Re: (ET) Battery charger question

[Rob Brockway <GEtractor yahoo com>]

Dave,
This is a great summary.  One question why would the battery voltage drop 
as plates loose surface area?  I would think that would impact capacity 
not voltage.

I had an automatic Soneil charger for years.  I was vey happy with it but 
thought I may have had to add water more frequently.  It was also 5 amps 
so would never have added the C20/10 you explained is beneficial.  Without 
any hard data I concluded the built in charger is very effective.
Thanks,
Rob

Sent from my iPhone

> On Oct 9, 2021, at 2:17 AM, David Roden <etpost drmm net> wrote:
> 
> The GE charger is actually not an awful choice for the flooded golf car 
> batteries that most people use.  It has some voltage regulation because 
> it's a ferroresonant design.  Left to its own devices, it would probalby 
> overcharge, but the timer (if it's working) puts something of an upper 
> limit on that overcharging.  
> 
> Think about it.  Golf car batteries typically are rated for a life of 
> around 700 80% DOD cycles.  If you use your ET every week, that's about 
> 14
> years.  From what I hear, most folks get around 10 years.  So - not 
> awful.
> 
> But suppose your GE charger is kaputt, or you just want something easier 
> on 
> your battery.
> 
> Below is a rather long article on smart charging, a piece that I wrote 
> for 
> the EVDL about 12-15 years ago.  Lead batteries haven't changed much in 
> that time. :-)  It might help you choose a smart charger for your ET.
> 
> One thing I want to underscore in it is the importance of ENOUGH 
> CURRENT.  
> You might be temtped to buy a small charger because it's cheap.  Don't 
> do 
> it!  As explained below, your charger should be able to supply 20-25 
> amps.  
> At the very least, don't go smaller than 10 amps.
> 
> Besides, charging at 5 amps takes about a day and a half just to get to 
> 80% 
> charged, and easily another day or so to finish.  If you have work to do 
> with your ET, you may not have that kind of time.
> 
> ----
> 
> Smart Charger Algorithms - How smart chargers "think"
> 
> Why You Need a Smart Charger: As long as you're using simple golf car 
> batteries, actually, you don't. Golf car batteries are relatively easy 
> to 
> charge, and fairly tolerant of a few charging errors now and then.
> 
> But you might WANT one. Charging manually takes a fair bit of work and 
> attention. These days, practically every other rechargeable gadget has 
> an 
> automatic charger, or at least one we don't have to pay much attention 
> to. 
> So most of us just aren't used to manual charging.
> 
> What a Smart Charger Does: It provides basic battery care and feeding, 
> automated, so you don't have to sweat it. It might have a microprocessor 
> "brain," or discrete logic, or just linear circuits; but in some way it 
> tries to figure out the battery's current state of charge, and how to 
> get 
> it to a full charge as quickly and safely as possible. The rules it 
> follows 
> in doing that we call a charging algorithm.
> 
> Alphabet soup: Probably the most common algorithms in smart chargers are 
> IU 
> and its variants, IUI and IUU. Here each letter represents one phase or 
> period of charging, so this is two phase charging or three phase 
> charging 
> The I stands for constant current, and the U stands for constant 
> voltage. 
> I'll explain those terms in a moment.
> 
> Charging Phase One: The first phase of the charge is what we call bulk 
> charging.
> 
> Initial Charging Rate: Theoretically, as long your charger is mindful of 
> the battery's temperature, in this phase it can stuff in the electrons 
> about as fast as the battery can dish them out when you discharge it. 
> This 
> can be in the hundreds of amps for golf car batteries, and some AGM 
> batteries can handle charging currents in four figures!  But the rule of 
> thumb on initial charge rate is somewhere between C20 / 10 and C20 / 4.
> 
> That looks like some kind of code, doesn't it? C20 is the battery's 
> 20-hour
> amp-hour rating -- that is, how many amp hours it can produce if you 
> discharge it over 20 hours' time. (The faster you discharge a lead 
> battery, 
> the fewer amp-hours you can get from it. Most battery manufacturers 
> specify 
> the battery's capacity at at least 2 different discharge rates.) 
> Amp-hours 
> are not the same as amps, but they're a handy way to express the size of 
> the battery and its current requirements, so in this case, we use them 
> that 
> way. Thus for your typical 220 amp-hour (20 hour rate) golf car battery, 
> you want to use an initial charging rate between C20 / 10 (22 amps) and 
> C20 
> / 4 (55 amps).
