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electrical:depth_of_discharge

Depth of discharge / State of Charge

Depth-of-discharge (DoD or DOD) refers to how low a deep cycle battery is taken between full charges. DoD is the inverse of State of Charge (SoC). Example: a battery at 30% DoD is at 70% SoC.

For solar powered systems the greatest DoD (and therefore lowest SoC) will be in early morning just before the panels start creating power again.

DoD has a significant impact on longevity of deep cycle batteries. For this reason Inverters and other high-load devices may have a low voltage cutoff.

effect of SoC on battery life

How deeply one regularly discharges lead-chemistry batteries will have a direct effect on how long the battery bank will last.1)

The most common discharge limit for deep cycle batteries is 50% DoD. This gives a good balance between usability and longevity. The lowest cost per Ah occurs around 30% DoD although this requires buying, installing, and moving dead lead or unusable battery capacity.2)

Based on the following data on the Trojan T-105:

  • lowest cost per Ah happens at 30% DoD
  • longest life happens at 20% DoD
  • least battery weight happens at 80% DoD

so make your DoD decision based on what is most important to you.

T105 Ah Cost weight per set Target Ah
225 $260.00 124 175
DoD State of Charge power per cycle Num. of cycles lifetime power in Kah3) levelled cost / Kah life in years Sets needed for target Ah Weight
10 90 22.5
20 80 45 3000 135 $1.93 8.2 3.9 482
30 70 67.5 2250 151.875 $1.71 6.2 2.6 321
40 60 90 1450 130.5 $1.99 4.0 1.9 241
50 50 112.5 1200 135 $1.93 3.3 1.6 193
60 40 135 1050 141.75 $1.83 2.9 1.3 161
70 30 157.5 900 141.75 $1.83 2.5 1.1 138
80 20 180 800 144 $1.81 2.2 1.0 121

One can choose to run the batteries quite hard in emergency or temporary conditions with the understanding that it will likely “hurt” the batteries to some degree. Consistently going past 50% DoD will greatly reduce the battery's usable cycles. Some studies suggest discharging to 80% yields 1/10th the number of cycles available at 20%.

estimatating SoC

SoC by specific gravity

“The truest measure of a [lead chemistry] battery's state of charge is the specific gravity of the battery acid.” – Rolls battery.4)

Only flooded-type batteries are practical for SoC assessment by SG of the electrolyte. Needed for this operation:

SoC by amps

A battery monitor, usually a amp/coulumb counter with a shunt, will measure the current going to/from the battery. It will often show real-time current flow and a state of charge percentage. It may also show voltage (see below).

Note that deep cycle battery capacities are given at the “20 hour rate”, roughly depleting the battery over the course of a day/night cycle. Heavier loads will cause apparent capacity to decrease. Lithium is much less susceptible and tends to deliver more predictable capacity.

SoC by voltage

100% SoC (~12.7v) is measured after the bank has been fully charged and then rested. It is measured “on the way down”; measuring “on the way up” while charging will lead to false confidence and leaving the battery less than fully charged.

The voltage level associated with 50% DoD is widely discussed. 12.1v rested is generally used as 50% State of Charge (SoC). A more conservative approach uses 12.2vdc rested.

Since resting is rare in most practical scenarios stopping at 12.2vdc under light loads would be a practical approach. DC expert SternWake5) says: “…those who are loading their battery and stop at 12.2v are treating their battery better, just not using all the capacity they could and perhaps seriously inconveniencing themselves by thinking they need to stop at this point, especially if the loads are fairly large, like while running a laptop and watching tv and while their fridge compressor is running.”6)

By this yardstick both of these uses should keep DoD from going beyond 50%:

  • constant light loads with measured >=12.2v
  • intermittent heavier loads that leave the system with measured >=12.2v when that load is removed

The more challenging task is judging when to kill the circuit based on voltage under heavier loads. It may take experimentation with your system to see where the battery voltage rebounds after removing the heavy loads. An approach might be:

  1. apply expected load
  2. run battery down to 11.5v (then 11.25, 11, 10.75, 10.5 etc until battery no longer rebounds to 12.1-12.2v)
  3. remove load
  4. observe battery voltage

If the battery rebounds to the desired voltage then repeat to deeper discharge. Stop when the battery no longer can rebound to the setpoint. The LVD voltage is the lowest voltage the system can drop to and still rebound to the desired setpoint when the load is removed.

partial state of charge

This section has moved.

electrical/depth_of_discharge.txt · Last modified: 2020/08/02 09:55 by frater_secessus