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.

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

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    \$\begingroup\$ what do you mean by "pulling load"? \$\endgroup\$ – Marko Buršič Mar 27 at 12:31
  • \$\begingroup\$ You can't charge a battery while taking a load from it. Current either goes in or out. \$\endgroup\$ – Finbarr Mar 27 at 12:48
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    \$\begingroup\$ I don't think the question is suggesting that it's possible to charge a battery while discharging it. The question asks: If I have a load connected to a battery while charging the battery, the load draws some of the current that's output by the charge controller. Does that interfere with charging the battery? \$\endgroup\$ – Kevin Kruse Mar 27 at 13:06
  • \$\begingroup\$ Kevin, you have a correct understanding of my question. By ‘pull load’ I mean that my compressor (by way of its inverter) is consuming current. It’s getting this current from the terminals on the battery. These are the same terminals that the charge controller uses to charge the battery. Sorry, I’m a software guy so my word choice may be inaccurate \$\endgroup\$ – anschoewe Mar 27 at 16:59
  • \$\begingroup\$ why exactly aren’t you holding it at a safe float of 12.8V ? \$\endgroup\$ – user2497 Mar 28 at 5:51

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

  1. 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.

  2. 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.


It is very common to have a charging source, battery, and load connected in parallel so that it may look like you are charging and discharging a battery simultaneously.

What actually happens is that if the charging source can supply more current than the load demands, the excess current will go to charge the battery.

If the load demands more current than the charging source can supply, the extra current will be supplied by the battery (discharging the battery).

The battery will change between charging and discharging automagically as the load demand and charging current vary.

  • \$\begingroup\$ From the way you describe it, it sounds like there is a decent chance the battery would not get all the current it would normally get during the absorption phase if there were no load -especially if it became cloudy. Given all of the responses it seems like I should just run the compressor at night at then stop it around 7am -before the absorption phase begins. \$\endgroup\$ – anschoewe Mar 28 at 3:15

The answer depends on your solar setup, charger, and inverter.

To charge the battery during a load condition the charger must supply enough current to satisfy both the load and the current demanded by the battery. If your solar setup can't do that then you must inhibit the load from coming on while the battery is charging.

Second, the inverter must be capable of accepting the higher voltage. Most likely it is fine but it does not hurt to check.

  1. If the charger is able to reach 14.8V with load connected then this means that there is an excess of solar energy. In such case the battery will be filled up to 100% while supplying the load.

  2. If the higher voltage can't be reached it means that there is no excess of solar energy and the battery is being charged and drained simultaneously.

As I understand your question: would it be better to switch of the load while charging? In case 1 certainly not, in case 2 you won't gain any benefit. If you switch the fridge off, then you just postpone the load demand, you will fill the battery and then drain immediately after you switch on the load. The energy will be wasted, because the efficiency charge/drain is not 100%. In my opinion it better to supply the load while charging.

Looking at your equipment I have an opinion that the fridge is an important element of the system. Unfortunately I would say that your choice of using inverter + AC compressor is not the optimal one. If the budged is less important than overall efficiency, then you should go with 12/24V compressor with built in inverter. Such compressor has a integrated inverter that turns on only on a demand. See Danfoss 35F, 50F.

  • \$\begingroup\$ The use case here is pond aeration. So there’s no delaying the load. I will just run it fewer hours in the day. This is also my first electrical project so I probably made some mistakes. I was trying to find an air pump (compressor) that used DC current but could still supply enough air pressure to overcome the water pressure at 10 feet of depth. I couldn’t really find any. \$\endgroup\$ – anschoewe Mar 28 at 11:32

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