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### Topic: check my battery math please (Read 933 times)previous topic - next topic

#### jointtech

##### Aug 07, 2012, 01:35 pm
I'm looking at a 6V 225Ah battery.
I'm trying to figure out if this is way overkill.
I have motors and servos that pull about 6amps at 6v plus the arduino. (I actually havent confirmed this but they pull about 3amp with 12v)
amps X volts = watts
6 x 6 = 36watts

and the battery has 6v x 225Ah = 1350watts.
1350watts / 36 watts = 37.5 hours??
that seems not right.  these motors are on powerwheels and this battery doesnt look that much bigger than the batteries that come with it and only last 30 minutes.

#### johnwasser

#1
##### Aug 07, 2012, 03:00 pm
Isn't 10.375" x 7.125" x 10.875" somewhat bigger than a PowerWheels battery?

If the motors draw 3A at 12V they are likely to draw 1.5A at 6V unless you re-wind the motors to draw more current.
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#### MarkT

#2
##### Aug 07, 2012, 03:20 pm

Isn't 10.375" x 7.125" x 10.875" somewhat bigger than a PowerWheels battery?

If the motors draw 3A at 12V they are likely to draw 1.5A at 6V unless you re-wind the motors to draw more current.

Or if you were using a 12V to 6V DC-DC converter to power them...

You need to confirm whether you can run them at 6V (we don't understand how they would work at both 6V and 12V - servos are often 6V only for instance).

Also for battery lifetime calculations you need to know the average current (servos often spend some time idle, so the peak load currents aren't sustained constantly).

Make sure you never over-discharge the battery (lead acid batteries cannot tolerate this without substantial permanent loss of capacity).
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#3
##### Aug 07, 2012, 05:08 pm

these motors are on powerwheels and this battery doesnt look that much bigger than the batteries that come with it and only last 30 minutes.

Looks to me like those batteries are nearly 8 times as large, and 6 times the weight. (double one dimension, and you are twice the size, two dimensions and you are 4 times the size, double all three dimensions and you are 8 times as large).  Add to that the fact that flooded lead acid batteries have better energy density (at the expense of higher upkeep/maintenance), and it's no real surprise to me that it has 10 times the energy capacity.

#### jointtech

#4
##### Aug 08, 2012, 02:00 pm
weird i dont get all my notifications.
Yea its a ton bigger.  I went and looked at the powerwheels battery at the office today and its only about 5" x 5" or so.  9.5Ah.
The battery i was looking at is huge lol.

My issue is I dont want to buy a convertor for my servo which can draw almost 4amps at stall (which costs a ton more than my motors).  So i was just going to run my motors at 6v and see what happened.  If they crap out ill look for real 6v motors.
Since I only dust off my arduino and electronics every so often I forget my amps to volts but I had it in my head from somewhere there was some inverse relationship.

#### pito

#5
##### Aug 08, 2012, 02:13 pmLast Edit: Aug 08, 2012, 02:19 pm by pito Reason: 1
..the math:
>and the battery has 6v x 225Ah = 1350watts.
>1350watts / 36 watts = 37.5 hours??

225Ah means AmperHours - that is the capacity of the battery (C=I*T). It does not indicate the max current you can draw out of it (it depends on the battery type).
So you may source 225Amperes for 1 hour long out the battery (if applicable ), or 22.5Ampers for 10hours, or 2.25A for 100hours.

The power P (in Watts) you can get out of the battery is P=U*I. So ie. while drawing 22.5Amp * 6Volt = 135Watt.

Or,
6v x 225Ah = 1350Wh
1350Wh / 36W = 37.5 hours

#### jackrae

#6
##### Aug 08, 2012, 08:18 pm
Assuming the 225AH battery is rated at C20  (that's 20 hours discharge) this equates to a discharge current capability of 11.25amps for 20 hours.
However if you want more amps, which means a quicker discharge rate and hence a shorter discharge time, a general rule of thumb is 80% capacity for every halving of the discharge period.
For example :-
At C10 the capacity will be 180AH  (18 amps for 10 hours)
At C5 the capacity will be 144AH    (29 amps for 5 hours)
At C2 the capacity will be 100AH    (50 amps for 2 hours)
At C1 the capacity will be 80AH      (80 amps for 1 hour)

In other words if you try and discharge your 225AH C20 battery in only one hour, you'll only get 80AH worth of energy.   certainly you can get several hundred of amps out of it but the time period will be severely reduced.

Disappointing ?  definitely but that's how batteries function.

#### jointtech

#7
##### Aug 08, 2012, 11:54 pm
How is that compared to a jump starting box.  The ones you carry in your car that have 450 cold cranking amps.  Are they not a C type battery?  Or are they batteries with some other thing that allows them to discharge massive amounts of amps at once?
Not relevant to the original question but got curious since I happen to have one sitting here.

#### johnwasser

#8
##### Aug 09, 2012, 03:27 am

How is that compared to a jump starting box.  The ones you carry in your car that have 450 cold cranking amps.  Are they not a C type battery?

That might be a discharge rate of C0.01 (450 Amps for 0.6 minutes) or 4.5AH.
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