Manual Voltage Pressure Relief Valve ?

[DerpsyDoodler]

I bought some 1ohm 25 watt resistors (shown in the picture) to be able to individually drain high cells across a battery. Essentially a manual balancer. I soldered some copper wire to the ends and put some heat shrink over it. Works well. drains ~3a (as expected) while its connected. If i leave it on for more than 5 seconds it starts to get pretty hot.

My question is should it get hot? if the cell was fully charged it would be 3.65v, so i run the calcs with that.

3.65v * 1 ohm = 3.65a

3.65v * 3.65a = 13.32 watts.

13.3 watts is well under the 25 its rated for. Why does it get so hot? Is my understanding mistaken somewhere?

Since i have 5 of them, i could do 2s2p with the resistors and have the same current flow but the power loss spread across 4 resistors instead of just one. Would this prevent them from getting so hot so fast?

I did measure the resistance of the resistor (confirmed @ 1 ohm) and I also attached a clamp ammeter to the resistor circuit and i saw ~3.2a going through.

Ideally, i'd like to just be able to put this in place and hold it for 20-30 seconds without worrying about it.

Can anyone offer some insight as to why this isn't working the way I expected it to?

 

[FilterGuy]

I bought some 1ohm 25 watt resistors (shown in the picture) to be able to individually drain high cells across a battery. Essentially a manual balancer. I soldered some copper wire to the ends and put some heat shrink over it. Works well. drains ~3a (as expected) while its connected. If i leave it on for more than 5 seconds it starts to get pretty hot.

My question is should it get hot? if the cell was fully charged it would be 3.65v, so i run the calcs with that.

3.65v * 1 ohm = 3.65a

3.65v * 3.65a = 13.32 watts.

13.3 watts is well under the 25 its rated for. Why does it get so hot? Is my understanding mistaken somewhere?

Since i have 5 of them, i could do 2s2p with the resistors and have the same current flow but the voltage loss spread across 4 resistors instead of just one. Would this prevent them from getting so hot so fast?

I did measure the resistance of the resistor (confirmed @ 1 ohm) and I also attached a clamp ammeter to the resistor circuit and i saw ~3.2a going through.

Ideally, i'd like to just be able to put this in place and hold it for 20-30 seconds without worrying about it.

Can anyone offer some insight as to why this isn't working the way I expected it to?

View attachment 64282

Yes.... they should get hot. That style resistor should be mounted on a heat sink if you are going to run it at the full 25W.

This is a picture of a 25W 1ohm resistor that does not need a heat sink.

It is made of ceramic in order to be able to tolerate all the heat that 25W will generate.

Remember the old fashioned 60W light bulbs? They were a lot larger and they got really hot even though some of the energy was escaping as light. Yes, you are running half the wattage but the resistors are a lot smaller and those 25 watts have to go somewhere.

BTW: This is also why you don't see BMSs with high current and passive balancing.... Dealing with the heat becomes a massive problem.

 

[time2roll]

Let it run hot. No smoke should come out in 30 seconds or 30 minutes. The heat still needs to be dissipated. A couple alligator clips can hold it in place as needed.

 

[FilterGuy]

No smoke should come out in 30 seconds or 30 minutes.

I have never let that style resister run without a heat sink and any significant wattage. It would be interesting to see if they survive at half the rated wattage. (I am pretty sure they would not survive at full wattage)

 

[DerpsyDoodler]

Thanks for the responses.

What would i expect if one should fail during use?

While they're 1ohm 25w, i did get them off amazon.

Assuming I can do it safely, happy to see if it can handle the load over a minute or so. It will only ever balance across single cells, so short of an oops, will never hit the 25w max. I can also track it with my thermal imager. determine its peak temperature (presuming it does peak).

Thoughts on how to do it safely? I guess I should fuse it? 5a should be good? Anything else? do I have to worry about it exploding?

 

[FilterGuy]

Thanks for the responses.

What would i expect if one should fail during use?

While they're 1ohm 25w, i did get them off amazon.

Assuming I can do it safely, happy to see if it can handle the load over a minute or so. It will only ever balance across single cells, so short of an oops, will never hit the 25w max. I can also track it with my thermal imager. determine its peak temperature (presuming it does peak).

Thoughts on how to do it safely? I guess I should fuse it? 5a should be good? Anything else? do I have to worry about it exploding?

View attachment 64300

I don't think it would explode. My *guess* is it would fail as an open circuit but putting a fuse on it is not a bad idea. If it failed as a short circuit, things could get very 'interesting' very quickly.

