# Need an electrical wiz

### Help Support HomeBuiltAirplanes.com:

#### TJay

##### Well-Known Member
Ok I ordered a Quartz clock for my build and I am trying to decide weather or not to hard wire it so it runs 24/7 or wire it into the master buss, Pros with running 24/7 I don't need to set it every flight cons may drain the battery if I don't fly for 6 months, So I need a electrical wiz to help me figure out how long it will take to drain the battery to 50%

My setup is a EarthX lithium battery EXT-680 AH (1 hr 1c rate)=12.4
The quartz clock has no internal battery and draws 10-11 Milliamps.

Hope I provided enough info thanks TJay

#### Dan Thomas

##### Well-Known Member
Ok I ordered a Quartz clock for my build and I am trying to decide weather or not to hard wire it so it runs 24/7 or wire it into the master buss, Pros with running 24/7 I don't need to set it every flight cons may drain the battery if I don't fly for 6 months, So I need a electrical wiz to help me figure out how long it will take to drain the battery to 50%

My setup is a EarthX lithium battery EXT-680 AH (1 hr 1c rate)=12.4
The quartz clock has no internal battery and draws 10-11 Milliamps.

Hope I provided enough info thanks TJay
Does this clock have a digital or mechanical display? 10 or 11 mA sounds way too high. 10 mA for an hour is 1/100 of of an amp-hour, and your battery is good for 12.4 Ah, so 1200 hours would drag it down a long way. 1200 hours is almost two months. The clock should take a lot longer than that to run the battery down.

#### lr27

##### Well-Known Member
If the plane lives outside, a small solar cell can keep the battery charged. Inside, for that matter, if you're willing to run wires outside. I'm not sure what kind of charging your lithium battery requires, though.

#### Dan Thomas

##### Well-Known Member
Why not simply add a separate battery for the clock if you are worried about draining the aircraft battery? 8 AA batteries in a commercial holder would do it, here's one for three bucks.

Or you might go for two battery clips and a 12v voltage regulator chip, a pair of 9v batteries, some wire and an Altoids tin.

Either solution would cost about \$10 or less total including the batteries.
At an 11-13 mA draw, those batteries will be dead in short order. http://www.techlib.com/reference/batteries.html

#### cluttonfred

##### Well-Known Member
HBA Supporter
Hmm, I must be missing something. 2400 mAh per AA battery x 8 batteries = 19,200 mAh ÷ 13 mAh max draw = ~1477 hours ÷ 24 = ~62 days. Changing batteries once every two months doesn't seem unreasonable to me.

#### Dan Thomas

##### Well-Known Member
Hmm, I must be missing something. 2400 mAh per AA battery x 8 batteries = 19,200 mAh ÷ 13 mAh max draw = ~1477 hours ÷ 24 = ~62 days. Changing batteries once every two months doesn't seem unreasonable to me.
Yes, you are missing something. Your adding up the mAh would apply if the batteries were all connected in parallel, but they're not: they're connected in series to get the 12 volts you need. A parallel system would give you a useless 1.5 volts and your 19,200 mAh; series gives you a workable 12 volts and a short-lived 2400 mAh. 185 hours, less than eight days.

#### akwrencher

##### Well-Known Member
HBA Supporter
Man, that's one thirsty clock. My wall clock in my shop takes one AA bat and lasts a year or more.

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