In order to prioritize the use of solar power it would be useful to know when your batteries are fully charged.
This would be useful for very low priority loads.
Is there a standard way that this is done?
Hi Richard, in short, no there is no set standard. It all depends on your inverter, charge controller and battery combination. When your inverter can communicate with a BMS, you rely on the BMS to report a full condition, when using lead acid or similar batteries, you can rely on voltage to indicate a full condition, if you want a more reliable method, it is best to install something like a Victron Battery Monitor. It physically measure the power flowing in both directions and calculate the remaining power of the battery, it also looks at tail current as well as voltage and time to decide when to report a 100% full condition.
If you don’t have the luxury of a BMS I would strongly recommend something like the BMV .
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In most cases you can rely on voltage. Whether it is lead acid, or Lithium, if the voltage pushes up, the battery is full, or close to full.
In the case of lead acid, it is even better. The battery reaches the charge voltage around 85%. At this point, you already want to start running those small loads, rather than have the MPPT throttle the power away as the battery starts accepting less current.
Where you cannot rely on voltage is on the discharge side. LiFePO4 is too flat, and Lead Acid similarly don’t have a clear correlation.