Battery Charger Power Draw Questions

[DotDun]

Originally Posted by colemj

Perhaps I didn't phrase that correctly. The charger is constantly measuring the DC system. It can crudely infer state of charge from that. I gave the example of where it skips bulk and absorption completely and simply goes into float when the charger is turned on if the batteries have not been drawn down much since the last time it was on.

I don't understand your question about trying to charge a small 40ah battery with the charger. How and why is that relevant here, and why is it directed at me?

I also gave the example of the charger not putting out full current when it (the charger) is hot (and it gets very hot at times).

Mark

If your Victron charger uses algorithms similar to Victron's BMV, they consider the battery charged when voltage is >13.2v (obviously a 12v battery) and accepting <.04C.

[DotDun]

Originally Posted by Dockhead

OK, but then we come right back to the beginning: so, the charger will be putting out its full power (and will be consuming a corresponding amount of mains power) until it reaches absorption voltage? You are both saying "yes"?

Let's ignore other DC loads; how that works is obvious.

I interpret your question as: "The charger is in bulk mode until it goes to absorption mode?"

I think you are quibbling too much between bulk and absorption, they are basically the same, some manufacturers may use slightly different voltage set points and timed vs voltage milestones to signal the end of bulk/beginning of absorption. I'm not sure if lowering the voltage set point .2v and calling it 'absorption phase' makes a lot of difference in the longevity of battery life. (that probably explains why I'm not a charger engineer!)

IMO, 'constant current' is a marketing term. They are basically stating their charger will put out it's maximum rated amperage to the bulk voltage set point. DUH! If it didn't do that, they would have to rate it differently! If they use the same voltage set point for both bulk and absorption phases, what's the difference between the phases? The difference can only be the internal resistance of the battery has increased such the batteries are taking less than maximum amperage from the charger. So what?

And, to your question, amperage going into the house/batteries will be reflected proportionally in the source feeding the charger.

[bornyesterday]

It almost embarrasses me to admit how much I use UK mains monitors, less than £20 from Maplin. Plug into shore power with an adapter to monitor total use in real time - plug the charger into another one and keep it next to the DC voltmeter to watch charger in/out activity.

They can display your choice of Volts, Amps, Watts, Hz., kWh. and more.

I do also have good quality test gear including scopes, data logger etc. - but these cheap monitors are really convenient, taking no time to set up.

Leaving them plugged in where I see them as I walk by for a week or two means I've become familiar with the normal range of power drawn by fridge, freezer, microwave, kettle, charger, TV etc. That knowledge might help to diagnose faults more easily in future and it allows me to use full shore power capacity without fear of overloading it.

I also used an infrared thermometer to establish baseline temperatures of everything I could think of when I first bought the boat. Same reason, helps with future diagnosis.

[sailinglegend]

Originally Posted by DotDun

...IMO, 'constant current' is a marketing term.....If they use the same voltage set point for both bulk and absorption phases, what's the difference between the phases?.....

You have totally missed the point that there IS a difference between "Constant Current" charging and "Constant Voltage" charging - Google it.

Bulk Phase is "Constant Current" charging, and when the absorption voltage is reached the "Smart" charger switches over to "Constant Voltage" charging. At this point the current supplied to the battery will slowly fall as the battery internal resistance increase as the SoC increases.

[conachair]

Originally Posted by sailinglegend

You have totally missed the point that there IS a difference between "Constant Current" charging and "Constant Voltage" charging - Google it.

I with dotdun, on this one, the " constant current" in bulk is a property of the charger because that's all it can supply , not some physical property of the battery.


Sent from my SGP312 using Tapatalk

[sailinglegend]

Originally Posted by conachair

I with dotdun, on this one, the " constant current" in bulk is a property of the charger because that's all it can supply , not some physical property of the battery.

At all points on the charging curve the battery determines how much current it will take. Mr Sterling has happily put 160 amps into a 100 Ah battery and let it go up to 50C.

My point is there are constant current chargers, constant voltage chargers, and SMART chargers which change over from "constant current" chargers to "constant voltage" chargers at the absorption voltage. If it didn't change over to a constant voltage charger at the absorption voltage then the voltage would go higher and higher - Ohms Law. Is that clear?

[Dockhead]

Originally Posted by colemj

It is the latter - Ohm's law with the battery as a variable resistor.

Mark

But you just said that "Yes, most of the time it simply goes flat out during bulk, but if the SOC is high, it skips bulk mode altogether and sometimes just goes straight into float."

Which sounds like the former -- subject to a few conditions (battery not too small; temperature not too high; etc.), the charger puts out a constant current at or near its maximum, and that is Bulk Mode. Current stays constant while voltage gradually rises. Correct?

