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#1
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"James Hahn" wrote in message
... 2) that same battery will alway be charged a little less than the other because the charger measures the voltage across both batteries in series. Incorrect. The charger measures the voltage across both batteries in series. The voltage it sees is the result of both batteries being there, not the one with the highest or lowest charge. The charger will charge until the voltage across the two batteries reaches the correct level. There are two possible scenarios: one battery wears more quickly than the other, eventually reaching a state where the cell voltages reach higher values due to increasing internal resistance. Thus the charges shuts off too early, leaving the better battery not topped up. The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Have you ever disassembled a broken battery pack of some device? In 99% of the cases you will find all cells in perfect condition (charged and all) except one, which was obviously the weakest in the chain, showing a reversed voltage. You have demolished our own argument. All cells were treated identically, yet one has collapsed! You can't use that evidence to support a claim that cells treated differently are more liable to collapse (which I assume is what you are trying to say). There is always one cell that collapses first (the weakesrt), due to tolerances in materials, production, etc. By introducing an extra load (wear) on part of the cells in a string, they are more likely to collapse first. Collapse of one cell is the most common mode of failure for wet cell batteries, but it is not associated with differential discharge rates. We all know that a lead-acid battery suffers from deep discharge. And in this setup, it is a plain fact that the battery "below" the tap is discharged sooner then the one "above" the tap. And if the charger is on "the outside" terminals, it can only see the charge state of the entire string, which is in this case, wrong information. For years battery manuafacturers have provided a 12v tap across the first eight cells of high voltage industrial batteries. There has never been any evidence that using the tap shortens the battery life, and it has never been the experience of the users. Uhm, 12V across 8 cells? You are talking about non-rechargable batteries here? Meindert |
#2
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I've read this thread carefully, and although I'm no sort of expert here, I
think that only Meindert is getting at the truth. Most of the rest of you are getting confused between voltage and current. Because both batteries are connected in series, IRRESPECTIVE of the charging mechanism (eg alternator or smart charger), then both receive an identical charge. If charging starts from a state where one battery is more discharged than the other, then the fuller battery will replenish its charge before the other one, and towards the end of that charging regime its terminal voltage will rise fairly suddenly from around 13V to around 14.5. Now, depending on the precise characteristics of the charging source (different for smart chargers and alternators), the ONLY way that the undercharged battery can continue to receive charge is to overcharge the now fully charged one. This implies applying more than 14.5 volts across the full battery, and this will, of necessity, cause to battery to gas, perhaps even boil in extreme circumstances. Enough said? Guy is probably right in saying that small additional current drain from one of the two batteries is lost in the noise, as it were. But as a concept, asymmetric current from series-connected batteries is bad news. Interestingly, one could get round the problem described above by putting a 14.5V zenner diode around the battery which is not being drained by your 12V circuit, but this isn't without problems either - the diode would have to be capable of handling the maximum charge current, lets say 30 amps. 30A x 14.5V = 420watts - thats a pretty big zenner on a pretty big heatsink!!!! HTH |
#3
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"Meindert Sprang" wrote in message
... snip There are two possible scenarios: one battery wears more quickly than the other, eventually reaching a state where the cell voltages reach higher values due to increasing internal resistance. Thus the charges shuts off too early, leaving the better battery not topped up. The charger does not monitor cell voltages. It monitors the voltage across all the cells in series. I don't know what you mean by 'wears', but the initial charge state of individual cells does not affect the voltage level at which the charger shuts off. The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Have you ever disassembled a broken battery pack of some device? In 99% of the cases you will find all cells in perfect condition (charged and all) except one, which was obviously the weakest in the chain, showing a reversed voltage. You have demolished our own argument. All cells were treated identically, yet one has collapsed! You can't use that evidence to support a claim that cells treated differently are more liable to collapse (which I assume is what you are trying to say). There is always one cell that collapses first (the weakesrt), due to tolerances in materials, production, etc. By introducing an extra load (wear) on part of the cells in a string, they are more likely to collapse first. It's not the extra load that causes cell collapse. Mistreatment of the batteries (either, or both) will shorten their life. If they are operated as they are designed to be operated then manufacturing differences between cells are more significant than the fact that some cells have been discharged to a lower level than others. If you do a proper study of those dissambled batteries you will find that the collapsed cell is not random - it has a strong bias towards the poitive terminal. Collapse of one cell is the most common mode of failure for wet cell batteries, but it is not associated with differential discharge rates. We all know that a lead-acid battery suffers from deep discharge. Who is talking about deep discharge? And in this setup, it is a plain fact that the battery "below" the tap is discharged sooner then the one "above" the tap. And if