Wire size matters

[JPS350]

Please help me understand. I thought 14.7V was bulk mode and 13.6V was absorption mode?

I think you may be getting things confused between the way a WFCO charger converter works and what is typical for a 3 stage smart battery charger.

Here's the description for the WFCO from their manual:



Heres a charging diagram for a typical 3 stage smart chargers, like what I installed:



I'm not sure why WFCO does what they do with the charging profile on their converter chargers but there must be some method to the madness. Maybe it has something to do with the fact that it has to power the trailer as well as charge the battery, but it doesn't do a real good job of battery charging.

The first part of the WFCO graph (bulk) does look kind of like a combination of bulk and absorption on a 3 stage charger. Then there's a lower absorption stage. Confusing to say the least because nowhere do they claim that they bring the battery up to 14.4 v (they seem to be unable to do this). They also don't supply a graph for current, which might help you make sense of the picture. A decent charger brings the battery voltage up to absorption voltage in the bulk stage and holds it there.

 

[JPS350]

13.6 is a converter mode rather than a charging mode.

The idea that there is a difference in charger behavior for bulk and absorption in a so-called smart charger is marketing by charger manufacturers. The charger makes only one change to its output during the charging process, and that is at the end when it sees a low enough current draw that it switches to float voltage.

The idea that the charger is in constant current or constant voltage mode is like saying a vehicle is in constant speed mode when the pedal is on the floor.

The charger is a DC power supply with output voltage in this case set at 14.7V. When it first comes on,it doesn’t have enough current capacity to raise the battery to 14.7V, so it runs at max current while battery voltage increases as it charges. Then, once the battery charge increases enough, the charger is able to hold it at 14.7V with less and less current, so the current falls. The charger never, ever changes its output in any way until it sees that low current draw and switches to float mode. The fact is, it is physically impossible for a charger to change voltage or current independently.

Converters have an intermediate mode where they see usage from the RV and jump up a little above float.

There is no such thing as bulk charge, and there is no such thing as absorption charge except as a description of what’s going on in the battery. At all times while the battery is charging the charger acts as a 14.7V fixed output DC power supply and never changes during the charge.

Good explanation. The WFCO unit makes more sense to me now, at least.

Concerning marketing hype on the part of the charger companies, I suspect you're right. Concerning distinct charging stages, I can say after watching the Victron charger as it hits absorption voltage, it goes straight from 30 a to a variable amperage and tapers down starting like someone flipped a switch. One could set the absorption voltage to to 14.4 instead of 14.7 (my setting) and the same behavior would happen at the lower voltage. So there's more going on than a power supply charging at a set voltage and the battery controlling everything when it reaches saturation.

 

[coglesby]

I think you may be getting things confused between the way a WFCO charger converter works and what is typical for a 3 stage smart battery charger.

Here's the description for the WFCO from their manual:

View attachment 191188

Heres a charging diagram for a typical 3 stage smart chargers, like what I installed:

View attachment 191192

I'm not sure why WFCO does what they do with the charging profile on their converter chargers but there must be some method to the madness. Maybe it has something to do with the fact that it has to power the trailer as well as charge the battery, but it doesn't do a real good job of battery charging.

The first part of the WFCO graph (bulk) does look kind of like a combination of bulk and absorption on a 3 stage charger. Then there's a lower absorption stage. Confusing to say the least because nowhere do they claim that they bring the battery up to 14.4 v (they seem to be unable to do this). They also don't supply a graph for current, which might help you make sense of the picture. A decent charger brings the battery voltage up to absorption voltage in the bulk stage and holds it there.

I have WFCO WF-8955. Since I moved the battery from the front bay to the basement right beside the converter, it does a great job of charging. In fact, it supplies about 57A in bulk mode now. Because of the added resistance between the converter and the battery before, it never went into bulk mode because the current was only about 10A when charging.

The only difference I see between the two graphs is the rate of change of the voltage in the bulk stage. The WFCO graph shows the voltage reaching the bulk charge voltage earlier than the other graph. The other graph shows the voltage reaching the absorption voltage right at the end of the bulk charging mode.

The WFCO graph does show the voltage reaching 14.4V after the battery is charged enough to reach that voltage (converter is no longer current limited)

I monitored mine for several days. After the battery reached 13.6V during bulk mode, the converter changed to absorption mode. Apparently if it doesn't reach that voltage within 4 hours, the converter will switch to 13.6V anyway. After 44 hours, the voltage dropped to 13.2V.

 

[coglesby]

Good explanation. The WFCO unit makes more sense to me now, at least.

Concerning marketing hype on the part of the charger companies, I suspect you're right. Concerning distinct charging stages, I can say after watching the Victron charger as it hits absorption voltage, it goes straight from 30 a to a variable amperage and tapers down starting like someone flipped a switch. One could set the absorption voltage to to 14.4 instead of 14.7 (my setting) and the same behavior would happen at the lower voltage. So there's more going on than a power supply charging at a set voltage and the battery controlling everything when it reaches saturation.

