diy solar

diy solar

Electronic Loads

That's entirely up to you but you may not have to. I suggest you record your deltas as reported by the BMS when you finish your pack. Then look at them in a year. If the deltas are close there will be no need to top balance. I think taking delta readings every so often will tell us what we need to know. That and doing a full capacity test every so often.
Considering how unlikely these cheap 280 AH cells are to be matched, I am using an 8S Deligreen 5A Active balancer in addition to my BMS. Hopefully this will maintain the cells in a fairly top balanced condition. I am curious to see how the capacity changes over time and if the change is consistent from cell to cell.

Don't worry, I will wear a white lab coat and record the results on a clipboard with a proper Victorian flair.

I have found my lab coat! This is so me.

Labcoat.jpg
 
Looks to me like that transistor should be safe for one cell, 4V at 35A. But not a 12V battery, 16V at 35A.

That's a lot of fet power dissipation. You can offload more than half the power to the resistor. If the cell gets discharged from 4.7V to 2.5V, then the op-amp can be programmed so that the current defining resistor has 2.5V. The maximum voltage across the fet is reduced from about 4V to 2.2V. As the cell discharges at 50A, fet power dissipation goes from 2.2V * 50A = 110W to 0W when the cell reaches 2.5V.

Since this is a DIY forum, you can make your own resistor. To make a 50A current sink you'll need a resistor:

R = 2.5V / 50A = 0.05 ohm

16 awg wire has about 4 milliOhm/foot resistance. You'll need12.5 feet of 16 awg cable. Throw the cable in a bucket of water and you got a tailor made, water cooled power resistor.
 
Since this is a DIY forum, you can make your own resistor. To make a 50A current sink you'll need a resistor:

R = 2.5V / 50A = 0.05 ohm

16 awg wire has about 4 milliOhm/foot resistance. You'll need12.5 feet of 16 awg cable. Throw the cable in a bucket of water and you got a tailor made, water cooled power resistor.

Keep the length of 16 awg from circuit to water bucket short (a couple feet max) so heat from 50A doesn't have to conduct very far.

For a 12V, 280 Ah battery (14 Mj), if half the heat goes to the wire resistor and water (other half to FET heatsink and air),
specific heat of water is about 4 joules/(gram degree C),
7 M joules / (4 joules x 75 degrees) = 23,000
23 kg or liters, 6 gallons of water could be raised from room temperature 25 degrees C to 100 C.

Did I get those numbers about right?
If so, the wire will end up too hot.
Need more water, or longer wire of larger gauge.
 
Is there anything wrong getting to 100C (other than small change in wire resistance)? OP wants a current shunt measurement anyways.

Your calculation assumes no convection cooling of water.

16 awg cable is cheap. Could also go with 2 buckets of water, each conducting 25A.

I would not allow any portion of the 16 awg wire out of water. Bigger cables need to connect to the 16 awg under water. Fusing current is 116A. All is safe!
 
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The limiting factor will really come down to heat sink capacity for the MOSFET. I was thinking about using a CPU cooler, but air cooled coolers top out about 250W. The Cooler Master 212 I used on my Ryzen 3600 CPU is rated for 120W of dissipation. If I use that one, I will limit max current to 35A.

Given the power density I'd recommend to use watercooling, not air cooling ;)

But I would recommend this (far better) solution too:
If you can size a fixed resistor placed in series with the transistor, able to take most of the power dissipation across the entire range of battery voltage, you'll greatly reduce stress on the transistor.

And/or this solution to have a better suited I/V curve:
Speaking of diodes - another way to shift some/most power dissipation out of the transistor.
Series connection of heatsinked diodes would serve as as non-linear resistor. Jumper to the voltage drop you want depending on what you're testing.
Forced air will provide much more cooling than convection.


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'd say ideally a C rate of 0.5 C. But of course that's a lot of power to dissipate. To me the bare minimum to have a useful tester would be 0.1 C. NB: those values are only my personal opinion based on educated guesses.


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.

Yes, they will be fine given plenty of derating ;)


Based on this, it doesn't sound as though my middle charge stage is needed. I should be able to use a CV/CC charge source of 0.2C then simply monitor for when the charge current drops below 0.01C.

