Reddit mentions of RioRand™ Upgraded 6V-90V 15A DC Motor Pump Speed Controller

Here you go

This?
https://www.amazon.com/RioRand-trade-Upgraded-6V-90V-Controller/dp/B00F839VNQ

You don't really have to adjust voltage. A PWM speed controller can be used for the same purpose (adjusting average voltage) and they're much cheaper. A 10A PWM speed controller can be purchased from Amazon for $11 shipped. Most of them will output in the same voltage you supply.

https://www.amazon.com/RioRand-trade-Upgraded-6V-90V-Controller/dp/B00F839VNQ

using this: https://www.amazon.com/gp/product/B00F839VNQ/ref=ox_sc_act_title_1?smid=A298K45OP416LP&psc=1 between the input and the relays would work?

I like the simplicity of the resistor idea but It's hard to tell how hot its going to get or what speed a particular ohm value is going to reduce it to until its all said and done.

Something like this I would think, you will just need to search out one rated for your voltage req. RioRand™ Upgraded 6V-90V 15A DC Motor Pump Speed Controller https://www.amazon.com/dp/B00F839VNQ/ref=cm_sw_r_cp_apa_i_TuI3Db0B6842P

Something like this should work. Just don't turn it past 60% or so.

This motor controller!

That's helpful. The relays are rated 10A t0 12A so the current is less than I'd assumed. I mean it should be less than the rating of the relay... big assumption, perhaps. It puts the current in the range of some low voltage PWM speed controllers sold on Amazon. My concern is the lower voltage is 6V and you need it to run down to 5-5.5V will it work? https://www.amazon.com/RioRandTM-Upgraded-6V-90V-Motor-Controller/dp/B00F839VNQ Better than a resistor if i works, depends on how much of a project you want to make of it.

Something like this should work.

If your battery is 100 amp hour, and your fan draws 5 amps it will last 20 hours(in theory).