# Project Power Requirements



## JPigg55 (Sep 7, 2015)

I'm working on a project that I need some help with power supply requirements.
The motor I'm considering using has a 12 vdc, 12 watt rating. I'd like to be able to power it from either batteries or AC plug source. Mathematically, power being I*V, it would draw 1 amp @ 12vdc.
So, for example, a 'D' sized NiMH rechargable battery is rated at 1.5 vdc & 11.4 watt/hrs. That calculates to 8 .atteries for 12 volts and 91.2 watt/hrs (11.4 ea). Therefore, this example battery pack should power motor for 7.6 hours theoretically. Realistically, I'll SWAG it at between 5 & 6 hours, math being correct.
So, is my math correct and would a power supply like this on (http://www.amazon.com/12-Volt-Power-Supply-Standard/dp/B009ZZDE0O) be adequate for powering and recharging ?


----------



## Terrywerm (Sep 7, 2015)

If this motor will see intermittent use with light loads, I would give the wall wart a try and power the motor directly. On the other hand, there are 2 amp switching versions of these lowly power supplies available as well, they sell for about $20. You might be better off with one of them if your motor will see heavier loads or quite a bit of use.

Let me guess: are you making a power feed for your 8520?


----------



## RJSakowski (Sep 7, 2015)

Remember that the 12 watt rating is for typical loads.  The current requirement for startup or stalling conditions will be considerably higher.  Some switching supplies do not like overloads so you may have to build in a suitable safety factor.  However, if you have a power supply available, it won't hurt anything to try it.

Bob


----------



## JPigg55 (Sep 7, 2015)

Actually, it's a 12 vdc powered vacuum pump (https://www.sparkfun.com/products/10398).
I'm trying to make a small, portable surge type goat milker.
Wanted to dual power it so it could be used without plug-in or combination of battery and direct if power available.
Under this use, it would run full time for 30-45 minutes. Due to scfm rating, may have to run two in parallel.
If battery pack would run it for 2 hours or 1 hour with 2 pumps, I'd be good with that.
I have a full sized cattle surge milker, but too big, heavy, and ackward for a couple three goats.

Power feed for 8520 comes later. LOL


----------



## Terrywerm (Sep 8, 2015)

Sounds like a cool project. Are you going to do a build thread on it?  I've dealt with milkers more than I care to think about but only for cows, not for goats, so this will be new.


----------



## JPigg55 (Sep 8, 2015)

terrywerm said:


> Sounds like a cool project. Are you going to do a build thread on it?  I've dealt with milkers more than I care to think about but only for cows, not for goats, so this will be new.


I'm planning on it, if it actually works.
Based on what I've seen on other forums I belong to, I think many people would be interested.
If so, planned on seeing if anyone here would like a small money making project to help support their hobby.


----------



## Zanfar (Sep 30, 2015)

JPigg55 said:


> I'm working on a project that I need some help with power supply requirements.
> The motor I'm considering using has a 12 vdc, 12 watt rating. I'd like to be able to power it from either batteries or AC plug source. Mathematically, power being I*V, it would draw 1 amp @ 12vdc.
> So, for example, a 'D' sized NiMH rechargable battery is rated at 1.5 vdc & 11.4 watt/hrs. That calculates to 8 .atteries for 12 volts and 91.2 watt/hrs (11.4 ea). Therefore, this example battery pack should power motor for 7.6 hours theoretically. Realistically, I'll SWAG it at between 5 & 6 hours, math being correct.
> So, is my math correct and would a power supply like this on (http://www.amazon.com/12-Volt-Power-Supply-Standard/dp/B009ZZDE0O) be adequate for powering and recharging ?



EE here, on paper, yes, your math is correct.

Realistically, the watt-hour ratings of batteries are taken at their most efficient current draw (a spec that the manufacturers don't readily disclose) so I would count on 30-50% of that number at a 1 A draw. That's a pretty high current for standard batteries. Also, you will need an actual charger--NiMH batteries can't be charged just by putting a voltage across them like lead-acids can--and the charger usually requires 2-5 V more than the battery pack. So that wall wart is probably OK for running the system, but using it at 100% capacity is not what I would recommend.

If it were me: I'd get a small lead-acid (motorcycle battery), a maintenance charger (battery tender Jr.), 12V 1.5A (18W) power supply, and an extension cord. All these are relatively cheap, robust, readily available, and well known quantities. Put the whole thing on a cart, plug it in when not in use to charge the battery, or run the extension cord and power supply out to run on mains. Ballpark, the whole thing costs about $100.


----------

