Do you beleive these stats on Battery Voltages?
in [Tech]
|
From: Bob on 28 Nov 2009 06:26 Battery Voltages Because batteries are always referred to as 12-volt (or 6V or 24V), it is often assumed that the normal voltage is 12V. In fact a 12V lead-acid battery only producing 12.0V is either almost flat or is delivering a large current (under a heavy load). In fact, a healthy 12V battery, when not being charged, should always show 12.2 - 12.8V. Very conveniently, the relationship between a battery's state of charge and its voltage is linear (10% per 0.10V) between about 90% (12.70V) and 20% (12.0V). However, when a battery has been on charge, even if its not fully charged, the voltage will be up around 13.8V. This will slowly drop to around 13.6V over the next few hours, but even if left overnight it shouldn't drop below 13.0V unless the battery was only partly charged or is on the way out. But, as soon as a reasonable sized load is switched on, the voltage drops quite quickly until it equates to the battery's actual state of charge. Thereafter, the rate of voltage change (under constant load) becomes linear. Thus, in practice, the voltage of a freshly charged battery is a poor guide to its state of charge until some of the charge has been used. ================================================== If the part about (will slowly drop to around 13.6V over the next few hours) is still true for small m/cycle batteries, and is not RC/Ah specific, then I've been getting ripped of on some of the "wet' sla GS/Yuasa batteries I've been buying.. I know, even the "agm" Chinese batts can hold these stats, but even 13.6 volt "agm" is a drag to start, whereas even a 12.6v "wet" will snap start better. The Chinese "agms" seem more like a "gel" battery rather than a Starting Battery, I think an AGM with cell vent cap holes is pretty unreal and may just be a bunch of sponge mixed with a sulphuric acid solution, rather than a real AGM,which is much different...and Concorde pricier. Bob
From: paul c on 28 Nov 2009 13:12 Bob wrote: > Battery Voltages > Because batteries are always referred to as 12-volt (or 6V or 24V), > it is often assumed that the normal voltage is 12V. In fact a > 12V lead-acid battery only producing 12.0V is either > almost flat or is delivering a large current (under a heavy load). > In fact, a healthy 12V battery, when not being charged, > should always show 12.2 - 12.8V. > > Very conveniently, the relationship between a battery's state > of charge and its voltage is linear (10% per 0.10V) between about 90% > (12.70V) and 20% (12.0V). However, when a battery has been on > charge, even if its not fully charged, the voltage will be up > around 13.8V. This will slowly drop to around 13.6V over the next > few hours, but even if left overnight it shouldn't drop below > 13.0V unless the battery was only partly charged or is on the way out. > But, as soon as a reasonable sized load is switched on, the voltage > drops quite quickly until it equates to the battery's actual > state of charge. Thereafter, the rate of voltage change (under > constant load) becomes linear. Thus, in practice, the voltage > of a freshly charged battery is a poor guide to its state of charge > until some of the charge has been used. > > ================================================== > > If the part about (will slowly drop to around 13.6V over the next > few hours) is still true for small m/cycle batteries, and is not RC/Ah > specific, then I've been getting ripped of on some of the "wet' sla GS/Yuasa > batteries I've been buying.. > > I know, even the "agm" Chinese batts can hold these stats, but even > 13.6 volt "agm" is a drag to start, whereas even a 12.6v "wet" will snap > start better. > > The Chinese "agms" seem more like a "gel" battery rather than a > Starting Battery, I think an AGM with cell vent cap holes is pretty unreal > and may just be a bunch of sponge mixed with a sulphuric acid solution, > rather than a real AGM,which is much different...and Concorde pricier. > > Bob Is the first part a quote? If so I'd like to know where it came from. Some of the sentences are a bit murky to me, eg., 'healthy battery implies 12.2 to 12.8 V'. It's logically patent that you can't conclude '12.6 V implies good battery' from that but the last sentence about voltage being a 'poor guide' gibes with experience. Even though there are grade-school kids who know more about chemistry and physics than I do, I find it helpful to remember that voltage is not a consumable quantity, rather it's a ratio of energy to charge, eg., joules per coulomb. So my no doubt over-simplified arithmetic tells me that there are only so many coulombs in a battery. Personally I find this to be a more tangible quantity, ie. more useful than the usual voltage metaphors such as water in pipes or waterfalls. I know they have some water inside but otherwise I try to keep water away from batteries! It seems that a low surface charge can drive a multimeter just as well as a high charge, but when it comes to high current draws such as electric starter motors, a lot depends on just how many coulombs one needs to expend. Even that doesn't encompass all the variables such as the ratio of acid to water, plate thickness and quality and a bunch of others I don't know much about that all affect internal resistance but it helps to explain why I couldn't start a 1500 cc thumper with a 12.2 V battery when the same battery could afterwards start a 50cc scoot. On the other hand, I've been fooled more than once by a 12.8 V battery that wouldn't start an engine until it was charged to 13.0 V, then noticing some ground wire corrosion and after replacing that the same battery now 'depleted' to 12.4 V started the engine immediately. The first time I made this mistake, I measured resistance of the ground wire wire with a fairly high precision ohmmeter. This seemed an easier test to do than voltage drop. I saw 0.24 ohms and thought that was pretty darn low resistance and couldn't possibly be the problem. But it seems that the effective resistance of a starter motor in good condition is much less than that.