the charger is on "the outside" terminals, it can only see the charge state of the entire string, which is in this case, wrong information. It is not "discharged sooner". It is discharged to a level slightly below the other. The charge state of the 'entire string' is not 'wrong' information. It will indicate the amout of charge required, and any differential between individual cells, or sets of cells, is not relevant. For years battery manuafacturers have provided a 12v tap across the first eight cells of high voltage industrial batteries. There has never been any evidence that using the tap shortens the battery life, and it has never been the experience of the users. Uhm, 12V across 8 cells? You are talking about non-rechargable batteries here? No. I am talking about standard industrial lead acid wet cells, so I guess the number is 10. If you need to know it exactly you can take yourself down to any nearby warehouse and look at the batteries used in a forklift battery system, and count the number of cells set aside for the 12v tap. |
#4
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"James Hahn" wrote in message news "Meindert Sprang" wrote in message ... snip The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Go and read my last post again. The charge doesn't magically share itself according to need between the two batteries connected in series. Instead, the charging current supplies equal charge to both batteries. If one becomes fully charged before the other, then the only way that the partially charged battery can get more charge is by overcharging the other. That overcharge is "energy" which cannot be stored in the battery, but has to go somewhere, and that somewhere is either/or elctrolysing the battery water to oxygen and hydrogen, and heat. If the discrepancy is small, it probably doesn't matter, but if not, then it certainly does. |
#5
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Forget for a moment the issue of connecting a 12 volt load
to a 24 volt system. Consider only the problem of properly charging two 12 volt batteries in series with a single 24 volt charger. My understanding is that unless the two batteries are identical (goes way beyond same make or capacity) they will not be charged to the same voltage. Two electrically non-identical batteries will have different internal resistances (by assumption)and thus charge to different voltages. As I mentioned in an earlier post, Ample Power shares this understanding and offers a 24 volt charger that is in reality two 12 volt chargers, one connected to each of the two 12 volt batteries (which are in series, of course). If my understanding is incorrect, that would mean that a 12 volt marine battery and 8 AAA's in series would all be properly charged by a 24 volt charger (with current limited so as not to destroy the AAA's). An attempted reductio ad absurdum. My point is simply that with a real-world installation of two 12 volt batteries in series, the two batteries will probably measure different voltages. The greater the differences between the batteries, the greater the potential problem (no pun). Without knowing the internal resistances, etc. of the two batteries, it is not possible to know the precise effect of placing a different load on one of the batteries. Doing so would effectively place the added load resistance in parallel with the internal resistance of the affected battery. That could make the two batteries either more or less alike, depending on the values involved. All of this is hand-waving, of course, because a battery's internal resistance is really not a constant, but a function of the battery's chemistry, state-of-charge, etc. It is a non-linear resistance, in that it varies with current. I don't believe we miss the mark here by ignoring these factors, however. Regards, Chuck Pete Styles wrote: "James Hahn" wrote in message news "Meindert Sprang" wrote in message ... snip The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Go and read my last post again. The charge doesn't magically share itself according to need between the two batteries connected in series. Instead, the charging current supplies equal charge to both batteries. If one becomes fully charged before the other, then the only way that the partially charged battery can get more charge is by overcharging the other. That overcharge is "energy" which cannot be stored in the battery, but has to go somewhere, and that somewhere is either/or elctrolysing the battery water to oxygen and hydrogen, and heat. If the discrepancy is small, it probably doesn't matter, but if not, then it certainly does. |
#6
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On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote: "James Hahn" wrote in message news "Meindert Sprang" wrote in message ... snip The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Go and read my last post again. The charge doesn't magically share itself according to need between the two batteries connected in series. Instead, the charging current supplies equal charge to both batteries. If one becomes fully charged before the other, then the only way that the partially charged battery can get more charge is by overcharging the other. That overcharge is "energy" which cannot be stored in the battery, but has to go somewhere, and that somewhere is either/or elctrolysing the battery water to oxygen and hydrogen, and heat. If the discrepancy is small, it probably doesn't matter, but if not, then it certainly does. Pete is right on the mark. Reread his prior post. Also go back and read Bruce's post on the subject. It is bad news to tap a battery bank. You can get away with it for a time but it will shorten battery life because the two batteries do not get charged equally. As Pete says, "in order to get the battery that is used the most charged fully you must overcharge the other". There is no way around that. Overcharging a battery not only cause water loss but it can warp the plates in the battery among other things. Overcharging is the biggest killer of batteries. Just ask anyone that still uses a feroresonant charger. They will tell you batteries last only a couple of years. Do it properly and use the inverter. regards Gary |
#7