> 
> Why Initial Rate Matters: Lead batteries aren't like the nickel cadmium 
> and 
> nickel metal hydride batteries you use in flashlights and cameras. Those 
> little batteries appreciate being charged slowly, and they'll last 
> longer 
> (for more charging cycles) when they're treated that way. However, lead 
> batteries actually LIKE and NEED an initial charging rate of at least 
> C20 / 
> 10, some even more.
> 
> I'm just a hobbyist, not an electrochemist, so I don't know the 
> electrochemical reasons behind this. What I do know is that that lead 
> batteries lose capacity (wear out) faster if they're not given this high-
> current jolt for at least a few minutes at the start of the charge cycle.
> 
> Hawker Genesis AGM batteries from the 1990s were poster children for 
> this. 
> They could lose half their capacity in under a year without it.  
> However, 
> all lead batteries benefit from high initial current. The engineers know 
> what they're doing when they recommend C20 / 10 as a minimum.
> 
> Constant current: When a battery is flat, its voltage is low. This means 
> it 
> can take (and wants) a huge charging current. As it charges, its voltage 
> rises, so a fixed-voltage charger's current falls. This slows down the 
> charge.
> 
> But one of your smart charger's missions in life is to charge the 
> battery 
> as fast as it can. To do this, it sets its own voltage so the charging 
> current is as high as it and the battery can tolerate. Then, as the 
> battery's voltage rises, the charger keeps bumping up its own voltage so 
> the charging current stays high until the last possible minute (we'll 
> see 
> when that is soon). This process is called constant current charging.
> 
> During the bulk charging phase, nearly all of the charging energy goes 
> into 
> the charging reaction. As the bulk phase proceeds, with the current held 
> constant, the battery's voltage rises. When the battery is about 80% 
> charged, it reaches the gassing voltage. From here on, more and more of 
> the 
> charging energy goes into heating the battery and dissociating the 
> electrolyte's water into hydrogen and oxygen. We'll soon see why this 
> heat 
> matters.
> 
> Gassing voltage depends on the battery's design -- the composition of 
> the 
> positive and negative grids, and the chemical makeup of the electrolyte. 
> For a typical flooded golf car battery, it's 2.4 volts per cell (VPC) at 
> 25° Celsius. Other battery types will vary from 2.35 VPC to 2.5 VPC. 
> Check 
> your batteries' datasheet.
> 
> Temperature Compensation: A good charger will adjust this voltage, and 
> all 
> the voltages that follow below, for battery temperatures significantly 
> higher or lower than 25° Celsius. You get the adjustment factor from 
> your 
> battery manufacturer, but a typical one is -3mv or -4mv per cell per 
> Celsius degree deviation from 25° C.
> 
> This isn't ambient (air) temperature, but rather battery temperature. 
> The 
> ideal way to read it would be to immerse a temperature sensor in the 
> battery's electrolyte. However, the usual way is to bury a sensor 
> between 
> two batteries in the middle of the pack. I've also heard of attaching a 
> sensor to a battery terminal post.
> 
> Temperature compensation (TC) is more important for valve regulated (AGM 
> and gel) batteries than for flooded ones, but if you have it available 
> on 
> your charger, there's no reason not to use it with flooded batteries, 
> too.
> 
> Charging Phase Two: Reaching the gassing voltage ends the bulk phase and 
> begins the absorption phase. The battery is now about 80% charged. 
> Depending on the charger's algorithm, the remaining 20% may take about 
> as 
> long as the first 80% did!
> 
> In the absorption phase of an IU charging algorithm, the charger holds 
> the 
> voltage steady (constant voltage charging) at the gassing voltage. 
> Remember 
> how the voltage rose when we held the current steady? Now the charger 
> holds 
> the voltage steady, so the charging current falls. The charger sits 
> tight 
> until the charging current has declined to about C20 / 50. For our 
> example 
> 220ah golf car battery, that would be 4.4 amps.
> 
> At this point the battery is essentially full. The charger can shut off 
> now, or you can pull the plug manually.
> 
> But maybe you shouldn't, at least not every time. That's because 
> although 
> the battery is full, some of its cells are a little fuller than others.
> 
> Cell Imbalance: The cells in a battery vary a bit in how fast they 
> charge. 
> Part of this is down to slight differences in their manufacturing 
> tolerances.
> 
> A larger factor in cell imbalance is that the cells vary in temperature, 
> sometimes a lot. In each battery, the inner cell or cells will usually 
> be 
> warmer than the ones toward the outside of the battery. In a large 
> battery 
> pack, the inside batteries will also be warmer than the outside ones. So 
> it 
> isn't unusual for cell temperatures to differ by 10 or more degrees.
