[Anthony]

- If the DCDC is connected to the same busbars as the motors and HV battery, how do we prevent the DCDC drawing current while regenerative braking? Is there any other, simpler way to connect the DCDC to the HV Battery?

-How important is an HVIL system? How do we go about making it simple and easy to control?

 

[john61ct]

Why would you want to power down the DCDC just because you're braking?

 

[Anthony]

1. The plan was have all the regen go back to the battery.
2. I want to ensure electrically that the DC/DC will not receive more current than it can handle. My hunch is that it won't draw more than it needs, but not sure if that is true.

I am thinking if regen generates say 50A, the battery takes 30A and DC/DC draws 10A, where does the rest of the 10A go? Would this be the only case where DC/DC would drain more current than it can handle and an electrical system to prevent this is necessary?

I apologize if any of my thinking/assumptions are wrong, as electrical is not my major.

 

[frodus]

What DC-DC converter? Some have a build in Diode to protect. That being said, you wouldn't force current into it, it only draws what it needs.

In your example, the battery takes 40A, and DC-DC draws 10, as long as the controller dictates that, and there's no other controls in the inverter causing a reduction in current. i.e. when you get to the upper voltage on the controller setting, it usually reduces output current accordingly, so it won't overcharge.

 

[john61ct]

DCDCs usually limit current themselves, in fact that's often the main reason to install one even if the Vin = Vout

So should be no worries about OCP there.

DCDCs often work in scenarios where charge current is coming in while loads are being carried, one may varying, greater current than the other, swap places where going positive vs negative direction at the bank from one second to the next, no worries.

Not to mention the loads carried by the DCDC may be essential to operations, even safety, no?

I'm thinking you're looking for a solution in search of a problem.

I am thinking if regen generates say 50A, the battery takes 30A and DC/DC draws 10A, where does the rest of the 10A go? Would this be the only case where DC/DC would drain more current than it can handle and an electrical system to prevent this is necessary?

Remember, sources do not "push" current, just make it available.

Loads "pull" the actual amount they need.

So hypothetically, if regen is capable of putting out 50A, but the battery is at such a high SoC that it "can't accept" (won't draw) that much, then some power must be shunted to resistance or otherwise burnt off, or the braking power might be too low?

Think of a big 300A alternator in an ICE vehicle; with no loads it offers barely any friction resistance. Turn up a big enough stereo and suddenly engine power to the wheels is noticeably reduced, maybe a 20HP of physical load diverted.

 

[brian_]

As already explained, the DC-to-DC converter's current consumption has nothing to do with what is happening on the supply side, whether the battery is charging or discharging, or what is charging it.

There is no dumping of power anywhere. A battery doesn't determine or limit charging current - it just has a voltage which determines how much voltage is required to charge it. The battery is protected by a BMS, which probably won't cut off charging based on current while the vehicle is in driving mode. It is also protected by the controller in a properly integrated EV, which will not allow regenerative braking to charge the battery at an excessive rate. If the controller limits regeneration, there is just less braking effect. It is possible to dissipate power in a huge resistor, and some vehicles without batteries (such as locomotives) do that, but it would be nuts to do in an electric car.

 

[john61ct]

A battery doesn't determine or limit charging current

I have no idea how anyone can think this is true.

It is exactly the battery chemistry that determines the charge rate, when the source makes available more than it can accept.

For example quality AGM will draw up to 0.8C, while FLA can struggle to pull 0.2C.

LI pulls so hard it can easily burn out a stock alternator setup, in fact a current limiting device is often required in order to protect both the source and the bank.

 

[brian_]

I have no idea how anyone can think this is true.

Apparently you don't understand what I'm saying, so I'll try to express it more clearly.

It is exactly the battery chemistry that determines the charge rate, when the source makes available more than it can accept.

For example quality AGM will draw up to 0.8C, while FLA can struggle to pull 0.2C.

LI pulls so hard it can easily burn out a stock alternator setup...

Yes, the characteristics of the battery determine the rate at which the battery will charge for a given applied voltage, just as the resistance of a heater element determines how much current goes through it when plugged into a constant-voltage power source. The design and construction of the battery determines what is acceptable, but it doesn't have any way to control the charge or limit it to a safe level. It's like adding air to a balloon: the size and thickness of the balloon determines how much will fit, how big it gets at a given pressure, and what pressure can be tolerated, but the balloon has no way to keep someone from over inflating it and popping it.

... in fact a current limiting device is often required in order to protect both the source and the bank.

So, we agree that something external to the battery must control the charging to avoid charging at an excessive rate (expressed as current or power), and to avoid charging to an excessive voltage. During normal plugged-in charging, that's the onboard charger or (in the case of fast DC charging) the EVSE, possibly with instruction from the BMS. During regenerative braking, that's the controller.

If you use a large motor capable to producing a thousand amps and spin it fast enough to drive 500 volts, run it in regeneration with a controller having a current limit set to that thousand amps, and connect that directly to a salvaged Leaf battery than can handle a couple hundred amps and less than 400 volts, then apply full regen braking, the battery will be destroyed... because the battery itself has no way to control charging rate or voltage. As the designer of the vehicle, it is your responsibility to ensure that all of the components work together properly so that each component only handles the voltage and current that is safe for it.

