5
$\begingroup$

I'm looking for a power source that is able to deliver 12V at 15-30A (yes, amps). I was looking at high capacity batteries, but looking at the specs, they either say they have a lower discharge voltage than 12V, or they don't say anything at all.

Do such power sources exist?

I'm hoping to use this in a hydraulic system. The pumps I've seen require 12V/24V and consume around 10-15A minimum. I've been looking at Hydraulic Power Units (HPUs), and their specs say that power consumption is really high.

Reading around, it seems most, if not all, hydraulic systems are powered by small diesel engines. I would really like to avoid that, if it is possible.

If such high capacity power sources do not exist, what would be my alternative?


Apologies for the above sounding like a "find things for me" question. This was not the intention. Maybe I haven't actually understood fully how hydraulic systems actually are designed. Also, I'm not very good with words.

I am new to hydraulic systems, and I am in the process of designing a portable hydraulic system, similar to ones used in vehicles, like construction vehicles. However, my hydraulic system will not be part of a vehicle. I realise vehicles use their engines and some form of petrol/diesel to electricity generator to obtain the power required, but I cannot see a way of obtaining that same power without using a vehicle's engine.

How would I go about getting that same power without the use of a vehicle? Ideally, the system can be moved around without it being attached to AC wall electricity, but that can be left until a later date.


Details/Background of my project:

I would like to build a hydraulic exoskeleton that uses as little electrical components as possible. I have gone for hydraulic over pneumatic as hydraulic packs a lot more punch than pneumatic. Most of the sites online sell the valves, pumps, pistons etc, but I have previously found it difficult to find a way of powering these pumps, up until I asked on here a while back, where I was introduced to HPUs. From what I have understood (do correct me if I'm wrong), I need a way of powering these HPUs to generate the necessary RPM and flow rates.

I have found the pistons I would like to use, the hydraulic fluid, how much to use, the reservoir tanks, the filtering system, etc, but I am stuck on the power source. I just have no idea how to harness the power I need for this. This is where I started looking at ultra-high capacity batteries.

To be honest, I don't even know which would be best, a DC- or AC-input motor. My thought was that the constant forward drive of the motor required a DC source, hence the batteries. The link I provided in the comments below points to AC motors.

I'll keep adding details if and when they are required. As I said, I'm not good with words and I'm new to hydraulics.

$\endgroup$
  • $\begingroup$ We often use hydraulic power packs powered by 400V AC motors, mostly 22kW. Can you provide links to the pumps you mean? also, you are talking about 180-360W, that's quite low for a Diesel I think. Something seems off. $\endgroup$ – mart May 6 '15 at 12:55
  • $\begingroup$ @mart Yes, the amperage might be low, but that's from reading the flow-rate to amp graphs I've seen. The HPUs I was looking at came from HydraProducts. $\endgroup$ – Tom May 6 '15 at 13:11
  • $\begingroup$ @Tom I'm guessing that your real question is about how to power a hydraulic system designed for use on vehicles when your power source is something other than a vehicle. This is a difficult problem since pumps designed to go in vehicles have very high inrush demands, based on the assumption that there is an engine starting battery to pull from. If you can frame the question as how to design a power system for such devices, with only your local AC wall voltage as an input, I think it would be acceptable, and clearly not just a finding-things-question. $\endgroup$ – Ethan48 May 6 '15 at 13:30
  • 1
    $\begingroup$ @Ethan48 I have added details to my original question. I hope this clarifies is a bit. $\endgroup$ – Tom May 6 '15 at 13:45
  • 1
    $\begingroup$ The link you provided points at AC motors. You ask for a DC source. I think you are seriously off track, I think at this stage no answer would be helpful because your question is probably based on wrong assumptions. expand the question, state your goals, state your means, show yourresoneing so far so we can poke holes in it and help you better. $\endgroup$ – mart May 6 '15 at 13:46
5
$\begingroup$

Pretty much all electric motors demand a lot more power for starting from a dead stop than for generating motion in a steady state. There are a whole lot of schemes for smoothing out this 'inrush current' demand, either from the electrical side or from the mechanical side (by decoupling the load and gradually re-connecting it once the motor spins up.) The problem with HPUs designed for use on vehicles is that they are designed with the assumption that the power source has a battery already there to provide the starting current for the engine starter. These starting batteries can typically provide hundreds of amps for a few seconds - much more than a modest HPU will require.

Because they usually have such a large power source available, and because all of the strategies for reducing inrush current have some cost and complexity associated, vehicle HPUs are generally designed with no intention to minimize the inrush current requirements. This is made further more complicated as solenoids and relays are often used for the control systems on mobile hydraulics, which have inrush currents of their own, often applied at the same time as the pump starting.

While automotive batteries can provide extremely high currents (for a short time) most AC to DC power supplies can only deliver maybe 2 times as much power momentarily as they can in the steady state. This means that if you connect a vehicle hydraulic system to a AC to DC power supply rated for the same current that the HPU needs in the steady state, you likely won't have enough current to start the system. As a result, the pump will be starved for power and you may hear a whine or just notice it getting very hot without turning. Additionally, any relays of solenoids will likely 'chatter.' This is because on their own, there is enough power form the power supply to actuate the solenoid, but as soon as the motor is connected, the voltage drops too low and the solenoid opens again, restarting the cycle. Both of these conditions will damage your HPU and should not be sustained for more than a second.

The simplest strategy is to provide an automotive battery (typically lead-acid,) like the HPUs are designed to run off of. Depending on the size of your HPU and the length of time you need it to run on a charge will dictate the size of the battery you need. Automotive batteries are produced in a range of sizes designer for everything from starting very small cars to powering entire RVs for days without running the engine. They can also easily be combined in parallel. You mention that voltage form these batteries can sometimes drop below their 12 volt nominal value. That is true as they loose charge, but most HPUs can handle a significant voltage drop below nominal (10-20%) before performance is seriously effected. If the use will be steady state, a battery can be provided in addition to an Ac to DC power supply, which will maintain the voltage and keep the battery charged, just like a battery in a car is fed by an alternator once the engine is running.

If you must power your HPU without a battery at all, you need to plan for your AC to DC power supplies to have 2 or 3 times as much available current as the nameplate current demand of the HPU, or implement your own inrush limiting power supply (typically some combination of chokes and capacitors, but I don't know much about how they work.) Power supplies in the range of 100-200A at 12VDC are commercially available. If you need more than this, you can choose power supplies designed to be put into a parallel master-slave configuration and connect two or three power supplies together. One manufacturer of such supplies is MeanWell.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.