# Battery Yikes!



## major (Apr 4, 2008)

So 36V * 150Ah = 5400Wh (energy stored in the battery). 667W (full load power). 5400Wh / 667W = 8.1h.

So does your machine run continuously at full load for 8 hours straight?

If so, you need to recalculate with the increased battery mass and increase the budget


----------



## Karter2 (Nov 17, 2011)

Also, if this robot is going to alternate up and down, you might consider using regen on the down run to control speed and recover some power.


----------



## dragonsgate (May 19, 2012)

How far does this robot travel before it make the return trip? Is it great enough distance that it requires a battery? Could it possibly use a power cord and plug in? Is the robot going to perform any task other than just go up and down? Is it for a competition of some sort? I sound like a panelist on some old quiz show like what's my line don't I?


----------



## Vanbot (Mar 17, 2015)

This is great. A few hours after posting my comment and I already have more responses than I did at the best robotics forum I could find. Thanks everyone. 

No it's doubtful it will run at full throttle for 8 hours and you may have found an error in my calculations that will help out a bit. The motor and battery estimating tool that I used asked for hours of operation but didn't specify that that would describe full throttle operation or how that number affects the result. 

Likely it will operate at close to full throttle for a couple of minutes at a time and will then operate for maybe 30 minutes at much less throttle/effort as it descends. This pattern will be repeated several times over an 8 hour period. 

So...the big questions is, how would this affect my calcs and, most importantly, the cost and size of battery required?

I love the idea of regenerating during descent but that would seem to add quite a bit of expense and weight to the design, no? Again you would all know more about that than me. 

Do my other numbers seem reasonable?


----------



## Vanbot (Mar 17, 2015)

Sorry dragonsgate. I forgot to address your point. Yes it is possible that under some circumstances it will be run using a power cord. My understanding of the problem there is that DC power experiences significant voltage drop over the potentially 100 metres that the robot might travel. So I might see better performance with an AC motor but that would mean either swapping the DC motor for an AC one, converting to DC onboard or having two different devices, one DC and one AC.


----------



## major (Apr 4, 2008)

You need to define the duty cycle and calculate the average power to use for the energy calcs. And regeneration does not require any additional hardware, maybe software, but there would be no mass penalty. Actually it may save mass as you can reduce the friction brake size and maybe the battery.


----------



## Vanbot (Mar 17, 2015)

OK so by duty cycle you mean percentage of time operating vs percentage of time completely off?

To estimate, the robot will run at close to full throttle for maybe 2 minutes, followed by approximately 30 minutes of operation at very low throttle/effort as it descends, followed by probably 15 minutes of zero operation while the rig is reset.

If regeneration is mostly a matter of software and is reliable then I would absolutely include it.


----------



## dragonsgate (May 19, 2012)

Vanbot said:


> OK so by duty cycle you mean percentage of time operating vs percentage of time completely off?
> 
> To estimate, the robot will run at close to full throttle for maybe 2 minutes, followed by approximately 30 minutes of operation at very low throttle/effort as it descends, followed by probably 15 minutes of zero operation while the rig is reset.
> 
> If regeneration is mostly a matter of software and is reliable then I would absolutely include it.


Curiouser and curiouser as to what function this thing is for. Maybe you could reduce the battery pack to minimum size and have stationary charging stations at strategic points along the line. Do you have any pictures or drawing of what this setup might look like?


----------



## major (Apr 4, 2008)

Vanbot said:


> OK so by duty cycle you mean percentage of time operating vs percentage of time completely off?
> 
> To estimate, the robot will run at close to full throttle for maybe 2 minutes, followed by approximately 30 minutes of operation at very low throttle/effort as it descends, followed by probably 15 minutes of zero operation while the rig is reset.
> 
> If regeneration is mostly a matter of software and is reliable then I would absolutely include it.


