Charge termination "drinking bird"-style based on SoC

eXodus said:

You can always design a system to deal with anything. Victron is good about that - I wouldn't trust others.

You don't know how the SCC MPPT is designed internally. Does the computer-chip takes it power from the Batterie side or the PV side? Both?


many MPPTs have large coils inside - so you got inductive loads.


that should work, I'm just advising the threat author to not disconnect the battery from the MPPT without knowing what they are doing - there are risks.

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Never do anything you don’t understand. That’s fine. That’s rules out about 90%

And yes I do know how mppt controllers are designed and no they are not inductivve

Don’t listen to forums people. Just follow the manufacturers instructions. They designed the damm thing.

FyKnight said:

That is all good except for the last point, it assumes like a lot of "charging procedures" that unlimited power is available. It's not. We've barely had a bright sunny day since Covid where I am. It's been really frustrating just trying to test second-hand panels to see what their max output is! And loads come and go, e.g. fridge or aircon cycling on and off. That would all affect net current. So that's why I was thinking of looking at SoC (best guess by BMS) and net current to terminate the CV phase, instead of just net current.


It'd be nice if my budget could stretch to Victron gear. Maybe for my next build.

OK, thanks for this info. I was worried it'd be affected by loads too..

Option 1 is the go-to, I understand, and I kinda expected it to be suggested in the first reply. However I'd like to use the MPPTs in the much-less-capable all-in-one inverters I can currently afford.

Option 2 is also problematic with less expensive MPPTs, e.g. the MakeSkyBlue units that I have. They don't have any comms at all.

Option 3 is probably what I'll do then. I already have a BMS for insuring the battery against faults in other equipment. This "charge director" is somewhere between a BMS and a SCC in concept. (And directly between them physically, hah!). It'll only cost about $20 in hardware to implement, plus a relay of some kind — definitely under $100 anyway. And it might be better and cleaner overall too.

Thanks very much for your comprehensive reply! Excellent overview of my options.

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again you just need mppt with tail current monitoring. Then arrange to get the current sense info to each charger.

Each mppt will then terminate absorption as required. You don’t really care about the exact sequence

FyKnight said:

As others have mentioned the dam metaphor isn't entirely relevant,

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it is relevant if you got a Coil transformer based MPPT. It is not relevant for high frequency.
Opening up a coil circuit is how you produce spark in car ignition.

FyKnight said:

If the SCC's control loop checks the target voltage every millisecond,

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doesn't really matter since you can't turn off a coil within one millisecond. You capacitors would need to be able take all energy stored in the transformer.

FyKnight said:

I can see an 80 V, 22 mF capacitor on Aliexpress for $6.50

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You need to calculate how current and voltage you are expecting from an opening coil - could be hundreds of volts.

toms said:

When i set up systems with multiple chargers that don’t communicate, i set all bar the one that gets the last sunlight to lower setpoints for absorb and float.

This works well as the balancing phase requires less current in any case.

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This is what I do and it works great so far. I have a Schneider MPPT 100 and two Midnite classic 150s. The 150's get last sun, utilize shunts, and are in follow me mode. The setpoints on the 150's are the system target setpoints. The setpoints on the MPPT100 are slightly lower. Setpoints are determined by the voltage at the battery. It works well. I have it so the MPPT100 drops to float right when the system enters balance. Essentially what happens is the MPPT100 operates as a bulk only device with very little absorb. Then it will provide amperage in float if the 150s can't handle the load.

eXodus said:

it is relevant if you got a Coil transformer based MPPT. It is not relevant for high frequency.
Opening up a coil circuit is how you produce spark in car ignition.

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It’s not how mppt works they are either synchronous or asynchronous switched mode power systems. These systems can be disconnected at the output with no deleterious effect . This is not the reason some mppt controllers don’t like being removed from the battery , this is more because the overvoltage systems and spike protection relies on the battery acting as a big capacitor.

