Internal battery function (from another thread)
nsaspook Solar Expert Posts: 396 ✭✭✭
The meter is reset when the battery is at full charge. At this point the amount of electric charge accumulated by battery is roughly equal to its 100-hour rate usually measured in amper-hours. Physically this is a number of electrons displaced between plates. One amper-hour is equal to 2.225 x 1022 electrons. The meter then measures current in and out of the battery and accumulates these numbers over time. This way it is know exactly (with measurement errors of course) how many electrons moved between plates. The ratio of the number electrons left to the number of electrons at full charge gives you SoC.
Determining battery SOC in non-linear profiles is the subject of one of my pet projects. https://github.com/nsaspook/mbmc/tree/master/swm8722
The Victron is a very good meter when calibrated but as others have said take it's SOC reading with a large dose of salt until it's tracking is matched with good data. As someone once said: 'Trust but verify'.
That tells you the electrical charge moved by the electric field generated by the chemical reaction inside the battery but it only provides some of the information about the intricate non-linear electrochemical redox reactions inside the battery that really provides the information about true SOC. Reaction temperature, charge rates, relaxation recovery effect, level of charge, charge efficiency, Peukert effect and a host of other factors can completely invalidate charge in/out counting when you have non-steady charge/discharge rates (PV or wind profile and/or pulsed loads like motors).
Charge acceptace (end amps) should be the primary factor on when to float but I like to extend the cycle a bit when it's met before going into float.
0 · Share on Twitter
Interesting project ...
Electrons are the charge. The amount of electrons displaced between plates is SoC by definition. Some of the charge might be lost. Besides normal discharge, there are two ways - self-discharge, which encompass several bad reactions, but for the purposes of amper-hour counting is negligible, and hydrolysis, which doesn't occur at below-gassing voltages.
These factors do affect the "willingness" of the battery to release the charge, that is voltage. For example, if you heat up a battery, voltage drop caused by the same load will be smaller, so you'll get higher voltage and thus more energy. You also can discharge the battery longer because voltage will drop slower, so you get more useful capacity.
But none of this has any influence on the validity of amper-hour counting. Do you really believe that if you increase a temperature of the battery, the SoC increases?
There is nothing wrong with the validity of amper-hour counting as a practical way to 'guess' the SOC but a battery does not store charge. A battery operates on ion charge seperation generating an EMF due to a chemical redox half reaction process that moves ions across both plates, producing acid when charging and producting water when discharging. You don't pack electrons into a battery that remains there until it's discharged as any electron conduction inside the battery will cause self-discharge. I use temperature as one of many SOC correction factors when measuring voltage when running internal resistance calculations by cycling a combination of loads to measure voltage drops at different current levels, I never said anything about SOC increases but temperature does affect capacity and I also use that as a SOC factor during charge/discharge cycles to measure charge efficiency and Ah capacity.
It certainly does. Would be completely useless if it didn't.
First, there's a static charge. Battery plates are charged pretty much like a capacitor. One plate has a deficit of electrons, the other has an excess. A voltage potential exists between plates because of this static charge.
Then, there's a chemical reaction. Lead can be in three forms inside the battery
metallic lead - no deficit of electrons
Lead sulphate where lead lacks 2 electrons
and Lead oxide - the most oxidized form where 4 electrons are missing.
In discharged battery, both plates consist of lead sulphate.
When you start charging, you pump electrons from positive to negative plate. There's some excess of electrons on negative plate, therefore lead gets converted from sulpate to less oxidized metallic form. One atom of lead requires two electrons for that. When the conversion is complete, two electrons are "stored" in metallic lead. This decreases the excess of electrons on negative plate.
On the opposite positive plate there's a deficit of electrons. There, lead gets converted into oxide. As a result two electrons get supplied to the positive plate. This diminshes the deficit of electrons.
After this chemical reaction took place, static charge decreases by two electrons, but chemical reaction took place. When/If this reaction gets reversed, you get the two electrons back.
