Car accu as powersource: how to load the best and how to maintain on power while using accu's ?

[bstikkel]

Hello fellows :),

I am trying to get in motion my homecockpit. Therefore I have three 24v dc motors of 550 watt, which I want to make run at 2 12volt accu's in serial or ofcourse one 24 volt accu.

There are many accuload equipment, but what is the best way to keep the accu's in condition while using the motors ?
Should I buy a drip-charger, or a normal charger?

(Sorry for my restricted english)

Thanks for your advice.
Bram Stikkel

[Roland]

As far as I know, lead acid car batteries are best kept fully charged for long lifetime.

I run my platform from three 12V batteries each 7Ah. In a balanced 150kg platfrom, avererage power is low, around 100W. That gives me at least 2 hours before batteries are getting low. In those cases I run a lab power supply that has max voltage set at 3x13.5V=40.5V and max current limit at 3Amps. This setup keeps the batteries at almost full condition continuously, without risk of overcharging.

Not sure what average power your rig is going to consume. Lets assume 100W: In that case you'd need CC/CV (constant current / constant voltage charger that can deliver about 4A at 27V. This would work. Other chargers probably also work, as long as they have a max voltage limit.

[bstikkel]

Thanks for your advice, Roland !
Bram

[iwik]

Hi Roland,
Are you using Deep cycle SLA or just convential car batteries?. I was going to use standard car ones(cheapest) and leave an intelligent charger permanently
connected. Although i have been told that they should be Deep cycle as conventional ones are not designed for discharging like we would in a motion platform. It would be good to hear from someone like yourself who have used motion platforms for a while and to see what has been used and how long batteries have lasted.
Thanks and look forward to your comments.
Regards
Les

[bstikkel]

I have recently learned, there are also semi-traction accu's, a in between version between the car accu and the expensive deep-cycle accu. A semi-traction accu is a third of the price of a deep-cycle accu. The diiference betwee normal accu, semi-traction and deep-cycle is, that the lead elements in the accu are vulnarable to become bended in the normal accu, and become less vulnarable to that in a semi-traction and deep-cycle accu.

By the way. With all kinds of accu's you need to take care of very good ventilation, since dangerous gasses are produced anyway with lead and acid accu's. Jellyaccu's do not have that problem, but they are more expensive again.

Consider to do what I now choose to do.
Try to find used but good transformer electricity supplies on Ebay or something like that.

Greetings and succes !
Bram

[iwik]

Hi Bram,
Thanks for the reply. Had been considering a main operated supply.Have some 10amp transformers here, will use 3 to maybe achieve a regulated 30amp supply.Hadnt heard of Semi-traction ones,will do some research.
Best regards
Les

[Roland]

I'm using three Yuasa NP7-12 see spec http://www.houseofbatteries.com/pdf/NP7-12
They are not expensive, can handle big surge currents, and still run fine after 3 years of regular use.

I had also thought about power supplies, but the nature of a motion platform requires VERY large sudden peak powers, and in some cases, the driver tends to dump some current into the supply. Batteries handle these requirements w/o difficulties, whereas an AC supply would have problems with both. I need 100W average, and ~1500W peak!

Adding a huge electrolitic capacitor to the output of a beefy AC supply could be a possibility, but it then resembles my setup: a 40V / 3A CC-CV supply that is connected to three 12V batteries.

[iwik]

Hi Roland,
Thanks for replying.Im really in a quandry as i have just built Ian Hoppers mini platform and dont know what to buy in the way of batteries. I know i should be going for SLA but these are expensive for 38ahr ones.I really dont even know if i need this high a capacity ones.I have a intelligent 5amp
charger that i was going to connect in parallel with the battery. My thoughts were with buying two cheap 12v 38ahr standard ones to see what current i will be running then deciding what i need. Your power requirements seem very low.
I do like your latest design,how is it comming along?.
Any further comments you can make would be gratefully appreciated.
Regards
Les

[Roland]

Hi Les,

Ian's mini platform is similar to mine: it relies on bungee for weight balancing. Correct weight balancing is really the key to low power consumption. So if you do it right, you'd get similar power levels as I have. 38Ah batteries sounds a bit overkill to me. It won't hurt (except your wallet), but 15Ah would probably do just as well. Of course, it depends on the motor types.

