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Old 23-02-2007, 14:55   #1
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How do charge controllers work?

I have always wondered how regulators and/or charge controllers determine the battery's state of charge - especially the multi stage ones. Seems to me that the only thing it can measure is voltage at the battery terminals, but if the source is charging, then the terminal voltage is the charging voltage (kinda like trying to make your boat move by putting a fan in the cockpit!). Secondly, I have read in several places that you can't accurately determine a bettery's state of charge from the voltage unless it has been sitting with no charging or drain for at least 24 hours.
Then, as has been mentioned in several threads, if you have more than one charging source each with their own controller, they would interfere with each other, but it seems to me that they would ALL go into "float" mode since they would all "see" a battery terminal voltage above "fully charged" voltage of 12.8 or so from some other charge device.
Clearly I am missing something here since these things do work, would just like to know how...
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Old 23-02-2007, 15:09   #2
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Rick in Seattle would be the best to get that answer. We'll see if he responds.............................._/)
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Old 23-02-2007, 18:43   #3
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Lots of infomation on the web if you browse around a bit.

Heres one place.


http://www.fourwinds-ii.com/v2/libra...24i_manual.pdf
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Old 22-03-2007, 10:39   #4
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Multistage Charge Controllers, etc.

It's fairly simple in concept. A three-stage charger keys off the battery voltage, but throttles the current to hold a preset voltage. The more discharged a battery happens to be, the more current it will take to reach and hold the set charge voltage. It's important, because of wire resistances, for the charger to sense voltages at the battery terminal (from a low current, sensing circuit) and not at the charger output terminals (which see a higher voltage because of wire resistance in the high current circuit).
Each battery type (flooded, gel, AGM) and model (Exide, Trojan, etc.) has a manufacturer recommended bulk charge voltage and a float charge voltage. (Some batteries can take an infrequent equalize charge, but that's not important here.)
In the bulk charge mode, the charger feeds the battery as much current as the battery will take until reaching the bulk charge voltage limit. Total current is limited by charger capacity or programmed limit -- usually no more than 25% of the battery's amp-hour rating. Once the bulk charge voltage limit is reached, the charger throttles back the current in the absorption charge mode to keep from exceeding that voltage.
In the absorption mode, current gradually drops off (as voltage is held constant by throttling current) as the battery charges. Once current hits a pre-set level for a pre-set time where voltage doesn't change, the charger changes to float mode.
In float mode the charger maintains the battery's recommended float voltage essentially forever. The voltage is low enough that the electrolyte won't boil off, nor will sulfates form from discharge. If you load the system in float mode, battery voltage drops, the charger sees the voltage drop and increases current to feed the load and raise battery voltage to the float level.
With more than one charger on line, the one with the highest voltage setpoint will carry the load. The other will throttle back thinking the battery has reached the requisite voltage. If both are set to the same voltages (i.e., bulk or float), they'll both share the load. Actually, depending on internal resistances and such, one will share more than the other, but they shouldn't fight.
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Old 22-03-2007, 17:01   #5
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Just today, I got my "Outback 60" hooked to the 2, 160 watt solar panels ... almost like a bad joke, this was the first day of rain & clouds we have had in a very long time. To my complete amazement, in spite of the cloudcover/rain it still put a whopping 80 amp hours in the battery bank!!!!!! How does it work? Well it's a Multiple Power Point Tracking contoller "MPPT" for short, and what does that mean? Well ... could be VooDoo for all I know! If a lack of knowledge exists, simply throw lots of dollars at the problem ... this will most likely solve the problem!
FWIW, had read good things about the Outback 60 here on the forum, bought mine (and the solar panels too) from BP, Xantrex & Evergreen Residential Solar Panels, Batteries, Inverters & Water Pumps ...they treated me right & I can highly recommend them.
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Old 22-03-2007, 18:03   #6
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Jamacevoy,
Thanks for that explanation. My biggest point of confusion was hov a controller can sense battery voltage when it is applying a voltage to the battery at the same time, but if I understand it correctly from your explanation, the controller seems to be measuring the internal resistance of the battery to control the voltage and current it should apply?

