Edit to say I haven't posted here in a long while, so I figured I would just mention my latest project.

My buddy has an old Bruno stair lift with the chair. It worked for several years, but stopped working a few months back. It is only used for moving heavy band gear up and down from/to the bsmnt. It uses a 24VDC motor with 2 12V batteries in series. When we connect voltage directly to the motor, it moves like normal. The batteries are in good charge. The motor works fine. The limit switches check out as working. It would seem to be a PCB issue, and a replacement board is priced in the high multi-hundreds.

The plan is to bypass the PCB and use a couple 40A rated FWD and REV contactors with 24VDC coils and a mechanical interlock. The motor is capable of pulling up to 35 amps, even though it's using only 12awg wires to it and from the batteries. That's the factory design. The limit switches can still be used as well as the toggle switch for fwd and rev operation. He has a stand-alone charging unit that can be connected to the top and bottom charging points. This charger has always been good.

This will soon be wired almost exactly like some small panels that I build at work. The contactors will have some N/C aux contacts for control voltage. The main contacts will have 24v directly from the batteries, with a fwd/rev jumper on the line side, then a parallel jumper on the load side wired to the motor.

The control path (going in the up direction) is 24v from batt to the up/down toggle switch, through the upper limit switch, through the (down) N/C aux contacts, then to the up contactor coil A1. A2 of the coils returning to the batt neg. Obviously a similar, logical path for down operation.

This should be a good $200 solution instead of closer to $1K.

 

I seem to think that I should install a fuse inline from the battery. Possibly a 20A sloblo? I know the 12 gage wiring is good for a continuous 20A.

 

I would prefer a MOSFET H-Bridge,
but it sounds like you've got it figured out.
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The 12-ga. Wire,
if not in a packed Conduit-Pipe,
will handle ~30-Amps without issues,
but may have a small Voltage-Drop,
which might actually be a good thing for reducing the Current, and Mechanical,
Shock-Loads on Motor-Startup.
Heavy-Duty MOSFETs can easily be setup to reduce the Startup Shocks.

A Fuse is always a good idea when working with Batteries.
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The 12-ga. Wire,
if not in a packed Conduit-Pipe,
will handle ~30-Amps without issues,
but may have a small Voltage-Drop,
which might actually be a good thing for reducing the Current, and Mechanical,
Shock-Loads on Motor-Startup.
Heavy-Duty MOSFETs can easily be setup to reduce the Startup Shocks.

A Fuse is always a good idea when working with Batteries.
.
.
.

It probably had a HD transistor on the pcb...attached to a good sized heat sink. We do want to avoid most silicon components, except for the diodes at the coils of the contactors.

 

The wires that go to the limit switches are probably 24AWG, and the contactors require 4.5 amps. So I'll add a couple of slice relays that can operate on small current. They can handle 6A through the contacts, which will be enough to drive the contactor coils. I'l use the original 25CB that is on the side of the lift housing.