Please help me understand something in a vehicle electrical system.

Thread Starter

Tweaked250r

Joined Jul 1, 2021
4
Hi. I’m not an engineer or anything like that. But I am losing sleep over something, trying to understand it.

I am installing an aftermarket ECU module (engine control unit) into a car. It is called a Holley terminator.

So, Holley explicitly states that the main power wire and ground both, for the ECU absolutely MUST be connected directly to the battery cable lug, right on the battery. They claim that it can not be connected to a power junction or buss bar, that has anything else attached to it. Holley claims that this is because of “flyback and transient voltage”. But the thing is, no other factory car manufacturer connects their ECU directly to the battery. They all connect their ECU’s to a power junction stud in the under hood fuse box, which is directly connected to many other loads. No manufacturers are having a problem by doing it this way.

so I have been researching transient voltage in 12v dc systems for days, but I simply can not see why there would be any difference at all from a direct to battery connection, over a junction stud or bus bar connection. How would connecting directly to the battery “protect against flyback or transient voltage” what-so-ever? It’s still the same circuit, and the same main power wire. I could find no reason for this, after searching for days. Even if the Holley ECU did not have TVS devices protecting the ECU, how would hooking it to the battery protect it at all?
I have connected two diagrams. One Holley says is good. The other Holley says absolutely do not do.
 

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drc_567

Joined Dec 29, 2008
1,156
A possible alternate modification, for the off-battery installation, would be to install a pair of 'Decoupling Capacitors' directly on to the power leads of the ECU module, typically a 10 μF electrolytic, and a 100 nF ceramic capacitor. ... Be sure to observe the polarity mark for the electrolytic capacitor, and allow for a sufficient voltage rating.
... A call to the Holly support line might confirm this idea.
 

Ian0

Joined Aug 7, 2020
3,754
My guess is that after they sold the first few, they discovered that they had not designed it with adequate power supply transient protection, and in order to avoid having to spend money retrofitting suppression components, they are leaving that job to the user. There are probably fewer destructive voltage transients the closer you get to the battery.
The car manufacturers use ECUs that have been tested properly - makes me wonder what other "features" yours has that have not been adequately tested. Does your insurance company know you are fitting it?
 

Thread Starter

Tweaked250r

Joined Jul 1, 2021
4
My guess is that after they sold the first few, they discovered that they had not designed it with adequate power supply transient protection, and in order to avoid having to spend money retrofitting suppression components, they are leaving that job to the user. There are probably fewer destructive voltage transients the closer you get to the battery.
The car manufacturers use ECUs that have been tested properly - makes me wonder what other "features" yours has that have not been adequately tested. Does your insurance company know you are fitting it?
It’s for a dragrace car. So it’s going to be off-road.
 

shortbus

Joined Sep 30, 2009
8,952
OEM's know what has been used in their systems. As an add on to something old that may have been modified by some unknown way, which most cars using these have, there is no way of knowing what can happen electrically.

Also it is more for the reason to wire right to the battery so Holley or any of the other makers know that the "constant on" power is a constant on. To keep the memory settings in the ECU. Most of the cars I'm familiar with do have a direct connection to the battery in the under hood fuse box, whether you believe it or not. Most or at least GM cars that is also called a "direct buss" fuse box. Heavy gauge bare copper wire is routed in them, through passages in the plastic base. I worked where they were made.
 

LowQCab

Joined Nov 6, 2012
1,308
Hot-Rodders have a bad habit of doing some really stupid stuff,
especially when it comes to Electrical-Installations.
This is just "Anti-Stupid", "You didn't follow the instructions, so there's no Warranty", stuff.

