Should be fine for what you want to do. If you need a thermal analysis for a heatsink/fan I run Sauna here and would be happy to run it for you. I buy heatsinks off Ebay for pennys on the dollar. I got a bunch over here that ate about 4" tall and very high density. I can part with one no problem. Bought a half-dozen on Ebay..... Thought about it some more and bought all he had. Still have a couple left.Looks to me like that transistor should be safe for one cell, 4V at 35A. But not a 12V battery, 16V at 35A.
View attachment 33513
The FET is going to end up being a voltage controlled resistor.
Here is a bare bones example of the circuit (not my design). There is a mistake in that schematic. R3's value is whatever resistance is required to develop 75mV at the max desired current. I will use a 50A/75mV current shunt (0.0015 ohm). I will design the control voltage circuit to do whatever I end up deciding I need to. I could generate it using a DAC from a microcontroller, although the first version will just be a pot followed by a voltage divider and an op-amp voltage follower. I will use comparator circuits to manage the voltage cutoffs. All that is just the fiddly bits. Right now I just want to make sure I understand what I need to do, not how I will do it.
View attachment 33535
-Edit-
There will be additional circuitry to switch between charging and discharging modes. Plus a timer of some sort to make measurement of AH more automatic. Like I said, the final version will very likely end up having a small microcontroller running the show. For now I just want the electrical circuitry worked out.
Decided to move forward with the 300W buck DC-DC converter and some big honking resistors instead. Got a box full of 1 ohm, 200W resistors in the mail today.Have you tried to simulate this in LT Spice?
You do realize what the input current to a buck converter looks like? it is anything but constant current.Not going to work for single cells, but for a 24V battery pack, that is decent.
I might buy an extra DC-DC converter and build two of them (I have a total of 4 resistors). That would let me discharge at a 0.07C rate.
Can you explain what you mean by negative temperature coefficient with IRFP fets? When a fet heats up the RDS_on goes up. This is what makes them so easy to parallel. If you are running the fets in the linear mode then RDS_on is of no consequence.I understand your concerns. Those IRFP FETs have a negative temperature coefficient which means when they overheat, they go into thermal runaway. Linear Trench FETS have a positive temperature coefficient (resistance increases with temperature) so they are not subject to thermal runaway. Like I said, these parts are designed to be operated in the linear region without burning up.
What is an appropriate charge/discharge profile to use for testing single cells? I can certainly do CC followed by CV if this is a requirement. If a soak stage is required, then how long is appropriate? Since this is a single cell I don't need to hold near full while cell balancing is taking place.
I am going to be using these cells in a solar power application so high C rates for prolonged periods of time are not of interest to me. Does it matter which C rate I use for my testing? LiFePO4 cells are not supposed to suffer from Peukert effects to any significant degree and its not like I am going to be cycling the cells more than once or twice. For my cells, 0.05C = 14A, 0.1C = 28A, 0.2C = 56A. Obviously higher C rates will let the testing completes faster, but I also don't want to make this tester more expensive than necessary.
I have no idea how anything other than a CC discharge profile is appropriate, am I supposed to tail off the discharge current as I reach the low voltage cutoff? I have never heard of a load that does that. Nothing I power from these cells is going to behave in that manner.
These parts ARE used in electronic loads in precisely the way I intend to use them. This is what the manufacturer designed them to do (read the attached document. I have seen 100A electronic loads designed with 3 of these in parallel (actually the 90A version). I expect that one 110A part should have no problem handling 35A.
These are nice parts. I think they would work great for a discharger. I don't think any special discharge curve would be necessary. I would think the major point is that the cells are balanced and remain balanced over time. What benefit is testing the cells one at a time with a discharge tester rather than putting them into a real life situation and monitoring cells?Would anybody consider answering any of the the questions I have asked?
Qualification of each individual cell capacity. There has been a lot of commentary about these 280AH cells being unmatched. I am curious to quantify just how unmatched my cells are are and if this changes over time.These are nice parts. I think they would work great for a discharger. I don't think any special discharge curve would be necessary. I would think the major point is that the cells are balanced and remain balanced over time. What benefit is testing the cells one at a time with a discharge tester rather than putting them into a real life situation and monitoring cells?
Damn! Here I am about to pull the trigger on a string of 280Ah units and I haven't been paying attention to the chit-chat. Thanks for the heads up. I wonder too about cells changing as the cycles go up, and if those wimpy BMS resistors are going to keep up over time. I'm thinking your linear approach might work better - that is putting a linear transistor in parallel with each call. Transistors are actually better at getting the heat down to the chassis than a resistor.Qualification of each individual cell capacity. There has been a lot of commentary about these 280AH cells being unmatched. I am curious to quantify just how unmatched my cells are are and if this changes over time.
What is an appropriate charge/discharge profile to use for testing single cells? I can certainly do CC followed by CV if this is a requirement. If a soak stage is required, then how long is appropriate? Since this is a single cell I don't need to hold near full while cell balancing is taking place.
I am going to be using these cells in a solar power application so high C rates for prolonged periods of time are not of interest to me. Does it matter which C rate I use for my testing? LiFePO4 cells are not supposed to suffer from Peukert effects to any significant degree and its not like I am going to be cycling the cells more than once or twice. For my cells, 0.05C = 14A, 0.1C = 28A, 0.2C = 56A. Obviously higher C rates will let the testing completes faster, but I also don't want to make this tester more expensive than necessary.
I have no idea how anything other than a CC discharge profile is appropriate, am I supposed to tail off the discharge current as I reach the low voltage cutoff? I have never heard of a load that does that. Nothing I power from these cells is going to behave in that manner.
These parts ARE used in electronic loads in precisely the way I intend to use them. This is what the manufacturer designed them to do (read the attached document. I have seen 100A electronic loads designed with 3 of these in parallel (actually the 90A version). I expect that one 110A part should have no problem handling 35A.
Would anybody consider answering any of the the questions I have asked?
Those are the exact inverters I use for my real-world capacity test (2 of them).Some sort of grid-tie inverter (as another member suggested) is starting to sound like a really good idea.
Amazon.com : 12 volt grid tie inverter
www.amazon.com
Instead of dissipating hundreds of watts (or thousands), just pour them back into the ocean.