 

[DerpsyDoodler]

I don't think it would explode. My *guess* is it would fail as an open circuit but putting a fuse on it is not a bad idea. If it failed as a short circuit, things could get very 'interesting' very quickly.

Lol...I'd rather things not get interesting unless its in a controlled manner. I'll fuse it, to be safe. do resistors usually fail open or closed? is it just a random possibility with any resistor failure?

 

[FilterGuy]

Lol...I'd rather things not get interesting unless its in a controlled manner. I'll fuse it, to be safe. do resistors usually fail open or closed? is it just a random possibility with any resistor failure?

In my experience resisters fail open......The resistive element just burns up and breaks apart. However, I don't have experience with this style resistor failing. I can imagine oddball scenarios where it fails as a short. What if the heat breaks down the insulation and caused a short through the aluminum case? That seems unlikely but keep in mind that the infamous Mr Murphy is always looking for the unlikely.

If you really want a safe test, hook it up to a current controlled power supply for an hour to see what happens.

.I'd rather things not get interesting unless its in a controlled manner.

But then it's not near as 'interesting'. ?

 

[DerpsyDoodler]

...seems unlikely but keep in mind that the infamous Mr Murphy is always looking for the unlikely.

If you really want a safe test, hook it up to a current controlled power supply for an hour to see what happens.

Aha! duh, brilliant idea. Will do.

But then it's not near as 'interesting'. ?

Haha...while I don't disagree, i prefer to watch electro-boom to see those interesting things. ?

 

[Hedges]

Here's a data sheet for similar spec resistor (found on DigiKey)
It says 25W with heatsink (assuming 25 degrees C ambient), 12.5W without heatsink.
You wanted to run it without heatsink and around 12.5W, so expect case to reach maximum operating temperature - enough to burn your finger.
You could bolt it to a heatsink or sheet of aluminum, preferably with heatsink compound under it.
Or just dunk in a glass of water, not like we're worried about a little extra leakage current. But don't touch wet parts with your finger, since some cells in a pack would be around 48V.

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=1773277&DocType=DS&DocLang=English

 

[DerpsyDoodler]

Here's a data sheet for similar spec resistor (found on DigiKey)
It says 25W with heatsink (assuming 25 degrees C ambient), 12.5W without heatsink.
You wanted to run it without heatsink and around 12.5W, so expect case to reach maximum operating temperature - enough to burn your finger.

ahhha haaaa hahaaa. i'll keep the water away from it all. lol. How would any cells be 48v? I dont have any 48v batteries. My cells are arranged for 24v (8s) but I will only be "draining" from a single 3.2v nominal cell. Is there something I don't know?

 

[carlrx7]

i have those but 12v Lightbulbs work better to drain a cell

 

[time2roll]

Here's a data sheet for similar spec resistor (found on DigiKey)

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=1773277&DocType=DS&DocLang=English

Also note at the bottom shows a temperature rise of 100C is normal. (with sink at max watts) That is going to be very hot to the touch.
In the end maybe a 100w rating was needed.

 

[DerpsyDoodler]

I'd have provided more pics, but the cheap sd card didnt want to cooperate well. Also, power supply was set for 3.65, max current set to the same, the actual it pushed was 3.42a and the watt reading was 12.48.

5 minutes




10 minutes

 

[740GLE]

Looks like it was earning its keep

 

[FilterGuy]

Also note at the bottom shows a temperature rise of 100C is normal. (with sink at max watts) That is going to be very hot to the touch.
In the end maybe a 100w rating was needed.

The spec you referenced was for a wire wound resistor. Assuming the OP is also using a wire wound resister, the normal failure mode will almost always be an open circuit.

 

[DerpsyDoodler]

The spec you referenced was for a wire wound resistor. Assuming the OP is also using a wire wound resister, the normal failure mode will almost always be an open circuit.

Yes it is a wire wound resistor.

 

[DerpsyDoodler]

So last time I did that test, I'd done it multiple times in near succession (pics failing troubleshooting) and got a whole 2 pics. I redid it with a working sd card in the imager and was able to document more data. Here's the data:

1 test, 10 minutes long, resistor started at about ~76 degrees F.

13 pics total. 1 of the ps and the resistor configuration at start, 11 imaging thermal pics starting at ~0 seconds and taking 1 pic about every minute for ten minutes. Then a pic of the ps and resistor at the end (documenting the current went up by ~2mA over the course of the test.

When/if browsing through the thermal pics, note the high temp reading on the right side. I didnt always have the cross hairs on the hottest part of the resistor, so the left side reading will be misleading.

Next post for the remaining images (10 file limit per post).

 

[DerpsyDoodler]

remaining images:

 

[FilterGuy]

remaining images:

That sucker got HOT!