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electrical:depth_of_discharge [2019/02/07 13:13] frater_secessus |
electrical:depth_of_discharge [2023/08/17 21:36] frater_secessus [effect of DoD on lead battery life] |
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- | ====== Depth of discharge / State of Charge ====== | + | ===== Depth of discharge / State of Charge |
+ | |||
+ | //Depth-of-discharge// (DoD or DOD) refers to how low a deep cycle battery is taken between [[electrical: | ||
+ | |||
+ | DoD is the inverse of //State of Charge | ||
+ | |||
+ | Note: This information is primarily relevant to lead-chemistry batteries. | ||
+ | |||
+ | |||
+ | ===== knowing when SoC is 100% (fully charged) | ||
+ | |||
+ | |||
+ | With **lead batteries** we know the battery is 100% when | ||
+ | |||
+ | * we are holding Absorption voltage; and | ||
+ | * current acceptance has decreased to 0.02C, or about 2% of the rated capacity | ||
+ | |||
+ | So for a 225Ah Trojan T-105 that might be when current acceptance drops to **4.5A at 14.8v**. | ||
+ | |||
+ | Mythbusting: | ||
+ | |||
+ | |||
+ | With **lithium batteries** humans might use use amp-counting with [[electrical: | ||
+ | |||
+ | * at moderate charging rates like 0.2C((20A for a 100Ah LFP)) SoC will be ~100% when voltage rises to 14.0v | ||
+ | * at moderate charging rates SoC will be ~100% when voltage rises to 13.8v and we add perhaps 30 minutes of Absorption duration. | ||
+ | * at moderate charging rates and voltages between ≥13.4v and <13.8v SoC will be ~100% after some amount of Absorption. | ||
+ | |||
+ | ==== soft and firm charging ==== | ||
+ | |||
+ | |||
+ | Solar is typically a moderate (or " | ||
+ | |||
+ | So while we can say with confidence that a 100Ah Li battery charged at 20A to 14.0v will be ~100% SoC, the same battery charged to 14.0v at 80A might only be at 75% SoC. And it **could get damagingly overcharged** if charged to 14.0v very gently at something like 5A.((the BMS cannot detect this scenario)) | ||
+ | |||
+ | The amp counter will probably help here during charging although even it can be thrown off; see the battery monitor article for more on this. | ||
+ | |||
+ | |||
+ | ===== estimating SoC while resting ===== | ||
+ | |||
+ | {{ http:// | ||
+ | A rested (no load), fully charged, unFloated lead battery will be 100% around 12.7v-12.8v; | ||
+ | |||
+ | The famous chart to the right is used to estimate SoC of a rested battery after a full charge. | ||
+ | |||
+ | A rested (no load), fully charged, unFloated lithium battery will be 100% around 13.5-13.6v. | ||
+ | |||
+ | As we will see below SoC-by-voltage will appear to be **artificially high during charging**((voltage rise)) and **artificially low during discharging**((voltage sag)). | ||
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+ | ==== SoC by specific gravity ==== | ||
+ | |||
+ | > "The truest measure of a [lead chemistry] battery' | ||
+ | |||
+ | Only flooded-type batteries are practical for SoC assessment by SG of the electrolyte. | ||
+ | |||
+ | * temperature of the battery | ||
+ | * SG chart from the battery manufacuturer | ||
+ | * [[https:// | ||
+ | |||
+ | |||
+ | |||
+ | ==== SoC by amps ==== | ||
+ | |||
+ | A [[electrical: | ||
+ | |||
+ | Note that deep cycle battery [[electrical: | ||
+ | |||
+ | |||
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+ | ==== SoC by voltage ==== | ||
+ | |||
+ | Note: read [[https:// | ||
+ | |||
+ | 100% SoC (~12.7v) is measured [[electrical: | ||
+ | |||
+ | The voltage level associated with 50% DoD is widely discussed. | ||
+ | |||
+ | Since resting is rare in most practical scenarios stopping at 12.2vdc under light loads would be a practical approach. | ||
+ | //" | ||
+ | |||
