Common Smart Charger Algorithms How smart chargers "think"

But if you read Lee Hart's Basic Battery Charging Instructions, you can see why 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.

Besides, a proper charge on your EV is pretty important. If your mobile phone isn't fully charged in the morning, it might be a little inconvenient plugging in at work, but you'll do OK. If your EV isn't charged up, though, you might not get to work at all.

What a Smart Charger Does: Pretty much what Lee recommends, 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! So if you really need to charge your EV in an hour or less, you can, but you'd better check your bank account first. A charger that big can cost as much as a house.

For the rest of us, 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.

Some batteries are especially touchy about this. The Hawker Genesis Electric Vehicle Application Handbook recommended an initial bulk charge rate of at least C10 / 3, and strongly suggested 1 * C10 to 2 * C10. They weren't just blowing smoke. When the Massachusetts DOER were testing mid-1990s Solectria Force EVs, they found that their Hawkers' capacity fell by more than half in less than a year. Their report blamed this on a lack of proper equalization, but the real problem was Solectria's 3.3kW chargers. They were using a double string of 42 amp-hour batteries, so they should have been charging them at 84 amps. However, Solectria's chargers could only deliver about one-quarter of that current. And those were the high-power chargers; Solectria's earlier chargers had only managed about 7 amps.

Hawkers are poster children for sock-em-hard charging, but all lead batteries benefit from it. 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 more or less 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 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.

Other Charging Algorithms: There are probably as many other charging algorithms as there are battery engineers. Most of them aren't often used, but I think a couple are worth mentioning.

Amp-Hour Tracking: This is an especially accurate method, and it doesn't depend on voltages that can change over time. Though it's fairly common in laptop computers, as far as I know not many EV hobbyists have tried it. It was one of the algorithms Hawker recommended for their Genesis AGM batteries in EVs, and Saft specified it for their STM5 EV Ni-Cd batteries.

In principle, amp-hour tracking is pretty simple, though it requires a charger that lives on board the EV rather than in the garage. As you discharge the battery, the charger stays awake, monitoring the number of amp-hours being used. When it recharges the battery, instead of stopping the charge when the absorption phase current falls to C20 / 50 or DI/DT approaches zero, the charger stops when it's replaced the number of amp-hours that the EV used, plus some to account for charging inefficiency. For a lead battery, the charger usually puts back 105% to 110% of what the EV used. Nickel-based batteries are a little less efficient, so they typically get a 120% or 125% recharge.

One sticky complication of this method is battery self discharge. All batteries gradually lose charge just sitting. If you don't drive your EV for a few weeks, the charger needs to realize that, and add some self discharge amp-hours. This isn't quite as straightforward as it seems, because the amount of battery self discharge depends on such factors as battery design, temperature, and age, and vehicle parasitic loads.

Amp-hour tracking should also have the safety backups I mentioned above, and then some -- a maximum battery voltage, negative DV/DT, total amp hours added or charging time, and maximum battery temperature.

Valve Regulated Charging: This is a historical curiosity. I don't know of anyone actually using it today. However, I think it's interesting.

Maybe you've seen the term "valve regulated" used for gel and AGM batteries (I used it above, in fact). When "sealed" or closed batteries on charge reach gassing voltage, the hydrogen and oxygen gas they produce raises their internal pressure. It turns out that a charger can use this pressure change to regulate the charging. The earliest closed batteries had pressure sensing release valves in them for this purpose, so they were "valve regulated."