If we have a look at the chart above, 12.32V falls within the 70% to 80% range for resting voltage which would lead us to think that the battery is at around 70% SoC and that we can still continue discharging. In that test, after resting the battery for 16 hrs, the resting open circuit voltage rebounded to 12.32V. In that test, an AGM chart like below was compared with the Actual Voltage under load.Īt the end of the test, we notice that the actual voltage under load is quite different from the resting voltage in the real world: To understand the difference between Resting voltage and Voltage-under-load, let’s look at the test that was done by Rod “RC” Collins and the inference. The chart below will give you an idea of state-of-charge for various battery conditions in flooded cell lead-acid batteries: In fact, the battery has three different voltages – Resting voltage, Charging voltage and Discharging voltage depending on whether it is sitting idle, being charged or discharged.īattery voltage will vary for the same state-of-charge depending on whether the battery is being charged or discharged and what the current flow is in relation to the size of the battery. The answer is a resounding No because the voltage in Table I is the Resting voltage and not the Voltage-under-load or Discharge voltage which is what the inverter would be seeing. So should we refer to Table 1 and set the LVD to 12.06V? Now the question begs, “How do we ensure that we don’t discharge below 50%?”įor this we need to set the Low Voltage Disconnect (LVD) parameter in the Inverter to a voltage value that indicates 50% DoD. If you need more information, you can refer to the scientific paper “ Internal resistance and temperature change during over-discharge of lead-acid battery” by Balázs Broda and Gyorgy Inzelt.
Basically, it will impair the battery’s ability to accept a charge. This has a serious influence on aging specifically on the loss of capacity because there is high temperature increase and gas flow at the same time. This process and the increasing gassing caused temperature to rise inside the electrode undergoing the recharge. In fact, studies have shown that the internal resistance highly increased during the cell recharge after an over-discharge. Internal resistance increases because of the decrease in the specific gravity, a depletion of the electrolyte as it becomes more diluted. So why does the internal resistance increase? Internal resistance is basically how hard it is to put electricity in and out through that electro-chemical reaction within the battery. Note that the Open Circuit Voltage (OCV) reduces as we keep on discharging and in the below chart, you can see that the internal resistance increases as we keep on discharging. The lead acid battery’s internal resistance becomes higher the deeper it is discharged as shown below. So why should we not discharge more than 50% for lead acids? This is because if the DoD is more than 50%, it would reduce the life of the battery. Therefore, 50% represents a good balance between capacity and cycle life, also taking into consideration the cost of replacement. Clearly, the red region past 80% should be avoided unless absolutely necessary, say in case of emergency.
The yellow region past 50% is not overly damaging to your batteries, but consistent cycling to this level will shorten their lifespan. This cycling zone is also illustrated in the below table: In the above chart, AGM battery’s performance in a moderate climate (average temperature of 25☌) has been depicted wherein if the DoD is 80%, the battery would provide only 500 cycles or just about 1.5 years of life (if cycled daily) whereas with 50%, you can get 1000 cycles or ~3 years of life. However, they can be discharged for up to 80% as per below chart: In this blog, I will provide reasons as to why this is so. So if you have a 100Ah battery, you can only use 50Ah. So it follows that the usable capacity of a lead acid battery is only 50% of the rated capacity. This means that we should cycle them in the 100% to 50% window as shown below in the Typical state of charge window parameter. “Lead acid batteries should be discharged only by 50% to increase its life” – is an oft used phrase.
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