I have an off-grid solar panel setup. The solar panels send current to an MPPT charge controller. The charge controller controls current to a lead acid battery. On these same battery terminals I have wires leading to my load, an inverter and then AC compressor/motor.
I’m most concerned about the absorption charge phase, when it dumps most of the current into the battery at an elevated voltage. My controller operates in the absorption phase for 3 hours. It then drops into the float stage sending current at a lower voltage.
Should I avoid pulling load from the battery during the absorption stage so that the battery receives all of the current during this crucial phase? I worry if the load is also pulling current during this time that the battery will enter the float stage not having fully recharged.
My goal is to maximize long term battery capacity, through shallow discharges and daily (full) recharges.
# Update
Some people raised some very good questions/comments. I'll add some more details here to give a fuller picture. And yes, because this is an off-grid solar setup, the variability of the weather/elemeents make it hard to say with certainty that there will be enough sunlight to complete the absorption phase uninterrupted.
- Here's a
diagram
of my setup. - I have 2 100W solar panels (200W total) wired in series.
- I have 2 55Ah AGM batteries wired in parallel.
- I use this charge controller
- I use this inverter
- I use this compressor for pond aeration in 10ft of water.
On sunny days I’ve measure the voltage from the combined solar panels totaling 45V. The absorption phase charges the battery at 14.8V. The float phase is at 13.7V.
From my calculations I can only run the compressor for 4-5 hours before depleting the batteries to 70% of their capacity, which is the lower limit I’d like to sustain. I was seeing if I could maybe cheat and run it a little longer once the sun came up. From some of the answers it seems like that may put the battery at risk of not fully being recharged daily. I’m not sure it’s worth the risk
Best Answer
This is a very specific case of the more usual "can I charge and operate a load simultaneously"
In this case the answer is clear: If you value correct absorption phase charging then
The solar system must be able to supply at least enough current at the desired voltage to allow this. You need to be able to specify what this requirement is.
An additional load can only be operated if the PV output is able to provide it without preventing 1.
The above is "almost obvious" but introduces some extra questions.
If the available PV energy falls below that needed to perform absorption mode charging in the middle of an absorption cycle, what should you do? You can continue at a lower "float" rate, or stop charging until enough energy is available to continue absorption charging, or ... . If you stop and then recommence absorption charging are the periods simply additive, or is there a more complex algorithm involved? If so, does it matter where in the absorption cycle the pause(s) are or how many there are or how long between them? What fun!
If you can 'spare' surplus energy for another load, is it able to handle the variation from eg clouds, or complete cut off if the charging demands all the energy. eg a water heater would usually happily consume excess energy as available. A television (or its viewer) would not be so happy. An AC compressor is intermediate.
Unlikely - food for thought: You MAY be able to split the battery into two (or more) portions to allow largely uninterrupted boost charging in two stages - with the risk of having an unbalanced battery if the solar output fades sooner than expected on a given day.