Bulk charging stage: What determines amps in?

[VanGuy6678]

Hi all - What is the factor that determines how many amps are delivered to my 12V Lifeline AGM battery bank during BULK charging stage? I use a 12V, 120A 3-stage charger, but during BULK stage, my battery bank is only receiving ~50 amps. Thanks!

 

[BradKW]

50 amps is actually pretty high, especially when talking in the realm of 110v chargers. If you could link the charger specs it'd be helpful.
In bulk, the battery can accept more current at higher amps/lower volts...this changes as the battery starts to fill up...

 

[John61CT]

How large a battery bank, total AH?

What model batts and charger?

Multiple bank outputs on charger?

How many volts and amps on the shore power circuit?

How depleted the bank as SoC %age?

 

[jonyjoe303]

The battery itself determines how many amps it will accept. A fully charge battery might only take 5 amps, while one that is at 11 volts might take all 120 amps. Once the battery reaches 14.4 volts it will switch to absorb and the voltage stays the same but the amps slowly go down. Also the size of the battery will determine how much amps it will accept a 200 ah battery might actually be able to take all 120 amps, but a smaller 50 ah battery will only accept half that. The charger only makes the amps available the battery by its condition takes what it needs.

 

[Ticklebellly]

120 Amps is a generic figure saying something about the capabilities of the charger under specific design conditions. Any and all batteries have an optimal charging rate, usually some multiple of C. Because charger designers do not know the exact situation the charger will be hooked up to, what an individual charger puts out includes some safety factor so that a really small AH battery bank does not get fried and a really large AH battery bank gets a lot to minimise charging time.

Three stage you say. If the bulk charge happens at a fixed, constant voltage, then the instantaneous charging current measured will be a function of the charger output voltage compared to the SOC of the battery. A really flat battery will be charged at a high rate and a nearly full battery will get less charging current as, technically, the voltage differential is what determines the instantaneous charge rate. Many chargers monitor charge current and switch to float at some determined current rate. If a charger is only putting one or two amps into a battery, the battery is likely fully or very close to fully charged.

I suggest you read charging current several times across a charging cycle to get a better sense of what happens across a charge profile.

 

[John61CT]

>> Because charger designers do not know the exact situation the charger will be hooked up to, what an individual charger puts out includes some safety factor so that a really small AH battery bank does not get fried and a really large AH battery bank gets a lot to minimise charging time.

No. They just regulate voltage, Amps acceptance rate just depends on the battery's chemistry and SoC.

> If the bulk charge happens at a fixed, constant voltage

By definition Bulk (constant current) stage means voltage varies, trying to get up to the Absorb setpoint.

After that Absorb stage is constant voltage current falling until full, then Float.

 

[Zil]

I determine the parameters of my charger. I researched what my batteries charge voltages should be as recommended by the manufacturer. I selected the parameters by that research.

 

[VanGuy6678]

> If the bulk charge happens at a fixed, constant voltage

By definition Bulk (constant current) stage means voltage varies, trying to get up to the Absorb setpoint.

This is an interesting point of confusion: I always thought BULK charging stage meant Voltage is what is constant (in my case, 14.3V for my Lifeline AGM 440Ah battery bank). But you say the voltage varies in Bulk stage? 

My charger is a Sterling Power BBW12120, 3-stage charger. With this charger, I can program the Bulk Voltage, Absorption Voltage, and Float Voltage. If, as you say, Bulk Voltage varies, why am I able to program the Bulk Voltage to a precise 14.3V? Thank you!

 

[Spaceman Spiff]

[font=Verdana, Arial, sans-serif] ". . . I can program the Bulk Voltage, Absorption Voltage, and Float Voltage. If, as you say, Bulk Voltage varies, why am I able to program the Bulk Voltage to a precise 14.3V?"[/font]

[font=Verdana, Arial, sans-serif]You are programming the voltage at which the charger switches to absorption stage.[/font]

[font=Verdana, Arial, sans-serif]Bulk Charging:[/font]


Absorption:


Float:


 -- Spiff

 

[highdesertranger]

nice spiff, remind us what controller you have. I forget. highdesertranger

 

[John61CT]

>>I can program the Bulk Voltage, Absorption Voltage, and Float Voltage. If, as you say, Bulk Voltage varies, why am I able to program the Bulk Voltage to a precise 14.3V

Yes there are some charge sources that allow a "Bulk target" V separate from Absorb.

