Parallel 2 DC Power Sources

[tkc100]

This is probably a very simple question but I can't seem to get it through my head at the moment.
If I have two dc power supplies:
12 volts at 50 amps and another 16 volts at 10 amps
and I connect them in parallel what is the final output
Voltage - Amperage

 

[carbonzit]

This is not a simple question. Therefore I cannot give you an answer. (Others here probably can.)

Since you have two different voltages, the internal resistances of the supplies come into play. One would have to, presumably, Thévenize the whole thing to give a definitive answer.

I can tell you this: in such a situation, the two supplies will be "fighting" each other because of the differing voltages. Not a good situation.

 

[KeepItSimpleStupid]

One will "let out the magic smoke" per say.

Supplies can and are designed to operate in a master/slave arrangement, but the supplies have to be the identical model.
So, two 15 V, 50 A supplies could be connected in series and/or parallel, but the supplies have to be capable of doing so especially for the parallel situation.

In the series arrangement, one knob or whatever will adjust both supplies equally.

In the parallel arrangement, the setpoints again are the same, but one of the supplies operates in a manner that it's output becomes the setpoint to the other supply.

If you set the voltage to 11, the other supply will follow the 11V. If the master current limits, then the voltage of the master will decrease and, in tern, the setpoint to the slave will become the output of the master.

So, you can see how this could work.

If your batteries were as you had said, one of the battieries would charge to the higher voltage and might explode.

 

[dougy83]

If the 12V supply has a forward/sourcing/high-side (obviously I don't know what it's called) regulator only, then the output will be 16V/10A.

 

[Gary B]

Assuming that the power supplies are diode blocked so the 12 supply is not short circuiting the 16 volt supply, you will have 16 volts at 10 amps. The 16 volts on the output of the 12 supply will effectively turn it off.

 

[cowana]

As the above replies say, there are two possibilities:
1) The 12v supply has a built in diode.
Result: It is essentially disconnected, so you have a 16v, 10A supply.

2) The 12v supply has no diode.
Result: 16v will be forced into the 12v supply, meaning it will either:
- Shutdown
- Overheat
- Catch fire
- Destroy the transistors
or something else bad.

 

[tkc100]

Thanks for all the responses.
Perhaps I need provide a bit more information as to what generated this question.
I have a power converter designed for a recreational vehicle that works in parallel with one or more 12 volt batteries to delivery 12 volts at 50 amps.
Unfortunately it uses a ferroresonant transformer and is a good stable power supply but a lousy charger. See attached diagram. It just never attains a voltage adequate for completely charging a lead acid battery. Further is seems to be sensitive to any variation in the supplied electricity. Running off a generator which invariably has some cycle variation it hardly maintains the batteries at all.
When I hook a small 10 amp battery charger (see attached diagram) up in parallel at the batteries they begin to charge. It eventually achieves the maximum output voltage of the small charger. I know this is not a good solution but I was wonder just exactly what was happening.

 

[dougy83]

I was wonder just exactly what was happening.

If the 50A supply if producing sufficient voltage to charge the batteries, its output will be combined with the 10A supply (at the battery voltage). As the battery terminal voltage rises, the 50A supply won't be able to supply a high enough voltage to forward-bias its rectifiers and will no longer be supplying any charging current; all the charge current will come from the 10A supply. This is all fine, as the 10A battery charger is apparently designed to work over the battery voltage range; all that is different is that it is assisted at lower battery terminal voltages.

 

[tkc100]

dougy83
Thanks for the response
The output of the converter is rated at 50 amps 12.0 volts min @ full load, 14.1 volts max @ no load although I never saw an output voltage that high with the batteries in the circuit.
I am not an electrical engineer or any thing like that but I do know that for a lead acid battery to fully charge it needs an input voltage between 13.8 and 14.5. I can leave this converter hook up to shore power for days and never see a counter voltage within that range. That’s went I hooked the smaller battery charger up and things got markedly better. The battery actually started coming up to full charge and within a very reasonable amount of time.
Now let me see if I understand what you are saying. As long as the terminal voltage at the battery is lower than the max voltage output of the converter the two units will work in tandem. Theoretically at this point the charging current could be 60 amp.
As the battery terminal voltage matches the maximum output voltage of the converter only the smaller charger would be supplying current but the charging voltage could go as high as the smaller charger’s transformer would allow.
Is this right?

 

[dougy83]

Yep that's right.

The diodes in the supplies stop the current from the batteries or the other supply from feeding back into either supply, and therefore allows the 2 supplies to be connected in parallel without issue.

You could measure the current from the 50A supply to see when it stops providing useful power if you wanted.

 

[tkc100]

Would you consider this a fix or a patch?
I have considered changing out the transformer in the converter to standard reduction transformer with a secondary output somewhere in the range of 16 to 24 volts. I could then feed the DC output through a Xantrex voltage controller I already have. It would provide both a three stage battery charger and a low voltage cut out to protect the batteries from being too deeply discharged.
The problem with this idea is that I have not been able to locate such a transformer. Perhaps it is because I don't know exactly what to call it and therefore it's difficult to Google. I know they are out there because they are used in practically ever 12 volt battery charger. So far looking for a 16 to 24 volt transformer with an amperage rating between 50 to 60 amps has not been fruitful.

 

[dougy83]

If it's working well enough for you then it doesn't matter whether it's a fix or patch.

You can't just search for 1kw transformer or something?

You could have a look at rectifiers, though they are generally a little pricey. e.g. https://www.aegps.com/en/products/industrial/RectifiersChargers/
https://www.sbsbattery.com/subpage_index.php?_subp_=133

 

[tkc100]

Thanks for all you assistance