Charging an Electric Car · 20 April 07
Most of the time when someone considers converting a car to electric the last thing that probably springs to mind is the charger. The car itself garners the most attention since it’s the biggest and most visible aspect of an EV, followed by the motor. Somewhere down the list, after the batteries or controller, the charger choice lies.
Batteries are all but useless (or will soon be rendered so) without a good charger. Everything else in an electric car is there to use the batteries, with little thought to their well being (and that often includes the driver). It’s the charger that gets tasked with taking care of and getting the most out of the batteries. Without a good charger you could easily reduce battery life by 50%, which is kind of an expensive on-going design flaw.
There are a number of approaches to charging batteries, from do-it-yourself “bad boy” chargers to computer controlled, high voltage fast chargers. Before we start talking about the different approaches let’s take a bit of a detour and cover some basic electronics to get everyone on the same page. (click here to skip to the charging info)
To your charger a battery is just a resistor. A simplified schematic might look like:
Power source (charger) on the left, resistive load (battery) on the right. In a simple circuit like this the amount of voltage and resistance determine how much current flows. Let’s say we have 14 volts “E“ and the resistor “R“ is 10 ohms. OHM’s law states that current “I“ is:
I = E/R
I = 14/10
I = 1.4 amps
1.4 amps of current flowing through the circuit. Increase the voltage or decrease the resistance and current flow goes up, increase the resistance or decrease the voltage and there’s less current flowing through the circuit.
The difference between a resistor and a battery is that the battery is also power source, as well as having internal resistance. In order to charge the battery we’ll need to apply a voltage that is higher than the battery’s. You can illustrate this by measuring the voltage across your car battery when it is hooked up to a 12v charger.
Let’s think about batteries as a bunch of resistors again, just to illustrate a few more concepts. Whenever you combine resistors there are two configurations you might see: either parallel or series.
An EV battery pack is usually in series so let’s discuss that first. Resistors in series add up. So if the resistors R1 through R4 were all 10 ohms each the total resistance would be 40 ohms. Using our earlier equation:
I = 14/40
I = 0.35 amps
We get a current flow of 0.35, significantly less than with the previous circuit. We’d have to increase the voltage to 56 volts to see the same 1.4 amps of current we saw when using a single 10 ohm resistor.
A good thing to remember about a series circuit is that the current is the same through each resistor. If we are feeding the circuit 1.4 amps at 56 volts, then we know each resistor has 1.4 amps flowing through it. On the other hand the voltage varies across each resistor. Let’s say our power supply is putting out 56 volts at 1.4 amps. To figure out the voltage across each resistor you use the formula:
E = I * R
E = 1.4 * 10
E = 14
The voltage across each resistor is 14 and if we add them all up we get back to our total applied voltage of 56 volts. Cool! The resistors don’t all have to be the same value, which in the case of batteries they probably seldom are, but the formula works just the same.
So in a series circuit the current is the same throughout, but the voltage is different across each resistor.
Another configuration you might see is a parallel circuit, where the resistors are parallel with each other. Right off the bat you might notice that every resistor is tied to the power supply: therefore they must all have the same voltage applied to them. True, and that means the current flowing through each resistor and how the resistors combine is going to be different from a series circuit.
Let’s stick with R1-R4 being 10 ohms each, just to keep the math straightforward. The total resistance formula for parallel circuits is a bit messier:
Rtotal = 1/ (1/R1 + 1/R2 + 1/R3 + 1/R4)
Rtotal = 1/ (1/10 + 1/10 + 1/10 + 1/10)
Rtotal = 2.5 ohms
If you are a math wiz you’ll probably notice that if all of the resistors are the same you can just use a shortcut of:
Rtotal = Resistance/# of resistors
Rtotal = 10 / 4
Rtotal = 2.5
When resistors are added in series the total resistance goes UP and in parallel it goes DOWN. Less resistance means our parallel circuit is allowing more current through:
I = 14 / 2.5
I = 5.6 amps
Woo hoo, look at all of that current! It’s a little deceptive though: unlike a series circuit the current through each resistor when in parallel is not the same, it’s cumulative. Remember each resistor is 10 ohms and each resistor is hooked straight to the power supply, so the only thing that CAN change in our formula is current:
I = 14 / 10
I = 1.4 amps (across a single resistor)
Current in a parallel circuit is cumulative, so you add up the current flowing through each resistor to get the total:
I = 1.4 + 1.4 + 1.4 + 1.4
I = 5.6
Finally there are series-parallel circuits. Dr. Larry’s EV is a good illustration of this arrangement as his battery layout looks like:
At this point our heads explode into a thousand pieces (all having a specific ohms value, of course)! But, if we divide and conquer it’s not too bad. Again we’ll assume each resistor is 10 ohms
First off we figure out what the combined resistance of a parallel “leg” is (i.e. R1 & R2 are a “leg”):
R = 10 / 2
R = 5 ohms
Then add up each of these six legs for total resistance and figure out the current from the supply:
Rtotal = 5 + 5 + 5 + 5 + 5 + 5
Rtotal = 30 ohms
I = 14 / 30
I = 0.47 amps
Now, armed with the total current flow in the circuit we can figure out each leg’s voltage drop:
E = 0.47 amps * 5 ohms
E = 2.33 volts (per leg)
I = 2.33 / 10 (to get current per resistor in leg)
I = 0.23 amps
So even though we have 14 volts and 0.47 amps coming out of the charger, each resistor is only going to be seeing 2.3 volts and 0.23 amps.
