Author Topic: heavy duty diodes  (Read 5097 times)

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Offline larryf

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heavy duty diodes
« on: February 15, 2012, 05:26:41 pm »
If I understand a diode correctly, it limits current flow to one direction only.  Any I have researched seem to be in the miliamp range.  Are there diodes large enough to go between a battery bank and an inverter so that two ISOLATED battery banks could be connected to the same inverter?

Offline rossw

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Re: heavy duty diodes
« Reply #1 on: February 15, 2012, 05:58:34 pm »
If I understand a diode correctly, it limits current flow to one direction only.  Any I have researched seem to be in the miliamp range.  Are there diodes large enough to go between a battery bank and an inverter so that two ISOLATED battery banks could be connected to the same inverter?

There are more kinds of diodes than there are kinds of motor vehicle.

The basic theory of a diode is as you say - to pass current in one direction and block it in the other.
However that's an ideal diode.
Real diodes have leakage - they will pass some current in the reverse direction. The amount depends on a range of things - temperature, diode construction, voltage, and many others. Usually it's an almost negligable current. But sometimes it's used deliberately - like in some temperature sensors.

Small signal diodes are designed for small currents only - but will switch much more quickly than large power diodes. Such signal diodes typically have a maximum forward current in the  low milli-amps range.

Larger rectifier diodes are often rated to many tens of amps. Some specialist diodes will handle thousands of amps.

Schottky diodes have much lower forward voltage drop than equivalent standard diodes - which means they waste less energy as heat, but they have other downsides. More specific information on your application will help us give you a more specific answer.

Offline larryf

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Re: heavy duty diodes
« Reply #2 on: February 15, 2012, 06:22:38 pm »
I have 4 6v in a 24v configuration bank powered by pv panels and am planning a different bank  {same configuration} to be powered by a small pma wind generator.  Each  bank would have its own separate charge controller.  I think I want to connect both banks to the same 3500 watt inverter, but I want to keep the banks from being connected.  If my understanding is correct, the current would flow from the higher charged bank, but one bank would not interact with the other.  Does this make any sense at all?

Offline rossw

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Re: heavy duty diodes
« Reply #3 on: February 15, 2012, 06:51:43 pm »
I have 4 6v in a 24v configuration bank powered by pv panels and am planning a different bank  {same configuration} to be powered by a small pma wind generator.  Each  bank would have its own separate charge controller.  I think I want to connect both banks to the same 3500 watt inverter, but I want to keep the banks from being connected.  If my understanding is correct, the current would flow from the higher charged bank, but one bank would not interact with the other.  Does this make any sense at all?

Thanks, a far more detailed question!

Yes, if you have a charge source and controller connected directly to one bank, and a second source and controller connected to the second bank, then two diodes can be used to let either/both banks discharge into your inverter.

There are a few pitfalls with the plan though. One is the voltage drop. You will lose close to 1V across either diode when pulling any non-trivial current. So if your batteries are 24.8V, you will have around 23.8V going to the inverter.

That will increase the current the inverter needs to pull to make its given output, which makes things work that little bit harder. You also need to watch the low-voltage dropout setpoint of your inverter. But lets assume thats all ok for now.

At a nominal 24V, and assuming 90% efficiency in your inverter, you are going to draw 162 amps from your batteries.
If they're perfectly balanced (they won't be), that's 81 amps per bank. Your diode will need to handle at LEAST that much current, plus a decent safety margin for surges, motor starting etc.

At 81 amps, you will be dissipating at least 80 watts from each diode. Thats some serious heat. Large heatsinks and very likely fans.

1N5162 is rated for 150A  - but thats based on a half-cycle, sinewave. You will be treating it much more harshly than that. Even this guy might not be up to your task. Even if it were ok, it has a thermal resistance junction-to-heatsink of 0.35 degC/W, so with an infinite heatsink you can expect a 30 degree temperature rise on the junction.