My new platform design is progressing steadily: the system has been tested with 150kg payload, and performance looks OK, see http://www.youtube.com/watch?v=OfI-_RTExBc
From the video you can also see that average power for the slow 0.15Hz heave is indeed around 100W. Could even be lower, as I still have to fine tune the bungee force a bit.

Currently re-doing the complete motion cockpit with improved visuals, flight yoke, new throttle, and a more modular setup for easier assembly/dissassembly. Sufficient stuff to keep me busy!

[iwik]

Thanks Roland,
Im using 24volt 350watt motors with a max current of 18amps.I intend to balance the platform as closely as possible.
I have seen the video of your new platform and WOW it performs beautifully.You really put some thought into the mechanics
and only wish i had your skill level.Im following your new platforms progress with interest.Look forward to more updates,your
web page has been bookmarked for a while now and i visit it quite often.
Regards
Les

[Roland]

Thanks Les,

Your 24V 350W motors should work fine. What type of gear do you use? Are they worm gear or straight spur gear? The latter seems to be more efficient. If you have similar payload as mine, balanced heave should be very close to my power levels.

Note that pitch and roll may strain the motors a bit more, since Ian's platform design uses single centerpost weight balance. As the center of gravity of the platform will be some distance above the rotation point, there will be an extra offset force straining the motors during pitch / roll angle. Ian's motion platform software includes a power calculation function that can predict this extra power.

In my design, each actuator has its own weight balance counter force: Therefore there is little power increase during pitch / roll compared to Ian's design.

[iwik]

Thanks Roland,
My motors have a spur gear box built in to the motor assembly.Thanks for loading imfo,all new to me but looking forward to trying it out.
Could you further explain (Ian's motion platform software includes a power calculation function that can predict this extra power.)

Regards
Les

[Roland]

Small correction: Ian has written a small add-on software: Motion Platform Power Estimator. You can run this add-on in combination with BFF 3DoF driver V2.4 and above. It will also run in demo mode.

The software is designed for 3-point platforms with centerpost for weight balance (like the mini platform). It will ask you to enter the platform weight distribution properties (i.e. CG offset in YXZ directions) , you can also tweak the accuracy of weight (im)balance.
You can then let FS generate real-time motion cues, or apply user defined sinewave motion (heave/pitch/roll)
The software will calculate actuator position, velocity , acceleration and force , and derive the mechanical power from these properties.

As an electrical guy, it took some time to get my head around: Ian helped in explaining the basics:

For sine wave motion the maximum accelerations occur when the displacement is approaching and leaving the top and bottom of the displacement - ie as the velocity trace passes through the zero position. The instantaneous power is the product of force and velocity so the power will be zero whenever either the force or the velocity is zero - this will happen 4 times per cycle because the velocity and force are not at zero at the same time. The velocity is zero when the acceleration force is at its maximum, and the acceleration force is zero when the velocity is at its maximum!

I'd say start with a perfectly balanced setup, then apply simple slow heave sinewaves, and see what happens when you increase the sinewave speed. Then add some imbalance, compare the results.
Then do the same with pitch and roll, and see the effect with added CG height. It may take a while before it all starts to make sense, but it will finally help you in understanding power requirements better.

I did comparison with actual heave battery power with my #3 platform design: The actual electrical power drawn from battery differed a bit compared to the mechanical power calculated by the tool (due to driver/motor/gear efficiency, and some other effects not yet fully understood). I will make a comparison again with #4 platform design: it is more efficient, less friction losses, and should thus give more accurate results.

Info and download of the power estimator tool can be found on http://buggies.builtforfun.co.uk/Sim/calculator.php

[iwik]

Hi,
Yes i saw the software on his site but didnt really appreciate what it could do. Thanks for the explanation.
Regards
Les