Need 400W diode advice please

[motorhomer]

I need a diode to block reverse voltage from a charging source to a battery. When he charging source is off the battery will power all the load which will not be above 30A. The voltage source is 12v so the diode will need to be able to take at least 360 watts.

I was looking at BZW50-18 diode for this. Would this be a good choice seeing as it can take 5Kw. What would be best way to mount this. Raised on two post to allow ait flow seeing as this is a lot of power i assume this will get hot and I assume these do not require any form heat sink.

[Siwastaja]

The diode doesn't see or know anything about your load wattage.

The diode only sees it's own voltage drop, and current through the diode. For example, if I=30A and Vf=1.0V, then the diode dissipates 30W, and doesn't have any idea about the input voltage or load voltage.

[Siwastaja]

BZW50-18 is not a diode but a TVS diode. Well it's a special kind of a diode, but not what you want. You want a normal rectifier diode, probably want a Schottky type for such low voltage and high current.

[wraper]

BZW50-18 is not even a diode in a way you think about it. It's a transient suppressor. 5kW is something it can handle for only a few microseconds.

[motorhomer]

Thanks forgot about that, so the power would be voltage drop across the diode multiplied by the current

[Siwastaja]

Yes, so for reverse polarity protection where most other parameters are insignificant, you want a diode with lowest possible Vf, so that dissipation is as low as possible. At 30A, you can't avoid heatsinking, though. A very large schottky type might offer as low as Vf=0.5V, but even that is 15W already, this means an aluminium heatsink. Not a massive one, and no fan required, but still a substantial heatsink.

[floobydust]

The heat generated by an ordinary diode is around 1W/amp so up to 30W of heat dissipated which is a huge heatsink.
Schottky diodes are more efficient, about half that heat generated but still a good sized heatsink required.
People usually want the option to emergency start the vehicle with the aux battery, which is 100's amps so design for that or rely on a fuse.
Vishay offers Schottky diodes to 175A, USD $7 for 100A 45V VS-100BGQ045HF4 for example - minus heatsink.

Common on motorhomes or caravans, is to instead use MOSFET's as an ideal diode for the "battery isolator". They have very low losses and no heatsink or a tiny one is needed.

[motorhomer]

Good idea to look at MOSFETS will look into this. There will be no requirements to start any engine off the battery.

Basically the battery is a Lithium and will power all the motorhome equipment when no electric hook up is on which will be below 30 amps. When the 240v is connected the original charger will power the load and charge the vehicle battery but it cannot charge the lithium as it is an incorrect profile. So the diode was to prevent any charge voltage back to the lithium battery

[james_s]

Is there a good reason to try to roll your own rather than just use an off the shelf battery isolator? I remember my grandpa installing one in his motorhome back in the mid 80s so they've been around for a while. I think they're just a pair of diodes in a potted block on a large heatsink but they're designed and tested for the job.

[motorhomer]

Not sure they do diode battery isolators, I don't want any switch isolator. Only isolators I see are Victron but they are £30+ and I do not think they are what I want.

[james_s]

The one my grandpa installed was a blue anodized heatsink about 4" square as I recall. It had no user controls, just three terminal posts, one to each battery and one to the alternator. It was very similar to this. https://www.amazon.com/Sure-Power-1202D-Battery-Isolator/dp/B009XU8EQU

[motorhomer]

This would be same as what Victron sell. This is not what I am trying to do. I am trying to prevent the electrical charger charging the lithium battery by blocking the the voltage source feeding into the lithium batt, but allowing the battery to power the loads when the mains is not connected.
My lithium battery cannot be charged by the vans mains charger due to the profile charging is not lithium setting. So I have at the moment removed the mains lead from this system.

By removing this charger I loose the function that it would charge the vehicle battery when on mains and the system would parallel power all the load with the leisure battery (lithium battery)

I was thinking of putting in the blocking diode on the lithium line to the load control box so that it will still power all the loads but when on mains the charger will not increase the lithium voltage and try to charge it. i now see this may not be possible due to the max 30 amp load. Now looking at a way to do this with Mosfets.
Hope this makes some sense.
Picture showing very basic setup
 Need to add the load and mains charger are all part of one control unit an EBL-211

[motorhomer]

I was wondering would the attached circuit work using a Pmos. I could easily parallel two or more to share the load on a heat sink, but will it block the source 2 voltage feeding into source 1

[Vovk_Z]

TS needs a diode rated for needed rated current (or up to twice more) and about 3x-4x of a battery rated voltage (>40 VDC rated).
It can be a Shottky type, or "an ideal diode" (Mosfet with controller).

[floobydust]

I was wondering would the attached circuit work using a Pmos. I could easily parallel two or more to share the load on a heat sink, but will it block the source 2 voltage feeding into source 1

I would say no, because a mosfet always has a parasitic body-diode from D-S. It's not always drawn on the device schematic symbol. It will allow current to flow back from Source 2 to 1. So a mosfet is not an on/off switch for current that could flow in either direction, in or out of the battery. I think you would instead use the usual two-mosfet circuit(P-ch) giving an ideal diode plus backfeed switch, which is used in battery BMS systems.

There is a 12V high-current battery isolator project in Silicon Chip magazine Dual Battery Isolator for 4WD, RVs, Caravans, etc - July 2019 (errata Sept. 2019).
It was also reprinted in Everyday Practical Electronics July 2020 and the two PCB's available. Maybe take a look, it explains things in good detail.
The circuit is discussed and there is simpler logic as far as automatically switching ON when the vehicle is detected as running (over 13.4V sensed) to then charge the aux battery.
Key off it switches the mosfets off when voltage falls to 12.6V or so.