Then when for whatever reason (reaches absorption voltage; times out) the charger goes to Absorption Mode, then the voltage is held constant (at the set absorption voltage) and the current falls off naturally according to how much the battery can take.

So that would indicate the former thesis, not indeed the latter.

[Dockhead]

Originally Posted by DotDun

If they use the same voltage set point for both bulk and absorption phases, what's the difference between the phases?

If that is what it is, then my whole understanding of how multistage battery chargers work is wrong.

As I understood previously, there is no voltage set point during Bulk Phase. The voltage rises during the Bulk Phase. It is the current which is constant. And the voltage rises constantly as the SOC increases, because the higher the SOC of charge of the battery, the higher the voltage needed to keep the same amount of current flowing.

Then when that process achieves a certain pre-set voltage, the charger stops increasing the voltage and rather holds it steady at that pre-set voltage (which is the Absorption voltage), and allows the current to fall off naturally as the battery accepts less and less current as the SOC increases.

So you are saying that all this is wrong?

[DotDun]

Originally Posted by sailinglegend

At all points on the charging curve the battery determines how much current it will take. Mr Sterling has happily put 160 amps into a 100 Ah battery and let it go up to 50C.

My point is there are constant current chargers, constant voltage chargers, and SMART chargers which change over from "constant current" chargers to "constant voltage" chargers at the absorption voltage. If it didn't change over to a constant voltage charger at the absorption voltage then the voltage would go higher and higher - Ohms Law. Is that clear?

It's a given that a battery charger built to charge a 12v/24v/etc. battery will operate within the correct voltage range required by the battery chemistry.

Hence, there is a voltage set point the charger will not exceed, no matter what 'phase' the charger is working in. For the sake of conversation, let's assume a 12v battery and a 70a charger with a 14.6v bulk/absorption set point. If the battery is 50% depleted (~12.2v), the charger will start out dumping 70a into the battery. Since the internal resistance of the battery is too low for the charger to maintain 14.6v, as it would need to exceed 70a to do so, yes the voltage will be lower than 14.6v and gradually rise as the SOC rises. It is this naturally occurring event that charger manufacturer marketing types have capitalized on to call their charger 'constant current'. Yes, the 70a amp charger maintains 70a until the pre-chosen voltage set point, but it does so as it can't do anything else, not because some smart engineer programmed it to do so.

If you want to think of this initial charging stage (bulk) that your 'constant current' charger is doing something different than any other charger, that's fine by me. My only point is this is the way battery charging works.

[DotDun]

Originally Posted by Dockhead

If that is what it is, then my whole understanding of how multistage battery chargers work is wrong.

As I understood previously, there is no voltage set point during Bulk Phase. The voltage rises during the Bulk Phase. It is the current which is constant. And the voltage rises constantly as the SOC increases, because the higher the SOC of charge of the battery, the higher the voltage needed to keep the same amount of current flowing.

Then when that process achieves a certain pre-set voltage, the charger stops increasing the voltage and rather holds it steady at that pre-set voltage (which is the Absorption voltage), and allows the current to fall off naturally as the battery accepts less and less current as the SOC increases.

So you are saying that all this is wrong?

Let me try another way.

Battery charger marketeers have renamed 'bulk' to 'constant current'. Unless your charger is way oversized, constant current is simply a natural phenomenon of bulk charging.

[Zanshin]

Dockhead - I have a Victron Multi inverter/charger (5000W, 24V, 120A charging) and have been closely monitoring my charging patterns for the past 2 months of living aboard.

When my bank is down to about 60% SOC and I fire up the generator and charger, the Victron will ramp up to about 100A charging current (which is the limit I set) and I watch the voltage voltage quickly climb to about 26V and then it very slowly goes up, while the Victron is charging 80-90A. This is the bulk phase, and after about an hour or so the voltage reaches 28.1V and the charger switches from bulk to absorption, but still puts out those 80A. As the SOC approaches about 90% I can see how the charger's amperage goes down, relatively quickly, from that high amount and when it gets to 20A I usually terminate charging because I don't like to run the generator with a low load.

I have a Xantrex monitor, but have gotten the Victron monitor adapter which lets you plug your Victron directly into the USB port of your PC and it will show you a LOT of detailed information - I use this to cross-check the Xantrex and also the instrument panel voltage. See below where it tells you the output voltage and amperage (plus the genset details) on the left and shows the charging algorithm on the right.

[Maine Sail]

Originally Posted by Dockhead

If that is what it is, then my whole understanding of how multistage battery chargers work is wrong.

Your understanding is spot on..

Originally Posted by Dockhead

As I understood previously, there is no voltage set point during Bulk Phase. The voltage rises during the Bulk Phase. It is the current which is constant.

