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When I have a high load on my system the battery voltage on the MPPT display will go down. It usually sits between 12.8v - 13.1v however if the load is high (it is currently on 8amps) the battery level will drop down to 12.4 or something. This will then go back up if I turn the load off.

Why is this? I am worried about discharging the batteries too much and ruining them.

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  • \$\begingroup\$ I think its obvious, the load is causing voltage drop to your battery. \$\endgroup\$ – dpw Nov 15 '16 at 6:35
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All batteries have an internal resistance which drops more and more voltage as the load draws more and more current.

In your case, let's say the battery reading is 13 volts unloaded and 12.4 volts with 8 amperes into your load.

Under those conditions, the battery's internal resistance would be:

$$ R_{INT} = \frac{V_{BAT} - V_{LOAD}}{I_{LOAD}} = \frac{13V - 12.4V} {8A} = 0.075\text { ohms} $$

As for discharging a battery, the terminal voltage depends on your battery's chemistry, capacity, and how much current you take from it. Rule of thumb for sealed lead-acid is to never let the voltage fall below 10.5 volts.

Assuming yours is sealed lead-acid, here's a pretty good link.

There's also a lot of information here

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  • \$\begingroup\$ This looks exactly right as the more I increase the load the more the voltage drops. Have just spent the last few hours researching Internal Resistance, man batteries are difficult. \$\endgroup\$ – Nectar-Bomb Nov 15 '16 at 12:08
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Car Lead acid Batteries are supposed to drop to 12.5V with a light load after charging.

  • This indicates they are at 100% SoC
  • When they rest at 11.5V they are near 0% SoC

SLA's have a slightly different profile for both charging and SoC or Residual Capacity enter image description here

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