On 2/18/2016 2:36 PM, Snuffy "Hub Cap" McKinney wrote:
[Is there a switch that tells the motor to turn on? As such, can you tie
into that to cause the heater to turn OFF when motor is ON?]

There is a switch. Good point. I'll look into that. If the chair got
stuck mid-way for some reason, the resistor could possibly let the batteries
run down.

Exactly. And, you (or "they") are likely not to consider that this
is happening. If you use a *light* as a load, then they MIGHT notice
the light still being on -- but, what are they going to DO about it?

One could argue that the chair now risks "getting cold" and being
hard to start. But, this is an exceptional condition -- one where
you expect to have to "do something more" to get things back to
normal...

Note that even without modifying/augmenting the switch (to turn the
heater off when traveling), you could possibly mount a magnetic
reed switch in series with the load and a magnet affixed to the
stairs (top and bottom "parked" positions). So, when the chair
is in those *positions*, the circuit closes and provides heat.

You can also design something to sense power being APPLIED
(from the contacts) and engage a relay coil letting the
relay contacts engage the heater load. E.g.,

C
O + --+---||----- to relay coil
N |
T |
A +---||----- to motor circuit
C
T - -------------- to relay coil and motor circuit
S

The || being diodes. The top one is a dinky diode as it
only needs to pass enough current to engage the relay coil.
The bottom one only has to pass enough current to charge the
battery! (unless the battery is DEAD at the end of the stairs
and the "charger" is effectively trying to power the motor
THROUGH that diode -- in each case, the charger's current
capability determines the size of the diode)

Relay contact only need to carry the load for the heater
(7W / 24V ~= 1/4 A). In practice, you'd size larger to
help resist contact fusing (DC being unfortunate)

Is the motor frame (reasonably) accessible? E.g., could you wrap some
heating tape around it KNOWING that operating the tape at 24V -- instead
of 120V -- will cut the heat output to something more desirable (I have no
idea what the nominal output "per foot" of those tapes is likely to be)

Thanks. That might be better than a resistor -- will check it out.

A higher tech solution would be to run a light "idle current" through
the windings -- use the *windings* as the heating element -- but you'd
need to be able to better characterize the motor in order to exploit
that (i.e., if this was a desirable feature, that's how a manufacturer
would approach it).