Battery replacement

  1. We have a battery operated chair lift for seniors at our church. I am looking to replace the on board batteries with a DC power supply. The unit has 2 small 12 volt batteries in series to provide 24Vdc to power the chair. The motor is a 24Vdc 4 amp motor. I had put in a 24Vdc 200 watt power supply and it does run the chair up and down the staircase without issue. However, this configuration will not take the load of a person sitting on the chair to bring them up. I am trying to determine what wattage power supply I would need in place of the two 12 volt batteries.

    This unit is not used that often and battery replacement is an expense the church cannot always support. I am seeking the DC power supply solution to have the chair available anytime it might be required. Any input from this forum would be appreciated to point me in the right direction. Please let me know if you require more information

  2. jcsd
  3. I would think that the manufacturer would have the specs on what it takes.
  4. Manufacturer was my first stop. They do not have a supported solution for this since it is not within their design spec and they want you to continue to buy batteries. Like I said, I have tested this with a 200 Watt power supply and it works fine but not with a load on the chair. I am not sure how to calculate what size wattage power supply to compare to the 2 batteries. The batteries are about the size of a motorcycle battery in series to provide the 24Vdc for the motor.
  5. The manufacturer's design spec doesn't include any load? Having a load is outside the manufacturer's design spec?

    I must be missing something. I don't understand what the point of the chair is if it can't carry a load. That is, why does the chair have batteries that won't do the job?
  6. nsaspook

    nsaspook 1,388
    Science Advisor

    Get a clamp-on DC amp meter with a peak function. You might be shocked at how high the start current is with the torque needed with a full load. I would try to find a rechargeable battery solution to your problem as the peak current requirements needed will requires a over-sized power supply for the starting current unless you buy a big unregulated supply designed to handle motor start overloads.
  7. Drakkith

    Staff: Mentor

    Phinds, I think the problem is that the batteries do provide enough power to run the chair, while the power supply doesn't seem to.
  8. I ill check the current draw with the batteries, the motor has a spec of 4amp max which is why I got an 8amp PS, but there is output wattage considerations and a 200 watt PS is not enough to drive the chair with a load on it. Not sure how to determine the proper output wattage required for a PS
  9. mfb

    Staff: Mentor

    Those 4 A max could be meant for continuous operation (where overheating is an issue), not for a short startup period (where heat is probably not an issue).
    If you can, measure the current during operation. I agree that a rechargeable battery would be a good solution. Capacitors should be possible, too, but that looks expensive.
  10. Drakkith

    Staff: Mentor

    What's the rated power of the motor? Or, for those more knowledgeable than I when it comes to motors, is the power of the motor the rated current times the voltage?
  11. The batteries for the unit should already be of the rechargable type. How often do you have to change them? If they are not charging up after use, there could be a problem with the charging circuit.
  12. If the motor is rated at 4amp max, then if it is running off of 24V, it will draw a max of 96Watts (forgetting about startup surge), so a 200Watt power supply should be way more than enough.

    I still think we're missing something here.
  13. No that's wrong. A motor rated at 24V 4A continuous will be capable of delivering a lot more for short periods. The chair lift makers rely on this because they know their chair lifts won't be used continuously - perhaps just a few times an hour.

    You need to do what mfb and naspook have suggested and measure the peak current during normal operation on batteries with a load. You might not need a power supply that can deliver that continuously but at the very least if it has a current limiting function it will need to be higher or the lift probably won't start with a load - and that's likely to be the problem you are seeing.

    I suspect it might be simpler to source batteries from another supplier than the chair lift maker.
  14. PS: How long does it take the lift to climb say 3m?

    If the chair with load weighs say 120kg and takes 20 seconds to climb 3m then the power required is at least..

    p = mgh/t = 120 * 9.8 * 3/20 = 176W

    without allowing for friction of losses in the drive train.

    The start up load could easily be a lot higher, perhaps five times that, and could be tripping out any short circuit protection in an off the shelf power supply.
  15. What is the 9.8 value in the formula? The rate is much slower more like 1 m in 20 seconds but will run some tests today when church is over. Thanks for all the input
    Last edited: Aug 3, 2014
  16. OK, THAT's what I've been missing. Thanks.

    I think of surges as being a second or so at startup but I'm thinking from the point of view of the source, not the motor, which in this case didn't work out.
  17. SteamKing

    SteamKing 11,013
    Staff Emeritus
    Science Advisor
    Homework Helper

    9.8 m/s^2 is the acceleration due to gravity.
  18. OK I ran a test today, the unit will draw 16amp with a load on it going up hill. That explains why the 8amp PS would protect it self and shut down. So I will look into a 25+amp PS which is should provide about 600 watts of power. Hoping this is the right solution for this application.
  19. nsaspook

    nsaspook 1,388
    Science Advisor

    It will work but I would be concerned about the duty cycle of the unit with that much steady current at full voltage overheating the motor quickly. I think one reason it's eating batteries is because the motor load is really too high for the type of battery and the cells are outputting reduced voltage quickly after starting at that current draw.

    Do you have a I/V plot of the lift during a loaded climb. It would be interesting to have the actual cell voltages at 16A so a true power curve can be created.
  20. I will go back and run some more tests, all I did was get peak load today. I will see what current draw is during a straight run. I know my basic electric theory but it is testing my memory skill set. I do appreciate all the feed back from this forum though :approve:
  21. AlephZero

    AlephZero 7,248
    Science Advisor
    Homework Helper

    My feeling is we haven't got to the root cause of the problem yet. It's not clear exactly what is "outside the design spec" and not supported by the manufacturer. Maybe the whole device is not designed for transporting adult-people-sized loads up slopes corresponding to a flight of stairs.

    You can certainly buy battery operated stair lifts that do work (and with rechargeable batteries), but if your nominal 4A motor is drawing 16A continuously for the full duration of the climb, that doesn't sound right.

    Can you post a link to the manufacturers web site for the particular device? Is the specification online?
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