Duty cycle is the power profile of the intended use. Time on the horizontal axis and power on the vertical. Seconds at power level 1, followed by P2, then P3, and so on until a repeated pattern. The take the average power over the complete cycle. Include all power levels even zero.

Your motor, controller and mechanical power transmission need to be capable of regeneration, often called 4 quadrant operation. + & - rotation direction and + & - torque. Should be plenty of tutorials on the interwebs about this stuff. http://myelectrical.com/notes/entryid/106/understanding-motor-duty-rating


----------



## Vanbot (Mar 17, 2015)

All I can tell you is that it's for the film industry.  Charging stations won't work as it will be moved around from job to job and from position to position on any given day. So battery and/or power cord only.

Ok my homework today is reading up on regen. Thanks for the link.

If I wanted a rough estimate of battery requirement (to make the nightmares about $3000 batteries stop) could I estimate percentage of time it will operate at anything close to full power and then apply that percentage to the full throttle number I posted before? E.g. if in an hour it's running at full power for ten minutes then I could take my original estimate of needing a 150 Ah battery and magically transform it into a 25 Ah battery? 

10/60 minutes = .167
.167 x 150 Ah = 25 Ah

How efficient is regen at recovering power? The more I think about it the more the idea is perfect for this application.The robot fights gravity like hell for short periods of time thereby earning the right to loaf on the way down and refresh its batteries.


----------



## pm_dawn (Sep 14, 2009)

Here is another idea for you.

Use 2 wires and feed DC on the wires.

Have a small high power battery that has just enogh energy to take the bot down, but the power capability to remove high current loads from the cables.

That way you could probably eliminate the voltage drop of the cables/wires

How does that sound ?

Regards
/Per


----------



## Vanbot (Mar 17, 2015)

Hi Per.

I had considered this idea but I think someone, somewhere convinced me there would be problems with EM interference and voltage drop. The cable could be 100 m in extreme applications although most of the time it would be much sorter, maybe 30 meters.

I should confirm that we're talking about the same thing. Are you imagining transmitting power through the support cable(s)?


----------



## dougingraham (Jul 26, 2011)

Vanbot said:


> How efficient is regen at recovering power? The more I think about it the more the idea is perfect for this application


If you do a really good job you will be about the same efficiency at regen as with the drive system. So if you can achieve 90% on the drive system you could get to within a few percent of that in the energy recovery. However I would guess it won't be nearly that good and estimating 50-60% might be prudent when doing your calculations.


----------



## Vanbot (Mar 17, 2015)

Being able to reliably recover 50-60% would be helpful. It should mean that I can use a smaller, cheaper, lighter battery?


----------



## Sunking (Aug 10, 2009)

Vanbot said:


> Hello.
> 
> I'm brand new to the forum and have some questions that pertain to a robot project I'm working on. I'm also pretty new to robotics and was surprised to find that there just don't seem to be any good, active robotics forums out there.


You need to search around because there are lot of forums out there for robotics.


----------



## Vanbot (Mar 17, 2015)

Hi sunking. 

I did search pretty extensively and asked the people who run my local robotics club. There is one associated with a company that sells robot parts that isn't bad but so far I haven't found a really active forum for people who are developing their own projects. A lot of them haven't seen new comments posted for weeks or months. 
If you know of any I'd be interested to know about them.


----------



## Duncan (Dec 8, 2008)

Hi Vanbot

13kg, - 2.235m/sec = 291 watts – 65% efficient means 448 watts
36v means 12.5 amps

Likely it will operate at close to full throttle for a couple of minutes at a time and will then operate for maybe 30 minutes at much less throttle/effort as it descends
Over 8 hours

as you descend you should need effectively zero power, you will be re-gening 
Worse case is you can’t re-gen so say 10% of power when descending

With re-gen to at least mean zero power on descent
16 x 2min x 12.5 amps = 400 amp-minutes = 6.7 Amp hours – you should only use 80% of capacity so you need 8.4Ah

