Hi all.

I want to make a simple circuit that powers a load from a 12V battery and this circuit switches the power to the load off when the battery's voltage decreases under 10.5V. I prefer to use a MOSFET to switch the load ON and OFF instead of a relay.

I've built a simple circuit using the LM358 comparator and MOSFET IRFZ44. The current that the load draws is few amperes so IRFZ44 is ok as it's price is also good (0.3$).

The problem that's facing me now is when the battery's voltage becomes approximately 10.5V. The preset pot. is set to 5V on pin 3 when the source/battery voltage is approximately 10.5V. So at that point the output if the comparator starts to decrease to become LOW. This decrease means that Vgs is decreasing and so the voltage across the load decreases instead of switching off directly. This also makes the MOSFET get very hot if the period is long...

May be I need to add hysteresis to the comparator but I don't know how to get the value of the resistor and I don't now if his may help solve the problem.

Any help is strongly appreciated.
Regards,
Hazim

 

Try adding about 1MΩ between pins 1& 3 for about 50 mV of hesteresis.

 

A 12v battery is normally considered 100% discharged when it reaches 11.4v. Discharging even a deep-cycle battery 100% will result in its' service life being reduced by about 2/3. An automotive battery will very soon fall apart after such a deep discharge.

You really should not use an automotive battery for a deep-cycle purpose. They won't last long at all when used in such a manner. They are designed to produce a heavy current for a short period of time, and then to be immediately recharged.

If you want a deep-cycle battery to last for a reasonable amount of time, do not discharge it below about 12.2v.

 

I expected you will tell me about battery discharging and that its service life will be reduced if discharged more than it should. Actually I don't know what type off battery is used for whom I want to make this circuit. But I know that they connect the load directly to the battery and this means the battery discharges to about 0V if the load still connected and the electricity is still off. Here in Lebanon the electricity service is really bad.... UPS is used very commonly and the UPSs are set to inverts 12V DC to 220V mains until the battery voltage decreases to 10.5V approximately.

reducing the battery life to 2/3 is better than reducing the time we have electricity in our homes at night to 2/3 .......... Anyways I simply wants to make a circuit as I described above

Thanks SgtWookie, and I will try that Bernard, thank you too.

 

Please believe that I am giving you the information in your best interests, as batteries are expensive, so you should try to maximize their useful lifetime.

In another thread, JJ Alukas is having a problem with a 700va inverter/battery combination; the battery is ~2.5 years old and is going bad already.

The person who you are building this circuit for should know that minimizing the load on the inverter during power out times will help reduce the strain on the battery. Using LEDs will be much more efficient than using incandescent lamps.

 

Thanks SgtWookie. I understand your recommendations. The battery is used to power internet antennas/station for a local ISP. The network devices drains something between 1 and 3 amperes at max. They use an automatic battery charger to charge the battery. The devices are connected directly to the battery.

Anyway, the circuit is working fine for me now. I used a 22k resistor for hysteresis and not 1M ohm one. I tried 1M but didn't worked, so I tried decreasing the value till it worked fine at 22k. At less values the circuit switches at much high voltage ( >12.5V ) while now it switches back on if the voltage increased above approx. 12V which is fine. It would be better to have it switch back on at voltage near 11.5V but it's ok.

Thank you all and regards,
Hazim

 

A 12v battery is normally considered 100% discharged when it reaches 11.4v. Discharging even a deep-cycle battery 100% will result in its' service life being reduced by about 2/3. An automotive battery will very soon fall apart after such a deep discharge.

You really should not use an automotive battery for a deep-cycle purpose. They won't last long at all when used in such a manner. They are designed to produce a heavy current for a short period of time, and then to be immediately recharged.

If you want a deep-cycle battery to last for a reasonable amount of time, do not discharge it below about 12.2v.

This should be a sticky. I can't count how many times I've read this (usually posted by sgtWookie) on this forum.