 

[john61ct]

Your statement does certainly hold true for the LI chemistries currently in use with mainstream EVs. But not all chemistries! Assuming voltage is well regulated and the source has a reliable stop-charge algorithm, just talking here about current rates.

we agree that something external to the battery must control the charging to avoid charging at an excessive rate (expressed as current or power)

It is true that bare cells do not explicitly "control" the amps rate the way a current-limiting source or intermediary circuit does.

And most LI chemistries do require such circuitry to avoid pulling current at a too-high rate harmful to longevity.

But I believe LTO does not, and GEL is the only **lead** chemistry that does.

AGM and FLA batteries are indeed self-limiting, do not need any other current limiting other than their internal resistance, will not be harmed by a source "offering" even a 50C rate.

 

[cts_casemod]

The only reason you would want to disable the DC-DC converter was if the regen voltage exceeded the maximun input for it.



For example: My DC-DC was rated for 450V operational and 500V survival. So it would go off automatically during periods of heavy regen. But this was automatic. I didn't had to turn it off.



Another thing you need to take into account is how to stop regenerating if the voltage exceeds a certain amount. You either have to stop (and in this case think how the sudden loss of braking power would affect the vehicle) or dump the excess energy somewhere, like a PWM controlled resistive element.



Also mind you. On my car regen only ever accounted for up to 4-6% gain, when used in town. I found it more useful as a brake assist that from any gains obtained with it. Often one can save more battery by predicting the traffic and keeping a smooth constant speed.

 

[brian_]

AGM and FLA batteries are indeed self-limiting, do not need any other current limiting other than their internal resistance, will not be harmed by a source "offering" even a 50C rate.

I think it's very confusing to suggest that a source "offers" a charge rate. Yes, any source has limits of both current and voltage, but those limits don't matter until they are reached; at any moment, one of those two settings or capability limits is controlling what the source provides.

If you have a light bulb plugged into a socket in your house, it doesn't matter if the house if powered by a small portable generator or the entire regional grid, the same current flows, because the current limit of the source (what it "offers") is irrelevant - the source voltage and load impedance determine the current.

A typical automotive starting battery has a capacity of about 80 amp-hours. A 50C charging rate for that battery would be 4,000 amps. I suggest not trying to charge one at 4,000 amps, which would require a substantial voltage to overcome internal resistance. 5C would be a routine starter motor current requirement; that's not a problem, at least for discharge.

 

[john61ct]

I think it's very confusing to suggest that a source "offers" a charge rate.

Nope it is clarifying reality.

The load determines the rate, only pulls a certain amount. There may be 500A available, but that 80Ah FLA battery will accept / draw no more tha 140A or so,

it is 100% impossible for FLA to ever go higher than 0.2C

and that only if well depleted, and then only for a short time befor rising resistance lowers the current rate as SoC rises.

Now an LFP bank will draw a much higher rate, more than is healthy for longevity or even safety. That is why current limiting VR or a B2B charger is required, perhaps also to protect the alternator or power supply.

It is relatively rare for a source to be current limiting, especially so for that limit to be adjustable.

 

[brian_]

The load determines the rate, only pulls a certain amount.

That's why what the load "offers" is confusing - it encourages people to think that a charger can force a specific current independent of the charging voltage.

There may be 500A available, but that 80Ah FLA battery will accept / draw no more tha 140A or so,

it is 100% impossible for FLA to ever go higher than 0.2C

and that only if well depleted, and then only for a short time befor rising resistance lowers the current rate as SoC rises.

"Available" is a much better term.

In this example, the FLA battery charges at 140/80 = 1.75C... so clearly more than 0.2C is possible. Is that just a typo?

Rising state of charge reduces charging rate for a fixed charging voltage, but to assume that this is some self-regulated mechanism also assumes a specific charging voltage. Common automotive and recreational battery chargers step the charging voltage down after a specific voltage and time combination is reached, because at their highest charging voltage the battery would eventually be destroyed... not by high current, but by excessive voltage. Apply 100 volts to a nominally 12 volt flooded lead acid battery, and this notion of nicely controlled and low charging rate will go out the window.

It is relatively rare for a source to be current limiting, especially so for that limit to be adjustable.

On the contrary, it is completely normal. All but the very cheapest automotive battery chargers not only limit the bulk charging stage current, they have a configuration switch to set it to a current appropriate for the battery size. EV chargers not only limit the rate, but they can routinely change the current limit to suit the available power supply and battery conditions.

Here's the first reasonable discussion of EV charging rate control that I found in a few seconds of searching:
How DC Fast Charging Really Works and an Intro to Charging Curves

 

[john61ct]

That's why what the load "offers" is confusing - it encourages people to think that a charger can force a specific current independent of the charging voltage

bulldust. I offer you 5 kg of raw oysters every 10min. Up to you how many you eat.



> FLA battery charges at 140/80 = 1.75C... so clearly more than 0.2C is possible. Is that just a typo?

Yes, my bad, 12-14A max, for maybe 15-20min, then declining.

AGM maybe 50-60A, some lithiums 400+A.



Any charging setup that does not cap voltage to a fixed maximum is faulty, even dumb PSUs do that. Spurious issue, obviously nothing to do with the topic at hand.


The context of my correction here was much more general than EVs, I already noted that it does apply to current mainstream EV tech, just not true about charging batteries in general.