Now Dr. Larry doesn’t suffer from one single power supply trying to feed the entire pack, instead he has a dedicated 12v power supply hooked to each leg (i.e. R1 & R2, R3 & R4). Using these formulas we know that if the charger is capable of supplying 8amps maximum, that the most each battery will see is 4amps.
One of the nice things about having batteries in parallel is when the car wants 100 amps, each battery in a leg only has to supply 50 amps (assuming they are the same type of battery), so there’s less wear and tear on an individual battery. On the other hand it can be tricky to monitor each leg for a bad battery. A pair of batteries is only as strong as the weakest one.
Those are some of the basics behind what happens when you hook up resistors (or batteries) in parallel or series. If you think of the batteries in these terms it can help to visualize how things work electrically, especially when you start adding a charger to the circuit.
How Much Charger Do You Need?
Our 120vac Zivan (144vdc output) charger delivered up to 8 amps and could charge the pack in 3-6 hours, depending on how far we’d driven. A 240vac Bycan charger we had for a while could deliver almost 20 amps and charge the pack in half the time. But, if you are charging at the end of the day and have all night to do so is it worth springing the extra $$‘s to speed it up?
How much amperage will you need out of your charger? It depends on how much power your EV uses each day and how fast you want it recharged. Let’s say we use 36 amp hours in one day of driving. For reference, 36Ah is equivalent to pulling 36 amps for a full hour, or 72 amps for half an hour, or 216 amps over a ten minute period. However you do it the Ah meter will say 36Ah.
And however you use the power the charger has to replenish it … and some. Remember, whenever you move energy from one place to another or from one form to another, you inevitably lose some along the way. The heat you feel on the charger, wires and batteries during a charge is all lost energy. Simply stated this means we’ll need to put more than 36Ah back in to bring the batteries to full charge.
Let’s pick a number and say it’ll take 40Ah to top the batteries off. You’d think an 8 amp charger would take about five hours to do this, but in actuality it may be six or more. Check out a typical charging curve chart to see the suggested charge cycle for a lead-acid battery. As this site so nicely puts it, the charger has three key functions (which they cover in detail):
- Getting the charge into the battery (Charging)
- Optimising the charging rate (Stabilising)
- Knowing when to stop (Terminating)
I’d add that on top of doing all of this a good charger is also taking care of:
- shutting down if batteries overheat
- establish limits; both in voltage/current and TIME
- ability to run conditioning charges (for lead acid)
The second point is something I’m very familiar with. Let’s say the charger is at the first phase (constant current) of charging and is waiting for the battery pack to reach a certain threshold voltage so it can switch to the second phase, a slower, constant voltage top-off. What happens if the pack never reaches the cut-over voltage? We once awoke in the middle of the night to our CO2 detector beeping and found the charger essentially boiling the battery pack. Why? One (or two) of the battery cells had gone bad enough such that the phase one threshold voltage was never reached. Without a default timeout the charger merrily cranked eight amps into the pack.
This is one of the downsides of a full-pack charger. Without additional monitoring equipment or control it really has just a rough idea how things are going: as far as it is concerned you only have “one” big battery. There are ways around this of course, which we’ll talk about later.
All of these features, both safety and performance, start pushing the cost of a charger higher and higher. Considering that the EV market is quite small we don’t benefit from the price reductions of mass-produced products. Instead you get a few folks pouring their heart and soul into making a quality charger in the hopes that they’ll make a small profit along the way.
Types of Chargers
Bad Boy Charger
Eventually you’ll read about “Bad Boy Chargers” (BBc for short) so we’ll get that one out of the way first. There’s a few approaches to this, but the basic concept is to more or less plug your battery pack into the AC outlet … with a few diodes and a variac (variable transformer) tossed in for good measure. Some folks even use a FUSE! ":^)
When the charger died on my first EV I put together my own BBc after spending a couple of days trying to charge the pack one battery at a time with two cheap 12v chargers. It works, but is more in the category of a hand-grenades as far as finesse goes.
The main issue with this type of charger (and I don’t think anyone with something more expensive than flooded lead-acid batteries would dare use one) is that there’s no automatic control or regulation. You need to manually monitor the pack and as the batteries fill up adjust the charging level. No safety circuits and no fancy formulas, just brute force.