From: Bob on 28 Nov 2009 17:08 On Sat, 28 Nov 2009 18:12:54 GMT, paul c <toledobythesea(a)oohay.ac> wrote: |>Bob wrote: |>> Battery Voltages |>> Because batteries are always referred to as 12-volt (or 6V or 24V), |>> it is often assumed that the normal voltage is 12V. In fact a |>> 12V lead-acid battery only producing 12.0V is either |>> almost flat or is delivering a large current (under a heavy load). |>> In fact, a healthy 12V battery, when not being charged, |>> should always show 12.2 - 12.8V. |>> |>> Very conveniently, the relationship between a battery's state |>> of charge and its voltage is linear (10% per 0.10V) between about 90% |>> (12.70V) and 20% (12.0V). However, when a battery has been on |>> charge, even if its not fully charged, the voltage will be up |>> around 13.8V. This will slowly drop to around 13.6V over the next |>> few hours, but even if left overnight it shouldn't drop below |>> 13.0V unless the battery was only partly charged or is on the way out. |>> But, as soon as a reasonable sized load is switched on, the voltage |>> drops quite quickly until it equates to the battery's actual |>> state of charge. Thereafter, the rate of voltage change (under |>> constant load) becomes linear. Thus, in practice, the voltage |>> of a freshly charged battery is a poor guide to its state of charge |>> until some of the charge has been used. |>> |>> ================================================== |>> |>> If the part about (will slowly drop to around 13.6V over the next |>> few hours) is still true for small m/cycle batteries, and is not RC/Ah |>> specific, then I've been getting ripped of on some of the "wet' sla GS/Yuasa |>> batteries I've been buying.. |>> |>> I know, even the "agm" Chinese batts can hold these stats, but even |>> 13.6 volt "agm" is a drag to start, whereas even a 12.6v "wet" will snap |>> start better. |>> |>> The Chinese "agms" seem more like a "gel" battery rather than a |>> Starting Battery, I think an AGM with cell vent cap holes is pretty unreal |>> and may just be a bunch of sponge mixed with a sulphuric acid solution, |>> rather than a real AGM,which is much different...and Concorde pricier. |>> |>> Bob |> |>Is the first part a quote? If so I'd like to know where it came from. |> A quote at: Useful Info Very conveniently, the relationship between a battery's state of charge and its ... up around 13.8V. This will slowly drop to around 13.6V over the next few hours, .... Visit http://www.adverc.co.uk or http://www.sterling-power.com for ... www.attfield.dircon.co.uk/info-indx.html |>Some of the sentences are a bit murky to me, eg., 'healthy battery |>implies 12.2 to 12.8 V'. It's logically patent that you can't conclude |>'12.6 V implies good battery' from that but the last sentence about |>voltage being a 'poor guide' gibes with experience. |> That's reassuring, I was beginning to think I was getting ripped-off all over on those readings... |>It seems that a low surface charge can drive a multimeter just as well |>as a high charge, but when it comes to high current draws such as |>electric starter motors, a lot depends on just how many coulombs one |>needs to expend. Even that doesn't encompass all the variables such as |>the ratio of acid to water, plate thickness and quality and a bunch of |>others I don't know much about that all affect internal resistance but |>it helps to explain why I couldn't start a 1500 cc thumper with a 12.2 V |>battery when the same battery could afterwards start a 50cc scoot. |> Surface charges are easily fixed with recharging or topping for a longer time, to put a difference between Quick "surface" Charge, and a real holding voltage.. |>On the other hand, I've been fooled more than once by a 12.8 V battery |>that wouldn't start an engine until it was charged to 13.0 V, then |>noticing some