Why is bulk/absorption voltage used?

[hwy17]

I charge to 55.6v, hold for 60 minutes, then float at 54v.

So my questions would be:

1. Is your charging voltage ever actually rising above 54v. Not the parameter, but the reality.

2. Is it actually any faster than just charging to 54v.

 

[TomC4306]

3.4, 3.45, 3.5, you can pick your final target voltage separately.

The question is do you need a higher stage before float, do you need to charge at 3.5 and then float at 3.45. If the actual charge curve is never exceeding 3.45 the 3.5 stage is pointless, it's not changing the charge curve.

What?
So you want to charge at say 3.45, then insert the stage at 3.5, then float at 3.4?

 

[TomC4306]

So my questions would be:

1. Is your charging voltage ever actually rising above 54v. Not the parameter, but the reality.

2. Is it actually any faster than just charging to 54v.

If I just charged a 54 volts the battery will not be full and I will not receive full capacity from the battery

 

[hwy17]

What?
So you want to charge at say 3.45, then insert the stage at 3.5, then float at 3.4?

I am doing 1 stage, charge to 3.437, and that's it. And I'm probing for justification that any other stages in the process are necessary.

 

[hwy17]

If I just charged a 54 volts the battery will not be full and I will not receive full capacity from the battery

This statement would rest on the theory that two batteries floating at 3.4 can have materially different states of charge. I think that probably is true but only very slightly.

 

[hwy17]

Better illustration of a single voltage CC/CV charge profile:



Multi stage charging implies that there is something to improve about this profile by using multi stage voltage targets. Getting the final 10% of SOC into the battery faster is a possibility that I am open to.

 

[TomC4306]

I am doing 1 stage, charge to 3.437, and that's it. And I'm probing for justification that any other stages in the process are necessary.

Okay, 3.437 is not high enough to give you a reliably 100% charged battery.
3.437 is an unnecessarily high voltage to expose the battery to once it is full. Float setting with lfp keeps the battery fully charged but exposes it to a slightly lower voltage and allows the mppts to cover your loads.

 

[TomC4306]

deleted

 

[TomC4306]

This statement would rest on the theory that two batteries floating at 3.4 can have materially different states of charge. I think that probably is true but only very slightly.

It depends on whether or not they are charging up through 3.4 volts, or coming back down to 3.4 volts in a float scenario. In the first instance, absolutely they can have different states of charge.
Charging typically is an algorithm that has to follow a process.

 

[hwy17]

It depends on whether or not they are charging up through 3.4 volts, or coming back down to 3.4 volts in a float scenario. In the first instance, absolutely they can have different states of charge.
Charging typically is an algorithm that has to follow a process.

That also depends on how long they float for. Two batteries held at 3.4v float, for 3+ hours, would have nearly identical SOC, within a few percent, as far as I understand it.

Like you can see in the profile above, a battery that hits float still gains SOC after that, the whole CV period. It doesn't have to exceed that float at any point to gain that SOC, although the rate it gets there is in question.

 

[TomC4306]

 

[hwy17]

I'd really like to know what grid scale ESS operators and large EV makers are doing, like what is Tesla's CATL profile. But that data is hard to find and probably secret.

 

[TomC4306]

That also depends on how long they float for. Two batteries held at 3.4v float, for 3+ hours, would have nearly identical SOC, within a few percent, as far as I understand it.

Like you can see in the profile above, a battery that hits float still gains SOC after that, the whole CV period. It doesn't have to exceed that float at any point to gain that SOC, although the rate it gets there is in question.

True, but typically in a solar charging scenario, you won't have good sun long enough to charge AND absorb at a depressed voltage for 3 or 4 hours.

 

[hwy17]

View attachment 199516

This is great data for me thank you, I will look it over more carefully later to digest it but it does look like a charge profile that is indeed well exceeding float voltage at multiple stages.

If you were really game, would you try setting all your stages to the float voltage for a day and then making these same graphs? Not if you're not interested of course, I have my battery now and plan to have the solar this spring to test it more myself.

 

[TomC4306]

Here is the same period as the other graph showing bulk, moving to absorption, and then the dip at the right end is moving into float which only requires 2 or 3 minutes.

 

[TomC4306]

This is great data for me thank you, I will look it over more carefully later to digest it but it does look like a charge profile that is indeed well exceeding float voltage at multiple stages.

If you were really game, would you try setting all your stages to the float voltage for a day and then making these same graphs? Not if you're not interested of course, I have my battery now and plan to have the solar this spring to test it more myself.

Suggest you pick up Solar Assistant so you can observe the behavior with pretty fine granularity.
Excellent product. Great value. For me.

 

[hwy17]

Suggest you pick up Solar Assistant so you can observe the behavior with pretty fine granularity.
Excellent product. Great value. For me.

None of my equipment is compatible.

 

[Partimewages]

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

Probably should look at this.

 

[hwy17]

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

Probably should look at this.

Thanks. Yeah that's a single stage charge curve, voltage never drops. But maybe I need to read further.

 

[Partimewages]

I think he got to 99% SOC. It just took longer.

 

[TomC4306]

Good video also. Watch the other one first.
In this one, if your in a rush, see minute 8 to minute 11.

 

[sunshine_eggo]

Thanks. Yeah that's a single stage charge curve, voltage never drops. But maybe I need to read further.

Two stages. Bulk and absorption. Conveniently labeled on the chart:



Achieved 99.7% SoC with a 0.02C tail current.

 

[hwy17]

Two stages. Bulk and absorption. Conveniently labeled on the chart:

View attachment 199561

Does the charger target voltage change or you just mean the CC to CV transition?

 

[sunshine_eggo]

Does the charger target voltage change or you just mean the CC to CV transition?

Not sure I understand your question. It's a standard two phase charge. Bulk (max current, 20A) until absorption voltage (13.6V), hold voltage and taper current.

 

[hwy17]

Not sure I understand your question. It's a standard two phase charge. Bulk (max current, 20A) until absorption voltage (13.6V), hold voltage and taper current.

So then maybe what I was calling single stage is better referred to as two stage.

One target voltage setting the whole time though, CC up to that voltage then CC onwards.