Can I use batterys in place of power supplies?

[grinderdust]

This is a cnc question as well as a general question so here goes. I was wondering what steps must be taken to get rid of power supplies and use d.c. directly from batteries on various electronic devices. Take for example a small d.c. motor driver running off of a 12v power supply. Assuming the batteries are of sufficient capacity to do the work and have a sufficient charge, what if any steps must be taken to regulate the electricity from a lead acid battery or a bank of nicads?

[Al_The_Man]

Batteries are an ideal DC source if you can keep them charged!
You do not necessarily need regulation, depends how you are controlling them and what the max voltage rating of the motor. As long as the supply does not exceed the motor rating by any large degree, no regulation is required.
Generally though it is not considered practical because of the discharge rate.
Al.

[grinderdust]

Would using batteries in place of wall transformers on smaller devices also work without a regulator? Other than keeping the batteries charged, are there any other pitfalls when it comes to doing this type of thing?

[Al_The_Man]

In most case the pitfall is the necessity to constant recharge.
And most WallWart's are un-regulated anyway, the come down to the rated voltage when on load.
Maybe if you indicate what devices you want to run, a better assessment may be possible.
Al.

[Geof]

Your little discussion interests me because I am planning on doing something similar; running a 12 volt device from a battery that is on constant charge.

The device is a hydraulic pump, actually a Toyota power steering pump that draws 80 amps full load, that will be opening a large gate at the end of my drive. The reason for using a 12 volt pump is because I don't want the hassle of running a regular 120volt AC supply through an underground conduit and having weather proof switches, etc. With 12 volts I can run a single wire just below the surface and use the earth for the return. Because the current is 80 amps I would need a hefty wire and to avoid this I figured to put a regular automobile battery out at the gate location. This gives me oodles of current capacity for a short while but plenty long enough to open the gate before my battery voltage declines noticeably. The 12 volt supply to the battery will come from a charger that has the ability to float when the battery is fully charged.

As far as I can figure it the battery is going to be seeing the type of use that a battery sees in normal automobile; occasionally a large current draw for a short time followed by a long period when the battery is being recharged.

Am I missing anything, are there any reasons this will not work effectively?

[grinderdust]

Thanks for your help. I am on the road alot with my band so I'll start with some of that stuff and work my way back to cnc. The first device I would like to run is a shure wireless mixer/transmitter that I use to mix sound for my in ear monitors when I'm performing. The reason is to get rid of ground loops and make the equipment faster to set up and tear down. It has a wall transformer that reads 12v 400 milliamp. the back of the unit itself says 12-18 vdc 170 ma.
This device would appear to have a very low drain, intermittent use and is not to picky about voltage. I'm thinking two rc car battery packs wired in series at 14.4v and having 3000 mah of power in each of them would power this for 3 sets of an hour each and then some. I also want to split the signals going to the house mixer and send them to my yamaha mixer for a seperate mix but that sets up ground loops unless I use splitter/transformers which are bulky and expensive. Going with battery power should solve this problem. Not to mention there are never enough outlets at the places we play.
Someday I would also like to live off the grid and have small cnc machines so using batteries would make more efficient use of the limited power available. Thanks.

[Al_The_Man]

Originally Posted by Geof With 12 volts I can run a single wire just below the surface and use the earth for the return. Because the current is 80 amps I would need a hefty wire and to avoid this

Consider this, the ground path will have to be very low resistance at that current, you may have to put the hose on it every day
At 80 amps 12v supply the load resistance is .15 ohms, if the ground resistance approaches this, you can see what is going to happen!!

Actually re-reading your post, if you are locating the battery out by the Gate, why not run a feed and return?
Or is that what you meant and only the charger use the earth return?

Originally Posted by grinderdust It has a wall transformer that reads 12v 400 milliamp. the back of the unit itself says 12-18 vdc 170 ma. This device would appear to have a very low drain, intermittent use and is not to picky about voltage. I'm thinking two rc car battery packs wired in series at 14.4v and having 3000 mah of power in each of them would power this for 3 sets of an hour each and then some.

Also you could look at sealed lead-acid type used in emergency lighting, they are not that small, but are rugged and cheap 12v @ 5aH.
Al.

[Geof]

Originally Posted by Al_The_Man Consider this, the ground path will have to be very low resistance at that current, you may have to put the hose on it every day ..... Or is that what you meant and only the charger use the earth return?

Just the charging current will use the earth return.

But anyway where I live even in the summer the water table is no deeper than 3 feet; I can easily reach it with a metal stake .

[gar]

080102-1946 EST USA

Geof:

Yes the car battery is a good means. Suppose 20 seconds operates the gate, then per operation you have consumed 80/180 AMP-HR. Suppose you do this 10 times per day. Then we have about 4.5 AMP-HRS per day, and the continuous charger will need to supply 4.5/24 = about 0.2 A.

