Few entry level PHEV questions.

CA - Cranking Amps. Measured at a warmer temperature than CCA, Cold Cranking Amps, a winter spec.

The others are more complex that I can describe now. Explanations require more than Ohm's Law, and currents are deliberately controlled by the car's brains.

 

I think "C" stands for capacity.
With the 40Ah capacity battery, you are correct that the 1C charge is 40A, 2C charge is 80A, 0.5C charge is 20A and so on.
I have never seen the "CA" unit. Maybe, someone creates the very local "CA" unit by himself.

There is approx 0.35 ohm internal resistance on the Prius 215V nominal voltage pack.
When it is charged by 100A current, the voltage increment is 0.35ohm X 100A = 35V (Ohms Law), then the battery voltage becomes 215V+35V=250V.
When it is discharged by 100A current, the battery voltage becomes 215V-35V=180V.

I have no clue what your question is.

Ken@Japan

 

Dan,
Ken is correct about CA. It is a standard unit ued by battery manufacturers when referring to charge or discharge rate. This is because they make batteries with varying capacities. It is easier to publish charge and discharge curves using CA because the same curve would apply to all battery sizes. a 40AN battery with a max discharge rate of 3CA would be capable of 120A discharge rate.

To answer your question about work, you are correct that the magic is in the inverter. The inverter is a variable speed motor controller, outputting the sine wave at a desired frequency. The motor will turn at a rate dictated by the output frequency.

 

For automobile starting batteries only, I'm sticking with CA being Cranking Amps, and it is measured at 32F. CCA is measured at 0F. Here is a reference.

 

Correct.
Please be careful when you connect the 300V and 200V battery together.
Assuming the both batteries have the same 0.35 ohm internal resistance, the charge current will be 100V / (0.35+0.35) ohm = 143A.
That's huge!

Please be aware that "ALWAYS wear high-voltage insulated gloves when diagnosing the Hybrid System."
http://www.autoshop101.com/forms/Hybrid15.pdf

Ken@Japan

 

You can buy insulated gloves at $100.
You can not buy your life at any price.

Ken@Japan

 

Ken,
You will not be connecting a 300V battery to a 200V battery. The Enginer battery is 51.2V Nominal. The converter boosts the voltage to 240V. The two systems are isolated by a transformer in the converter. The connection to the DC bus is made with the service plug disconnected. The voltage at that time is zero, confirmed with a Voltmeter.

Dan,
To answer your two questions:

When using a voltmeter, you are not closing a circuit, you are measuring a voltage difference. Many voltmeters have settings up to 600V. I have used a $5 voltmeter to measure the DC voltage on the Prius Battery. I recommend spending more for a better one however.

See posts #12 and #13 by miscrms.

 

Voltmeters are designed with very high internal resistances, usually on the order of 1Mohm. As long as the impedance of the circuit under test is much lower than this it will appear as if what you say is true. 600V/1MOhm < 1mA, which is a very reasonable amount of current for even a cheap meter to deal with.

On the question of whether ~50V is low enough to not need to take safety precautions, consider what happens when you place a screwdriver across the terminals of a 12V car battery. You will get a very impressive fireworks show, weld the tool to the battery terminals until the terminals melt off, spray boiling sulfuric acid all over the place and set any flammable objects in the vicinity alight. Alternatively, you could place the same screwdriver across the terminals of a small capacitor charged to 600V, and see nothing more than a small spark. The Voltage of a system is important, but so is its capacity and resistance. In other words its ability to supply current. Li-ion cells are inherently dangerous by virtue of the very large amount of energy they pack into a very small package.

It is true that it generally takes a higher voltage to kill a person, but remember its current that kills, not voltage. It takes less than 100mA through the heart to kill. The voltage required to produce this current is a function of where the two contact points are, humidity, how hydrated you are, salinity of your blood/fluids, how sweaty you are, etc. Here is a brief discussion of this issue:

Electrical Safety: The Fatal Current

Also bear in mind that while the supplemental side of the system is only ~50V, you are working in an area where 110V AC line voltages and a 240V traction battery can be present. Insulating gloves, and insulated (or dipped) tools should always be used to prevent serious damage to your vehicle or yourself. Better safe than sorry when it comes to electricity IMHO.