Wahoo Sails,
If you look at the specs of your solar panels, you will find that its output voltage is somewhere between 17 and 21 volts, and an amp rating around 8-9 amps. In simple terms, a "standard" controller will drop that voltage to around 14v and the amps stay the same essentially wasting a significant portion of the solar panel's output. The MPPT controller somehow recaptures that lost energy and increases the amp output at the 14v charging voltage. This is all under ideal conditions though, if the sun isn't perpendicular to the panels, the amp output is lower, it a small portion of the panel is shaded, its output voltage will be lower (I believe this is why the panels a designed with such a high output voltage), and if your batteries are only slightly discharged, then you probably won't be able to put the full output of the panel in there.
Got my stuff at Northern Arizona Wind & Sun, their website has a lot of good info on this kind of stuff.
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Old 22-03-2007, 19:01   #7
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Charge controllers do not, in general, "know" when a battery is full. They do not "know" the battery state of charge. You cannot "know" a battery state of capacity by terminal voltage alone either by charging or measuring "standing" voltage. You need to know state of capacity not state of charge in order to make any intelligent decisions regarding battery charging. Assumptions regarding state of charge using battery standing voltage has always been crude.

One needs a battery monitor that tracks energy in and energy out along with some form of an internal model for the battery in order to determine state of capacity and use that information to control a charge controller or any other charge source.

The most sophistocated charger regulation used to determine when to go to a float voltage without the use of a "real" battey monitor uses an algorithm that still monitors charge current and battery terminal voltage to determine a decreasing charge acceptance indicated by a rise in terminal voltage with constant current charge. Not many chargers do this.

Peak-power-point pv trackers merely allow the voltage on the pv panel reach a value higher than battery voltage would be for maximum source power. This requires a "buck" regulator and control algorithm. The control algorithm is relatively trivial as long as the controller truly monitors power (current X Voltage) and varies one or the other to keep the power at a peak value. One way is to control the buck output voltage by allowing it to increase step-by-step and looking for the resulting power out of the pv panel. As soon as the power starts to decrease the voltage is then decreased incrementally etc. etc. This is done today with switch-mode power conversion as opposed to the crude PWM switched FET controllers that have no inductors and cannot deliver peak power from a pv system, only a regulated output voltage on the average.
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Old 23-03-2007, 06:17   #8
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More on Battery Charging

Thermal,
Almost. A good charger has a voltage sensing circuit that carries no (minimal) current and connects the charger directly to the battery. A second, heavy duty circuit, connects the charger output to the battery and carries the charging current. As heavy charging currents flow, voltage drops across the heavy duty circuit wires cause the charger output voltage to be slightly higher than the batter terminal voltage. This is where the voltage sensing circuit comes into play. The charger monitors the sensing circuit voltage to guess charge state of the battery. When sensing circuit voltage reaches a preset value, at a preset charging current, for a preset time, the battery is considered charged. Our forum moderator < Rick >, whose name I can't see from here, and I apologize for omitting it, wrote an excellent message on algorithms for calculating actual charge states. I'm honestly not sure if those algorithms are applied inside the average 3-stage digital charger, or if they are used to calculate the charge parameters applied by the charger. Our moderator, who is obviously well-versed on the subject, may be able to shed some light on that.

Cheaper chargers have no voltage sensing circuit, and sense voltage at the charger power output, which is always slightly higher than the battery terminal voltage, but the two voltages get closer and closer as charging current decreases. The major drawback is a slower charge and failure to hit the bulk charge voltage until later in the charge.

The classical (i.e., pre-digital charger) approach for measuring charge state, and which I adhere to, is to use flooded cells and measure charge state by electrolyte specific gravity using a hydrometer. Knowing the s.g., you can tell when the cells are deteriorating because of sulfation and you can then equalize charge to restore the batteries to their highest possible charge state. Equalize charges occur at such a high voltage that the electrolyte boils off hydrogen and oxygen, and therefore should not be done on a sealed, e.g., gel or AGM, battery unless expressly approved by the mfr, and adequate ventilation must be provided because H2 and O2 can explode. The higher voltages can damage d.c. equipment not designed for the higher voltages. Also, your basic single stage charger does not have an equalize mode, so you'd need to get a more expensive 3-stage charger and have the proper batteries.
-John

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Originally Posted by Thermal
Jamacevoy,
Thanks for that explanation. My biggest point of confusion was hov a controller can sense battery voltage when it is applying a voltage to the battery at the same time, but if I understand it correctly from your explanation, the controller seems to be measuring the internal resistance of the battery to control the voltage and current it should apply?
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Old 27-03-2007, 12:42   #9
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We're going to be replacing all of the wiring in our new Westsail, and installing an entirely new battery bank/regulator/alternator system.

I've been trying to research different regulators. I'm erring toward the Xantrax Link 2000R. This is higher up in the line, providing monitoring and regulation in one system. Does anyone have experience with this series, and if so, what are your thoughts?

I'll install a second alternator, like a 150W Series-6 Balmar. All will be hooked to either a series/parallel Trojan T-105 bank of 450ah (4 batts), or 2 of the 12v 225ah Trojan deep cycles.

Thanks for your thoughts/opinions!
Aaron N.
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