Ok, I think I get it now. You can adjust your absorption voltage whereas the WFCO is set at 13.6V with no ability to adjust.

 

[jkwilson]

Good explanation. The WFCO unit makes more sense to me now, at least.

Concerning marketing hype on the part of the charger companies, I suspect you're right. Concerning distinct charging stages, I can say after watching the Victron charger as it hits absorption voltage, it goes straight from 30 a to a variable amperage and tapers down starting like someone flipped a switch. One could set the absorption voltage to to 14.4 instead of 14.7 (my setting) and the same behavior would happen at the lower voltage. So there's more going on than a power supply charging at a set voltage and the battery controlling everything when it reaches saturation.

The reason the current starts to taper is that the current has been maxed the entire time, but when the battery reaches a high enough state of charge it no longer needs full current.

At a given voltage, the charger can neither increase nor decrease its output current under any circumstances. The only way for the current to change is for the load or voltage to change, which in this case is the state of charge of the battery. Similarly, at a given output current, the charger cannot change its output voltage. The increasing voltage in the early part of charging, and the decreasing output current in the later part are a direct indication of battery state of charge, not setting changes made by the charger..

At a given battery state of charge (and condition) the charger current and voltage will always be the same. This isn’t Ohm’s law, but it’s a similar relationship.

 

[JPS350]

At a given voltage, the charger can neither increase nor decrease its output current under any circumstances.

Victron's charge algorithm would disagree with that statement.

"The battery is charged at the configured absorption voltage, with the charge current slowly decreasing as the battery approaches full charge.

The default absorption stage duration is adaptive and intelligently varied depending on the battery’s level of discharge (determined from the duration of the bulk charge stage).

Adaptive absorption stage duration can vary between a minimum of 30 minutes, up to a maximum limit of 8 hours"


The charger could not do this at a fixed voltage without controlling current.

Charge algorithm

www.victronenergy.com

 

[Scott'n'Wendy]

Victron's charge algorithm would disagree with that statement.

"The battery is charged at the configured absorption voltage, with the charge current slowly decreasing as the battery approaches full charge.

The default absorption stage duration is adaptive and intelligently varied depending on the battery’s level of discharge (determined from the duration of the bulk charge stage).

Adaptive absorption stage duration can vary between a minimum of 30 minutes, up to a maximum limit of 8 hours"


The charger could not do this at a fixed voltage without controlling current.

Charge algorithm

www.victronenergy.com

The battery state of charge controls the current. The charger applies the voltage.

 

[JPS350]

The battery state of charge controls the current. The charger applies the voltage.

That's correct to a point. If you are talking about the old single stage chargers or a power supply that would be true.

In this case, we have a charger that has a bulk current selection of either 15a or 30a. The battery is not determining the bulk charge current. The charger instantaneously switches from 30a charge current steady to a charge current of less than that and ramping down EXACTLY at the set absorption voltage. This voltage can be configured to 14.7 v or 14.4 v (defaults) or what the user wants to configure it to. The behavior is the same at any set voltage.

The charger algorithm states that the absorption time is "adaptive and varied" based on the amount of time that it took for the absorption voltage to be reached. One could also set things up using tail current, but that is not the default. Obviously, this all takes place within the confines of how a lead acid battery can be charged, but do you still want to stick with the battery, and the battery alone, controls the current and the rate of charge?

 

[coglesby]

The charger can only charge the battery at a constant current if it has unlimited voltage.

 

[JPS350]

The charger can only charge the battery at a constant current if it has unlimited voltage. Are you saying a completely charged battery can be forced to take 15A or 30A by the charger?

No.

 

[coglesby]

No.

Physics

 

[JPS350]

Physics

No argument there. My no reply was to the second sentence in your post that must have been edited out while I was replying.

 

[coglesby]

We’re probably all saying the same thing, just in different ways. At least that’s my take.

 

[jkwilson]

Victron's charge algorithm would disagree with that statement.

"The battery is charged at the configured absorption voltage, with the charge current slowly decreasing as the battery approaches full charge.

The default absorption stage duration is adaptive and intelligently varied depending on the battery’s level of discharge (determined from the duration of the bulk charge stage).

Adaptive absorption stage duration can vary between a minimum of 30 minutes, up to a maximum limit of 8 hours"


The charger could not do this at a fixed voltage without controlling current.

Charge algorithm

www.victronenergy.com

It is 100% physically impossible for a charger to change current without changing voltage. It cannot be done. The only way to increase current is to increase voltage. The only way to decrease current is to decrease voltage. No algorithm can change this.

The current decreases because the battery state of charge increases, not because of any action by the charger. The idea of “charging profiles” is just marketing jibberish to make chargers seem special. There is no such thing. A charger is a fixed DC power supply and the output Victron describes is exactly how a fixed DC power supply behaves when connected to a varying load like a charging battery.