Insulating gloves can be found online for $50 or so, there are a few pairs on ebay for $19 at the moment that would probably be fine. There are also rubber dips you can buy to insulate the handles of your existing tools inexpensively, rather than having to buy new tools.

Rob

 

That sounds about right. To say it another way, a coulomb counter computes the integral of current, with respect to time or the area under the Amps vs. time curve. With lead acid, Voltage is a fairly linear function of SOC. For that reason you can pretty much just get away with charging and discharging to certain preset voltages, maybe just adjusting a bit for temperature if you want to get fancy. Lead acid is also very tolerant of, and in some senses likes overcharging. Neither of these aspects are true of NimH or Li-ion. They maintain a very flat voltage curve vs. SOC, which is generally very desirable, but it does cause problems when trying to determine SOC. Both are also very unhappy about being overcharged, and can go into a runaway mode where voltage actually decreases beyond 100% SOC. This mode was the cause of the famous Sony li-ion laptop battery fires. These are just a few of the reason that Coulomb counters are employed in many battery management systems. The idea is if you know how much charge was taken out, then you know how much to put back in. In general it works quite well.

Yeah, this is another example of different industries inventing their own measurement units. Battery vendors often only show one representative curve for a whole family of batteries, and so they use relative Cs instead of absolute Amps. Its also a handy figure of merit for comparing different batteries. For example, the Prius NimH can put out 100A which doesn't necessarily sound that impressive until you realize that this is >15C! Many Li-ion cells max out at 3C discharge, good ones often are ok up to 10C but likely with some reduction in lifespan.

Thats the basic idea. The two disparate voltages equalize because neither is infinitely powerful. Think of the simpler case of a car alternator. When it is running free with no load applied, its putting out maybe 15-16V and has minimal drag on the engine. This is referred to as the open circuit voltage (Voc). You'll see a lot of generators, including solar panels spec'd this way. As soon as an electrical load is applied, things change. The alternator is now having to put out current, or having to do work. Correspondingly its load on the engine increases, and the engine computer has to increase fuel injected to maintain idle rpm. This increase in fuel consumption is where the extra energy comes from to supply power to the load. Lets say the load applied is the battery, which is sitting at 12V. Ignoring second order effects, the basic situation is Voc of the alternator, minus outgoing current * internal resistance of the alternator, minus current * resistance of the interconnect bus = the voltage at the battery. We also know that the Voltage at the battery should be its Voc, often called resting voltage, plus the incoming current * the internal resistance of the battery. Combining these two equations, assuming you know all the resistances, you can solve for the equilibrium voltage at each end of the bus and the current flow through the bus.

The same basic idea holds in the Prius, depending on whether the voltage is higher at the battery (drive) or at the MG (regen). What makes the Prius more complicated is the inverter in the middle in both directions. In a simplistic sense you can think of the inverter as a transformer with a variable ratio. When the SOC of the battery gets low, it can adjust the ratio such that the voltage at the battery side is high even when the MG voltage isn't. Similarly it can make the voltage at the motor side as high as 500V during electric drive, even when the loaded battery voltage is under 200V. While it may seem like magic, energy must be conserved. In other words V1*I1 must equal V2*I2 as in a transformer, so when voltage is boosted, output current is lower than input current. This is basically how the Prius HV controller can "decide" how to manage the battery system (IE when to allow EV mode, when to go into warp stealth, etc) somewhat independently of what else is going on. In reality of course its much more complicated than this, but the analogy is a decent one I think. Pulse width modulated AC controllers are very complex little beasties, similar in many basic respects to a multiphase switching power supply. Essentially it generates a variable frequency AC sine wave (or multiple sine waves with an exact phase offset), and then superimposes a high frequency variable duty cycle on top of that to modulate its effective voltage. The Prius inverter also incorporates whats referred to as a boost converter, so that it can convert voltages up, not just down. I believe the Prius inverter is also bidirectional, so that it is functional both during drive and regen. There is some basic info at the wiki page below, as I recall the Prius inverter is a three phase most similar to the "space vector modulation" case. The MGs are of a permenant magnet type.