Yes. But this does not stop "marketing departments" from trying to confuse the masses. Bulk is not a voltage limited stage of charging it is constant current. Despite companies like Balmar etc. BASTARDIZING terms, that have been around longer than they have, it sounds good to the naive to have a "bulk voltage" and "absorption voltage" and a "float voltage"..

Any charger, regulator or solar controller that has a "bulk voltage" is selling you marketing BS. It sounds "smarter" at boat shows I guess.. It is an ABSORPTION/CV stage not bulk. The device may have two stages of absorption/CV before entering float. Call them Absorption #1 and Absorption #2 or CV #1 & CV #2 but bulk is not voltage limited despite what the marketing departments want you to believe..

BULK = CC Charging

Absorption, Float, Equalize, Bulk #1, Bulk Voltage = CV Charging

All voltage regulated/limited charging devices do at least TWO stages, bulk/CC & absorption/CV. Even the dumbest of dumb alt regulators does bulk/CC and absorption/CV. This has been this way since voltage regulation was created.

This applies to chargers, controllers, and alt regulators:

A charge source with no voltage limit is a CC only charger - If left connected too long the batteries will cook. These are very rare today.

A charge source with voltage limiting is a CC/CV charger - Pretty much all battery chargers, solar regulators and alt regs do CC/CV, even the dumb ones.

A charge source with multiple voltage limited stages (absorption, float, equalization) is still a CC/CV charger. - These are usually considered "smart".

Originally Posted by Dockhead

Then when that process achieves a certain pre-set voltage, the charger stops increasing the voltage and rather holds it steady at that pre-set voltage (which is the Absorption voltage), and allows the current to fall off naturally as the battery accepts less and less current as the SOC increases.

So you are saying that all this is wrong?

No you have it spot on! Once the voltage is held steady or "limited" the battery can only take so much current and it not the charge source determine how much current the battery will take at X-SOC and X-Voltage. As the SOC increases beyond where the battery hit CV the current continues to decrease.

Here is a recorded example of CC/CV:


125Ah Group 31 "Deep Cycle" battery fed a 20% of "C" charge rate, or a fixed 25A, after being discharged to 50% SOC. It took 1.7 Hours in bulk or constant current charging to hit absorption voltage with a charge rate of 20% of battery capacity.

2 Min = 12.59V / 25A = BULK / Constant Current
10 Min = 13.0V / 25A
20 Min = 13.12V / 25A
30 Min = 13.22V / 25A
40 Min = 13.33V / 25A
50 Min = 13.45 / 25A
60 Min = 13.6V / 25A
70 Min = 13.76 / 25A
80 Min = 13.95 / 25A
90 Min = 14.16V / 25A
100 Min = 14.39 / 25A
102 Min = 14.4V / 24.5A = ABSORPTION VOLTAGE / Voltage Limiting
110 Min = 14.4V / 20.5A
120 Min = 14.4V / 15.0A
130 Min = 14.4V / 11.9A


As can easily be seen the charger was in constant current for 101 minutes before it entered CV/Voltage Limiting and the current immediately dropped from 25A to 24.5A the minute the battery voltage needed to be CV limited.

[goboatingnow]

I think everyone is frantically agreeing with everyone.

Doesn't answer his original question though

Dave

[Maine Sail]

Originally Posted by goboatingnow

I think everyone is frantically agreeing with everyone.

Doesn't answer his original question though

Dave

Ideally it needs to be clamped with an AC clamp... I have about 10 chargers in my shop and each one has a different efficiency. Really pretty hard to say without physically measuring the charger in question....

[colemj]

Originally Posted by Dockhead

But you just said that "Yes, most of the time it simply goes flat out during bulk, but if the SOC is high, it skips bulk mode altogether and sometimes just goes straight into float."

Which sounds like the former -- subject to a few conditions (battery not too small; temperature not too high; etc.), the charger puts out a constant current at or near its maximum, and that is Bulk Mode. Current stays constant while voltage gradually rises. Correct?

Then when for whatever reason (reaches absorption voltage; times out) the charger goes to Absorption Mode, then the voltage is held constant (at the set absorption voltage) and the current falls off naturally according to how much the battery can take.

So that would indicate the former thesis, not indeed the latter.

In the bulk mode, the charger puts out as much current as the batteries will accept - up to the limits of the charger. The current the batteries can accept changes with SOC and things like age, sulfation, etc.

So the charger will not "work flat out producing maximum output" if the batteries cannot accept that output. That is why it is the latter of your two posits.

To test this, does your charger produce the same current throughout the entire bulk charge phase until the battery voltage reaches the absorption point? Ours does not - it starts out pretty much flat out at 100-110A, but slowly tapers down to ~90A by the time the absorption set point is reached. It is the variable internal resistance of the batteries during the charging that controls this output.

Mark