Is this an accurate summation?

Yes.


Do you have any experience measuring IR of a cell? All of my cells came with a vendor applied label claiming the IR was 0.14 milliohms. I am a bit suspicious at them all have exactly the same IR. Makes me wonder if they just have a role of pre-printed labels that they slap on the cells before putting them in the box.

I assume my load will also make testing IR pretty trivial. Just use Thevenin to determine IR from the open circuit and loaded cell voltage. Does cell IR have a reasonably linear function? Is there a standard load (C rate) or SOC point to use when measuring IR? How long should I let a cell settle after charging before measuring IR?

There's two methods: measuring the impedance using an AC current and assuming IR ~= the impedance (that's what the cheap chinese meters do, and why they aren't very accurate), or measuring the true IR using dV / dI (very accurate result with very simple equipment).

I wouldn't use the true unloaded voltage because of the settling problem, but rather a very low current (like 0.5 or 1 A).

And for the higher current measurement, 0.5 to 1 C would be ideal (not too hard to achieve as you only need to load it long enough to do the voltage and current measurements so the MOSFETs should be able to take the pulse no problem if your ADC sampling doesn't take too long) even if anything above 0.1 C should give good results anyway (again, rough educated guess).


Unfortunately the YR1035+ is unable to accurately measure IR below 0.3 milliohms. The manufacturer specifically states it is for cells of 100 AH capacity or less.

And even 0.3 mOhms is optimistic at best... Those are the testers I was talking about; don't do what they do.


The manual also emphasized the importance of making 4 wire measurements (Kelvin connection). I will certainly build that into my design. I wasn't concerned about that for measuring battery capacity since all I really need to measure for that is charge current.

Yes, you already have a shunt with kelvin connections in your design, not a problem.


The HRPG-450-3.3 can be adjusted up to 3.8V output so it will work just fine to charge LiFePO4 cells. It also does CC up to 90A which is more than I need. It would make capacity testing of cells a lot faster and I could parallel my cells for top balancing instead of having to individually charge each cell for top balancing.

Yes. Charge at max current with the PS no load voltage set at 3.65 V (at the cells, modifiy the PS to add remote voltage sensing wires ideally) and cut at 3.65 V (or hover here for 20-30 min with a timer if you really want to shove 0.x Ah more, and then cut the charge).


Hum, paralleling 3 FETS and water cooling would get me up into the 100A discharge range. And I happen to have some water cooling equipment from another project that is currently stalled...

Yes, exactly what I'm saying just above... :D


Need more water, or longer wire of larger gauge.

Not really needed, water will self regulate at 100 °C, that's one of the main advantages of a water based dump load :)
 
That's a lot of fet power dissipation. You can offload more than half the power to the resistor. If the cell gets discharged from 4.7V to 2.5V, then the op-amp can be programmed so that the current defining resistor has 2.5V. The maximum voltage across the fet is reduced from about 4V to 2.2V. As the cell discharges at 50A, fet power dissipation goes from 2.2V * 50A = 110W to 0W when the cell reaches 2.5V.

If you want fancy make a DC-DC converter that feeds the energy back to grid-tie inverter or charge your battery bank with it! ;)

Probably wouldn't be too hard to hack chinese inverter to accept input voltage range down to 2 volts or so. Efficiency is not great but who cares if your efficiency is only 70% in case that the alternative is 0%
 
If you want fancy make a DC-DC converter that feeds the energy back to grid-tie inverter or charge your battery bank with it! ;)

Probably wouldn't be too hard to hack chinese inverter to accept input voltage range down to 2 volts or so. Efficiency is not great but who cares if your efficiency is only 70% in case that the alternative is 0%

Have you forgotten the objective was to drain the swamp? (I mean battery?)
;)

(But that would be good to roll through testing one battery after another. Or just back to grid, as you say, to avoid screwing up battery charge control with a poorly conceived DC/DC converter.)
For a production environment, power would add up to real dollars, also size of utility connection.
So actually quite a good idea. (y)
 
Just started the first charging cycle on the first cell using the Mean Well HRPG-150-3.3.

Resting cell voltage was at 3.3V (so no idea what SOC these cells were when shipped to me).

