Battery Charger Power Draw Questions

[Dockhead]

Originally Posted by DotDun

It's simple ohm's law. For a charger to be 'constant current' it would need to raise voltage level as the internal resistance of the battery goes up.

One can only assume that 'constant current' implies if your batteries accept 70 amps at say 25.5v, the charger will raise the voltage up to the maximum (26 as an example) to keep 70 amps flowing. Once the maximum voltage is reached, assuming it won't go any higher, current flow will depend on the internal resistance of the batteries, the charger can't do anything to change that.

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.

[colemj]

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.

That's how ours works after accounting for temp compensation and state of charge of the battery.

Mark

[Dockhead]

Originally Posted by colemj

That's how ours works after accounting for temp compensation and state of charge of the battery.

Mark

I don't under this phrase:

". . .after accounting for . . . state of charge of the battery."


That's the whole question. Either the current changes during the bulk charging phase, according to state of charge of the battery, or it's constant current -- i.e. full output -- until it reaches absorption voltage, despite the constantly changing state of charge of the battery (that's what battery chargers do -- change the state of charge of the batts, right?). So which is it?

And what does temperature compensation have to do with it? Temperature compensation, as far as I understand, will either throttle back in case of overheating of the batts (lets leave that case to one side), or it will change the absorption voltage point. The latter has no relevance to the question of whether current changes prior to reaching the absorption voltage point (whatever that is), or not. That is to say, during the bulk phase.

[btrayfors]

Here's how I understand the way most "smart chargers" and "smart regulators" work.

Charging during the bulk acceptance phase is generally governed by three distinct factors:

1. the state-of-charge (SOC) of the battery (its internal resistance and capacity relative to the output capacity of the charger);

2. the output capacity of the charger (also relative to the capacity of the battery bank under charge); and

3. any limitation by virtue of a temp sensor on the charger itself, the batteries or the alternator.

Given a large battery bank relative to the size of the charger, the charger will work flat out until the internal resistance of the battery rises (as does its SOC) to a pre-determined cutoff point, either time-related or voltage-related or, possibly, temperature-related.

The "constant current" label isn't really totally descriptive of what's going on in the bulk phase.

In the absorption phase, the voltage output from the charger is limited. That means that the charger will put out all the current the battery will accept at the set absorption voltage, provided that the charger is large enough relative to the size of the battery bank. With a very large bank and a smallish charger, it may take quite some time to reach this voltage.

Individual chargers and regulators may work a bit differently, but the bottom line is that the battery bank itself will determine how much charging amperage it will accept at any given SOC and applied voltage.

Bill

[Dockhead]

Originally Posted by btrayfors

Here's how I understand the way most "smart chargers" and "smart regulators" work.

Charging during the bulk acceptance phase is generally governed by three distinct factors:

1. the state-of-charge (SOC) of the battery (its internal resistance and capacity relative to the output capacity of the charger);

2. the output capacity of the charger (also relative to the capacity of the battery bank under charge); and

3. any limitation by virtue of a temp sensor on the charger itself, the batteries or the alternator.

Given a large battery bank relative to the size of the charger, the charger will work flat out until the internal resistance of the battery rises (as does its SOC) to a pre-determined cutoff point, either time-related or voltage-related or, possibly, temperature-related.

The "constant current" label isn't really totally descriptive of what's going on in the bulk phase.

In the absorption phase, the voltage output from the charger is limited. That means that the charger will put out all the current the battery will accept at the set absorption voltage, provided that the charger is large enough relative to the size of the battery bank. With a very large bank and a smallish charger, it may take quite some time to reach this voltage.

Individual chargers and regulators may work a bit differently, but the bottom line is that the battery bank itself will determine how much charging amperage it will accept at any given SOC and applied voltage.

