12v cabin battery and maxxfan

[wollip]

I know zip about DC current - but that's never stopped me before.

But I do need some help.

I have installed a small agm 12v mainly to run the roof vent fan when parked in the hot sun. The fan has 10 speeds and I am running it on 3. So far it has run 25 hours and pulled the battery down to 12.03.

I think I know that a battery should never be pulled below 8.50.

My question concerns the fan - will I damage it if I were to ever drain the battery down to anywhere near the 8.50 point?

[DennisV]

Wollip

 

You need to calculate your usage in amp hours

 

Check the amperage rating for the fan and try to interpolate to figure amperage at #3 speed.

 

Let's just call it 2 amp hours for this exercise. ( It's probably less than that)

 

Your battery will have an amp hour rating also. If it's a wheel chair size AGM, it will be about a 35 amp hour +/- battery.

 

A good rule for batteries is to not discharge past 50% for best longevity of battery life.

 

 

So for this example:

 

Battery Ah (35) / Fan Ah (2) = 17.5 hrs run time to 100% discharge or 8.75 hrs to 50% discharge.

 

Plug your real numbers in there to find your run time in hours to 50 % discharge.

 

 

You might want to consider a dedicated onboard charger that you just plug in at night to take the hassle out of charging.

 

Very convenient for not much money.

[Mc2guy]

DennisV is correct.  You need to understand the amperage draw.  You won't necessarily hurt the motor at lower voltages, but you will hurt the battery if you let it run all the way down.  Lead acid batteries (including AGM), are not designed to be drawn down beyond 40-50% of their maximum state of charge.  If this is done repeatedly, it will dramatically shorten their operating life and the number of charge cycles you can expect to see.  Each battery has it's own unique characteristics, but keeping the amp-hour draw limited to no more than 50% of total capacity before charging is a good rule of thumb.  

 

You can expect to see 400-600 charge cycles on a lead acid battery if you treat it well and limit the draw to 50%.  If you are taking it down to 0% SOC, you may only get 50 charge cycles before you see sulphation of the cathode plates and significant performance degradation.

[wollip]

Thanks to DennisV and Mc2guy for responding with some good tips and info.

 

I have also had some time to troll the Internet and come away from that with a consensus that 12.18 volts is about as low as I will want to draw down the battery. 12.18 will have it at 50%.

 

My battery is a small 35 amp/hr AGM. During my experiment, it took 20 hrs to reach the 50% draw down when running the fan on speed #3. This relieves my anxiety about running the fan for 8 to 10 hrs at a time.

 

My van conversion is for travel and I do have a small Progressive Dynamics unit that recharges the battery whenever hooked to shore power.