Need cheap 12V current dump

[Skypower]

I need a good cheap convenient method of dumping loads from a 300 Ah 12V Lifepo4 battery. 40 to 75 amps continuously. The battery has an Overkill 130amp bms that’s enclosed so there’s no sense in pushing it. It’s for the trolling motor in 16’ Lund and there’s been a more than a few times that it’s come back with full charge and I don’t want to store it for months at a high state and I’d also like to run a capacity test with a shut that I already have. I’ve been kicking around the idea of using a carbon pile, the kind that they use for automotive battery testing. I was thinking of getting the 500 amp model and install a blower for continuous use at the lower amp range. Anyone got a better idea, I’m all ears.

 

[42OhmsPA]

Bunch of headlight bulbs wired in parallel or a cheap harbor freight inverter and heat gun are 2 ideas that come to mind quickly.

 

[Bossrox]

If you have 2kw inverter, just hook 1500 watt space heater to it & there ya go.

 

[Skypower]

If you have 2kw inverter, just hook 1500 watt space heater to it & there ya go.

Including the inverter loss/load I’ll be pulling more than i should even with the heater on low approximately 92 amps. I’ve been toying with a 2kw inverter idea.

 

[Bossrox]

Including the inverter loss/load I’ll be pulling more than i should even with the heater on low approximately 92 amps. I’ve been toying with a 2kw inverter idea.

Well you could swap a heater for something less consuming.

 

[webbbn]

A few years back I made an high-current MOSFET tester by bolting several high-power resistors to an aluminum plate. You can get 100W, low resistance (1-2 ohm or so) resistors on Amazon for around 1-2$ each. You could probably build a decent current dump that way for less than $20.

 

[Skypower]

Well gang, I pulled the trigger and bought the carbon pile tester above. It’s dumb simple and I’m pretty sure I can mod it to handle a 100+ amp continuous load with cooling fans that I already have. I’ll still be able to test batteries as intended but with just more options. The shunt controller that I have has dry contacts that I can rig a continuous duty solenoid to so that it’ll shut off before the bms low voltage protection triggers. So if I want I can set it discharge to 30% SOC and not worry about it. It’ll still be used in a fire safe area just incase it goes poof.

 

[Vigo]

I think that idea should be fine with enough fan cooling but it might take some pretty strong airflow because the carbon pile has some pretty poor surface area to mass ratio, so to speak.

 

[Skypower]

Yeah, I’m on the go slow approach. I’m actually more concerned about uniform heat distribution. If one side gets too cool it could actually be worse. Get out my FLIR camera. Diverter or shrouds could be in order. They claim 15 seconds load, one minute cooldown for 3 tests in five minutes. But under what load were they basing this on. I’m not planning on pushing it over 75 amps continuously with fan(s).

 

[Vigo]

Well, it will be making ~800w or so of heat. If you think about how strong the fan on a space heater is (most space heaters have a low setting around 700-800w) it's not very strong, HOWEVER it is blowing air over an actual finned 'radiator' type surface with a crapload of surface area compared to its mass. To get that same 800w removed from the carbon pile which is a stack of discs about the size of the palm of your hand, you would need a crapload of airflow. Im not 100% certain it's even feasible.

But a variable carbon pile load tester is still a cool thing to have for numerous possible uses, so as long as you don't destroy it during your attempt to use it as a dump load, i think you will not regret the purchase because you will probably find other uses for it afterwards.

 

[Skypower]

Well, that was a fail!
It worked and the air out never exceeded 200F at 70 amps. The problem was stability. I could set the current and it would constantly creep up. It appears that when the pile heats up it expands, when it expands it compresses itself and makes lower resistance then increasing the current. I move it down and the cycle repeats. I was hoping for it to settle in or break in but that wasn’t to be. It got to a point that it just disconnected because I had backed off so much. At least I now have a battery tester that I can still use for original design but just cools down a heck of a lot faster as long as you release the current and leave it connected for a while. Thinking of buying a cloths dryer heater unit, install a fan and pick a point of contact. I’ll choose the same model that fits my dryer, at least I’ll have a spare if this one takes a dive ?

 

[Vigo]

In retrospect that seems like a predictable result, but heck i didn't think of it so.. whoops!! We'll all do better next time, we hope. ?

Glad you agree it is a cool doodad to just have around, regardless. The cheaper style of 'fixed' load tester usually has a 'wire element' that would not suffer that variability due to expansion, but they usually pull around 100amps (mine does) and you already said that was high, so.. in my mind the simplest thing is just to get a 12v inverter specifically for this purpose since you probably already have some kind of AC load of appropriate wattage just in your general belongings. You can definitely get smaller space heaters, usually you would find them under search term of 'desk' or 'desktop' or 'personal' heaters, anywhere from ~150-500w or so. I have a 500w under my desk as i type here.

Im not sure what other kinds of electrical equipment might be on this 16' boat but if you had some kind of 12v DC load onboard that could serve as this dump load, you could use the load ports of a ~$10 pwm solar charge controller to trigger a relay or contactor with sufficient current capacity, and simply turn on that dump load until voltage dropped to X volts. That could be a ~$20-30 solution IF you have such a DC load available already.