Parallel Up-Converters

Thread Starter

BillNole

Joined Oct 15, 2020
6
Greatings!

I have been using an upconverter to boost voltage from a 12V deep cycle battery to 24V to power a 500W motor used on a hobby device I've built. The upconverter can output 40A continuous, which works well with the application. The motor itself is rated at 24A max at 24V, but I measure peak draw of 38A to 40A for a couple seconds at startup. (This is for hobby use and non-critical...)

I have active ventilation blowing across the motor and upconverter, and the device is used for a consistent 20 seconds, followed by a 100 second wait for the next cycle. I've never felt any heat beyond just perceptable warmth on any components in the device, even after 100 cycles over a time period of just over 3 hours.

The question... I also have two smaller upconverters onhand (12V to 24V) that can output 20A continous each. Can these be wired in parallel to use in place of the single larger upconverter? Would this result in output capability of 40A (or thereabouts...) at 24V?

The honest reason I'm asking is that I want to build a second device as a backup to my first and the larger upconverter costs about $160 while the smaller ones are about $30 each and I have them onhand already... The two of them combined are also much smaller than the larger one and simply more convenient to mount.

Thoughts to share?
Upconverter Images.png
 

Papabravo

Joined Feb 24, 2006
15,724
Wiring SMPS in parallel can be dangerous if they are not specifically designed for that purpose. What happens is that the one with the slightly higher voltage will take most of the load. You probably wont get the result you expect. What is wrong with two batteries in series to make 24V?
 

Thread Starter

BillNole

Joined Oct 15, 2020
6
Wiring SMPS in parallel can be dangerous if they are not specifically designed for that purpose. What happens is that the one with the slightly higher voltage will take most of the load. You probably wont get the result you expect. What is wrong with two batteries in series to make 24V?
Cost and weight of the added battery. I'm already carrying three 12V batteries for the necessary equipment and backups for the purpose. I'm succesfully using an upconverter now and very much like not having to carry and recharge an additional battery. Then, there is the cost of an upconverter being substantially less than the cost for a large deep cycle battery...

It's that dangerous part you mention that I want to avoid... These devices are used outdoors and I have a 50A marine circuit breaker attached and a fire extinguisher nearby (good friend is a Fire Chief and insisted I take his advice and keep one onhand due to the amps involved...) If a meltdown occurs, along with a fire, I can quickly disconnect if the breaker doesn't kick in and reach for the fire extinguisher. Still, I'd rather not burn up my toys and have to start from scratch, if I can avoid it...
 

Papabravo

Joined Feb 24, 2006
15,724
If you need more power from the Boost Converters you are better of with a single unit that can handle the load. If you think about it, when you wire two of them together you will have two independent control systems trying to control the output voltage and current while the measuring point is being affected by an outside influence. This is IMHO a recipe for disaster. Don't do it. Buy what you need or fugeddaboudit. Take my advice or don't - it really doesn't matter to me.

The other thing to know about any DC-DC converter is that the power out will always be less than the power in. Sometimes it will be a great deal less. For example:

24 Volts Out at 40 Amperes is 960 watts - geez almost a kilowatt!
Assume the converter is 90% efficient - aggressive but not out of the question.
How much input power is required? Glad you asked:
it is 960 watts / 90% = 1067 watts -- over 1 kilowatt
How much current is coming out of the battery?
Is 1067 watts / 12Volts ≈ 89 Amperes -- Now that is some serious current
How long will the battery last assuming you you quit using it before it drops below the "I cannot recover and hold a charge" level?
Don't let your batteries get to less than about 10.5 volts. Going lower will force the converter to try to pull more current with the possibility of permanent damage.

Whatever you try to stay safe.
 
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Thread Starter

BillNole

Joined Oct 15, 2020
6
Appreciate your position and you offering it Papa. I'm already aware of the facts you've stated and it makes sense.
I've done a lot of unadvised things in life and sometimes they work out and sometimes they don't. Testing this entails connecting everything and seeing if something pops, or smokes. Not complicated, nor will it be beyond my budget for this hobby if it does. Neither would buying the additional upconverter, but I also wouldn't personally learn whether using the two I have would actually work. I suspect it likely won't really work, at least not once everything is under load, and I have to decide whether to risk blowing some components to find out myself. Who doesn't get a thrill out of succeeding at something other said wouldn't work?!
It might lead to a cooked component (or a few...) and cost more to try out, or it might work out fine. Maybe, somewhere in between too! I'm interested in hearing others thoughts on it and don't disagree with you at all, except possibly the last three sentences... I can be convinced not to do this and it won't take much to do so, but I'd like to hear another opinion or two. Thank you!
 

Papabravo

Joined Feb 24, 2006
15,724
Maybe one of the other members would like to opine on why connecting two SMPS in parallel is a bad idea. I suspect the TS is only partially willing to take my word for it.
BTW connecting them in series to get a higher voltage is also, generally speaking, a bad idea.
 

Thread Starter

BillNole

Joined Oct 15, 2020
6
Thank you Papa. You have made your point well and I look forward to you following along, in the case anyone else does respond. Maybe I'll proceed with the test just because... or maybe not! :)

Best to you! ;)
 
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