A mppt controller bears no relation to a automotive coil. Please a “ little knowledge “ is a dangerous thing is appearing here.

eXodus said:

doesn't really matter since you can't turn off a coil within one millisecond. You capacitors would need to be able take all energy stored in the transformer.

You need to calculate how current and voltage you are expecting from an opening coil - could be hundreds of volts.

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Again this is not relevant. It’s not how SMPS works.

Again to address the OPs issue ( which he’s overthinking )

If you have multiple different charge sources to a bank , while it’s “ of some use “ to coordinate charging phases it doesn’t “ really “ matter that much.

1. In bulk ( ie CC ) all charge sources will contribute in proportion to their power ability and in essence their “ equivalent “ series output resistance.

2. As the terminal voltage rises during the CC stages. Currents will fall distributed across all capable charge sources. Clearly those with no power source will terminate.

3. Assuming the CC-CV setpoint is roughly similar across all charge sources. All charge sources will then transition ( it doesn’t really matter if they don’t ) at that point the next issue is when to terminate the CV stage.

4. All half decent Li chargers should have a maximum time in CV irrespective of all other metrics. If this is not present then your BMS will have to provide that functionality.

5. Hence even if tail current measuring is not present , it largely doesn’t matter. You can set the termination voltage and max CV time. Your battery may or may not reach exactly 100% but who cares.

6. What you must achieve is “ charge stop “ occurs not a reduction to some notional “ float voltage “ floating Li causes Ion movement and SEI layer growth and early degradation it’s best avoided.

Hence a charge stop point unlike LA should result in absolute charging disconnection. End of story. People telling you they reduce the voltage simply don’t understand what they are doing and are justifying their equipment choices.

Hence my view is actually you really don’t need to do “ anything “
iF :
(A) you have hard stop charging setpoint

(B) the charge stop is not a float

Otherwise if these don’t exist as say you need a “ remote “ charge stop controller the key again is to ensure the chargers are electrically disabled not floated

Goboatingnow said:

Again to address the OPs issue ( which he’s overthinking )

If you have multiple different charge sources to a bank , while it’s “ of some use “ to coordinate charging phases it doesn’t “ really “ matter that much.

1. In bulk ( ie CC ) all charge sources will contribute in proportion to their power ability and in essence their “ equivalent “ series output resistance.

2. As the terminal voltage rises during the CC stages. Currents will fall distributed across all capable charge sources. Clearly those with no power source will terminate.

3. Assuming the CC-CV setpoint is roughly similar across all charge sources. All charge sources will then transition ( it doesn’t really matter if they don’t ) at that point the next issue is when to terminate the CV stage.

4. All half decent Li chargers should have a maximum time in CV irrespective of all other metrics. If this is not present then your BMS will have to provide that functionality.

5. Hence even if tail current measuring is not present , it largely doesn’t matter. You can set the termination voltage and max CV time. Your battery may or may not reach exactly 100% but who cares.

6. What you must achieve is “ charge stop “ occurs not a reduction to some notional “ float voltage “ floating Li causes Ion movement and SEI layer growth and early degradation it’s best avoided.

Hence a charge stop point unlike LA should result in absolute charging disconnection. End of story. People telling you they reduce the voltage simply don’t understand what they are doing and are justifying their equipment choices.

Hence my view is actually you really don’t need to do “ anything “
iF :
(A) you have hard stop charging setpoint

(B) the charge stop is not a float

Otherwise if these don’t exist as say you need a “ remote “ charge stop controller the key again is to ensure the chargers are electrically disabled not floated

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Kind of correct. You don’t need to stop charging, setting the float voltage below the upper voltage knee has the same effect.

toms said:

Kind of correct. You don’t need to stop charging, setting the float voltage below the upper voltage knee has the same effect.

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No it doesn’t. With lithium any remaining impressed terminal voltage cause lithium ion movement. This contributes to plating and SEI layer growth and hence overall degradation

The best strategy is proper charge termination. Most modern chargers like Victron have remote disconnects etc.

