Basic Power Electronics Question

[raschemmel]

Given two dc sources, drawn as battery symbols in series, one being a 160V/220A EV battery, the other a 160V HV DC current limited power supply, with a load (INVERTER) drawn as a 1.6 ohm/80kW resistor across the two sources in series , is there any reason this wouldn’t work, given that the power supply , by itself can power a 360V/220A/80kW load by itself, without the battery ? The question is whether the two sources are incompatible and therefore not able to work in series or whether it makes no difference as long as both sources can handle the 220A current ? Is there anything that would prevent you from connecting these two very different dc sources in series ? For that matter, does it even make any difference what the voltages of the two sources are as long as they total 360V ? Is there any reason the current limiting wouldn’t work in this configuration ? (given that in order to source more current , the battery pack has to have a path to ground that allows more current and the power supply current limiting should prevent that) Not that it matters, the inverter’s load is a 3-phase EV motor, which it drives just fine with the dc supply by itself. The purpose of the question is to understand WHY this would not work, if that is the case. I believe it has something to do with the difference in the internal resistance of the two sources, which , while both dc sources drawn as equivalent dc sources, are , in actual fact, far from equivalent.

 

[crutschow]

does it even make any difference what the voltages of the two sources are as long as they total 360V ? Is there any reason the current limiting wouldn’t work in this configuration ?

Two problems I see:

One is that, if the power supply is turned off, it has to withstand the battery voltage across it terminals without damage.

The other, is that the current limit circuit will have to withstand the full 360V to limit the current.
It likely can't do that without failure.

 

[rjenkinsgb]

Any attempt to draw more current from the battery than the PSU can provide will result in a reversal of polarity at the PSU terminals.

It would worse than short circuited.
And a current limit within the PSU would just cause that to happen at a lower current.

Hypothetically, an appropriately rated reverse diode across the PSU output could limit the reverse voltage but the short circuit effect would still happen.

 

[raschemmel]

"One is that, if the power supply is turned off, it has to withstand the battery voltage across it terminals without damage."
They are in series , not parallel.

 

[Nigel Goodwin]

Given two dc sources, drawn as battery symbols in series, one being a 160V/220A EV battery, the other a 160V HV DC current limited power supply, with a load (INVERTER) drawn as a 1.6 ohm/80kW resistor across the two sources in series , is there any reason this wouldn’t work, given that the power supply , by itself can power a 360V/220A/80kW load by itself, without the battery ? The question is whether the two sources are incompatible and therefore not able to work in series or whether it makes no difference as long as both sources can handle the 220A current ? Is there anything that would prevent you from connecting these two very different dc sources in series ? For that matter, does it even make any difference what the voltages of the two sources are as long as they total 360V ? Is there any reason the current limiting wouldn’t work in this configuration ? (given that in order to source more current , the battery pack has to have a path to ground that allows more current and the power supply current limiting should prevent that) Not that it matters, the inverter’s load is a 3-phase EV motor, which it drives just fine with the dc supply by itself. The purpose of the question is to understand WHY this would not work, if that is the case. I believe it has something to do with the difference in the internal resistance of the two sources, which , while both dc sources drawn as equivalent dc sources, are , in actual fact, far from equivalent.

In order to work, both sources would have to be perfectly balanced, at all loads, all states of charge, and at all times. Otherwise - BOOOOMMM!!

Silly idea, and while theoretically possible, it would need considerable electronics to make it work, and work safely - you couldn't just 'chuck' the two together.

To go a little further - NEVER, NEVER, NEVER mix old and new batteries, or different makes and types of batteries.

 

[raschemmel]

"The other, is that the current limit circuit will have to withstand the full 360V to limit the current.
It likely can't do that without failure."

Since they are in series the HV P.S. only sees the
voltage across it's terminals and the current passing through it.

 

[raschemmel]

"In order to work, both sources would have to be perfectly balanced, at all loads, all states of charge, and at all times. Otherwise - BOOOOMMM!!"

Define 'balanced'
As stated , both battery and PS. are rated for the
load on the inverter. Also, the purpose of the
experiment is not to run it at any significant
load rating but rather to find out if it works at
any lower loads (lower rpm) first.

 

[raschemmel]

"Hypothetically, an appropriately rated reverse diode across the PSU output could limit the reverse voltage but the short circuit effect would still happen."

Theoretically, if the battery became short circuited,
(which I can't imagine happening for no reason),
then the inverter would only see the dc P.S. input
voltage (A+0=A).

 

[raschemmel]

"To go a little further - NEVER, NEVER, NEVER mix old and new batteries, or different makes and types of batteries."