You can always count the number of metallic lead atoms on the negative plate (or the number of lead oxide molecules on positive plate, which is roughly the same), multiply it by two, then add the number of excess electrons on the negative plate. This will be the number of electrons that were displaced during the charge, and, at the same time, the number of electrons available for discharge. That's your SoC.
To do so, you do not need to know any details on how the chemical reaction took place, how the molecules move, and other complexities. You just need to know that the number of electrons cannot change unless they travel by electric wires between plates. Amper-hour counting does exactly that - counts the number of electrons in and out. It is therefore the exact measurement (not a guess) of state of charge. It measures the entity directly.
As I said before, there are two exceptions - self-discharge and hydrolysis during absorption stage. Otherwise, amer-hour counting is exact.
Voltage certainly depends on SoC, so you can try to estimate SoC by measuring voltage then using empirical formulae such as Peukert equation, correct this for temperature, also add mesurements of internal resistance etc. and thus get some estimate of SoC, but this will not be a measurment, but only a guess.
I hate to be dogmatic about this but lets get the cause and effect in the correct order. The 'static' charge you describe is caused by chemical reactions generating free electrons. Take a glass of dilute sulfuric acid and add two lead plates with one oxidized, bingo you have a voltage difference between the plates. A capacitor operates on electrostatic charge separated by a dielectric or vacuum, that dielectric could be just about anything that DOESN'T" conduct current (as ions) like a battery electrolyte does. Batteries can be modelled (poorly) as a capacitor with infinite capacitance but they are NOT capacitors as capacitors "store" electricity whereas batteries "generate" electricity.
I'm sure we all remember this from grade school.
What you miss in pure electron counting during charging is the need to overcome the equilibrium of the cell, it's pushing back so we need to have a delta in voltage to continue to move ions across the electrolyte to deeper in the plates once we are past the surface charge stage. This energy from the flow of charge into the cell is normally seem as heat/gassing(dissociates water molecules) as it passes through the electrolyte and ready reacted material and is not used or stored (The electrical energy of the electron flow is lost) inside the cell to generate an electric field later. We normally correct the difference in Ah in to Ah actually stored with a 'charge efficency factor' (The key to accurate SoC charge tracking) that changes from close to 100% for a fully discharged cell to much lower as we increase voltage to overcome the equilibrium of the cell to fully charge it. SO yes, current is important but what's even more important is how much 'power' is converted from that current into chemical energy for later use.
I don't think a battery can be modelled as a capacitor.
It does, however, has an electrostatic charge on plates. The amount of the static charge controls the chemical reaction. If you start removing charge by discharging the battery, the chemical reaction starts and works to restore the charge that you're taking out. If you stop, the charge gets quickly restored and the reaction stops. If you connect a charger and thereby increase the static charge, the reaction reverses and works on removing the charge that you supply from the charger. When you stop charging, it takes long time for the chemical reaction to reach an equilibrium with the amount of static charge. That's why you need to wait until you measure the resting voltage.
Power is a different think. As you know:
power = voltage x current
or, if we multiply by time
energy = voltage x charge
You can reasonable tell how much charge is stored in the battery (say, you can say this battery has 100AH in it), but you cannot tell how much energy you're going to get because it also depend on voltage. Depending on the discharge conditions, the voltage may be different. Therefore, you can get more or less energy from it. For example, if you cool down the battery and then discharge it fast you'll get less voltage (and less energy) than if you heat it and discharge slow.
Using the car analogy, you can measure how much gas it is in the tank, but you cannot really tell how many miles you can drive on this gas because it all depends on driving conditions. If you drive at low gear in a rugged terrain you will get less miles than if you drive on highway at 60 mph for the same amount of gas.
Voltage varies mostly because batteries have internal resistance. That is rather abstract quantity that is useful to quantify things. This internal resistance causes a voltage drop. By Ohm's low V = I x R. The voltage you get during discharge will be decreased by I x R. So if you have higher I (discharge faster) or higher R (discharge when it's cold, or when the battery is already substantially discharged), you get a higher voltage drop and you get smaller amount of energy for the same amount of discharge.