The next step up in price, functionality, and safety is to use a bunch of individual 12v automotive-style chargers. Dr. Larry uses this approach with his 72v EV. His EV has six chargers (and I think a 7th for the acc battery), one for each battery pair, and from what he’s told us they are doing a great job.
Some things to take into consideration when using this approach:
- charger outputs must be isolated from input AC and Ground
- depending on how many you use there may be:
- large start-up current surge, which could trip the breaker
- heavy AC load during charging: be careful to properly size extension cords and power strips
- may not be waterproof and/or do well in a jostling, dirty environment
- more parts, more things to go wrong
- ensure the charger doesn’t DRAIN your battery when it is off
On the plus side each of your batteries gets individual care and attention, assuming that the charger you are using is of good quality and matched to the type of battery you are charging.
An off-the-shelf *mart charger isn’t the only way to approach this either. There’s a few companies that make “ganged” chargers, in dual, triple, ten and other configurations (and various current outputs).
You’ll notice that as the current output goes up, so does the price.
Next to the sticker shock of buying the motor and controller for an EV I suspect most converters get a lump in the throat when they start pricing out chargers. Admittedly so, like the motor it is a big investment. One thing that may help to deaden the pain a bit is that it’s a long term investment. When your current EV body rusts out or you outgrow it, it is a simple matter to pull the motor, controller, and charger out of the old vehicle and pop them into the new one.
You could probably do this for decades, especially since the controller and charger have no moving parts. And, if you’ve purchased a quality charger, it should be able to be reprogrammed to handle new types of batteries (although check before you buy, especially if you are leaning towards a certain battery type).
A few of the EV specific chargers on the market these days include: Manzanita Micro, Russco, and Zivan. There have been others, like the Bycan and K&W, but these are the ones I was able to find web pages for and seemed to be for sale.
The advantages of these types of systems is that A) they are engineered specifically for charging EV batteries, B) the companies are typically small and available for questions specific to EV systems, and C) they usually have a number of performance and safety features built in, along with additional options (temp probes, boosters, etc..).
The downside, somewhat, is that they see your battery pack as a whole and typically can’t monitor individual batteries. Although, to be fair, a twelve volt battery is composed of six cells and individual chargers can’t monitor each of these. But if we go down this path we’ll never make much headway in deciding on a charger!
Which leads us to …
So far the choice has been lots of chargers (i.e. lots of points of failure/equipment) with tighter individual battery control vs. single dedicated EV chargers with fewer parts but less individual battery control.
Bypass regulators are small circuits that you hook to each battery in your pack as a companion to a single EV charger. Each regulator keeps an eye on its battery and automatically bypasses it when it senses a full charge. In that way you don’t have a bunch of batteries boiling away while one laggard takes its sweet time reaching full charge.
There’s been a few types out there over the years but it seems like the only ones currently in production are the Rudman regulators Mk II and Mk III, the later sporting a digital interface to talk to a smart charger. Lee Hart has posted a couple of designs for do-it-yourself regulators including a relatively simple zener-lamp regulator. And here’s a schematic of someone else’s clamper (pdf) (original here).
There’s one more from Lee, which I’ve linked to a couple times before: the TMSI Battery Balancer. It’s part battery charger, part monitor, and part regulator. And, because you build it yourself, it is extensible to do even more!
You’ll have to excuse the crude block diagram, it is by no means complete or accurate, but hopefully enough to explain the concept behind the design. Before you freak out, the thing in the upper right is supposed to look sort of like a rotary switch, with about twelve outputs. Yes, instead it looks like a bald guy with bad hair plugs!
Think of the relay signals line coming in from the CPU as a control signal which switches a virtual rotary switch (which in actuality is a big board of relays) such that one battery is hooked to the signal lines below; to the ADC (analog to digital converter) and DC-DC.
Once it has a battery hooked up there’s a pause to let things settle down and then the ADC (a digital voltmeter in this case) measures the voltage across the battery and sends the reading to the CPU. It repeats this for each battery in the pack and then the CPU picks the lowest reading of the bunch and goes back to that battery and turns on the small DC-DC converter to provide a “booster” charge to the battery.
So, unlike the regulators which bypass charged batteries this system goes around and tries to help out the weaker/slower batteries. You’ll still need a charger for the full pack, this unit merely “walks” around and helps things out. In addition to these functions it can also monitor a couple temperature probes, turn on LEDs, send signals out an RS-232 port for logging and/or a display on the dash.
The surprising feature is that it also performs the balancing while the system is discharging. It’s a little hard to wrap your head around at first, but basically it uses the full pack voltage, by way of the DC-DC converter, to help out a weaker battery, even though the battery is being used to help power your EV.
A number of folks have created their own chargers and regulators over the years. One that comes to mind is Peter’s BMS (which he’s redesigning). If you know of others post a link in the comments or send me an email. Same goes for any corrections or additions to the article.