The purpose of the question is not to learn dos and don'ts. but to understand why it will or won't
work. If you make a statement, you need to explain your reason. ie, WHY mixing different batteries
(with obviously different internal impedances)
is bad. ie; What will happen if you do that ?

 

[Diver300]

Simple models can be too simple, and when it gets into the sort of power involved in EV batteries, then there can be hidden dangers, and significant dangers.

When putting power supplies in series, you have to consider what happens when one fails. If you have some small mains power supplies, say <1 kW, it's quite easy to put a big diode across each one so that if it stops, the other one can't drive a negative voltage across it.

At 64 kW, just engineering diodes like that is a real problem.

Just looking at things on a very superficial level, I can see a few potential problems, and with that amount of power available, any problem can be really serious.

1) What happens if the power supply shuts down? Will the battery drive the voltage negative on the power supply terminals?

2) What happens if the battery shuts down? An EV battery has its own battery management circuits, so if it loses the 12 V supply, it will turn off the contactor. At that point, the contactor will be subjected to 320 V, and breaking the current under load, and it may not be rated to that. With the contactor open, will a negative voltage across the terminals cause problems?

3) What happens when the batteries discharge?

4) What happens if the inverter short out or otherwise overloads the circuit? Does the power supply turn off, the battery management system shut down or a fuse blows?

5) Can the inverter ever generate power? If it's an inverter driving a motor, that motor has inertia, and slowing that motor means the energy in the inertia has to go somewhere. If it goes back onto the 320 V supply, will the power supply and the battery be OK with that?

There are probably a load of other potential problems that will need to be looked at.

 

[Diver300]

"To go a little further - NEVER, NEVER, NEVER mix old and new batteries, or different makes and types of batteries."

The purpose of the question is not to learn dos and don'ts. but to understand why it will or won't
work. If you make a statement, you need to explain your reason. ie, WHY mixing different batteries
(with obviously different internal impedances)
is bad. ie; What will happen if you do that ?

If you put cells in series that are different, they will get fully discharged at different times. If the load isn't disconnected, the cell that has gone flat will be charged in reverse by the other cells, which will damage it.

Also, when charging cells in series, if one cell is fully charged, there is no way to charge the other cells. If charging continues, the fully charged cell may be damaged.

If cells are put in parallel, different types of batteries will not discharge at the same rate, so it is easy for one battery to discharge first. When the load is removed, battery voltages often rise a bit, and at that point there will be one cell in a pack charging another.

These problems can happen even if the cells are identical. Much of the time, putting cells in series and parallel relies on them being very similar to minimise imbalance. It's quite common to get problems where one end of a multi-cell battery is hotter than the other, and then the hot cells behave differently from the colder ones and the imbalance causes failure.

It is never a good idea to make mix battery types as that will almost always lead to imbalance being worse.

The least dangerous arrangement is paralleling of individual cells of the same chemistry, as there will be some balancing that happens anyhow. However, any differences in the different cells in the relationship between charge and voltage will mean some cells taking a bigger share of the current than expected.

 

[rjenkinsgb]

"One is that, if the power supply is turned off, it has to withstand the battery voltage across it terminals without damage."
They are in series , not parallel.

So, with the PSU turned off, the PSU output is in series - with reversed polarity - with the load, across the battery output.

If the PSU did not conduct under those conditions, the full battery voltage would be across its terminals.
(It would in reality almost certainly conduct to some extent, but in a manner not intended by the designers).

 

[raschemmel]

Diver300 .
That's a lot to process. I need tome to think about it.

 

[raschemmel]

"So, with the PSU turned off, the PSU output is in series - with reversed polarity - with the load, across the battery output.

If the PSU did not conduct under those conditions, the full battery voltage would be across its terminals."

The PS. would not be turned off.
They are in series so the battery voltage can never
be across the P.S. terminals.

 

[raschemmel]

Diver300,
Some good points. Not sure yet if they apply
to a PS in series with a battery that still has a
good charge.

 

[Pommie]

The PS. would not be turned off.

And in the event of a power cut? (I'm assuming PS= power supply).

Mike.

 

[raschemmel]

What are the odds ?

 

[rjenkinsgb]

Is there any reason the current limiting wouldn’t work in this configuration ?

If the PSU ever goes in to current limit, the output voltage will collapse and have the same effect as it being turned off - and result in the same reverse polarity.

 

[crutschow]

They are in series , not parallel.

Yes.
I should have stated the reverse battery voltage would appear across power supply if it were off (with a load attached).
That would be minimized by a diode as stated.

The diode would also prevent the power supply current-limit from working for the battery.

 

[raschemmel]

Maybe I'm missing something here but if Dc source A is in series with dc source B and B reverses polarity then instead of A+B=C uou
have A-B=C.
How is that catastrophic ?