I say target, since its point is to set the V level the Bulk stage strives to reach at maximum current.

When it achieves the target, that is when it switches to Absorb stage, holds that voltage steady as current declines.

 

[VanGuy6678]

[font=Verdana, Arial, sans-serif] ". . . I can program the Bulk Voltage, Absorption Voltage, and Float Voltage. If, as you say, Bulk Voltage varies, why am I able to program the Bulk Voltage to a precise 14.3V?"[/font]

[font=Verdana, Arial, sans-serif]You are programming the voltage at which the charger switches to Absorption. 
[/font]
Thanks so much. Very very helpful!!

 

[Ticklebellly]

What is the factor that determines how many amps are delivered to my 12V Lifeline AGM battery bank during BULK charging stage? 

Does your particular charger use constant voltage or constant current during bulk charge?

From   http://www.centurybatteries.com.au/...alk/issue-2-battery-talk-battery-charging.pdf

"Constant Voltage Charging - This method charges a battery at a maximum voltage, typically from 13.8V to 15V and a maximum current depending on the charger specification. When a “flat” battery has been charged for a period of time and the maximum voltage is approached, the charge current drops off in proportion to the battery voltage increase"

 

[John61CT]

> Does your particular charger use constant voltage or constant current during bulk charge?

That is not something under the control of any charge source.

Even the simplest plain constant power supply, when hooked up to a depleted battery is a "2-stage" charger.

The battery simply does not allow full "target" voltage the charger is "seeking" during Bulk (CC) at low SoC, and neither the current absorbed nor the timing of the transition to Absorb (CV) has anything to do with the capabilities of the charge regulator.

Both are up the the chemistry of the bank, via resistance and SoC.

The only factor controlled by the charger is the voltage sought during CC and held during CV.

And maximum possible current of course.

 

[Zil]

Some chargers gradually increase the voltage to maintain their maximum amperage. Then switch to steady voltage. Then when amperage drops, switch to lower voltage to maintain the battery. This helps the battery to not off gas as much water, amongst other benefits. Other chargers just start with top voltage and let the increasing resistance internal to the battery slowly reduce the amperage into the battery. Never changing the voltage. Not the best way to maintain a battery.

 

[John61CT]

again, what you describe in the former is not the regulator but determined by the battery SoC during the CC Bulk stage

the second is impossible. If a battery is already near full, the CC to CV transition may happen very quickly, but no charge source can instantaneously "force" the target voltage on a depleted batt, the actual V is a compromise between the batt's resting V at that time and what the charge source is striving for

 

[VanGuy6678]

Does your particular charger use constant voltage or constant current during bulk charge?

Mine uses constant current, I just learned. Thanks for pointing out both types do exist!

 

[VanGuy6678]

 

[frater secessus]

Yes there are some charge sources that allow a "Bulk target" V separate from Absorb.
I say target, since its point is to set the V level the Bulk stage strives to reach at maximum current.

I've never understood what a Bulk voltage setpoint was for.  

A few minutes googling shows the term is used in different ways

• interchangeable term with Vabs.  Midnite does this and does not have a separate term for Vabs
• a separate Vbulk setpoint which is set identically to Vabs in deep cycle use
• the "rebulk" voltage below which the controller restarts bulk charging

If there is a use case for setting a separate Vbulk I am not familiar with it.

 

[John61CT]

Speculating here. Higher voltage means higher current flow during Bulk, faster charging, also healthier for AGM longevity.

Any danger from overcharging would come later in the cycle, so it would work to push the voltage to the high end early, get to Absorb stage faster, then back off, for a shorter gentler Absorb stage.

Note also some major vendors use the terms incorrectly in their documentation, the current definitions used here became (more of) a consensus in recent years.