drop battery voltage

[hydrojohn]

Hi folks, I need to drop a 12 volt battery down to 2.2 volts. The load will be 20 - 25 amps not sure yet but it will be consistent. I've been reading about using a series voltage dropping resistor and I am out for easy, I don't mind power spill as it's a one off test scenario (single cell electrolysis for respirator). Is this the easiest way to do this?

 

[James]

I would use a variable resistor, I expect you will need to adjust the resistance regularly as the electrolyte changes conductivity and the electrodes wear or tarnish.

 

[DPG]

It would be a very hefty variable resistor to handle 25 Amps.

 

[telectrix]

It would be a very hefty variable resistor to handle 25 Amps.

make toast in it, or design and build a suitable electronic witchcraft circuit. things like SCRs and Zener diodes are still available even after the demise of Maplins.

 

[hydrojohn]

I would use a variable resistor, I expect you will need to adjust the resistance regularly as the electrolyte changes conductivity and the electrodes wear or tarnish.

Thanks I figured i'd have to look into adjusting the voltage around the 1.7 to 2.2 mark any way. It's a PEM elestrolyzer and so doesn't go through the same aggressive electro chemical conditions as a normal hho unit. The reaction takes place on the membrane/catalyst layer. Further it's just distilled water

 

[DPG]

Thanks I figured i'd have to look into adjusting the voltage around the 1.7 to 2.2 mark any way. It's a PEM elestrolyzer and so doesn't go through the same aggressive electro chemical conditions as a normal hho unit. The reaction takes place on the membrane/catalyst layer. Further it's just distilled water

Sounds interesting. Just remember whatever you use to regulate/reduce the voltage needs to be of suitable power rating.

 

[hydrojohn]

Sounds interesting. Just remember whatever you use to regulate/reduce the voltage needs to be of suitable power rating.

Yeah It's got to carry 20 - 25 amps..
[automerge]1601826593[/automerge]

Yeah It's got to carry 20 - 25 amps..

Probably going to be hard to find

 

[stewartrossinv]

Dropping 10V at 25 Amps would be 250 Watts power dissipation - a lot of heat to cope with. The inefficiency of this approach will also exhaust your 12V battery - they are not designed for continuous current output of 25A (starting peaks are much higher but for short duration). I wonder how long a typical 40-70 Amp-hour battery will last?

Wirewound or thick-film 1 ohm resistors that you could put in parallel of about 250W rating are phenomenally expensive - like this thick-film one from Farnell at £78 - https://uk.farnell.com/vishay/rps0250dh1r00jbza3/resistor-250w-1r/dp/1141565.

Some form of switch-mode power supply delivering this is the only way I can see it would work in the short term, but I've not found anything suitable for 2.2V at 25A.

This reminds me of the power supplies for the first generation of British colour TVs in the 70s - the Thorn 2000 chassis (I think) used large dropper resistors to derive some higher-current supplies for line scan circuits before switched-mode techniques using thyristors became commonplace. The large dropper resistors regularly failed as a result of the heat dissipated, which kind of defeated their purpose.

-Stewart

 

[DPG]

Some kind person may design you a switch-mode supply with the right parameters. In fact there will be circuit suggestions on the web which you may be able to tweak. Obviously this would need a good knowledge of electronics though .

 

[Julie.]

It may be better to review your choice of battery.

Some batteries expose each cell, so it may be possible to use one cell at a time ~ 2V or so, or use a couple of cells and a correspondingly smaller resistance - you could even use a simple bipolar transistor with an adjustable voltage regulator if you are only dropping a volt or two 25-50W could be easier to disapate than 250W with a standard heatsink.

However even going to a 6V battery will significantly reduce the resistance and power lost

 

[Avo Mk8]

There are budget 3.3v PSU's about with 30A output, eg:
View: https://www.amazon.co.uk/Switching-power-supply-MeanWell-RS-150-3/dp/B00MWQE1QO/ref=sr_1_2?dchild=1&keywords=3.3v+30a+power+supply&qid=1601837220&sr=8-2

whether that's a genuine 30A continuous i don't know.

Assuming the o/p is not adjustable, you would still need a variable resistor to tweak the output, but not such high wattage, eg:

Electronics-Salon 50W 1 OHM High Power Wirewound Potentiometer, Rheostat, Var... | eBay

Quantity: 1 piece. Size: A=63mm, B=40mm, F=71mm, L=24 +/-1mm. Date First Available: 1 Nov. 2012. Manufacturer reference: VR-50W/1-1.

www.ebay.co.uk

How long would you like to run this for? Life of a 12v battery might be a few hours, or less, as said.
Good luck with the experiment.

Or maybe a lower voltage battery as suggested by Julie, with the above rheostat if you don't go the electronic route.

 

[Lucien Nunes]

Using a resistor will waste 82% of your battery energy as heat, and the resistor will be large and costly, more costly than the power supply above. That PSU is a decent brand (Meanwell) and will be continuously rated for the claimed 30A, but might require either a heatsink on the back of the chassis or a minimum air velocity - check the specs.

The rate of reaction in an electrolysis cell is proportional to the current, therefore it is helpful to regulate the current rather than the voltage. The 12V battery and dropper resistor make an approximation to a current source, since the voltage dropped greatly exceeds the unavoidable variations in cell voltage. Using a lower fixed voltage and adjusting the current with a lower resistor will not give quite such a stable cell current as any voltage variations will represent a much larger fraction of the supply regulated voltage.

I would probably use something like this:
DC-DC buck converter
Which will drop the battery to your desired voltage with only 10-15% energy loss as heat. There are many similar products, made down to a price but all the ones I've tried do what they say on the label. Don't push them to their limits! I usually change the electrolytic capacitors for good-quality low-ESR types before use, as the cheap capacitors fitted are the most likely point of failure.

A useful feature of some of these modules including the one in the link above is the option to run in either constant-voltage or constant-current mode, so you can have stable cell current without any further dropper resistor.