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Battery Meter
"Wayne.B" wrote in message ... On 21 May 2004 15:07:58 GMT, (Gould 0738) wrote: There is no logic at all in a position that says the battery is not "fully charged" until the reading declines .6 volt. ===================================== There is a great deal of logic however in saying that 12.6 is the normal resting voltage of a fully charged battery. That is the condition most people will be interested in. Evaluating the state of charge of a battery still connected to a charger/eliminator makes very little sense, and that is really what this whole discussion is all about. This is the point I was trying to politely make to Gould. If you listen to his story half the newbie boaters will be filing into West Marine to buy new batteries when the ones they have are likely to be perfectly fine. Eisboch |
Battery Meter
This is the point I was trying to politely make to Gould. If you listen to
his story half the newbie boaters will be filing into West Marine to buy new batteries when the ones they have are likely to be perfectly fine. Eisboch Your point is accurate, merely incomplete and also misleading if considered without taking important variables into account. If that "newbie" owns a battery that cannot be charged to a point above 12.6 volts on a functional charger he darn well just might be in need of a new one. Do most boaters disconnect the battery from the boat, and set it on the dock overnight, before evaluating the state of battery charge? If we are going to discuss testing a battery and the results that should be expected, it makes sense to frame that discussion around actual boating conditions. What happens when the "surface charge" bleeds off a battery that can only absorb 1.1 volts per cell? Probably drops down close to 12 volts in fairly short order- a marginal level that all of us will agree is getting rather weak. There's also a difference in the voltage one can expect if checking the batteries on a trailer boat sitting in the backyard under a tarp vs a boat that is connected to shorepower. But in either case, at the moment when the battery has absorbed a full and healthy charge or recharge it will read 2.2 volts per cell. I don't disagree with a statement that later on it may read less. |
Battery Meter
"Gould 0738" wrote in message ... But in either case, at the moment when the battery has absorbed a full and healthy charge or recharge it will read 2.2 volts per cell. I don't disagree with a statement that later on it may read less. Awesome! We agree. Thanks Eisboch |
Battery Meter
"Gould 0738" wrote in message ... Do most boaters disconnect the battery from the boat, and set it on the dock overnight, before evaluating the state of battery charge? If you want to establish state of charge based on voltage alone then that is what you should do. It doesn't have to be overnight, but an hour would be a good idea. If the battery is not at rest then you have to consider the current along with the voltage, which makes things a lot harder. A battery at rest will NOT be at 13.2 volts. A battery charger will "float" a battery at around 13.2 volts, and IF the battery is fully charged there will be little or no current flow into the battery. If you measure the battery voltage when it is connected to a charger then you need to verify that the current is near zero before you can say that the battery is fully charged. Rod |
Battery Meter
"Gould 0738" wrote in message
... This is the point I was trying to politely make to Gould. If you listen to his story half the newbie boaters will be filing into West Marine to buy new batteries when the ones they have are likely to be perfectly fine. Eisboch Your point is accurate, merely incomplete and also misleading if considered without taking important variables into account. If that "newbie" owns a battery that cannot be charged to a point above 12.6 volts on a functional charger he darn well just might be in need of a new one. A bad battery or a depleted battery may still read a high voltage when connected to a charger, and even for a while after being removed. All your reading of 13.2 tells you is that your charger decided to go into float mode. This may be a strong hint that the battery is fully charged, but it doesn't necessarily mean that. Do most boaters disconnect the battery from the boat, and set it on the dock overnight, before evaluating the state of battery charge? If you read the information I presented, you would know that a flooded battery will settle most of the way rather quickly, and that the surface charge can be removed by applying a load for a few minutes. Every boater should learn these simple facts, it isn't rocket science. If we are going to discuss testing a battery and the results that should be expected, it makes sense to frame that discussion around actual boating conditions. What could be more of an "actual condition" than checking the state of charge when you wake up after a night on the hook? Your scenario seems to be connected to shore power. Further, if someone is interested in getting a reliable State of Charge, they should use the methods described by all of the experts. It only takes a few minutes to remove a surface charge; failure to do so gives a meaningless answer. What happens when the "surface charge" bleeds off a battery that can only absorb 1.1 volts per cell? Probably drops down close to 12 volts in fairly short order- a marginal level that all of us will agree is getting rather weak. I'm not sure what you mean by "absorb 1.1 volts" - batteries absorb Amps, not Volts. But yes, if a battery is reading 12 Volts with no load, it is probably either discharged or in poor health. There's also a difference in the voltage one can expect if checking the batteries on a trailer boat sitting in the backyard under a tarp vs a boat that is connected to shorepower. But in either case, at the moment when the battery has absorbed a full and healthy charge or recharge it will read 2.2 volts per cell. I don't disagree with a statement that later on it may read less. This may be true with a given charge protocol, but it is not true in all cases. Further, the opposite is not true at all: if you get a reading of 13.2 without having any knowledge of the history, you can't say anything about the charge state or the general health of the battery. This is the essential point in this discussion. If a battery is discharged to 80%, and then you put it on a float charger at 13.2, you won't add much (if anything) to the charge state, but because of the surface charge you will get a reading of 13.2. Anyone interested in learning about this should read the links I've provided, or google on: "surface charge" battery |