Ground is not a good conductor in most places. In my area, two rod interfaces and ground, has 10 to 20 ohms resistance over a rod separation of 30 ft. These are vertical 8 ft long rods fully driven in the ground.

You probably have a longer path length than 30 ft. However if 0.2 A is enough charging current, then 100 ohms would produce a 20 V drop.

There is only a small saving in using the earth as a conductor, although copper is expensive these days. If you used two #14 wires for strength and the distance is 100 ft, then the loop resistance is 2*0.25 ohms = 0.5 ohms and the drop at 200 MA is 0.1 V and at the source you have good feedback on the battery voltage without interrupting the continuous charge.

You can play with other estimates based on your actual conditions.

A slightly different subject on batteries, or capacitors. Suppose you remove all the rectifiers from your CNC machines and connect all the 325 V busses of all your machines together. Leave the current capacators in the machines. Now you have a larger capacitor bank tied to all the machines. Maybe float more capacitors externally in parallel or a series of car batteries. Supply a common rectifier for this whole array.

Now machines that do a lot of regeneration will probably never activate the clamping regen circuit. In the case of a float of batteries the regen resistor would not exist. This would reduce your energy consumption by whatever amount would have been dissipated in the separate regen resistors.

If you derive power for the controllers from the 325 supply, then you can also float thru short power interruptions.

There are safety issues to consider, but these should be manageable.

Relative to regulators: batteries are a fairly stable voltage source in of themselves.

.

[epineh]

I know of one CNC machine running from batteries... link :

http://www.cnczone.com/forums/showthread.php?t=25895

It is my friends and I helped setup the batteries for him. He was looking to build a power supply and I happened to have some large quality batteries from machinery that were being de-commisioned after about 18 months. End result is ripple free power for his servo drives, we did go a little overboard with the power requirements but he is happy with the result.

Batteries supply very nice power, no transients, ripple, third order harmonics, nuthin but good old fasioned DC, but you have to allow for the recharge, with my friends machine we allowed for continual running without any charge for about 6 Hrs at least.

Geof for your setup surely solar would provide enough to charge the battery ? I guess the amount it is used will affect this but as an electrician I would either bury conduit in the ground at the required depth... blah blah blah or look seriously at solar. Then again you may not get as much sun as we do here, right now you could cook eggs on the roofing iron (I'm not joking, it works )

Russell.

[Geof]

Given that gar and epineh have aided and abetted in a thread hijack I don't feel too bad. Where were you two when I started this thread nearly 2-1/2 years ago and got very little response?

http://www.cnczone.com/forums/showthread.php?t=12391

Solar is a bit of a non-starter where I live, near Vancouver, B.C.

I like gar's ganging lots of machines together with a battery bank. It is virtually identical to a suggestion I have put to a young fellow I know who is finishing a degree in alternate energy systems. A system such as this may make it feasible to utilize solar or windpower to run industrial scale operations without needing a backup generation system capable of handling the peak loads. Really what the batteries are doing is clipping the peaks. One thing that I do wonder about, however, is whether the batteries can absorb the regen energy fast enough. There is an upper limit on how fast you can cram energy into a battery so with a pure battery/capacitor system you may still dissipate regen energy into the resistors. If I ever do manage to get something going to work on a system I will also have a flywheel system that can absorb regen energy very quickly and then bleed it back to the batteries at an acceptable rate.

[epineh]

Originally Posted by Geof Where were you two when I started this thread nearly 2-1/2 years ago and got very little response?

Join Date: 05-18-2006

I am pretty good... miracles take a little work but I am getting there

As far as ganging batteries together and charging goes, this is exactly what happens with a UPS, AC is fed in, rectified to a DC level that happens to be about the float voltage of the battery bank, anything in the order of 12-550Volts DC, then this is inverted back to 3 phase, single phase whatever the machine is designed for.

In the event of mains failure the incoming AC dissapears and since the batteries are already in circuit they keep powering the output stage and everybody is happy until the batteries go flat. (condensed version )

The killer is maintenance, battery life is generally 5 years in ideal conditions, we don't get much more than 2 here if they are not in airconditioned rooms. And of course the more batteries the higher the cost, people don't like hearing that their UPS battery bank consisting of 64, 500 Ahr 6volt batteries is now dead in the water and needs immediate replacement (been there, done that).

Yes it can be done but man is it expensive, and it keeps being expensive every five or so years. On a small scale it is not too bad, as soon as the load starts adding up, then it gets a bit more serious. I would love to get off the grid as such, but it keeps coming back to batteries, and I cannot see a better solution without drastic lifestyle changes.

There are better batteries that last longer and cycle deeper but of course they are way more expensive.

Russell.