+ | By this yardstick both of these uses should keep DoD from going beyond 50%: | ||
+ | * constant light loads with measured > | ||
+ | * intermittent heavier loads that leave the system with measured >=12.2v when that load is removed | ||
+ | |||
+ | ===== estimating SoC while discharging ===== | ||
+ | |||
+ | The more challenging task is judging SoC (including for LVD purposes) under heavier loads. | ||
+ | |||
+ | Consider this chart for Trojan FLA batteries: | ||
+ | |||
+ | {{ https:// | ||
+ | |||
+ | For a 200A bank 50% DoD would be 12.1v at rest, ~12.0v at C/10 (20A discharge), ~11.55v at C/5 (40A discharge), and 11.2v at C/3 (~70A discharge). | ||
+ | |||
+ | It may take experimentation with your system to see where the battery voltage rebounds after removing the heavy loads. | ||
+ | |||
+ | - apply expected load | ||
+ | - 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) | ||
+ | - remove load | ||
+ | - observe battery voltage | ||
+ | |||
+ | If the battery rebounds to the desired voltage then repeat to deeper discharge. | ||
+ | |||
+ | |||
+ | **Lithium batteries** also exhibit voltage sag under load but typically much less than AGM or especially FLA. An [[electrical: | ||
+ | |||
+ | ===== estimating SoC while charging ===== | ||
+ | |||
+ | Similar to voltage sag during discharge, batteries exhibit voltage //surge// or //rise// during charging. | ||
+ | |||
+ | |||
+ | ===== partial state of charge ===== | ||
+ | This section [[electrical: | ||
+ | |||
+ | ===== effect of DoD on lead battery life ===== | ||
+ | |||
+ | DoD has a **significant impact on longevity of lead deep cycle batteries**.((and, | ||
+ | |||
+ | |||
+ | The **most common discharge limit for deep cycle batteries is 50% DoD**. | ||
- | {{ http:// | ||
- | 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**. | ||
- | The **most common discharge limit for deep cycle batteries is 50% DoD**. | ||
Based on the following data on the Trojan T-105: | Based on the following data on the Trojan T-105: | ||
* lowest cost per Ah happens at 30% DoD | * lowest cost per Ah happens at 30% DoD | ||
* longest life happens at 20% DoD | * longest life happens at 20% DoD | ||
- | * least battery weight happens at 80% | + | * least battery weight happens at 80% DoD |
so make your DoD decision based on what is most important to you. | so make your DoD decision based on what is most important to you. | ||
+ | |||
| | **T105 Ah**| | **Cost**| | | | **T105 Ah**| | **Cost**| | ||
| | 225| | $260.00| | | | 225| | $260.00| | ||
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| | | | | | | | | | | | | | | | | | | | | | ||
- | | **DoD**| | + | | **DoD**| |
| 10| 90| 22.5| | | | | | | | | 10| 90| 22.5| | | | | | | | ||
| 20| 80| 45| 3000| 135| $1.93| | | 20| 80| 45| 3000| 135| $1.93| | ||
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- | ===== SoC by amps ===== | + | ===== lithium |
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- | A [[electrical: | + | |
- | + | ||
- | + | ||
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- | + | ||
- | ===== SoC by voltage ===== | + | |
- | + | ||
- | + | ||
- | The voltage level associated with 50% DoD is widely discussed. | + | |
- | Since resting is rare in most practical scenarios stopping at 12.2vdc under light loads would be a practical approach. | + | |
- | //" | + | |
- | By this yardstick both of these uses should keep DoD from going beyond 50%: | + | |
- | * constant light loads with measured > | + | |
- | * intermittent heavier loads that leave the system with measured >=12.2v when that load is removed | + | |
- | ===== partial state of charge ===== | + | |
- | This section [[electrical: | + | |
+ | Lithium chemistries have very flat voltage curves, making it notoriously difficult to gauge SoC by voltage. |