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opportunity loads

based on Solar power installations are generally designed as if the load[s] will be steady around the clock. This can lead new folk into buying too much battery.

In practice, actual loads may be heavier in the daytime (as when working with power tools) or at night (as when watching movies or running a forced-air furnace).

If loads are applied attentively (or automatically!) one can run electrical loads that do not affect the battery bank's State of Charge at all. Additionally, consuming loads at the right time can also increase the health and longevity of lead batteries.1)

how much surplus current is there?

It is common for Lead-Acid batteries to consume C/100 or C/200 amps to Float, meaning that during Float you will have at least 99% your panels' output2) available for loads.3)

During Absorption the charge current is tapering down as the battery accepts less; the more it tapers down the more is available for loads.4) At the very beginning of Absorption ~0% of “bonus” output is available. At the very end of Absorption 99%+ is available. Mid-way through Absorption5) ~49% of the panels' output is available for loads. See the image at the top of the page for an example of how current demand drops in Absorption.

There will generally be more “surplus” power available when using MPPT charge controllers v. PWM,6)7) though the difference is less dramatic during periods one would run opportunity loads (see below). Siphoning off power can actually help PWM controllers run cooler by reducing the ON-OFF switching activity that generates heat.

Overpaneled systems have even more excess power available.

when to run opportunity loads

The best time to run non-essential loads is when there is surplus current over what battery charging needs. This means during Float, or after current has started dropping off in Absorption. For LiFePO4 there is excess power any time after the bank has reached the intended charge state.

Opportunity loads should not be applied during Bulk stage since Bulk, by definition, means the battery is taking all the available current.8)

timing opportunity loads

You can start the loads manually (ie, start using the power when you have extra). This is error-prone but is free and requires no equipment (except your own memory).

The most precise way to do it automatically is to use a charge controller which turns on the LOAD output only when the batteries are in Float stage. Charge controllers with this feature tend to be expensive.

LVD only

A free way to automate opportunity loading is to set the normal controller's LOAD or external Low Voltage Disconnect to shut off below Vfloat.

Since the LVD only knows voltage and Vabs > Vfloat this approach will

  • correctly run the opportunity load in Float; we want this.
  • inadvertently run the opportunity load in the latter part of Bulk stage. This scenario occurs when voltage is >= Vfloat but < Vabs; we probably don't want this. It's better than running the loads wide open any time the system is above 50% DoD but if a given system struggles to reach Absorption mode further tweaking is possible.

To run loads heavier than the rating of the LVD or LOAD output use a relay between the LVD and the load.

This setup looks like: [LVD or controller LOAD output] –> relay –> load

LVD + timer

A 12v timer could give the system a chance to make some progress in Absorption before starting up opportunity loads.

In this approach the LVD is set just below Vfloat as above, but activation of the loads is delayed by some amount of time. Observation of the system during charging will suggest how long it normally takes takes charging amperage to drop off (ie, when the system has surplus current).

Suggested delay for conservative opportunity loading == the time from [passing Vlvr setpoint during Bulk] to [completed charging].9). This delay might be 2-3hours.

Suggested delay for aggressive opportunity loading == the time from [passing Vlvr setpoint during Bulk] to [far enough into Absorption that enough current is available to power intended loads]. This delay might be 30-60 minutes, depending on the charging rate.

This setup looks like: [LVD or controller LOAD output] –> timer –> relay –> load

uses for opportunity loads

1) by keeping them at 100% SoC more of the time
2) under present conditions
3) normal + opportunity
4) agaom. normal + opportunity
5) judging by current not time
8) assuming no charging limit has been configured in the charge controller
9) settling to Float
electrical/solar/nonessential.txt · Last modified: 2020/01/13 08:38 by frater_secessus