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--- electrical:autonomy [2024/07/16 12:08]
frater_secessus [TL;DR]
+++ electrical:autonomy [2024/08/23 21:12] (current)
frater_secessus [minimizing the required days of autonomy]
@@ Line 1: / Line 1: @@
+[draft]
 ====== Days of Autonomy ======
 ===== TL;DR =====
-  * Days of autonomy (or reserve) is the number of days of power one stores for periods of reduced or non-existent charging.
+  * Days of autonomy (or reserve) is the number of days of power one stores for days when zero charging is available.  It is planning for the Worst Case Scenario.
   * A system whose [[electrical:12v:battery_capacity|capacity]] == [[electrical:12v:dailypowerrequirements|daily power requirements]] has one day of autonomy;  24 hours after a full charge there will be no usable capacity left.
   * Depending on [[electrical:12v:power_mix|charging sources]] we might think in whole or fractional days
 [[opinion:frater_secessus:pareto|about these summaries]]
-  * the decision on how much capacity to purchase is economic as well as technical
+  * the decision on how much capacity to purchase is economic as well as technical.
+  * it is possible to reduce the days of autonomy required by reducing consumption on days with less or no charging
@@ Line 28: / Line 32: @@
       * lithium:  2,000Wh / 0.8 = **2,500Wh** of capacity required.  At 12.8v nominal this would mean a ~**195Ah** bank.
+He needs 2 days of autonomy to make it through the weekend before the bank is effectively empty.
-If he is on **shore power** at an RV park for one of those days then (from a power perspective) his off-grid is only 1 day, so his total requirements and required capacity is halved. This is true because shore power is effectively limitless;  you are not offgrid.
+If he is on **shore power** at an RV park for one of those days then (from a power perspective) his time off-grid is only 1 day, so his total requirements and required capacity is halved. This is true because shore power is effectively limitless;  you are not offgrid.
@@ Line 41: / Line 46: @@
 ==== alternator ====
-**Alternator charging** is a bit more arithmetically complex because it involves both **time the engine is running** and the bank's **current acceptance in Amps** from the alternator.  Let's assume Joe is driving 1 hour per day and is running a a 30A [[electrical:12v:b2b|DC-DC charger]] to charge his Li bank.  The DC-DC will produce ~400Wh of charging daily((assuming an average bank voltage of 13.5v //while charging//)).  So now the bank only has to cover 1,200Wh on the outing rather than the whole 2,000Wh. (2,000Wh - (2 days x 400Wh charging)).  So now he only needs ~**117Ah** of Li (1,200Wh / 0.8 usable / 12.8v nominal).
+**Alternator charging** is a bit more arithmetically complex because it involves both **time the engine is running** and the bank's **current acceptance in Amps** from the alternator.
+=== example ===
-==== solar ====
+Let's assume Joe is driving 1 hour per day and is running a a 30A [[electrical:12v:b2b|DC-DC charger]] to charge his Li bank.  The DC-DC will produce ~400Wh of charging daily((assuming an average bank voltage of 13.5v //while charging//)).  So now the bank only has to cover 1,200Wh on the outing rather than the whole 2,000Wh. (2,000Wh - (2 days x 400Wh charging)).  So now he only needs ~**117Ah** of Li (1,200Wh / 0.8 usable / 12.8v nominal).
+The fractional way to think about this is, after alternator contribution, he only needs 1.6 days of reserve rather than 2.0 to get through the weekend.
+==== solar ====
+In any given 24hr period [[opinion:frater_secessus:beginner_mistakes#believing_solar_makes_zero_watts_in_rain_overcast_etc|solar will make at least //some// power]] no matter the weather conditions or actions of the owner.
-;  in any given 24hr period [[opinion:frater_secessus:beginner_mistakes#believing_solar_makes_zero_watts_in_rain_overcast_etc|the setup will make at least //some// power]] no matter the conditions or actions of the owner.