ground wire corrosion and after replacing that the same You mean like a "dirty" Neg(-) terminal right, or something like that?...the Neg(-) terminal goes to the rectifier right?, which holds some up to 14.9v no load voltage, but allows you to desulfate/egualise charge at 16v/2Ah through it..... I don't really understand the Rectifier's 14.0-->14.9v no load/22A/withstand 200volt meaning.....what is NO LOAD and why does the rectifier allow 16volts/2ah then. I can't figure that, when I do that, I ground on the frame/terminal, and no main fuse blow...lucky maybe? If I turn the ignition on even at a 2Ah setting I'm sure to blow the 40amp main fuse for sure. Bob
From: paul c on 28 Nov 2009 18:46 Bob wrote: .... > You mean like a "dirty" Neg(-) terminal right, or something like > that?...the Neg(-) terminal goes to the rectifier right?, which holds some > up to 14.9v no load voltage, but allows you to desulfate/egualise charge at > 16v/2Ah through it..... I meant a copper ground wire where all the strands had turned green. As for de-sulphating, I was taught that you need a constant-current charger, and even then it's not always possible. The documentation for the small Ctek charger I use specifically says that it switches periodically from constant-voltage to constant-current mode. I'm pretty sure it has succeeded on three different batteries but once it took about a week. All of those batteries were MF Yuasa's, which I believe is a fairly high-quality brand. One of them was three years old and had sat in a neighbour's unused bike outside from October to June so it never got very warm. On the other hand, it couldn't save a one-year old cheap refillable import battery that sat unused just over the winter months, even with two weeks on that de-sulphating charger. Maybe a more expensive constant-current charger that can give larger currents for short periods would have worked, I don't know. Or maybe the guy I got the battery from had filled it with tap water that was full of various metals. Around here there is a lot of iron in the water. > I don't really understand the Rectifier's 14.0-->14.9v no > load/22A/withstand 200volt meaning.....what is NO LOAD and why does the > rectifier allow 16volts/2ah then. > > I can't figure that, when I do that, I ground on the frame/terminal, > and no main fuse blow...lucky maybe? If I turn the ignition on even at a 2Ah > setting I'm sure to blow the 40amp main fuse for sure. > > Bob >
From: Bob Myers on 1 Dec 2009 16:21
S'mee wrote: >> Battery manufacturers would disagree slightly. >> >> If you charge up a sealed AGM battery, you may read >> 14 ~ 15 volts when you first take it off the charger, but the resting >> voltage will drop down to 12.8 volts and stay there in about half an >> hour, assuming that the battery was in good condition and took a full >> charge. > > If you posted it it's either a LIE or you plagerized it... He no doubt did a cut-and-paste of this info, but at least this time he used a source that was correct. Per the basic chemistry, one lead-acid cell should provide a nominal voltage of somewhere around 2.1, so for a six-cell-in-series battery, you should see somewhere around 12.6V. The "13.X" volt reading that we're used to seeing in charging situations (including what you'd see on a voltmeter on your car dash, since presumably the engine is running and the system is in the "charging" condition when you'd see this) is because you have to use a slightly higher voltage per cell to charge the thing. But 14-15V is high; exceed about 2.4 V/cell (14.4V across a "12V" six-cell battery, and a lead-acid cell will start outgassing. Typically, a charger is set to deliver something like 2.2-2.3 V/cell, or 13.2-13.8V across the battery - and that's the highest you should ever be reading across the battery terminals. Bob M. |