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I have a 12V, 200Ah lead acid tubular battery which I charge with solar panels using a PWM or MPPT charge controller.

PV array specs are:

Max Power                840W
Voltage at Max power     23.70V
Current at Max power     35.46A
Open-Circuit Voltage     24.22V
Short-Circuit Current    41.90A

As we know, during the first bulk charging stage, chargers apply a constant high current to the battery.

To have a longer battery life, I want to charge the battery at maximum 20A (@10 hours).

If I use the PWM charge controller, it gives up to 35A current (@ 11V-15V) during peak hours. Controller's guidelines state:

The power of solar panels should match the battery capacity as much as possible, otherwise it will easily do abnormal charging. The best charging current for the battery is 10%-20% of the battery capacity.

So if I attach a 200W load (as shown in the diagram), which draws 15A or more, leaving 20A or less for the battery, can it still damage the battery? Or the battery cells will receive only 20A?

Or in other words, does the load (an inverter, to be more specific) draw the current (partially or fully) from the battery or directly from the controller?

If I use an MPPT charge controller, it gives up to 45A current at peak noon. There's no setting in the controller to set the maximum output current. So the question remains the same: if I attach a 300W load to the controller output, will it secure the battery cells from aggressive charging?

One more question related to the output voltage:

As we know that PWM charge controllers do not actually transform the voltage or current. Instead they only shorten or elongate the duty cycle - the time interval in which source is connected to the load/battery, and then disconnected for a short interval, thus sending a series of rapid on-off-on-off rhythm.

So in our case, even during the float stage, battery is actually getting pulses of ~22V (and 35A current during peak hours, when no load is attached) - in contrast to MPPT controller which actually does DC-DC conversion to lower the voltage to 14V-18V range (depending on the charging stage) and applies a continuous low current.

Is my understanding correct?

If yes, is it correct to say that MPPT controllers treat the battery more gently and politely as compared to PWM controllers (putting aside the efficiency and power output)?

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  • \$\begingroup\$ Lead-acid batteries are made specifically for starting internal combustion engines. That is, they must provide a very high current pulse for short periods. I would say that they are not suitable for loads that constantly absorb energy. More suitable, however, are AGM batteries which provide energy consistently for long periods of time. \$\endgroup\$ Commented Feb 28, 2024 at 12:37
  • \$\begingroup\$ @Franc you are correct. But I'm using a deep-cycle tubular battery, not flat plate, specifically designed for solar applications. \$\endgroup\$ Commented Feb 28, 2024 at 12:44
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    \$\begingroup\$ @Franc AGM batteries are lead acid, the acid is absorbed in a glass mat. Even flooded lead acid batteries can be designed for deep discharge and cycling applications. The main difference is plate thickness. Start batteries have lots of thin plate to provide lots of current quickly. Cycling batteries have thicker plates and will provide less current for longer periods of time. \$\endgroup\$ Commented Feb 28, 2024 at 13:00

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Some quality MPPT charge controllers e.g Victron Smartsolar range allow you to set a maximum charge current. This would be the best solution as it covers all use cases these controllers also have a separate port for the load.

If you use the setup shown in your diagram the charge current that your battery sees is the output from the solar array minus the load current by the simple application of Kirchoff's current law.

Pulse charging will not give the sharp edged waveform that you show the battery will tend towards averaging out the controller output, it will still pulse but not as drastically as you show. There are some people who think that this is beneficial to the battery because the pulses will help to reverse sulphation however my experience has been that it had very little benefit for my usage case.

In terms of treating the battery more gently there is very little benefit to a MPPT over a PWM charger. The big difference is the increase in efficiency that the MPPT gives you.

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  • \$\begingroup\$ Thank you for the answer. Your reference of Kirchoff's current law seems valid. But I asked this because an inverter directly connected to the charge controller's output won't work if a battery is not attached. So the battery has a role here; probably functions somewhat like a capacitor, a voltage reference, or a voltage stabilizer. That's why I've a notion that this behavior may put some strain on battery cells. Though I'm not theoretically strong enough to be sure about this. \$\endgroup\$ Commented Feb 28, 2024 at 15:08
  • \$\begingroup\$ The battery provides a stable power supply for the control circuitry. \$\endgroup\$ Commented Feb 28, 2024 at 15:10
  • \$\begingroup\$ Like this page states: "The Solar Charge Controller operates by regulating the flow of power from the solar modules to the batteries, charging them and finally sending the remaining power directly to the inverter. The charge controller is designed to use the batteries as reference voltage output, which is why it needs to have a battery connected." \$\endgroup\$ Commented Feb 28, 2024 at 15:11
  • \$\begingroup\$ So while "providing a stable power supply for the control circuitry" as you stated, battery cells are not charged/discharged at very high rate? I think this happens when we use a capacitor to stabilize the voltage. \$\endgroup\$ Commented Feb 28, 2024 at 15:13

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