better resistor, method for testing 12v lithium/lifepo4 batteries?

wattwatt

Member
Joined
May 21, 2018
Messages
55
I'm using a ZB2L3 battery capacity tester (all I have on hand at the moment) to test some 12v LiFePO4 batteries (4s8p A123 cells). First I used the included ceramic 5 watt 7.5 ohm resistor; it pulled ~1.5 A, got very hot (temp gun says 350 to 400+ F (177 to 204 C)), had a very unpleasant smell and after only two tests it broke apart. I did set up a fan to help cool it about a quarter of the way through the first test, but I guess it didn't matter. I then remembered there's a science to this resistance thing (d'oh!), realized this resistor was likely meant for 3.7v li-ion testing (rated for 5 watts, but was doing 20 watts.../).

So I calculated and looks like I could use a aluminum 25 watt 8 ohm resistor I had on hand. I started a new test with a new battery with the aluminum resistor. It's pulling 1.7 A, so it's doing ~23 watts, which is within 25 watts, but it also gets very hot; temp gun says 310+ F/154+ C without cooling, 228 F/109 C with cooling.

Is that amount of heat normal for a resistor and I just need to find better ways to keep it cool or am I still not using a more suitable resistor for capacity testing these 12v LiFePO4 batteries?
 

Redpacket

Well-known member
Joined
Feb 28, 2018
Messages
1,265
Yes, it's "normal" for power resistors like your & typical ceramic 5W/10W type to run very hot if dissipation is anything close to their "rated wattage". They probably won't last well at these levels & it's dangerous too (burns/fire, etc)
Manufacturers spec sheet list temps like 150degC rise, etc
You'd probably be better off using multiple (eg maybe 5x) resistors that add to the add resistance/load current you want.
You can fan cool them too.
Aluminium ones can be bolted to a heat sink (with some heatsink goop) & fan cooled if you're getting serious!
Once built a 5V, 50A load like this to test an industrial power supply, worked well.
 

Roland W

Member
Joined
Oct 9, 2017
Messages
91
Maybe you can find a so called "dummy load resistor". Those are designed to constantly burn off energy in for example electric vehicles. Bit bigger in size to easily radiate heat away, and typically cheap as well and easy to connect straight to wires.
 

Attachments

  • Screenshot_20210220-205900_Lazada.jpg
    Screenshot_20210220-205900_Lazada.jpg
    138.8 KB · Views: 19

Cypher618

New member
Joined
Jan 14, 2021
Messages
27
The heating elements in old space heaters are also an option. Bonus they come with built in heat sinks!

In north America the resistance will be around 10ohm, but they typically have connections for two heat levels, I think you could rewire that to put the elements in parallel for 5ohm... and since these are typically designed for 1500W add a DC fan and your good to go (probably wouldn't want the normal 120vac fan!).

Or spend the 30 bucks on the dummy load, that's probably a better idea.
 

wattwatt

Member
Joined
May 21, 2018
Messages
55
Dummy load resistors appear to start at 100W. Will a dummy load resistor try to pull 100W out of this tester even though it's only capable of 45W max (though I'd think it's best to keep it under 35W to help prolong the life of the tester)?
 

Roland W

Member
Joined
Oct 9, 2017
Messages
91
Dummy load resistors appear to start at 100W. Will a dummy load resistor try to pull 100W out of this tester even though it's only capable of 45W max (though I'd think it's best to keep it under 35W to help prolong the life of the tester)?

No no, the watts is always a function of Ohm's law. Its resistance will govern the current which will flow, while your source voltage multiplied by that current will result in the Watts. Watts written on the resistor is just the max rating what it can handle.

I do have a 50 Ohms/50W in my Powerwall. So they seem to start somewhere at that Watts ratings.
 
Last edited:

floydR

Well-known member
Joined
Aug 23, 2017
Messages
1,021
R = Vbat / Iset 12.8v/2.73= 4.68Ohm
P = Vbat * Iset 12.8v*I=35w 35/12.8v=2.73A divided watts by voltage to get I current
so look for a dummy load resistor with a ~4.6Ohm?
later floyd
 

wattwatt

Member
Joined
May 21, 2018
Messages
55
R = Vbat / Iset 12.8v/2.73= 4.68Ohm
P = Vbat * Iset 12.8v*I=35w 35/12.8v=2.73A divided watts by voltage to get I current
so look for a dummy load resistor with a ~4.6Ohm?
later floyd

Yup, 4.6 Ohm is what I got too. I can't find 4.5 Ohm resistors readily available, so I'm just going to grab some 5 Ohm, 50 watt resistors off Amazon.

Thanks!
 
Top