Voltage adjust topped out at 4.05V so plenty of range (spec is max = 3.8V).

I am using a 30A current shunt (0.0025 ohm). When I first turned on the power supply I saw a peak of 80 mV = 32A. Within a minute it dropped back to 75mV which is the rated 30A output of the power supply. After 1/2 hour the shunt voltage has dropped to 66.6 mV = 26.6A with 3.342V across the cell. I have the CV set at 3.630 V so I expect I can ignore this until the current drops below 3A (using the 0.01C charge rate = fully charged).

Nice to not have to worry about damaging the cell. I predict this is going to take a while. :cool:

PXL_20210121_222929894.PORTRAIT.jpg

P.S. That power supply is getting warm, I have it propped up on its side with lots of passive air flow around it. I am certainly giving it a stress test.

After I pointed a fan at it, the charge current increased back up to 27A.

That is a good power supply. So far I have to say that was $64 well spent.

 
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This is actually a perfect capacity rate for testing my system. Unless I am on shore power, 30A is about the max charge rate I should ever expect to see (solar plus alternator charging).
 
With a small fan pointed at the power supply and shunt, the power supply is no longer warm to the touch (it was not comfortably to touch without the fan). The battery hasn't warmed up much if any from ambient.

Power supply temp (degrees C)

PXL_20210121_231229997.jpg

Shunt temp (degrees C)
PXL_20210121_231345750.jpg

Cell temp (degrees C)
PXL_20210121_231413284.jpg
 
I know what you mean "no pun intended" when you say you don't have to worry about damaging the cell. I slept through my top balancing but I was awake when it finished. I was using the Riden. Looks like that Mean Well is an excellent choice and comes with a good price too. (y)
 
I know what you mean "no pun intended" when you say you don't have to worry about damaging the cell. I slept through my top balancing but I was awake when it finished. I was using the Riden. Looks like that Mean Well is an excellent choice and comes with a good price too. (y)
I am working from home so baby sitting the cells is no hardship. I will turn off the charger while I am asleep, just so I can be paying attention when we get to the end of charge.
 
Looks like you got a pile of work ahead of you.

FYI: max shunt current should be 60% of rated current.
 
Looks like you got a pile of work ahead of you.

FYI: max shunt current should be 60% of rated current.

You can drastically help this guideline limitation by putting the shunt in a shallow oil bath with a lot of surface area - really easy way to manage the temp rise.
Not a problem for bench bound test setup.
 
You can drastically help this guideline limitation by putting the shunt in a shallow oil bath with a lot of surface area - really easy way to manage the temp rise.
Not a problem for bench bound test setup.

Polychlorinated biphenyl oils have properties that make them particularly good for cooling of electrical components.
There are additional considerations, of course.


Forced air maybe?
You'd be surprised at how far a little air flow goes toward keeping things cool.
 
Just putting a PC case fan next to it will add 50% to its power dissipation.
 
Polychlorinated biphenyl oils have properties that make them particularly good for cooling of electrical components.
There are additional considerations, of course.


Forced air maybe?
You'd be surprised at how far a little air flow goes toward keeping things cool.
That is like beryllium copper. Fantastic material for making springs. Just that minor Berylliosis problem.

 
Looks like you got a pile of work ahead of you.

FYI: max shunt current should be 60% of rated current.
It is a strip of steel. The rated current is the current at which there is a 75mV voltage drop across it.

PXL_20210122_020730459[1].jpg

I don't think that strip of steel is going to have too much problem handling temperatures like 43 degrees C.
 
Well the charge current on the first cell has trickling down under 2 amps now. Voltage across the cell is 3.645V. Took about 6 hours

Got to get that constant current load going asap.
 
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It is a strip of steel. The rated current is the current at which there is a 75mV voltage drop across it.
I don't think that strip of steel is going to have too much problem handling temperatures like 43 degrees C.
Hopefully not steel but manganin. Max recommended service temperature is 140C if you want long-term stability.
Better shunts are 0.1% accuracy and if you don't want to introduce gross error to shunt accuracy it is better to keep the temperature rather small.
For manganin shunt 50C increase causes -0.1% error to measurement.

Although Your use with cooling fan is fine.
 
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