Bill

I must be really stupid today, because I STILL do not perceive an answer to my question in this

You say: "the charger will work flat out until the internal resistance of the battery rises (as does its SOC) to a pre-determined cutoff point, either time-related or voltage-related or, possibly, temperature-relate"


I presume that the "pre-determined cutoff point" is that point at which the charger switches over to the absorption regime, correct?

Do I also presume that "will work flat out" means that the charger is putting out its maximum current? Correct?

But then you say that "the battery bank itself will determine how much charging amperage it will accept at any given SOC and applied voltage"

But this is a contradiction!! (Either a contradiction, or not relevant during bulk phase). If the charger is "working flat out" until it goes into absorption phase, then it is increasing the voltage all the time until the battery bank accepts the full output of the charger, not so?


So I ask again, which is it? Does the charger "work flat out", that is produce maximum output, during the bulk phase, or not? Or does it put out some current between 0 and max depending on how "the battery bank itself determines how much charging amperage it will accept"? Which is it?

[goboatingnow]

It's important to point out that very large chargers on small battery's cause problem with overly aggressive bulk phases.

It's also with pointing out that real life battery charge curves are somewhat less well defined then the theory graphs suggest.

Dave


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[colemj]

Originally Posted by Dockhead

I don't under this phrase:

". . .after accounting for . . . state of charge of the battery."


That's the whole question. Either the current changes during the bulk charging phase, according to state of charge of the battery, or it's constant current -- i.e. full output -- until it reaches absorption voltage, despite the constantly changing state of charge of the battery (that's what battery chargers do -- change the state of charge of the batts, right?). So which is it?

And what does temperature compensation have to do with it? Temperature compensation, as far as I understand, will either throttle back in case of overheating of the batts (lets leave that case to one side), or it will change the absorption voltage point. The latter has no relevance to the question of whether current changes prior to reaching the absorption voltage point (whatever that is), or not. That is to say, during the bulk phase.

Our Victron is constantly measuring the DC system and uses the SOC and temperature to determine how it will shape its charging regime when the charger is operated. Because of that, the current and time profiles for any given charging period are variable.

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. For example, I had it unplugged the other day for several hours with no significant draw on the system (the reefer may have kicked on for a bit). When I plugged it back in, it went straight to float at 13.2V even though the battery voltage was 12.7V.

Also, it can only put out what the batteries can take. In our case, we usually are seeing 105-110A maximum output even though the charger is rated at 125A.

Mark

[colemj]

Originally Posted by Dockhead

So I ask again, which is it? Does the charger "work flat out", that is produce maximum output, during the bulk phase, or not? Or does it put out some current between 0 and max depending on how "the battery bank itself determines how much charging amperage it will accept"? Which is it?

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

Mark

[btrayfors]

Dockhead,

I think a stroll to the nearest pub for a couple of pints will sort you right out :-)

There's no contradiction.

Every charging source (battery charger, alternator, solar panel, generator, etc.) has a maximum capacity output at the rated voltage. "Working flat out" is my non-technical way of saying that the charging source is putting out all the amperage it possibly can at a given voltage.

Let's take a 40-amp charger trying to charge a 200AH AGM bank. At 50% SOC, that 200AH AGM bank can accept nearly 400 amps charging current! But there's no way in hell the 40-amp charger can put out that much. So, what happens then? The charger puts out all the amperage it can ("flat out") until one of three things happens:

1. it overheats, blows its fuse, or a temp sensor on the alternator or batteries cuts it off.

2. the battery SOC increases, its voltage goes up, and it eventually reaches the preset "absorption voltage", whence the charger switches to voltage regulation and holds at that voltage while it continues to put out all the amps it can deliver AND the battery will accept.

3. a time limit for bulk charging is reached, and the battery charger dials back. Depending, again, on the size of the charger relative to the size and type of battery bank, the charger now in the "absorption" mode might just continue pumping out all the amperage it can, so long as the voltage is still below the designated absorption voltage.

Bill

[colemj]

I forgot to mention that the Victron also lowers its current output if it (the Victron) is too hot. In this case, bulk charge will not be the rated current.

