Does a charger *really* have three stages?

ekarlson

New Member
All the product manuals and literature out there talk about the charger having three (sometimes four) "stages" - bulk, absorption and float (sometimes by different names). But here is the piece that I am puzzling about. For DC circuits, V=IR. The charger cannot change that rule and "R" is the internal resistance of the battery, which increases as the battery becomes more fully charged. The charger cannot control the "R" value - that is determined by the battery. So when the literature says "the charger maintains the voltage and reduces the current" this simply cannot be true - the charger cannot control *both* the voltage and the current at the same time. The charger can set a target voltage, in which case the resistance of the battery determines the current. Or the charger can set a target current, in which case the resistance of the battery determines the voltage.

So my question is this - is this three stage process simply a matter of the charger trying to maintain a particular voltage level without exceeding some amperage output and the varying resistance of the battery governs whether we are current-limited (in which case the voltage drops and we are in the "bulk" stage) or voltage-limited (in which case the current drops and we are in the "absorption" stage). The "bulk" stage simply reflects the fact that the resistance of the battery is low enough that the current draw exceeds what the charger can deliver, and the "absorption" stage simply reflects the fact that the battery's resistance has become high enough that the charger can now deliver enough current to obtain the fixed target voltage. The charger isn't really doing anything different in these two "stages" - it just wants to output a certain voltage but it has a limit on how much current it can deliver. The charger's target voltage and current limit do not vary - the only piece that varies is the resistance of the battery.

I could see the charger having two stages - stage 1 is try to obtain a target voltage with an amperage limit and stage 2 is once we are at the target voltage and the current has dropped to a certain point, then the charger decides to lower the target voltage (this would be the float stage). So I can see a charger having two stages of operation (essentially two different target voltages - one for the bulk/absorption phases of the battery and a second, lower target voltage once we hit the first target voltage and the current drops below some threshold). But I do not see the charger doing anything different during the bulk and absorption phases - the observed behavior in terms of varying voltages and currents is the result of the battery's resistance changing and the charger having a limit on how much current it can source.
 

smoothJoey

Honeyguide
Sorry I didn't read your whole post.
The charger controls the current by controlling the voltage differential.

1. During constant current(bulk) phase the charger determines the current by controlling the charge voltage.
2. During constant voltage(absorption) phase the battery determines the current flow as its voltage converges with the charge voltage.
3. During the float phase, The charger decreases voltage to allow the battery to cycle down and then shares the load.

Hope that helps.
I guess you could say that 1 and 2 are the same mechanism.
 
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HRTKD

Boondocker
The converter for my LiFePO4 battery bank has two phases. The converter for my lead acid bank has four phases, with the fourth phase being desulfation. I would never connect this converter to my LiFePO4 battery bank. Bad mojo.
 

carbon60

Solar Enthusiast
CC/CV confuses people, including myself, until I realized that we're talking voltage and current sensing, combined with PWM voltage controlling. Only voltage is ever "controlled" in the sense most people understand it. Someone needs to explain this better than I can.
 

smoothJoey

Honeyguide
CC/CV confuses people, including myself, until I realized that we're talking voltage and current sensing, combined with PWM voltage controlling. Only voltage is ever "controlled" in the sense most people understand it. Someone needs to explain this better than I can.
I thought I already did.
Used bold font even. ;)
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Ummm.. wee correction.
FLOAT is Constant Voltage Floating Low amperage, when locked at a set voltage it will stay there but the amperage will decrease to 0A depending on the battery pack. While Floating from an SCC, it will "Top" the battery pack(s) but can & will feed a running inverter and even supply sufficient amperage to meet demands if possible without "injecting" more than demanded at the battery packs.

BULK is High Amperage with preset max voltages. Considered as Constant Current (relative to solar input on an SCC) The Current (Amps) will float up & down relative to how much solar is incoming to SCC.

ABSORB is High Amperage & Higher Voltage - NOT needed for LFP. I set mine to 15 Minutes.

Equalize/Desulphate is OFF not needed for LFP. This is High Voltage High Amperage which "boils" FLA for 1-3 hours typically to descale the plates.
LFP ONLY REQUIRES CC/CV

AC->DC Chargers can maintain Constant Current as they have fixed input but solar will vary because a cloud goes by and the sun angle is changing throughout the day.

See PAGE-13 in the Luyuan Tech Doc (Link in my signature)
 

smoothJoey

Honeyguide
FLOAT is Constant Voltage Floating Low amperage, when locked at a set voltage it will stay there but the amperage will decrease to 0A depending on the battery pack.
Wee correction to your correction.
Float is lower voltage but the loads and/or battery can draw up to the charger's rated amperage.

The battery doesn't draw any current from the charger though until its voltage is a tiny bit lower than float.
 
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HRTKD

Boondocker
Just for grins I Googled "battery charge stages". It's hard to find two sites that agree 100%. Some don't even use the same name for the stages. Different battery manufacturers use different voltage and amp amounts (makes sense with different sized batteries). The battery charger manufacturers don't always agree either. CTEK has a charger with 9 stages (I have two of those).
 

ekarlson

New Member
Just for grins I Googled "battery charge stages". It's hard to find two sites that agree 100%. Some don't even use the same name for the stages. Different battery manufacturers use different voltage and amp amounts (makes sense with different sized batteries). The battery charger manufacturers don't always agree either. CTEK has a charger with 9 stages (I have two of those).
Yeah, it is kind of amazing that so many terms were created for the same thing, and none of them entirely consistent. I've read in several places that "float" does not even apply to LiON batteries - as long as it drops down to something sufficiently below the battery's nominal voltage you're good. I've got a SOK Battery coming (hopefully) and in its specs it talks about a "Charge Voltage" and a "Max Charge Current" - no mention of bulk, absorption, float. Renogy chargers talk about "boost", "float" and "equilization". Victron chargers have "Max Charge Current", "Absorption Voltage", "Float Voltage" and "Equilization voltage", but nothing about "bulk". Progressive Dynamics has LiON compatible electrical panels (with an AC/DC converter/charger). For lead-acid batteries it talks about "boost", "normal" and "storage" modes - but for LiON it doesn't even have modes - it just produces a steady 14.6V with a max current of 35A.

It was the wealth of inconsistent terminology plus the fact that many units did not seem to have a distinction between "bulk" and "absorption", plus the fundamental V=IR rule, that made me start to wonder if the charger really operated any differently in these different "stages" or if was all being driven by the combination of the battery's resistance/voltage changing and a limit on how much current a charger can deliver. The only piece that seemed like it was an altered operating condition for the charger was the "float" stage where the charger decides that the battery is full and it should reduce its voltage.
 

HRTKD

Boondocker
Many converters also throw in a time element. They'll stay in one stage for x number of hours/minutes and then switch to the next stage. Here's a link to a discussion about the charge profile for an IOTA Engineering converter.

 

jnrhome

New Member
constant voltage, constant current, direct voltage/current or mediated voltage/current.
Electrical engineers... These are actually the only possibilities, right?
Constant voltage refers to a consistently fed voltage, constant current is when the voltage matches and the saturation (absorption) brings that constant voltage up to 0a in. Direct application is unmitigated voltage and amperage. Mediated voltage/current is when the controller reduces the incoming voltage or current.

I know there are some big brains in here. What are the formal terms and explanations for each item?
 
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