I'm building a MOSFET controlled DC motor circuit with a 6V battery as the supply. Although when I turn the MOSFET on, the voltage at the battery positive drops to about 2.5V. I thought it could be back emf from the motor, but it stays at the lower voltage. Maybe it's the way I set up the circuit? Can anyone see what might be causing this problem?


simulate this circuit – Schematic created using CircuitLab

Note: That the inductance in the schematic for the motor is made-up. And that I am using an IRLB8721 MOSFET, not the one in the schematic.

Thank you for helping.

Edit: The solution it seems that the batterys I'm using either cannot supply enough current or they are low on charge. Thanks!

  • \$\begingroup\$ Ye be needing a bigger battery.... \$\endgroup\$ – StainlessSteelRat Feb 25 at 18:09

Battery ESR

There is a relationship between Ah capacity of a battery and it's ESR.

  • For a 6V SLA , it's about ESR= Ah/0.1 and for Li-Ion , ESR ~ Ah/50 but is more dependent on C rate. It also adds with number of cells in series and divides by strings in parallel,


YOUR FET is rated at 8.7mΩ @ Vgs=10V and Vgs(th) 1.35 to 2.35 V only conducts 25uA so at 2.5Vdc it's probably < 50 Ohms so your battery has high ESR.

enter image description here


Your Motor also has DC resistance or DCR which you can measure with a DMM.

This causes a surge current of V+/DCR which normally reduces to 10% at its rated load and 1% at no load.

Voltage drop

Ohm's Law...


simulate this circuit – Schematic created using CircuitLab

Need I say more?

Battery DCR rises with age and State of Discharge (SoD) rapidly < 10% from sulphation or electrode oxidation etc.


If the battery voltage is falling to 2V then you're drawing more current than the battery is able to supply. The battery may be running low, or it may be too small for your motor. If the motor is stalled, it might allow too much current to flow and cause other problems in addition to this one.

Back EMF from a motor is a different thing. It will be less than the motor's supply voltage and will cause current into the motor to drop while the motor is spinning.

  • 2
    \$\begingroup\$ An easy test to verify this is to run the motor straight from the battery -- if the motor spins and the battery voltage doesn't drop, then there's something oddball about your circuit (and it doesn't match the schematic). If you can't run the motor from the battery without the electronic switching, you can't run it from the battery with the electronic switching. \$\endgroup\$ – TimWescott Feb 25 at 1:06

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