Pulse-width modulation - Wikipedia, the free encyclopedia
[ame="http://en.wikipedia.org/wiki/Motor_controller"]Motor controller - Wikipedia, the free encyclopedia[/ame]

TechOnline | Prius inverter/converter: A power broker
Prius inverter

Its ok for the meter due to its high internal resistance. What you do have to be careful of is accidentally shorting the high V to ground with the probe tip. That's where you could cause big trouble. The meter will be rated for a max deltaV, but its usually 250-1000V even for cheap meters.

You are absolutely right, battery based systems (like solar and other "always on" systems) present special challenges. There are a lot of safety precautions you can take, but the system can never really be "off". Breaking the circuit at multiple points is a good start. This limits the amount of voltage that can develop in any one area, and avoids having any one place where a single mistake can result in a large voltage being shorted. This is part of the reason why the Prius safety disconnect is in the middle of the pack, and why there are HV contactors (big honkin' relays) that disconnect both the positive and negative side of the HV battery. Its also part of the reason the HV system ground is isolated from the car chassis ground. Ideally if you accidentally were to short from an HV line or internal battery bus bar to a metal point on the chassis nothing should happen as there is no return path to the battery. I wouldn't recommend trying it though... In general, wear isolated gloves, use insulated tools (both to protect you and reduce the chances of accidentally shorting something), always use your meter to make sure lines you think are off really are and never hesitate to double check, avoid wearing any metal jewelry. As much as possible avoid situations where one hand is in contact with a high voltage, and the other near a low voltage. The biggest danger is current flow from one hand to the other through your chest stopping your heart. As mentioned above, the lower voltages of your pack probably aren't high enough to kill you, but why take a chance? Either way it still has the capacity to do a lot of damage. All connections in the string need to be tight, need to be double checked, and need to be protected against loosening due to vibration or thermal cycling (lock washers and/or thread lock are a good idea). You may be familiar with the failure of a Hybrids Plus conversion a while back that resulted in a fire destroying the car (no one hurt thankfully). The cause was found to be a fault in a single HV connection where the lockwasher had been installed incorrectly. Basically treat the HV system with the respect you would the gas tank.

Not silly at all. I'm an EE and this stuff is different enough that I've had to study it quite a while to understand it. I still get confused about things, particularly when you start getting into the details of compex components like the inverter, or the non-idealities of the batteries. Like how exactly a 20Ah lead battery can only put out 20A for about 30 minutes ;-)

Rob

 

How do you serially wire up 16 3.2V batteries safely? - I'm thinking the first few would be ok, but as you added each new battery to the chain isn't the voltage your dealing with getting higher and higher. Seems the batteries in a PHEV kit are always "hot" or "live". Any other precautions

Very carefully! I once had to connect 5 gel cell deep cycle batteries in parallel in my RV. Due to the physical layout and location I was unable to do only the hot side, then ground and instead had to do both on each battery as I went. The first and second weren't bad, but then I began thinking about the current available should I slip and short to ground. Considering that each battery was probably quite capable of putting out at least 1000A (5000A total, in parallel) the 4/0 wire would probably vaporise in my face, along with a good chunk of whatever I grounded to. Not a pleasant thought, and one that caused me to insulate all surrounding metal before continuing. The fact that I was only dealing with 12V wasn't much consolation.
Never forget that amperage as well as voltage can be dangerous. Amperage coupled with low voltage probably won't kill, but it can and does disfigure, burn and blind.