Renewable Energy Questions/Discussion > Automation, Controls, Inverters, MPPT, etc

heavy duty diodes

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larryf:
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?

rossw:

--- Quote from: larryf 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?

--- End quote ---

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.

larryf:
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?

rossw:

--- Quote from: larryf 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?

--- End quote ---

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!

larryf:
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

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