Mark

[btrayfors]

[QUOTE=goboatingnow;1511285]It's important to point out that very large chargers on small battery's cause problem with overly aggressive bulk phases.
......
Dave

Dave,

I think you're thinking of the old ferro-resonant battery boost chargers garages sometimes use on dead starting batteries.

Actually, the battery itself determines how much amperage it will accept at any given voltage.

So long as that voltage is limited to the proper level for charging, it doesn't matter a whit how big the charging source is.

Example:

A 100AH 12VDC battery at 50% SOC is accepting 25 amps @ 14.4 VDC from a 50-amp charger.

How much will it accept from a 500-amp charger @ 14.4 VDC?

Answer:

Exactly the same amperage, i.e., 25 amps.

Does it matter that you charge a flooded or AGM battery....or gelled battery for that matter....giving it all the amperage it will accept?

The evidence suggests that faster charging, i.e., near the limits of battery acceptance for any SOC, is actually good for the battery, so long as you keep the voltage at the proper limit for each stage.

At very low SOC an AGM battery can accept 5X it's rated AH in charging current. For example, a Concorde-Lifeline 100AH battery at a low SOC can accept over 500 amps charging current for a short time. And, their research shows that this is actually good for the battery's capacity and longevity!

Bill

[goboatingnow]

Originally Posted by colemj

Our Victron is constantly measuring the DC system and uses the SOC and temperature to determine how it will shape its charging regime when the charger is operated. Because of that, the current and time profiles for any given charging period are variable.

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. For example, I had it unplugged the other day for several hours with no significant draw on the system (the reefer may have kicked on for a bit). When I plugged it back in, it went straight to float at 13.2V even though the battery voltage was 12.7V.

Also, it can only put out what the batteries can take. In our case, we usually are seeing 105-110A maximum output even though the charger is rated at 125A.

Mark

Firstly , no charger can measure SOC.

Secondly, try charging a small 40 ah with that charger. ??

Typically it's rate to see chargers output the maximum rated charge.

Temperature is used to derate absorption and float levels only.

Dave


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[Pete7]

Originally Posted by btrayfors

Dockhead,

I think a stroll to the nearest pub for a couple of pints will sort you right out :-)

There's no contradiction.

Every charging source (battery charger, alternator, solar panel, generator, etc.) has a maximum capacity output at the rated voltage. "Working flat out" is my non-technical way of saying that the charging source is putting out all the amperage it possibly can at a given voltage.

Let's take a 40-amp charger trying to charge a 200AH AGM bank. At 50% SOC, that 200AH AGM bank can accept nearly 400 amps charging current! But there's no way in hell the 40-amp charger can put out that much. So, what happens then? The charger puts out all the amperage it can ("flat out") until one of three things happens:

1. it overheats, blows its fuse, or a temp sensor on the alternator or batteries cuts it off.

2. the battery SOC increases, its voltage goes up, and it eventually reaches the preset "absorption voltage", whence the charger switches to voltage regulation and holds at that voltage while it continues to put out all the amps it can deliver AND the battery will accept.

3. a time limit for bulk charging is reached, and the battery charger dials back.

Bill

Interesting numbers Bill has used because that is our set up but with FLA. In practice the Boost stage in Sailinglegends diagram is almost instant so the charger switches very quickly into the absorption stage. Max current I have seen is 42AH @ 14.8v. Just as the diagram shows the voltage remains constant and the amps then slowly and sadly drop down until after say 60 minutes the reading is in the high twenties. Eventually the amps reach single figures and the charger drops into a float stage at 13.9v.

There is a battery monitoring circuit, but I don't think in the UK we get the batteries hot enough for this to have an effect.

Interestingly the PWM solar regulators do a similar thing.

Pete

[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

[goboatingnow]

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

I'm just commenting , not inferring. It's all cool

Dave


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