Replacing Batteries in Chair Lift with DC Power Supply

[TurtleMeister]

I agree with AlephZero's solution, since the real problem is:

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.

 

[nsaspook]

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?

Yes, but the amount of power in the original circuit was limited by what could be supplied by a small battery (the battery voltage would drop during a excessive current draw) if the motor stalled or was very heavy loaded. With a 24vdc regulated 600W supply during the same conditions you might have 2x to 3x the power.

 

[Wes Tausend]

...

For economy, I would go with standard batteries and recover them by modern low voltage 3-stage chargers. A new single 24 volt specialty charger would slightly simplify the mod, but two common 12 volt units would actually cost less (about $20 each).

From the sounds of it's needing a specific charge station rest position, part of the problem here is to somehow supply a continuous heavy amperage to the motor which may be traveling in the lifted chair. Running 110vac to a riding power supply (or charger) on a flexible, spring-loaded follow-cable can be a bit dangerous if it gets pinched. Alternately, running full amperage 24vdc on a similar spring-loaded follow-cable will cause a significant voltage drop (therefore current) in spite of huge cables to transfer the increased amperage needed over the entire length. But... if just a 12/12 set of low-volt/low-amp tricklecharge wires must be run over follow-cables, then much smaller 3-wire cables can be used to gently recover onboard batteries.

If the motor is in a stationary drive position near the stairway, then wiring is much safer and simpler. That scene was not the impression I got, however. Still, batteries may be needed for current surge since it is a low volt system.

I would sincerely recommend the simplicity of good chargers instead of a giant power supply. For good battery life, it is imperative that at least 3-stage solid-state charger(s) be used. If the original charger is much more than 10 years old, it is likely a very crude older design. More efficient modern economic examples are small Schumacher designed/built smart chargers sold by Sears or Walmart.

The 3-stage charger units are computerchip controlled to supply a heavier current initially, a lower voltage as the battery reaches full charge and a periodic voltage-monitored on-off charge for storage. They will save your small batteries and make them last a long time. I have some of those for my seldom driven vehicles, bikes, boats and camper. For full size batteries, the $7 Harbor Freight (HF) solid-state battery chargers work ok, but for motorcycle batteries, the HF tend to slowly boil the water off over several months. These HF chargers are psuedo-2-stage in that they limit voltage to 13.8 so charge-current does drop as the batteries reach 100% charge, but the leakage current is enough to still lose water in small batteries. The worst chargers are old transformer driven diode units that overcharge with higher and higher voltage as the load drops off, perhaps like your aging original lift unit.

Modern RV (camper) units usually have built-in 40 amp+ 3-stage 12vdc chargers (converters) and both slide-out rooms and coupler tongue jacks use common 12 v motors that draw heavy current. In spite of the HD power-supply/charger, the motors still need good batteries to function because of heavy peak currents. After all, total parallel winding currents alone determine magnetic strength and therefore torque to prevent stalling.

Good luck.

Wes
...

 

[256bits]

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?

Not really. Power supplies have their own characteristics. It is more of a " can supply x current at y voltage and % ripple."

A typical transformer-diode(s)-capacitor power supply will have an open circuit voltage, 1/2 of the tranformer peak-to-peak AC voltage, and no ripple at no load. We have full wave, bridge, and half wave supplies.
( A transformer-diode(s) combination will just have the DC voltage of the AC voltage, and this is not the rms value of the AC voltage, but less, and a large ripple factor. )

To decrease the ripple voltage, and thus increase the DC level of the output voltage, one can add larger capacitors. Even so, with increasing load, output voltage will show a drop as the capacitors discharge more of their charge into into the load. Subsequentially, as the capacitors can only be re-charged only on a % of the cycle, the charge current through the diodes and transformer is higher than the nominal current through the load. The transformer windings then have to have a suitable gauge to accomidate the charge current as to avoid I-squared-R losses through the windings. No wonder then that transformer DC power supplies can get so huge.

So a 24v 200 watt power supply rating of these type of unregulated supply, is not all that indicative of how it will function in a circuit. Is the 24 v the rms value of the transformer or the DC value, or 24 volts at y current?

 

[mfb]

A power supply also gives a mechanical issue - how to make sure the cable is always connected to the device, does not get stuck in the drive mechanism, blocks anything else and so on?

If the issue is just

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.

then a spare battery should solve the problem. In case someone forgets to park the chair correctly, you replace it, and think of recharging both afterwards.

 

[puncheex]

Pretty generally the spec on the motor will be "xyz amps at abc rpm". At very near zero rpm, it will try to drain a lot more than that. Motors quite often will accept 10 or more times their ratings under those condition, and batteries can supply it, for a short time. Power supplies often cannot because they are speced to other requirements, like regulation of the voltage and lack of interference with rf devices and so on. As they say above, try a heavy duty un-regulated supply, such as a battery replacement supply. In fact, and automotive battery charger - replacer might be just what you want.

 

[NascentOxygen]

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.

I can appreciate the benefits long-term of eliminating battery dependence. In light of sundry issues that have been raised in consequence of this, perhaps an answer for the short-to-medium term might be a buzzer that sounds when the conveyance is parked but is not correctly connected to the charger?