Replacing Batteries in Chair Lift with DC Power Supply

[rknudson]

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

Rick

 

[phinds]

I would think that the manufacturer would have the specs on what it takes.

 

[rknudson]

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.

 

[phinds]

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?

 

[nsaspook]

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.

 

[Drakkith]

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.

 

[rknudson]

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

 

[mfb]

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.

 

[Drakkith]

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?

 

[256bits]

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

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.

 

[phinds]

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

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.

 

[CWatters]

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.

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.

 

[CWatters]

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.

 

[rknudson]

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.

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

 

[phinds]

... A motor rated at 24V 4A continuous will be capable of delivering a lot more for short periods ...

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.

 

[SteamKing]

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

9.8 m/s^2 is the acceleration due to gravity.

 

[rknudson]

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

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.

 

[nsaspook]

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.

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.

 

[rknudson]

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.

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

 

[AlephZero]

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 website for the particular device? Is the specification online?

 

[davenn]

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...

I don't think that's the problem. The chair lift as stated is already in place and being used to transport the seniors up and down the stairs

The OP just wanted to convert from the existing battery supply to a mains operated DC supply
Presumably to avoid the continuous hassle of recharging the batteries

The root of the problem is as some one else hinted at before ... his 24V 200W supply cannot supply the initial huge peak current needed to get the motor turningcheers
Dae

 

[AlephZero]

Presumably to avoid the continuous hassle of recharging the batteries

The OP mentioned "the expense" and "buying batteries" (not recharging them) in two separate posts.

Using non-rechargeable batteries as a power source makes no sense to me.

 

[256bits]

I don't think that's the problem. The chair lift as stated is already in place and being used to transport the seniors up and down the stairs

The OP just wanted to convert from the existing battery supply to a mains operated DC supply
Presumably to avoid the continuous hassle of recharging the batteries

The root of the problem is as some one else hinted at before ... his 24V 200W supply cannot supply the initial huge peak current needed to get the motor turning


cheers
Dae

I would rather have that hassle of charging or changing batteries, than offering an explanation to insurance or fire department why the church has had an incident from an overheated motor, through modification of the power supply.

He doesn't have all the parameters to change to a AC lines power supply safely.
The unit could be out of alignment, dirty screw, dried lubricant in the gearbox, motor bearings.
If the lift unit is old then these could be an issue. Adding an power supply, which may end up being oversized, is not a solution.

Nothing has been said about the mechanical parts being checked first. Or how often a maintenance schedule of cleaning the units working parts mentioned. Do they test the unit a least once a week to see if it goes up and down effortlessly, pinpointing possible malfunctions.

He still hasn't answered my question on how often he has to change the batteries.
Is it really as much of a hassle as he says it is.

 

[rknudson]

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.

OK motor will surge up to 16-17 amp to get started depending on where on the stairs it is. Once running in a straight line it seems to be a constant 10 - 11 amp draw. Again I think a 25 amp PS should work for this application less someone sees a flaw in my logic (or lack there of )

 

[rknudson]

I would rather have that hassle of charging or changing batteries, than offering an explanation to insurance or fire department why the church has had an incident from an overheated motor, through modification of the power supply.

He doesn't have all the parameters to change to a AC lines power supply safely.
The unit could be out of alignment, dirty screw, dried lubricant in the gearbox, motor bearings.
If the lift unit is old then these could be an issue. Adding an power supply, which may end up being oversized, is not a solution.

Nothing has been said about the mechanical parts being checked first. Or how often a maintenance schedule of cleaning the units working parts mentioned. Do they test the unit a least once a week to see if it goes up and down effortlessly, pinpointing possible malfunctions.

He still hasn't answered my question on how often he has to change the batteries.
Is it really as much of a hassle as he says it is.

To answer the battery change question is easy. These are rechargeable lead acid batteries. About the size of motorcycle batteries. The chair does not get used often enough since it is a small congregation but we do need it to work when it is required. Replacing the batteries with a PS takes away the question of will the unit work today? And yes the entire system needs a good cleaning and that will be done to the system but I want to ensure the PS will support the design as is worst case scenario.

 

[256bits]

OK motor will surge up to 16-17 amp to get started depending on where on the stairs it is. Once running in a straight line it seems to be a constant 10 - 11 amp draw. Again I think a 25 amp PS should work for this application less someone sees a flaw in my logic (or lack there of )

The flaw in your logic is that you are asking for advice on a modification to a device that carries people, without a mention of how old the device is, whether it is in proper mechanical condition.
There is no way that anyone here can absolutely know for certain if the added power supply is a match for the device.

 

[rknudson]

The flaw in your logic is that you are asking for advice on a modification to a device that carries people, without a mention of how old the device is, whether it is in proper mechanical condition.
There is no way that anyone here can absolutely know for certain if the added power supply is a match for the device.

Device is 12-15 years old and everything works fine on the battery system. The tracks need a good cleaning but all works. I want to eliminate the batteries to ensure the unit works when it is required to work. Sometimes the chair is not parked in the proper charging station and when they go to use it the batteries are dead. Other history is that the unit will not function when the battery gets dead cells. This forum was very helpful in pointing me to proper things to check and I did not expect such a surge on startup which is why the 8amp PS was not sufficient for this application. I appreciate everyone who took the time to even read this post

 

[nsaspook]

OK motor will surge up to 16-17 amp to get started depending on where on the stairs it is. Once running in a straight line it seems to be a constant 10 - 11 amp draw. Again I think a 25 amp PS should work for this application less someone sees a flaw in my logic (or lack there of )

The problem I (and others) have is we have no way to know if these are 'normal' values for a properly working device. If the battery type is the typical 12V 7 or 8Ah gel type, higher than normal currents can lead to much shorter battery life.

As this is a human transport device that in a malfunction might cause injury to a person I can't recommend this modification of just increasing power (with proper protection devices like fuses) unless you are absolutely sure it's in proper running condition, are sure of your ability to do it safely and are willing to assume liability. It's the equivalent of increasing the size of fuses on a device to keep it running until something smokes inside.

 

[AlephZero]

Isn't there a low tech solution to all this? If the building is a church, I guess it is used at least once a week. A lead acid battery should hold its charge for longer than that.

So, just give some responsible person who attends regularly the task of making sure the chair is parked correctly and the charger is on. Lead acid batteries can be trickle charged indefinitely without any damage.

Good quality lead acid batteries should last for years if you keep them charged, but leaving them discharged for long periods will reduce the life. If the manufacturers are selling you batteries that fail quickly, buy some from a specialist battery supplier instead.

 

[Drakkith]

As this is a human transport device that in a malfunction might cause injury to a person I can't recommend this modification of just increasing power. It's the equivalent of increasing the size of fuses on a device to keep it running until something smokes inside.

Correct me if I'm wrong, but won't a 24v power supply only provide as much current and wattage as the device requires, up to the power supply's maximum rating?