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Heat sink contact

Tulex

Tulex

Solar Wizard
Joined
Mar 30, 2023
Messages
1,057
Location
Finger Lakes NY
Installing Seplos BMS into a steel cabinet. In order to get the leds and dip switches even on the far side, the heat sink touches the cabinet. It appears to be isolated. I've read about putting thermal pads between the heat sink and steel. This has a fairly thick paint coating on it.

20230708_135048.jpg

1 - Can I count on the heat sink staying isolated?
2 - With the paint, will a thermal pad be worth it?
3 - Should I put an isolator between the two, or just let it touch?

I know I could cut an opening for the heat sink, but this looks very clean without it. I have room in the cabinet, barely, to put a fan, and this cabinet has an opening on the bottom, so if I do end up adding a fan, I would prefer air flow goes across the whole box and not just by the BMS.
 
Why is there a problem with the heatsink touching the steel cabinet, other than the cabinet putting pressure on the the heatsink? Grounding shouldn't be a problem. Cabinet sinks heat. Maybe blocks airflow.
 
If anything, the heatsink touching the cabinet could be an advantage as some heat will conduct into the cabinet.
Just make sure the cabinet is earthed and you should be fine.

Ps. I'm doing exactly the same with my Seplos BMS, but mine doesn't quite touch. I hope the bms doesn't generate too much heat as there won't be much airflow over the heatsink.
 

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Is the heatsink finned? That may not allow enough contact surface in any event. The heatsink is most likely isolated. From the construction it appears there is already a thermal and electrical barrier between the FET and the heatsink. A failure of that insulation from lightning could cause something catastrophic since this is a big battery. You can buy wide Kapton tape which is used as an insulator and thermal conduction and apply some thermal paste. Are the bolts electrically isolated from the heatsink? If not, you might as well make contact with the heatsink against the case.
 
Thanks all. I've dealt with heat sinks in computers, but never with one around so much power, so I didn't want to just assume anything.
I ordered some .5mm thermal pad. It is finned, but figure it can't hurt. If it gets too hot in the box, will add a fan. If the fan isn't enough, will open up the area in front of the heat sink.
 
Heatsinks are designed to do their job in "free air", preferably with the fins facing UP while the base plate is horizontal or else with the fins vertical while the base plate/ckt. board is also vertical.

What you have shown will do neither. No air flow at all.

DANGER, WILL ROBINSON!!
 
v_green57 said:
Heatsinks are designed to do their job in "free air", preferably with the fins facing UP while the base plate is horizontal or else with the fins vertical while the base plate/ckt. board is also vertical.

What you have shown will do neither. No air flow at all.

DANGER, WILL ROBINSON!!
Click to expand...
This BMS pretty much has to be installed the way I am installing it. Can't tell from the picture, but it is mounted on a cabinet door that is hinged on the side. So, you are seeing the cabinet tipped on it's back with the door open.
 
Right, I get how you've mounted it. And that sometimes you are forced (well, maybe) into a sub-optimal situation.

Just letting you know, as someone who has built power electronics, the effectiveness of a heat sink depends on the criteria I outlined above (and others, like ambient). And since heatsinks are a large, bulky and expensive component, they are often sized just large enough to get the job done according to that criteria. It's always about the $$$ and saving it.

Adding forced air cooling will obviously help, but it's difficult to tell how much.

Are you able to monitor the heatsink temp with the BMS software?
 
I used to have a manager who said, "The optics of a project are everything." It didn't have to work, it just had to look like it would work. Nobody ever really expected to work, we were just pushing technology before it was practical. After spending millions you'd think another outcome would be desired. So, I understand optics I do everything for airflow like cutting every other piece of metal finger safe protection and spacing equipment off wall so bottom can ventilate. I orientate so natural convection aids in cooling. I do hate to see heat sink fins mounted horizontally. In reality, the stuff that is going to fail will probably fail anyway.

Fun fact. Diodes actually work better when they are hot. A FET on the other hand increases internal ohmic resistance as it gets hot and an increase in resistance makes it get even hotter.
 
I figured it would be fine because Seplos mounts these BMSs the same way in their own batteries afaik.
 
efficientPV said:
Is the heatsink finned? That may not allow enough contact surface in any event. The heatsink is most likely isolated. From the construction it appears there is already a thermal and electrical barrier between the FET and the heatsink. A failure of that insulation from lightning could cause something catastrophic since this is a big battery. You can buy wide Kapton tape which is used as an insulator and thermal conduction and apply some thermal paste. Are the bolts electrically isolated from the heatsink? If not, you might as well make contact with the heatsink against the case.
Click to expand...
In the Mason kits they require a heat transfer pad to be installed..
My best guess it also isolates the BMS from the steel case
 
If a finned heatsink is mounted vertically (as it should be) placing a cover over the fins actually assists convective airflow, as long as the bottom and top are left fully open.
This creates mini flues or chimneys and aids convective airflow.
I think I would like to see a very thin sheet of electrical insulating material between the fins and the metal frame, even if the heatsink is grounded.
Probably does nothing, but something like that can be the root cause of some very mysterious intermittent faults in the future.
Better that something stops working completely, than something that starts and stops intermittently and drives you crazy.
 
Last edited:
Warpspeed said:
If a finned heatsink is mounted vertically (as it should be) placing a cover over the fins actually assists conductive airflow, as long as the bottom and top are left fully open.
This creates mini flues or chimneys and aids conductive airflow.
I think I would like to see a very thin sheet of electrical insulating material between the fins and the metal frame, even if the heatsink is grounded.
Probably does nothing, but something like that can be the root cause of some very mysterious intermittent faults in the future.
Better that something stops working completely, than something that starts and stops intermittently and drives you crazy.
Click to expand...
Fins are horizontally, and aren't ment for airflow, but for enlarging contact area to the heat transfer pad
 

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I've just ordered some 3mm thick thermal pad for my one. Lucky I read this thread before I commissioned by battery.
 
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