Battery Meter
"Jeff Morris" wrote in message ... A bad battery or a depleted battery may still read a high voltage when connected to a charger, and even for a while after being removed. All your reading of 13.2 tells you is that your charger decided to go into float mode. This may be a strong hint that the battery is fully charged, but it doesn't necessarily mean that. Exactly. The best way to tell (other than checking specific gravity of the cells) is to also monitor the charger current delivered to the battery. If it is at it's float voltage (13.2v - 13.5v) and is still indicating a small current flow, then the battery voltage - which is a reflection of it's apparent internal resistance - is less than the float voltage. A difference of potential must exist in order for current to flow. If the battery charge potential were the same as the charger float potential, the current meter would read zero. With due respect, I think this is where Gould's understanding is flawed. The battery behaves like a variable resistance as it is charged, much like a large capacitor. For a given charge voltage delivered by the charger, the current will vary (decrease) as it is charged). Not to start this debate all over again, but I think Gould might be surprised that while his voltage meter is reading the float potential of the charger, it is almost a certainty that there is still a small amount of current flow - probably an amp or 2. This can only mean that the battery has not come up to 13.2 volts. Eisboch |
Battery Meter
Just to maybe add more fuel to the fire, when I measure the current into the
battery (flooded lead acid) from my fixed voltage (13.3 volts) float charger, and the float charger has been floating the battery for days on end, the continuous unchanging current is around 20ma. I guess my battery is fully charged. The current is not going up or down and the voltage is not changing. I can then conclude the internal leakage current of the battery (while on float charge for days) is 20ma. Also, when I remove the float charger and wait 24 hours for the battery voltage to settle, it measures around 12.65 - 12.72 volts (depending on which battery I measure) This is as measured with a DVM. Have a nice day..... "Eisboch" wrote in message . .. "Jeff Morris" wrote in message ... A bad battery or a depleted battery may still read a high voltage when connected to a charger, and even for a while after being removed. All your reading of 13.2 tells you is that your charger decided to go into float mode. This may be a strong hint that the battery is fully charged, but it doesn't necessarily mean that. Exactly. The best way to tell (other than checking specific gravity of the cells) is to also monitor the charger current delivered to the battery. If it is at it's float voltage (13.2v - 13.5v) and is still indicating a small current flow, then the battery voltage - which is a reflection of it's apparent internal resistance - is less than the float voltage. A difference of potential must exist in order for current to flow. If the battery charge potential were the same as the charger float potential, the current meter would read zero. With due respect, I think this is where Gould's understanding is flawed. The battery behaves like a variable resistance as it is charged, much like a large capacitor. For a given charge voltage delivered by the charger, the current will vary (decrease) as it is charged). Not to start this debate all over again, but I think Gould might be surprised that while his voltage meter is reading the float potential of the charger, it is almost a certainty that there is still a small amount of current flow - probably an amp or 2. This can only mean that the battery has not come up to 13.2 volts. Eisboch |
Battery Meter
If a battery is discharged to 80%, and then you put it on a float
charger at 13.2, you won't add much (if anything) to the charge state, but because of the surface charge you will get a reading of 13.2. If a battery has discharged to 80% and you put it on a charger that brings it up to 13.2, nothing really happened. OK. Whatever you say. Guess one has to wait for the battery gods to bless the charger before there's any "real" change in the voltage. I should have been buying lotto tickets all these years. With frequent checks of battery electrolyte level, quarterly checks of specific gravity with a hydrometer, and periodic terminal cleaning I thought I could trust my voltmeter. Come to discover that my track record of never being stuck without battery power is nothing but dumb luck. |
Battery Meter
Just to maybe add more fuel to the fire,
