# MPPT display voltage vs actual battery level

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.

• I think its obvious, the load is causing voltage drop to your battery. – dpw Nov 15 '16 at 6:35

All batteries have an internal resistance which drops more and more voltage as the load draws more and more current.

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.