Of course. It is 100% if you start charging, and then when gassing starts it goes down because part of the charge is wasted on hydrolysis (bubbling). This numder should be the ratio of charge going into gassing to the total charge put into batteries. It does depend on SoC, but it also depend on other factors, such as speed of charge. If you charge at very low voltage to minimize gassing, it'll be close to 100% until the end. Or, if you hit a relatively discharged battery with really high current, it can start gassing at much lower SoCs. Of course, people do not do that, and charge at roughly the same speed. So, it appears to depend on SoC only.
At this point we now agree more than disagree about the gory details of energy production and physics of batteries. You points about taper charging and energy efficiency are very good. With a nice slow tapered charge we can increase the total charge efficiency to close to 90% (the reaction is exothermic so heat is always generated) but who does that with limited charging times due to solar panels. If you've got the power available for maybe only a few hours a day you push it to the limit to charge the battery as soon as possible without damaging it. That's really when the effects of coulomb and energy efficiency (Nernst equation) losses kick in and when it's much harder to calculate SOC, and (what's really important to most people) how much energy is being stored in the battery as chemical energy to be converted to electrical energy later in a accurate manner.
More gory details for those interested.
I'm going to use the monitor for two things:
- To figure out if batteries are full enough so that I could skip absorption
- To see if I have to run the generator to survive another day
Hopefully, once I decide to charge it goes to the end so I don't need monitoring. If it doesn't, I do some estimate of lost AHs, which is based on my measurements of termination voltages done last winter, but I don't expect it to be very accurate. So, after several unsuccessful attempts (not very likely scenario), the monitor may be way off. I do have a sefety net for this case - if voltage drops too low, I start the generator anyway.
This is very interesting as I've been trying to do something similar:
My usecase is for a motorhome with a 220Ah 12v battery bank, 64W solar panel, and 50A charging when the engine is running.
My problem is there is virtually *never* a time that the batteries are under no load. During the day they get solar charge, and I have a compressor fridge which kicks in every 5-10 minutes.
I tried several attempts to try and get readings based upon terminal voltage, but could never get anything accurate. The voltage pattern was just too difficult to track. So in the end I have dropped back to just counting current in and out and correcting for Peukert, charging efficiency and temperature. Whilst I know this will drift over time, I'm hoping toensure it can 'reset' to a known state of 100% charge when I do a long drive, or am not using the motorhome and solar panel is float charging the batteries.
Maybe a MOD could add the link below to the other links of my post.
It is a formal logic flaw here.
"capacitor stores charge" and "battery is not a capicitor". This two statementds do not prove that "battery doesn't strore charge".
"gorilla has eyes" and "crocodile is not a gorilla". Doesn't mean "crocodile doesn't have eyes".
Batteries produce electricity via chemical reaction. They do not store charge. For certain batteries the chemical reaction is reversible to a degree, allowing them to be 'recharged'. This recharging does not put electricity into a battery; it reverses the chemical reaction (never completely) that has taken place so that the process of producing electricity can begin again. This is why it always takes a bit more energy in than you will get back out.
Capacitors do not produce electricity, they store charge. They can be charged and discharged without affect on their capacity (unlike a battery which always deteriorates a little with every cycle even though it may not be noticeable). Essentially they do not have a 'cycle life' as a rechargeable battery does (although like everything in this universe they do deteriorate over time regardless of treatment).
You can leave a capacitor in either a charged or discharged state without ill effect. The same is not true for a battery.
No gorillas nor crocodiles were harmed in the making of this post.
That is a purely philosophical distinction. I put money in the bank. They invest it somewhere, but I can't really know. When I come to get money back, did they store it for me, or did they produce it for me. I don't know. The important thing is that quantity in is the same as quantity out (with some fees of course). Same thing with batteries.
Okay, just try interchanging them and see how well that works.
It isn't "philosophical", it's physics.
Your analogies are from hunger.
I have already exchanged a good portion of my bank account for batteries
Physics is simple. There's a chemical reaction which embeds electrons into molecules, where they're, for the lack of better word, stored until the chemical reaction is reversed and they're released to produce electricity.