Here's yet another reference stating that a battery cell has a capacity of 2.2 volts, not the 2.1 being trotted through the NG by those on the other side of this question: Illustrations to the text are available at: http://www.allaboutcircuits.com/vol_1/chpt_11/2.html Battery construction All About Circuits Volume I - DC Chapter 11: BATTERIES AND POWER SYSTEMS Battery construction -------------------------------------------------------------------------- ------ Battery construction The word battery simply means a group of similar components. In military vocabulary, a "battery" refers to a cluster of guns. In electricity, a "battery" is a set of voltaic cells designed to provide greater voltage and/or current than is possible with one cell alone. The symbol for a cell is very simple, consisting of one long line and one short line, parallel to each other, with connecting wires: The symbol for a battery is nothing more than a couple of cell symbols stacked in series: As was stated before, the voltage produced by any particular kind of cell is determined strictly by the chemistry of that cell type. The size of the cell is irrelevant to its voltage. To obtain greater voltage than the output of a single cell, multiple cells must be connected in series. The total voltage of a battery is the sum of all cell voltages. A typical automotive lead-acid battery has six cells, for a nominal voltage output of 6 x 2.2 or 13.2 volts: The cells in an automotive battery are contained within the same hard rubber housing, connected together with thick, lead bars instead of wires. The electrodes and electrolyte solutions for each cell are contained in separate, partitioned sections of the battery case. In large batteries, the electrodes commonly take the shape of thin metal grids or plates, and are often referred to as plates instead of electrodes. For the sake of convenience, battery symbols are usually limited to four lines, alternating long/short, although the real battery it represents may have many more cells than that. On occasion, however, you might come across a symbol for a battery with unusually high voltage, intentionally drawn with extra lines. The lines, of course, are representative of the individual cell plates: If the physical size of a cell has no impact on its voltage, then what does it affect? The answer is resistance, which in turn affects the maximum amount of current that a cell can provide. Every voltaic cell contains some amount of internal resistance due to the electrodes and the electrolyte. The larger a cell is constructed, the greater the electrode contact area with the electrolyte, and thus the less internal resistance it will have. Although we generally consider a cell or battery in a circuit to be a perfect source of voltage (absolutely constant), the current through it dictated solely by the external resistance of the circuit to which it is attached, this is not entirely true in real life. Since every cell or battery contains some internal resistance, that resistance must affect the current in any given circuit: The real battery shown above within the dotted lines has an internal resistance of 0.2 O, which affects its ability to supply current to the load resistance of 1 O. The ideal battery on the left has no internal resistance, and so our Ohm's Law calculations for current (I=E/R) give us a perfect value of 10 amps for current with the 1 ohm load and 10 volt supply. The real battery, with its built-in resistance further impeding the flow of electrons, can only supply 8.333 amps to the same resistance load. The ideal battery, in a short circuit with 0 O resistance, would be able to supply an infinite amount of current. The real battery, on the other hand, can only supply 50 amps (10 volts / 0.2 O) to a short circuit of 0 O resistance, due to its internal resistance. The chemical reaction inside the cell may still be providing exactly 10 volts, but voltage is dropped across that internal resistance as electrons flow through the battery, which reduces the amount of voltage available at the battery terminals to the load. Since we live in an imperfect world, with imperfect batteries, we need to understand the implications of factors such as internal resistance. Typically, batteries are placed in applications where their internal resistance is negligible compared to that of the circuit load (where their short-circuit current far exceeds their usual load current), and so the performance is very close to that of an ideal voltage source. If we need to construct a battery with lower resistance than what one cell can provide (for greater current capacity), we will have to connect the cells together in parallel: Essentially, what we have done here is determine the Thevenin equivalent of the five cells in parallel (an equivalent network of one voltage source and one series resistance). The equivalent network has the same source voltage but a fraction of the resistance of any individual cell in the original network. The overall effect of connecting cells in parallel is to decrease the equivalent internal resistance, just as resistors in parallel diminish in total resistance. The equivalent internal resistance of this battery of 5 cells is 1/5 that of each individual cell. The overall voltage stays the same: 2.2 volts. If this battery of cells were powering a circuit, the current through each cell would be 1/5 of the total circuit current, due to the equal split of current through equal-resistance parallel branches. REVIEW: A battery is a cluster of cells connected together for greater voltage and/or current capacity. Cells connected together in series (polarities aiding) results in greater total voltage. Physical cell size impacts cell resistance, which in turn impacts the ability for the cell to supply current to a circuit. Generally, the larger the cell, the less its internal resistance. Cells connected together in parallel results in less total resistance, and potentially greater total current. Back Forward |
Battery Meter
Yet another irrelevant link. Why do you keep posting links to high school
