Replace On/Off Seat Heater Switch with High/Low Plug-In Switch

I'm not quite sure from your question what other, plug-in seat heaters you have been looking at.

A system designed to have a high and a low output doesn't just have a different switch; something needs to be designed into it to produce a lower output when the switch is in the low setting.

One easy way to do that (and I think I've seen it done this way in some aftermarket kits) is if the seat has two separate heat elements, one for the bottom and one for the back. For 'high', you connect them both across the full 12 volts, in parallel. For 'low', you connect one to the other, in series, so each one sees half the 12 volts.

For example (I'm just making up some numbers here, not sure what typical seat heater wattages really are), suppose the bottom and back pads each have a resistance of 3 Ω. For high, you connect each one across 12 V, so each one passes a current of 12/3 = 4 amps. Each one makes 12V ✕ 4A, or 48 watts, of heat, for a total between your back and toches of a toasty 96 watts.

Now flip your switch to connect them in series instead. Now you have 12 volts flowing across a total of 6 Ω resistance, so 12/6 = 2 amps are flowing. Those same 2 amps are flowing through both pads. Each pad on its own still has a resistance of 3 Ω, so with 2 A flowing, you know each pad is seeing 3 ✕ 2 = 6 volts—they have split the supply voltage equally in this case. That means each one is making 6 V ✕ 2 A, or 12 watts of heat, so the total for both has been reduced to 24 W, one quarter of what you got on high. This is an easy trick to play, if you just add a DPDT relay somewhere and cross some wires around; you can google installation instructions/diagrams for some aftermarket seat heaters and you'll probably see an example of how to do it.

How easy it is to get individual access to the cushion and back heat pad wires in the Prius, I haven't checked, but I'd assume it's not too hard; the wiring diagrams are on techinfo.

-Chap

 

Aha! So dual-power seat heaters are available for the PiP, are they?

Jackpot. Just compare the wiring diagram for the PiP and your Five, and get a sense of how many differences there are. Might be fairly minor.

-Chap

 

Would it be simpler to add separate controls for the drivers seat back and bottom? I would love to be able to have the seat back on to help with my aching back without having to have my butt toasted as well.

 

Bit of wiring diagram for a 2013 Plug-In:



You can see they've gone and divided the seat heating pad into two paths of resistance heat joined in the "middle" (as shown schematically here, doesn't have to be the physical middle of the real heat pad). One of them is permanently grounded at the end labeled terminal 3, while terminals 1 (the "middle") and 2 (the other end) are wired to the switch.

Follow the diagram around and you see when the switch is in LO position, current flows out the switch terminal 1(LO), into the heat pad terminal 2, goes the long way through both lengths of heat wire in series, and to ground from terminal 3.

Put the switch to HI and now current flows out the switch 3(HI) terminal, into the head pad terminal 1 ("middle"), and passes through both lengths of heat wire in parallel, one to the permanent ground from terminal 3, the other to ground via the switch terminal 1(LO) which is now connected to ground when the switch is in the HI position.

If the story ended there, you'd get 4 times the heat on HI that you get on LO (same story I gave in post #2). But it doesn't quite end there, because the two little tie-fighters shown in the heat pad are thermostats capable of interrupting the current when the pad hits a certain temperature.

As the heat pad in a non-Plug-In has only two terminals, one path, and one thermostat, you don't really have a drop-in upgrade.

-Chap

 

Or for a whole 'nother way of thinking about it....

Drill a couple new holes and install a single or dual temperature controller intended for 12 volt heated motorcycle clothing. You should check the wattage capacity with the vendor and measure the draw of your seats first, but I'd think it pretty likely the wattages are similar.

I'm having trouble finding installation instructions for those controllers on that site (or specs), but it looks like you just drill a hole the size of the control knob shank for each one, and a smaller hole next to it for the LED to peek out.

Then, smugly flash a smile at all those Plug-In drivers, who have only two heat settings.

The above controllers are made offshore ... one of those unhappy stories, Gerbing was a long-known, proudly-made-in-US kind of heated motorcycle accessory operation, got acquired, got offshored, reputation suffered .... Now apparently the name of that company has been changed to Gyde Supply, while Gordon Gerbing has started over with a new, made-in-US operation called Gordon's Heated Clothing. They have a much smaller product line; there is a (dual?) heat controller too (made in Oregon), five levels rather than a knob adjustment. That one's in a finished package to wear on your belt, etc., which could make it more challenging to install nicely in the car.

... maybe if the dimensions were just right, there'd be some way to mount it behind the existing panel and have both rockers showing right through the rectangle cutout for the OEM switch....

-Chap