> 
> Temperature affects a cell's fully charged voltage, and also its charge 
> efficiency. See the problem?
> 
> In the short run, cell imbalance isn't a big deal. But over time, as you 
> charge and discharge the battery, the differences get wider and wider. 
> Eventually the lowest cells can end up chronically undercharged. That 
> will 
> limit how much energy you can get from the battery, not just because of 
> the 
> cells' lower charge, but because chronic undercharging causes permanent 
> loss of battery capacity.
> 
> You might think that the way to fix this is to charge every cell 
> individually. In fact, that's how many lithium EV batteries work. The 
> cells 
> are charged in series, as with any other battery, but each cell has its 
> own 
> bypass regulator. When the cell is full, the regulator diverts the 
> charging 
> current round the cell, so the charged cell can kick back while the rest 
> of 
> the cells finish charging.
> 
> Most road EV owners with long strings of AGM or gel batteries use 
> similar 
> regulators. However, they can't put a regulator on each cell, because 
> modern lead batteries aren't built so you can access the individual 
> cells. 
> Thus they can only regulate the charge to each individual battery. 
> That's 
> better than nothing, but they still need to somehow balance the 
> individual 
> cells in each battery.
> 
> Equalization (Charging Phase Three): Your charger fixes cell imbalance 
> with 
> equalization -- deliberately, but carefully, overcharging the battery. 
> The 
> fully charged cells dissipate the wasted energy through them as heat and 
> as 
> gassing, and the cells that aren't yet at 100% get topped off.
> 
> To do this, instead of shutting off at the end of the constand voltage 
> absorption phase, the charger switches back to constant current 
> charging. 
> This time, it uses much lower current -- the same C20 / 50 that signaled 
> the end of the absorption phase. Now our IU profile has become an IUI 
> profile. The charger holds that C20 / 50 current steady until the 
> voltage 
> rises to 2.5 VPC, temperature compensated.
> 
> Or not; some engineers say to go to 2.55 VPC. Some say to hold C20 / 50 
> for 
> 2-4 hours, no matter how high the voltage goes. Some suggest constant 
> voltage float charging (2.3 VPC) with no limit instead, which is an IUU 
> profile. So as you can see, some opinion is involved here. If your 
> charger 
> has configurable equalization options, you might want to ask your 
> battery 
> manufacturer which one is best for their batteries.
> 
> On the other hand, you might want to not to ask the manufacturer how 
> often 
> to equalize. Some of them -- US Battery is one -- will tell you to 
> equalize 
> on every single charge. That does give you the absolute maximum amount 
> of 
> stored energy, and thus the maximum range. However, overcharging 
> stresses a 
> battery, so too-frequent equalization is about as bad for your battery 
> as 
> too-infrequent equalization. My recommendation is to equalize only when 
> it's necessary.
> 
> How often is that? Ah, there's the rub. Small differences in cell states 
> of 
> charge (SOC) can be hard to detect until they become big differences. 
> Unfortunately, although I know of one high quality (large, expensive) 
> industrial battery charger that keeps track and equalizes every 7 
> cycles, 
> most smart chargers aren't all that smart about equalization. Typically 
> they either don't equalize at all, or they equalize every time.
> 
> If your charger is a never-equalizer, you might equalize your battery 
> every 
> 5 to 15 charges by restarting the charger after it's shut off. If yours 
> is 
> an always-equalizer, you could try to pull the plug on it when it gets 
> to 
> the equalization phase most of the time. The problem with these schemes 
> is 
> that now you're doing some manual charging. You paid good money for a 
> smart 
> charger so you wouldn't have to do that, no?
> 
> Voltage-Based Charging Problems: Equalization strategy isn't the only 
> weakness in smart chargers. Straight IU and IUI chargers have another 
> one 
> that not many charger and battery makers own up to.
> 
> A battery is a little like the water heater in your house. As your water 
> heater ages, it starts to build up sediment in the bottom of the tank, 
> so 
> it holds less water. Well, as a battery ages, it builds up sediment too. 
> This is not a joke; it really happens: the active material in the grids 
> crystalizes, falls off, and sinks to the bottom of the battery. With 
> less 
> active material in the grids, the battery's capacity to hold energy 
> declines. That also means that its fully-charged voltage declines.
> 
> If your charger is programmed for a new battery's fully-charged voltage, 
> it 
> may overcharge an old battery. In fact, an old battery might never reach 
> that magical 2.5 VPC above, so the equalization phase can go on too 
> long. 