If one battery bank is better charged, and is supplying the bulk of your current, you're almost certainly going to blow one diode, which will then transfer all the load to the remaining one - which will suffer the same fate!


NTE make some beefier diodes. NTE6365 for example, up to 300A. Again, based on 180 degrees (halfwave) sineusoidal input. You'd have trouble managing 162A continuous through it (the worst-case continuous current you need)

NTE6102 is 500A. We're starting to get up to a beast that is likely to survive.
No idea what they cost... but they won't be $3.50 each!


Offline larryf

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Re: heavy duty diodes
« Reply #4 on: February 15, 2012, 08:58:18 pm »
rossw
I really appreciate your input, and an explanation that even an old truck driver can understand!  It seems a lot of my ideas loose steam in the finance and real world results commitees finish with them.  Regards,  Larry

Offline Wolvenar

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Re: heavy duty diodes
« Reply #5 on: February 15, 2012, 09:25:23 pm »
I find many of my ideas end when the wife catches wind of them..  :-\
Trying to make power from alternative energy any which way I can.
Just to abuse what I make. (and run this site)

Offline WooferHound

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Re: heavy duty diodes
« Reply #6 on: February 16, 2012, 08:11:05 am »
You should hook the banks straight together like we all do, and get a bigger charge controller.
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Offline tomw

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Re: heavy duty diodes
« Reply #7 on: February 16, 2012, 08:53:08 am »
Larry;

You seem to be imposing unrealistic limits on this system.

As long as the banks are the same voltage of the same type battery a rectifier (diode) is just going to be a point of failure & consumer of energy. A hunk of suitably sized copper cable will last for millenia, is cheap and is all you probably need. But I don't know your whole plan or reasoning for this unusual approach.

Actually, I would not use cable but cable, a fuse / breaker and possibly a switch between them.

In fact, my 450 AH pallet jack battery is connected with my 850 AH fork lift battery with a switch a breaker and of course some cable. All charged by the same sources of solar and wind. Controlled with a single 24 volt Ghurd Controller adapted to drive a large IGBT dumping power into a Kilowatt of dump load.

Glen is a long time member and sells kits and completed controllers when he is not on vacation in the South of France! :D :D

http://ghurd.info

Maybe explain further what you think you need and I am sure someone here can point you the right direction.

Tom
Do NOT mistake me for any kind of "expert".

( ?° ?? ?°)


24 Trina 310 watt modules, SMA SunnyBoy 7.7 KW Grid Tie inverter.

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Offline Rover

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Re: heavy duty diodes
« Reply #8 on: February 16, 2012, 09:02:31 am »
Hi Larry,

As others have said, just create one big 24V bank (your 2 banks in parallel). The only reason I can think of trying to do what you suggest is if the battery banks are very different in charge/discharge capability, possibly due to age differential, and you have fears of undercharging or overcharging because of the variance in SOC when one bank is at full charge.

Even in that case I would still just connect them in parallel, or operate them entirely as 2 separate banks (possibly with a multibank battery switch so you can charge/discharge from one or all)

Rover

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Offline ghurd

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Re: heavy duty diodes
« Reply #9 on: February 16, 2012, 09:52:36 am »
Like they said, just get the 2 banks connected together.
It will be more efficient to charge, and more efficient during discharge (that gets complicated to explain).

Think about this...
Say it is windy.  The wind bank is full, and the controller is dumping the surplus power on one battery bank.
But the reason it is windy is a 3 day thunderstorm, so it is not sunny.  The solar charged battery is in need of power.
That means one battery is dumping power the other battery could use!

Can still use the solar controller for the solar power.
Wind needs a dump load controller.

Or-
Beef up the dump load controller to handle the solar too, and don't need a solar controller, because a dump load controller does not care where the surplus power came from, it just cares there is too much of it.
Make sense?
G-

PS- The South of France?
OK, we'll go with that.
To me it feels like 200 miles South of the South Pole, but I ain't in France quite yet.