drop below 50%, but how do u determine that?
Is there a voltage that can correlate to discharge percentage?
I remember i read somewhere that it should not fall below 11.8vdc. is that accurate?
Thanks everyone.
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Old 12-15-2019, 04:48 AM   #2
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Find a different table using resting battery voltage or just consider the smallest discharge from that table. Then, there is a temperature effect, also. In a simple sense just use 12.2 for 50% for a standard lead acid battery that has been resting for 30 minutes. Without a battery monitor, you won't know the discharge rate.
Larry
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Old 12-15-2019, 08:12 AM   #3
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Hi

Unless you measure right at the battery terminals, current in the wiring will mess up what you read. Since you are looking for tenths of a volt, it does not take much to throw things off. Toss in the magic voltages moving a half volt as temperature goes up or down and it is even more complicated.

You can get a shunt based meter that will be far more accurate than a voltage reading. They aren't cheap, but neither were those batteries .....

==============

The chart you show is for the battery terminal voltage at various discharge rates. If you *know* the rate, you can then figure the capacity left from the terminal voltage. In an RV that generally is not of much use.

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Old 12-15-2019, 08:37 AM   #4
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I use the 12/12 rule for all lead acid and AGM batteries. Meaning never discharge below 12.12 volts at the post with batteries at rest. Under a load they will drop below that number but will “recover” a little when the load is removed.

Easy to remember and fairly safe number to use.

Statement: The 12/12 rule was developed by me so there is very little science involved, just my experience...
Use as a general rule of thumb.
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Old 12-15-2019, 10:18 AM   #5
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Flooded Cell Lead Acid batteries can be safely and routinely taken down to 50% degree of discharge without compromising battery life. Your battery is an Absorbent Glass Mat (AGM). They can be discharged to a higher degree (60-80%) and achieve the same cycle life as a flooded cell battery. However they can last much longer if only infrequently or never discharged below 50%. So it's your call on how far to take them down, but below 80% should be avoided.

Okay you've decided to stick with 50% DOD routinely, great! Good call. The chart you have provides the (approximate) answer for cell temperatures of 77F. Estimate the discharge rate. Your batteries are rated for about 220 AH (depends on several factors but lets use it). If you are drawing 220 amps you're on the one hour rate, 27.5 amps corresponds to the 8 hour rate, 11 amps corresponds to the 20 hour rate and so on. Then the 50% DOD voltage is read from the chart but it is only accurate for 77F. You will need more charts for lower temperatures. However all this is a bit academic as Bob indicated because it is difficult to have accurate voltage, current and temperature readings.

So your 11.8 V fixed go by voltage is an okay start. It's good for rapid 1-2 hour discharges at 77F but excessive for slow discharges. It also happens to be a good go by for slow discharges at freezing temperatures.

You didn't ask, but as important to battery life as avoiding deep discharges below 50% (or so) is to routinely fully charge the batteries and also avoid prolong overcharging. Now fully charging a battery requires adjustment of the charge voltage based on the cell temperature. Higher charge voltages are required when the battery is cold in order to obtain a full charge, so chargers with temperature sensors and compensation for temperatures are worth a look, especially when you have expensive batteries.
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Old 12-15-2019, 10:20 AM   #6
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What this chart is showing you is that under different loads the battery voltage corresponding to x% differs. Lead batteries exhibit a distinct “sag” under load that increases with the load applied. When you then let the batteries rest again, the sag will snap back. If you load lightly, less sag. Load induced sag is not necessarily indicative of actual state of charge. This is what makes assess battery state by voltage alone challenging.
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Old 12-15-2019, 11:02 AM   #7
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Their 'given' of 77°F battery core temperature is important!

Anyhow -my point is be aware of temperature compensation both in charging and discharging. Our trusty hide-away secure battery lockers often lets battery overheat in summer and stay chilly in winter! Jump into the trailer and fire up the propane furnace YET have the battery sitting at 35-40°F so 25% of capacity is missing!

Some temperature variables to be aware of are discussed here: Victron Blog

and a graph from that article...
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Old 12-15-2019, 12:52 PM   #8
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Learn to understand amp-hours , and install metering that can tell you what your consuming / charging and accumulated of both !!!!
Do not forget to look up what your battery capacity is & what you can & cannot do .
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Old 12-15-2019, 05:02 PM   #9
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Hmnn - seems formatting was lost on my link, my apologies

https://www.victronenergy.com/blog/2...-cold-weather/
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