With using 10% on descent
28 min x 1.2 amps x 16 = 540 amp – minutes = 9 Ah, 80% = 11.25 Ah plus the 8.4 Ah = 19.65Ah

So you need a 20Ah pack at 36v

If you use Lithium Poly (RC planes) you will need a 10S (10 cells in series) pack to get the 36v
Four of these would do – at $80 each - $320
http://www.hobbyking.com/hobbyking/store/__21377__ZIPPY_Compact_5000mAh_10S_25C_Lipo_Pack.html
just an example not recommending these actual batteries

They would weigh about 5Kg – so your 13Kg could go to 18Kg

Allowing for the weight of the batteries you would use seven of these batteries – giving a total weight of 21.75Kg and a power requirement of 
19.65Ah x 21.75Kg/13Kg = 32.8Ah with 35Ah available


----------



## pm_dawn (Sep 14, 2009)

Vanbot said:


> Hi Per.
> 
> I had considered this idea but I think someone, somewhere convinced me there would be problems with EM interference and voltage drop. The cable could be 100 m in extreme applications although most of the time it would be much sorter, maybe 30 meters.
> 
> I should confirm that we're talking about the same thing. Are you imagining transmitting power through the support cable(s)?


Yes I was thinking about using the support cables as live, bare power cords.

The thing is that the voltage drop will only be there when high currents are drawn.

When the bot is near the feed there will be very little voltage drop.

And if you have a small powerful battery on the bot, that battery takes away the high current spikes from the cables.

Think about it. 
I would think that you could use a single 5000mah 10s lipo pack with that setup.

Also I guess that the stability of the bot would benefit from using two cables.

Regards
/Per


----------



## Karter2 (Nov 17, 2011)

Yes, in effect you could use the cables as a "charge" circuit, to constantly supply a low current charge to a small on board battery sized to power the peak load.


----------



## Vanbot (Mar 17, 2015)

This is great. Thanks for all the creative ideas. 

So between regen and potentially using support cables as power supply there seems to be potential to really cut down on the battery requirement and therefore weight and cost. 

But if we assume the most extreme operating conditions, say the robot is 100m from the power supply, then isn't that going to result in rather severe voltage drop? Again the robot wouldn't be that far from the power supply for too long so possibly it's not an issue. 

And such long power lines wouldn't cause problems with, for example, transmitting data from the robot to a ground station?

The other obvious question is the safety of using bare cable to transmit power like that. Is 36 DC volts potentially dangerous? I'm wondering if that design would ever get safety approval.


----------



## major (Apr 4, 2008)

I think it is a bad idea to use the support cables for electric power transmission.


----------



## Vanbot (Mar 17, 2015)

Transmitting power through the cables is certainly a fun and creative idea but I do think the inherent problems with safety and EM interference make it problematic.

Besides, now that I understand the process of calculating battery requirements better it seems it won't be too difficult to get the power I need onboard in a battery.

When I started out with this project I did some research into the space elevator project. Some interesting ideas there including transmitting power via laser from a ground station to PV panels on the elevator.

https://www.youtube.com/watch?v=zO1EV6A76ZE


----------



## Vanbot (Mar 17, 2015)

Another general question I've been mulling over is whether to use a stepper motor or a servo for this application. 
At the moment I'm considering using an onboard idler wheel with a magnet and a stationary Hall effect sensor to track the robot's position on the cable. This wheel would be made of urethane or some other material with good traction and would be held via a tensioner against the cable. I think this is necessary because it's possible that the actual drive wheel could slip under wet conditions and so this way I'd have a fairly accurate way of tracking the length of cable passing by which allows the robot to determine its position relative to a zero or home point.
So my understanding is that stepper motors are reliable, low maintenance, high torque but low power motors. Their max torque is at zero velocity and torque then falls off as velocity increases. So they have good braking/holding power but might not work well to pull a robot up a rope or cable.
Can anyone comment on this?


----------