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"Gary Schafer" wrote in message ... On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles" wrote: "James Hahn" wrote in message news "Meindert Sprang" wrote in message ... snip The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Go and read my last post again. The charge doesn't magically share itself according to need between the two batteries connected in series. Instead, the charging current supplies equal charge to both batteries. If one becomes fully charged before the other, then the only way that the partially charged battery can get more charge is by overcharging the other. That overcharge is "energy" which cannot be stored in the battery, but has to go somewhere, and that somewhere is either/or elctrolysing the battery water to oxygen and hydrogen, and heat. If the discrepancy is small, it probably doesn't matter, but if not, then it certainly does. Pete is right on the mark. Reread his prior post. Also go back and read Bruce's post on the subject. It is bad news to tap a battery bank. You can get away with it for a time but it will shorten battery life because the two batteries do not get charged equally. As Pete says, "in order to get the battery that is used the most charged fully you must overcharge the other". There is no way around that. Overcharging a battery not only cause water loss but it can warp the plates in the battery among other things. Overcharging is the biggest killer of batteries. Just ask anyone that still uses a feroresonant charger. They will tell you batteries last only a couple of years. Do it properly and use the inverter. regards Gary I disagree. Two 12v batteries in series to make a 24v battery will self-level with respect to charge. They will self level even if one is tapped to run a VHF. No harm will come to either battery. Voltage can be likened to pressure. I hope this helps. CN |
#8
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"Capt. Neal®" wrote in message ... "Gary Schafer" wrote in message ... On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles" wrote: "James Hahn" wrote in message news "Meindert Sprang" wrote in message ... snip The other possibility is that due to a deeper discharge of one battery, it takes longer for the total voltage to reach the point where the charger goes from bulk to float. So while for the more empty battery the charge is still ok (bulk), the other one still gets the full bulk current too while it should receive a float charge. Deeper discharge of some cells in the string will not, of itself, mean that the charge takes longer. The _difference_ doesn't matter. The amount of charge required controls the charge time. Whether OP is running his radio off one battery or both makes no difference to the ah used, and therfore no difference to the charge time (except, of course, that if he uses a converter he will be consuming more ah for a given usage than if he uses just one battery). Go and read my last post again. The charge doesn't magically share itself according to need between the two batteries connected in series. Instead, the charging current supplies equal charge to both batteries. If one becomes fully charged before the other, then the only way that the partially charged battery can get more charge is by overcharging the other. That overcharge is "energy" which cannot be stored in the battery, but has to go somewhere, and that somewhere is either/or elctrolysing the battery water to oxygen and hydrogen, and heat. If the discrepancy is small, it probably doesn't matter, but if not, then it certainly does. Pete is right on the mark. Reread his prior post. Also go back and read Bruce's post on the subject. It is bad news to tap a battery bank. You can get away with it for a time but it will shorten battery life because the two batteries do not get charged equally. As Pete says, "in order to get the battery that is used the most charged fully you must overcharge the other". There is no way around that. Overcharging a battery not only cause water loss but it can warp the plates in the battery among other things. Overcharging is the biggest killer of batteries. Just ask anyone that still uses a feroresonant charger. They will tell you batteries last only a couple of years. Do it properly and use the inverter. regards Gary I disagree. Two 12v batteries in series to make a 24v battery will self-level with respect to charge. They will self level even if one is tapped to run a VHF. No harm will come to either battery. Voltage can be likened to pressure. I hope this helps. CN Capt. Neal thinks I'm wrong. I rest my case! :-) P. |
#9
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In article ,
Capt. Neal® wrote: I disagree. Two 12v batteries in series to make a 24v battery will self-level with respect to charge. They will self level even if one is tapped to run a VHF. No harm will come to either battery. Voltage can be likened to pressure. I hope this helps. CN bullcrap.... Better to leave the Blackgang stuff to the engineers that know what they are talking about...... Me |
#10
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"Me" wrote in message ... In article , Capt. Neal® wrote: I disagree. Two 12v batteries in series to make a 24v battery will self-level with respect to charge. They will self level even if one is tapped to run a VHF. No harm will come to either battery. Voltage can be likened to pressure. I hope this helps. CN bullcrap.... Better to leave the Blackgang stuff to the engineers that know what they are talking about...... Me I'm an electrician. Don't insult me with this engineer crap. I know how batteries work and I know circuits - particularly DC circuits. You idiots are only correct provided there is no circuit to which the two 12v batteries in series are connected. In any yacht I've ever been on the batteries are part of a circuit of there would be no use to have them aboard other than as ballast. In Nigel's case, tapping off one of two batteries connected in series in a circuit to run his VHF will not cause only the tapped battery to discharge as the voltage in the circuit will be lowered and not just the voltage in the tapped battery. Only if the two batteries were disconnected from the circuit would the tapped battery discharge more than the untapped battery. C'mon, folks, it doesn't take a rocket scientist to figure something this basic out. CN |
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