physics experiments, rather than acknowledging the information from the leading manufacturers and experts? The issue is not the voltage from an "ideal" cell on a lab bench, it's how to measure the State of Charge for a real life battery, which has a different chemistry. The bottom line is that the method you're recommending is considered by all the experts to be flawed. "Gould 0738" wrote in message ... Just to maybe add more fuel to the fire, Here's yet another reference stating that a battery cell has a capacity of 2.2 volts, not the 2.1 being trotted through the NG by those on the other side of this question: Illustrations to the text are available at: http://www.allaboutcircuits.com/vol_1/chpt_11/2.html Battery construction All About Circuits Volume I - DC Chapter 11: BATTERIES AND POWER SYSTEMS Battery construction -------------------------------------------------------------------------- ------ Battery construction The word battery simply means a group of similar components. In military vocabulary, a "battery" refers to a cluster of guns. In electricity, a "battery" is a set of voltaic cells designed to provide greater voltage and/or current than is possible with one cell alone. The symbol for a cell is very simple, consisting of one long line and one short line, parallel to each other, with connecting wires: The symbol for a battery is nothing more than a couple of cell symbols stacked in series: As was stated before, the voltage produced by any particular kind of cell is determined strictly by the chemistry of that cell type. The size of the cell is irrelevant to its voltage. To obtain greater voltage than the output of a single cell, multiple cells must be connected in series. The total voltage of a battery is the sum of all cell voltages. A typical automotive lead-acid battery has six cells, for a nominal voltage output of 6 x 2.2 or 13.2 volts: The cells in an automotive battery are contained within the same hard rubber housing, connected together with thick, lead bars instead of wires. The electrodes and electrolyte solutions for each cell are contained in separate, partitioned sections of the battery case. In large batteries, the electrodes commonly take the shape of thin metal grids or plates, and are often referred to as plates instead of electrodes. For the sake of convenience, battery symbols are usually limited to four lines, alternating long/short, although the real battery it represents may have many more cells than that. On occasion, however, you might come across a symbol for a battery with unusually high voltage, intentionally drawn with extra lines. The lines, of course, are representative of the individual cell plates: If the physical size of a cell has no impact on its voltage, then what does it affect? The answer is resistance, which in turn affects the maximum amount of current that a cell can provide. Every voltaic cell contains some amount of internal resistance due to the electrodes and the electrolyte. The larger a cell is constructed, the greater the electrode contact area with the electrolyte, and thus the less internal resistance it will have. Although we generally consider a cell or battery in a circuit to be a perfect source of voltage (absolutely constant), the current through it dictated solely by the external resistance of the circuit to which it is attached, this is not entirely true in real life. Since every cell or battery contains some internal resistance, that resistance must affect the current in any given circuit: The real battery shown above within the dotted lines has an internal resistance of 0.2 O, which affects its ability to supply current to the load resistance of 1 O. The ideal battery on the left has no internal resistance, and so our Ohm's Law calculations for current (I=E/R) give us a perfect value of 10 amps for current with the 1 ohm load and 10 volt supply. The real battery, with its built-in resistance further impeding the flow of electrons, can only supply 8.333 amps to the same resistance load. The ideal battery, in a short circuit with 0 O resistance, would be able to supply an infinite amount of current. The real battery, on the other hand, can only supply 50 amps (10 volts / 0.2 O) to a short circuit of 0 O resistance, due to its internal resistance. The chemical reaction inside the cell may still be providing exactly 10 volts, but voltage is dropped across that internal resistance as electrons flow through the battery, which reduces the amount of voltage available at the battery terminals to the load. Since we live in an imperfect world, with imperfect batteries, we need to understand the implications of factors such as internal resistance. Typically, batteries are placed in applications where their internal resistance is negligible compared to that of the circuit load (where their short-circuit current far exceeds their usual load current), and so the performance is very close to that of an ideal voltage source. If we need to construct a battery with lower resistance than what one cell can provide (for greater current capacity), we will have to connect the cells together in parallel: Essentially, what we have done here is determine the Thevenin equivalent of the five cells in parallel (an equivalent network of one voltage source and one series resistance). The equivalent network has the same source voltage but a fraction of the resistance of any individual cell in the original network. The overall effect of connecting cells in parallel is to decrease the equivalent internal resistance, just as resistors in parallel diminish in total resistance. The equivalent internal resistance of this battery of 5 cells is 1/5 that of each individual cell. The overall voltage stays the same: 2.2 volts. If this battery of cells were powering a circuit, the current through each cell would be 1/5 of the total circuit current, due to the equal split of current through equal-resistance parallel branches. REVIEW: A battery is a cluster of cells connected together for greater voltage and/or current capacity. Cells connected together in series (polarities aiding) results in greater total voltage. Physical cell size impacts cell resistance, which in turn impacts the ability for the cell to supply current to a circuit. Generally, the larger the cell, the less its internal resistance. Cells connected together in parallel results in less total resistance, and potentially greater total current. Back Forward |
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