Goboatingnow said:

No it doesn’t. With lithium any remaining impressed terminal voltage cause lithium ion movement. This contributes to plating and SEI layer growth and hence overall degradation

The best strategy is proper charge termination. Most modern chargers like Victron have remote disconnects etc.

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For a battery that is not in use (eg stationary EV, backup UPS etc) i agree with you.

For an off-grid house there is always a load, setting float to 3.40V will not cause plating.

I have seen a few try to terminate charging rather than float. What ends up happening is the charger rebulks every time a significant house load is applied, and the battery ends up spending a lot more time in the 3.4V - 3.5V range where there is more degradation.

toms said:

For a battery that is not in use (eg stationary EV, backup UPS etc) i agree with you.

For an off-grid house there is always a load, setting float to 3.40V will not cause plating.

I have seen a few try to terminate charging rather than float. What ends up happening is the charger rebulks every time a significant house load is applied, and the battery ends up spending a lot more time in the 3.4V - 3.5V range where there is more degradation.

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Sure , in systems with high continuous loads often the chargers are on almost continuously. But not withstanding floating Lithium for any appreciable time should be avoided if at all technically feasible.

Rebulk needs to be carefully configured so as to avoid too much time near the upper “ knee”. People obsess with achieving 100% SOC this is a mistake.

Lithium is best maintained to 50 % DoD if charging sources are online. But very low C and hence long charging should be avoided

Agreed, one of the main reasons why parallel top balancing is such a bad idea.

toms said:

Agreed, one of the main reasons why parallel top balancing is such a bad idea.

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I agree fully. The obsession with extracting max SOC in lithium is just that. Better oversize the bank and worry less about capacity efficiency

Can you feed a single charge controller with the other charge controllers? That is, the other 5 SCCs all terminate into one controller that will decide what's best for the battery?

CoolWill said:

Can you feed a single charge controller with the other charge controllers? That is, the other 5 SCCs all terminate into one controller that will decide what's best for the battery?

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No you can’t. ( in general )

toms said:

When i set up systems with multiple chargers that don’t communicate, i set all bar the one that gets the last sunlight to lower setpoints for absorb and float.

This works well as the balancing phase requires less current in any case

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+1

Goboatingnow said:

I agree fully. The obsession with extracting max SOC in lithium is just that. Better oversize the bank and worry less about capacity efficiency

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Yep, if you hit 100% more than 2 months of the year with the longest sunshine you don't have enough batteries. The remainder of the year, the system should be hovering somewhere between 50-90%.

FyKnight said:

but from there to lower you're probably leaving a lot of solar on the table.

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I look at these scenarios like this: it’s the cost of doing business

You setup your system to optimize reaching full charge daily with headroom for less than optimal sunshine based on the typical variations of daily insolence for your locale. Thereby insuring full recovery after a cloudy day or snow day(s) etc.

Whatever “overages” you experience aren’t waste- they are a planned headroom for dependable service.

In my case, when I get the new panels set up, I’ll have just shy of 3000W of panels for a ~3kW daily need (in winter) which is way overpaneled. But in my locale, I’ll still be able to pull in 1kW (or maybe 1.5 or 2kW) in poor conditions extending my two-day guesstimated reserve. Granted those are wildhat guesses but they’re based on 4 years of experience with small solar and experimenting with arrays and mppts to observe what works in my favor in suboptimal conditions.

In your case, you are essentially doing the same thing but with multiple SCCs and so you aren’t ‘really’ leaving watts on the table- you’re just insuring you don’t “bounce any checks” with the watt balance in your (battery) bank.

If the unused potential is robust, a $15 relay setup could be used to turn on a $25 350W or 700W electric air heater or water heater element to ‘harvest’ the overage if you are in a northern/heating season locale.

It’s not wasted watts- it’s just the cost of doing business. Or like the cost of ensuring charge: like buying a 5000W generator to handle surge when you only use 1- 1.5kW on average for example.

Goboatingnow said:

Water has mass. electricity dues not. The analogy has limits.

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Not picking here but electrons technically do have mass…