They are charging 700 bucks for this so I'm motivated and only 2 known suppliers and one requires battery purchase from them. I'm a military trained electronics tech, not an engineer so the references I've read so far aren't sticking. I'm currently using a 11.1v 30ah Li battery, not warm enough and voltage drops as it gets depleted. Powering heated Drysuit undergarments and gloves for Scuba diving. I have 16.7v 25ah battery I want to use to a 12v load that will fail if over powered. Heated Undersuit is 1100 and gloves are 350 bucks. resistive load so dirty noisy output is no issue, average voltage is the goal. The battery canister is Delrin, so nothing bonds to it, my homemade ones are pvc, Space is very limited inside the cover where this contraption is easiest to place. any help appreciated. Commercially available bucks and pwm controllers are cheap but won't fit and not nearly as fun as building it myself. version 1 of this will have one setting 12v out. graphic shows canister, inverted cover, commercially obtained dropper for heated undergarments 100 bucks, it only provided 8.8v from 17, it makes no heat from the board and is meant for very long cave dives in fairly warm water, not the 40F water I dive in. pointers to reference materials welcome, dreaming of a schematic with 10 components on it.

 

Ebay have these...
DC 24V to 12V Step Down Power Supply Converter Reducer 20A 240W Regulator Module
They are around $20 each. Make sure you get the Step Down (buck) version as some that look the same are Step Up (boost).
The advert states 24V to 12V but they should well work on 16V too I would think.
Worth a try anyway.

 

If I understand correctly, you could probably use a 555 astable turning a mosfet on and off with the necessary mark/space ratio to get an average 12V. That’s about 72% on and 28% off.
For a smart solution which adjusts the mark/space as the battery voltage falls you could use a processor like the ATtiny85 to check the battery voltage (with a potential divider into an ADC pin) and calculate the corresponding on and off time to maintain the average 12V output

 

If I understand correctly, you could probably use a 555 astable turning a mosfet on and off with the necessary mark/space ratio to get an average 12V. That’s about 72% on and 28% off.

Or buy a dirt cheap PWM controller.

 

The device that BOBTPH indicated is probably your best option.
You mentioned that your quite expensive undergarments will fail if excessive voltage is applied to them. Failure of these PWM controllers is usually a shorted Mosfet, allowing the full battery voltage to be applied.
Thus I would only add a crowbar circuit to its output in case it fails. The unit has already a fuse that should blow up.

Where do you dive that the water gets so cold?

 

thanks for the replies. so far none of the pwm controllers or bucks that I can find will deliver enough watts/amps and fit, I ordered one anyway, maybe I can trim the mounting ears and grind it. I'll look into an astable against a mosfet and test against a resistor. I'll put a fuse in that is slightly higher than the required amps, 12. I dive off NJ mostly. it's probably 43F right now probably splash first weekend in May. Some fresh water year round, 41F top to bottom now, the top 30' warms seasonally. It gets frosty when the heat stops or there is a flood, when long deco stops are required. Technical dives can have some long deco requirements, shaking cold when you stop working hard on deco, even with the heat. The one company that made a good system abandoned it due to limited sales, the 3000 plus price tag probably, DUI offered Blue Heat 6v system, they abandoned the 11mm hood recently.

 

I tried to build the astable circuit and was presented with questions I had no clue about, resistors, capacitors, etc. I'll research the recommended frequency and try to work backwards to get the component ratings I suppose.

 

I would not bother with voltage conversion. PWM circuit is easiest, simplest and most economical. Since this is used under water, heatsink can be small and rather flat. Voltage current and temperature can be monitored to automatically adapt to situation but probably an overkill ..

As already stated, device that BOBTPH has proposed will easily do the job.

 

Now I am wondering if you could but different sections of the heated items in series. And I am also wondering about taps at different voltages from the battery pack. That would be the most effective and possibly the most efficient. Maybe the cheapest as well.