   * on average, the setup will make the power your wattage and [[electrical:solar:pvwatts|insolation models]] predict
@@ Line 59: / Line 65: @@
 Solar harvest is [[electrical:solar:output|highly variable]] based on time of year and geographical location.
-==== example ====
+==== examples ====
+There are a couple of approaches here:
+  * solar as a "best effort" supplement or crutch - easy and relatively inexpensive
+  * solar as the main or sole charging source for long durations - complex, space-intensive, and relatively expensive
+=== best effort solar ===
+Mount whatever you choose.  100w?  200w?  Sure, it's all gravy anyhow.  If the solar averages 400Wh/day then the math is the same as the alternator example above.
+=== reliable solar ===
+This scenario is completely different;   your solar implementation isn't a matter of choice but rather is driven by insolation models.  It requires **paneling for the worst average monthly conditions**((December for fulltimers in the northern hemisphere)).  The practical result is the setup will meet needs in the worst month and have a great deal of excess power in the best month. It also means that, //theoretically//, Joe only needs one day of autonomy because the panels will replace the Watt-hours every day.
+===== economic factors =====
+The decision between larger bank vs smaller bank + field charging is up to Joe.  The frequency and duration of his outings will affect the cost/benefit analysis.
+If his requirements are modest and outings are short and infrequent he might buy enough Wh to make it through his campouts with zero charging.
+If his requirements are heavier or outings longer, more frequent (or even full-time) then it is no longer practical to buy enough Wh to cover the outing;  he will need some amount of field charging.  If he is overlanding/roadtripping he might lean into alternator charging.  If he will be camping in place he might invest more in solar.
+In general, larger banks are cheaper by the Amp-hour.
+===== factors related to C-rates =====
+A [[electrical:12v:battery_capacity|C-rate]] is a fraction of the charge or discharge current bank capacity divided by the bank capacity. A 10A load on a 200Ah bank is 0.05C (10A/200Ah).
+Different battery chemistries have C-rates they can //tolerate//, and C-rates they //prefer// for performance and longevity.  We will use LiFePO4 for following examples.
+==== charging ====
+LFP prefers 0.2C charging (20A per 100Ah of capacity) and will tolerate 0.5C (50A per 100Ah).  This doesn't usually come into play with solar since it's such a "soft" charging source, but it can be critical with alternator charging since the possible currents are higher and the charging duration short.
+Having a larger bank could allow you to charge harder from the alternator((assuming the alternator can handle it)), Amp-wise, and still stay within the 0.5C  limit.   Using the alternator scenario above, we are now running a 60A DC-DC instead of the 30A unit.
+==== discharging ====
+If your LFP has a 0.5C constant discharge rating then doubling the Ah capacity doubles the load current you can apply to the bank.
+===== minimizing the required days of autonomy =====
+  * reduce optional power consumption during periods of low solar harvest.  For example, one's //average// consumption might be >2kWh/day but one might only //need// 1kWh/day when power is tight.
+  * think in fractional days of autonomy (see above)
+  * If your schedule is flexible you might adjust your plans for days where it is forecast to be rainy/overcast/whatever:
+    * plan your relocation drives for those days so you can take advantage of [[electrical:12v:alternator|charging by alternator]]
+    * if you schedule a day in a paid campsite from time to time to dump tanks and take on water, do so on projected days of poor solar harvest.  This way you can also [[electrical:converter|charge from the power pedestal]].
+  * assuming you have space, max out the roof with as much used [[electrical:solar:panels#panel_voltage|higher-voltage panel]] as will fit.  600w of used can cost the same as 200w of new, and power harvest would be tripled in poor conditions.
+  * a small portable array on its own controller can punch far above its weight:  sit it out in the sun angled correctly when you are hiding in shade or in winter when sun is low in the sky
+  * it's a bit extreme, but a [[electrical:12v:alternator_details#external_regulation|second alternator setup with external regulation]] might be cheaper than a a huge bank of premium LiFePO4.  Setups like this can yield maximal alternator charging without risking overheating.

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