Days of Autonomy

• 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 capacity == daily power requirements has one day of autonomy; 24 hours after a full charge there will be no usable capacity left.
• Depending on charging sources we might think in whole or fractional days

about these summaries

• 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

These charging sources lend themselves to planning in whole days because they are either ON or OFF, available or unavailable:

• shore power charging with a converter or other charger
• no field charging at all (charging at home only, no charging over weekend outings)

Joe Campervan requires 1,000Wh (1kWh) per day. He will fully charge his bank at home before leaving on a 2-day campout.

• his total requirements are 2,000Wh (2 days x 1,000Wh).
• the bank capacity required depends on the usable capacity percentage of his chosen battery chemistry, ~50% for lead and ~80% for lithium
  • lead: 2,000Wh / 0.5 = 4,000Wh of capacity required. At 12v nominal this would mean a ~333Ah bank.
  • 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 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.

Solar and alternator charging are different.

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 DC-DC charger to charge his Li bank. The DC-DC will produce ~400Wh of charging daily¹⁾. 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.

In any given 24hr period solar will make at least //some// power no matter the weather conditions or actions of the owner.

• on average, the setup will make the power your wattage and insolation models predict
• on clear, sunny days you will exceed the average²⁾
• on overcast days you might get 1/3rd your average harvest

Solar harvest is highly variable based on time of year and geographical location.

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

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.

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 conditions³⁾. 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.

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.

A 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.

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 alternator⁴⁾, 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.

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.

• 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 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 charge from the power pedestal.
• assuming you have space, max out the roof with as much used 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 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.