> As the battery ages more, eventually it might not even reach 2.4 VPC. If 
> that happens, the charger can get stuck in the bulk phase. Your battery 
> will get severely overcharged, aging it even faster.
> 
> OTOH, another symptom of a battery in its golden years is that its 
> internal 
> resistance increases. This can cause the exact opposite problem -- the 
> on-
> charge voltage rises very fast, and that fools the charger into stopping 
> the charge too early. Then the battery is undercharged.
> 
> One way to help this situation is to add a safety time- or 
> amp-hour-limit 
> to your charger. I'll talk more about this later.
> 
> DV/DT and DI/DT Charging: These are a more elegant solution to battery 
> aging. DV/DT and DI/DT stand for derivative of voltage (or current) with 
> respect to time. If you took calculus in college, this probably brings 
> back 
> memories. You might remember that derivatives calculate the slope of a 
> curve at a given point. In this case, the curves are the rising voltage 
> and 
> falling current in a charge cycle.
> 
> DV/DT charging takes advantage of the fact that as a battery charges at 
> a 
> regulated (constant) current, the voltage rise slows and eventually 
> stops,
> regardless of voltage. DI/DT charging is based on the similar idea that 
> at 
> a constant voltage, the decrease in current slows and eventually stops.
> 
> During the constant current bulk charging phase, in addition to watching 
> for gassing voltage, the charger's brain watches for the voltage rise to 
> slow down. A typical value to watch for is between 2.5mv and 5mv per 
> hour 
> per cell.
> 
> During the constant voltage absorption phase, in addition to watching 
> for 
> the current to fall to C20/50, it watches for the current decrease to 
> slow 
> down. A typical value here is between 0.2 and 0.4 amps per hour.
> 
> The battery makers usually specify DV/DT and DI/DT in hour increments 
> (if 
> they specify them at all). But IMO it's better to sample voltage or 
> current 
> more often than every hour. Also, the charger should look for the 
> specification (divided by samples per hour of course) to be met in 2 or 
> 3 
> consecutive samples, or for the delta (change) to fall to some much 
> smaller 
> amount. For example, Lester's Lestronic DV/DT chargers check the voltage 
> slope every 15 minutes.
> 
> Safety Limits: Whether the charger uses DV/DT or not, it should also 
> have 
> one or more backup methods that will halt the charge in case of really 
> weird or dangerous situations.
> 
> One of the pesky qualities of lead batteries is that their fully charged 
> voltage is lower at higher temperatures. This is called a negative 
> temperature coefficient. It explains why you should use temperature 
> compensation, but it's also a matter of safety.
> 
> I mentioned above that once a battery charger goes beyond the battery's 
> gassing voltage at 80% charged, an increasing amount of the charging 
> energy 
> goes into heating the battery and generating hydrogen and oxygen. Well, 
> when the battery reaches 100% charged, all the energy goes to waste this 
> way.
> 
> It's the heat that causes trouble. As the battery gets hotter, its 
> voltage 
> falls. That makes a constant voltage charger send more current through 
> the 
> battery, which heats it up even more, which increases the current more 
> ... 
> and before you know it, you have thermal runaway.
> 
> At best, the result is a hot, overcharged battery. At worst, the battery 
> can actually catch fire. (Yes, this has happened, though I'm glad to 
> say, 
> not to me yet.)
> 
> To prevent this, a smart charger should check for at least one of two 
> things. The first is negative DV/DT. If the on-charge voltage falls, the 
> charger should stop the charge immediately. The second is high battery
> temperature. If the charger's already using a temperature sensor to 
> carry 
> out temperature compensation, it should be able to keep an eye on this, 
> and 
> stop the charge if battery temperature exceeds something like 50° or 60° 
> Celsius.
> 
> If one of these conditions forces the charger to shut down, it's also a 
> good idea for it to let you know that something's gone wrong, maybe by 
> turning on a red warning light. That way, you don't find out the hard 
> way 
> that your EV isn't fully charged.
> 
> Another good safety backup: checking whether the amount of charging so 
> far 
> makes sense. The charger should keep track of the total amp hours and/or 
> charging time. If the charger knows what kind of battery it's charging, 
> it'll know if it exceeds, say, 150% of that battery's rated amp-hours. 
> If 
> it doesn't know, it can still make a guess at a reasonable amount. 
> Either 
> way, it should stop charging if things look odd, and warn you that 
> something might be wrong. 
> 
> 
> 
> 
> 
> David Roden - Akron, Ohio, USA
> 
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