 

[coglesby]

Here’s a real live example of the WFCO WF-9875-AD with a 600AH LiFePO4 battery starting with an SOC of around 60%.

TT plug into shore power at 1730 hours. To my surprise, the converter applies 83A. Converter goes into bulk mode (AKA charge mode in the owners manual) and attempts to reach 14.6V but cannot because it is current limited at 83A and can only raise the battery voltage to 13.8V.

As battery charges the battery voltage begins to rise.

According to the literature attached below, a timer starts and the converter will only remain in bulk mode for 4 hours max. If more than 4 hours of charging is required the power can be disconnected and reconnected to restart the timer so at 2000 Hours there a drop on the chart when that occurred.

At 2130 hours the battery begins to reach full charge and the battery voltage starts to rise and at 2200 hours the converter is finally able to reach 14.6V

Once the converter detects the drop in current to 5A, it reverts to attempting to force the absorption voltage of 13.6V (AKA the power mode in the owners manual) and the battery voltage begins to decline towards that voltage.

 

[huntindog]

The only comment I can add, is that none of my WFCOs ever switched modes, they were always at 13.6.
When I replaced them with either a PD, or a Boondocker they each switched modes as expected. These were drop in replacements. No changes were made to the wiring.

 

[coglesby]

The only comment I can add, is that none of my WFCOs ever switched modes, they were always at 13.6.
When I replaced them with either a PD, or a Boondocker they each switched modes as expected. These were drop in replacements. No changes were made to the wiring.

My WF-9855 changed modes at 48 hours to float at 13.2V and would drain my LiFePO4 battery down to 80% SOC so I upgraded to WF-9875-AD.

 

[huntindog]

My WF-9855 changed modes at 48 hours to float at 13.2V and would drain my LiFePO4 battery down to 80% SOC so I upgraded to WF-9875-AD.

I wish mine would gone into 13.2 float.
In Phoenix 13.6 will cook 6 volt batteries.
I really did not care about their failure to go into bulk.

 

[JPS350]

It is 100% physically impossible for a charger to change current without changing voltage. It cannot be done. The only way to increase current is to increase voltage. The only way to decrease current is to decrease voltage. No algorithm can change this.

The current decreases because the battery state of charge increases, not because of any action by the charger. The idea of “charging profiles” is just marketing jibberish to make chargers seem special. There is no such thing. A charger is a fixed DC power supply and the output Victron describes is exactly how a fixed DC power supply behaves when connected to a varying load like a charging battery.

Well, when you're wrong, you might as well come clean and admit it. So I'm doing that.

I did reach out to Victron. The charger does control current but only in bulk mode. But that's only the max so the charger doesn't self destruct. You can also limit it to 15a instead of 30a if needed for a smaller battery. What happens after that is due to the battery. Once absorption voltage is reached and limited by the charger, the current tapers. So no magic control, much to my dismay.

The "smarts" would be just the timing of the absorption phase either due to the algorithm paying attention to the amount of time it took to get to get to the set voltage, or by monitoring tail current, if it's configured by the user to do that. Then it just goes into a set voltage float stage, and a storage voltage after that. It jumps back up to absorption after 7 days for a bit just to keep the charge up if the battery is in storage. There's also a "recondition" stage that seems to be an equalization stage that can be set up.

So that's the life of a "smart" staged charger. No magic, just a decent price tag. Hopefully, with the price tag comes a dependable product. If it's like the Victron solar controllers and chargers I've bought for other applications over the years, it won't disappoint.

Thanks for setting me straight.

 

[jkwilson]

Well, when you're wrong, you might as well come clean and admit it. So I'm doing that.

I did reach out to Victron. The charger does control current but only in bulk mode. But that's only the max so the charger doesn't self destruct. You can also limit it to 15a instead of 30a if needed for a smaller battery. What happens after that is due to the battery. Once absorption voltage is reached and limited by the charger, the current tapers. So no magic control, much to my dismay.

The "smarts" would be just the timing of the absorption phase either due to the algorithm paying attention to the amount of time it took to get to get to the set voltage, or by monitoring tail current, if it's configured by the user to do that. Then it just goes into a set voltage float stage, and a storage voltage after that. It jumps back up to absorption after 7 days for a bit just to keep the charge up if the battery is in storage. There's also a "recondition" stage that seems to be an equalization stage that can be set up.

So that's the life of a "smart" staged charger. No magic, just a decent price tag. Hopefully, with the price tag comes a dependable product. If it's like the Victron solar controllers and chargers I've bought for other applications over the years, it won't disappoint.

Thanks for setting me straight.

Don’t feel bad. First, you learned, and any day you learn something new is a good day. Even many engineers are slow catch on to the idea that a battery is a fixed load at any instant in time.

Second, many, many people are in the same boat as you and they never catch on.

The downside is you’ll start to notice the bogus claims charger (I use that as a catch all term for chargers, converters and solar controllers) manufacturers make about their products. They aren’t “smart”, they just obey the laws of physics.