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electrical:solar:charge_controller_setpoints

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Solar charge controller setpoints

Solar charge controller, converter, and DC-DC charging behavior is guided by setpoints, or configured values stored internally. They may or may not be configurable by the user.

charging setpoints

Charging setpoints are configured in accordance with the battery manufacturer's recommendations. These recommendations are published in manuals, spec sheets, corporate blogs, etc. For example, Trojan's recommendations for charging their T-105 AGM batteries looks like this:

  • Absorption voltage (Vabs) - the voltage the battery bank is held at during Absorption stage. In this case the recommendation is 14.4v.
  • Float voltage (Vfloat) - the voltage the battery bank is held at during Float. In this case the recommendation is 13.5v.
  • Temperature compensation - how the voltage setpoints are adjusted for temperature. While this is sometimes configurable, generally the compensation factor is close enough between manufacturers that it can be left at the default.
  • endAmps - the amount of current (or percentage of capacity) accepted by the battery in late Absorption that indicates it's appropriate to Float. In the lower-right of this graph Trojan shows it as C/200:1)

Simpler charge controllers may only have preconfigured presets like “FLOODED” or “AGM” and don't let you set exact setpoints. If this is the case pick the closest preset to the recommendations2) and understand you will likely not get full performance or lifetime from the battery bank. Shunt controllers typically have only one configurable voltage setpoint. The simplest controllers may have no battery settings at all, and may or may not publish what those mystery settings are.

Less common setpoints:

  • Absorption duration - the amount of time Vabs will be held until dropping to Vfloat. May be used instead of or in addition to any endAmps above.
  • Absorption restart - voltage (or duration) at which Absorption stage after failing to maintain Vfloat

load setpoints

Only present on controllers with LOAD outputs.

  • Low voltage disconnect (Vlvd) - voltage at which the controller turns off loads to prevent excessive battery discharge. Typically something like 12.1v (the voltage at 50% depth of discharge). Heavier loads that cause voltage sag may require setting a lower Vlvd so the voltage pops back up to 12.1v3) when the load is removed.
  • Low voltage reconnect (Vlvr) - the setpoint at which the load is turned back on after recovering from Vlvd cutoff. Vlvr > Vlvd. If Vlvr is set too close to Vlvd the loads may continually toggle on/off.

non-setpoint settings

  • Maximum charge rate: While maximum charge rates normally are immaterial to solar charging4), Trojan specfies a maximum of C/5, or 1/5th of the battery's capacity in Ah.
  • Minimum charge rate: not stated in our example, but typical minimum charging values are ~C/10 for lead chemistries.
1)
actually C/20/200, but C/200 here for simplicity
2)
using Price Is Right rules: closest without going over
3)
or whatever
4)
lead batteries accept what they want rather than taking what they are given
electrical/solar/charge_controller_setpoints.1578848128.txt.gz · Last modified: 2020/10/11 19:48 (external edit)