A Big-Fat, 500,000uf, Car-Audio-Capacitor, works wonders,
they're cheap at Walmart, and some even come with a built-in Volt-Meter.
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GetDeviceInfo

Joined Jun 7, 2009
1,920
so I have been researching transient voltage in 12v dc systems for days, but I simply can not see why there would be any difference at all from a direct to battery connection, over a junction stud or bus bar connection. How would connecting directly to the battery “protect against flyback or transient voltage” what-so-ever? It’s still the same circuit, and the same main power wire. I could find no reason for this, after searching for days. Even if the Holley ECU did not have TVS devices protecting the ECU, how would hooking it to the battery protect it at all?
I have connected two diagrams. One Holley says is good. The other Holley says absolutely do not do.
Did you include starter operation. Many starters will put some nasty stuff on the line, just as the ECU is preparing to commence. Guess that's the difference between performance.
 

Thread Starter

Tweaked250r

Joined Jul 1, 2021
4
Did you include starter operation. Many starters will put some nasty stuff on the line, just as the ECU is preparing to commence. Guess that's the difference between performance.
this is what I’m not getting I guess.
Let’s pretend the starter puts noise or feedback, or dirty voltage or whatever you want to call it, back to “the line”.

why would it matter where the connection is, on “the line”? Why would it make any difference being on the battery lug, compared to 3 feet away, on the same exact heavy gauge wire. Or another way to think of it... the battery positive main wire hooks directly to the starter solenoid. What would be measurably different from hooking to the batt lug, compared to hooking to the starter solenoid stud. It’s the same exact wire. That’s what I don’t understand. Voltage spikes, drops, noise, etc etc will all be the exact same, on the same main power wire, no matter where you measure it on that main power wire, right? Or is that wrong?
 

Thread Starter

Tweaked250r

Joined Jul 1, 2021
4
All wires have Resistance, no matter how big they are.
When Current is drawn though a Wire, a Voltage-Drop occurs.
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Yes I understand. I had originally though that is why Holley has said to connect direct to battery, and were basically saying “we don’t have faith in you people to gauge the wiring correctly”, but they claim that isn’t why. In fact, they claim you can actually use a junction stud, to hook the ECU to, but “only if the ECU is the only load connected to that junction stud”, which doesn’t make sense.


This is a link to a video, where the Holley tech guy explains why “transient” voltage is the reason to connect to the battery. It starts at 7:15 minute mark.

But after reading about transient voltage and load dumps, I don’t see why there would be any difference, still. If there is a voltage spike for example at the main power wire junction stud, it will also be measurable directly on the battery lug correct?
 

LowQCab

Joined Nov 6, 2012
1,308
No, not correct.
The Battery is not only the source of the Power, it also has a huge amount of Capacitance.
It's probably up over 1-Farad, this absorbs Spikes.

If You can't wrap your head around this, just follow the instructions from Holley.
There are really good reasons why they tell You to go straight to the Battery, believe them.
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Ian0

Joined Aug 7, 2020
3,754
The battery also has impedance, probably a few tens of milliOhms, whereas the cable will probably have slightly more resistance, but also some inductance. It's a simple potential divider, the size of spike is reduced by Battery_Impedance/Cable_Impedance. It couldn't eliminate it completely unless the battery impedance was zero, but it does reduce it.
Would I also assume that for drag racing, you have stripped out a lot of extraneous electrical equipment? (Such as electric locks, electric window motors etc.) That will make a big improvement to noise on the electrical system.
 
It's hard to put every ground/return path wire to "this one screw". You have to make compromises.

You still want one point to be the reference, and that's like the negative teminal of the battery, but it really doesn; have to be there.

We play tricks and make sensors not dependent on ground.

You don;t want ground loops, so you design them out. it's EASY for Holley to say do it this way which may look like a mess.
They may not have put the effort into doing it so things also look nice.

You separate dirty from clean stuff.

The battery in a foreign car:

Negative to the engine block. Negative to the frame.

Hot on the battery has a cable tot he starter and another stud for the rest of the electronics. You can disconnect everything, but the starter cable by taking off one wingnut.

In a process system, I made a mistake. One instrument was really cool. It allowed you move the reference point o it was "at the computer" and no somewhere else.

It's so easy to do with isolated parts, but it gets expensive,but sometimes you need them.

You have 8 power supplies and they are controlled with a 0-5V signal. the 0 is relative to the positive terminal of the power supply and each supply could be at 0-40V relative to earth. It messes things up.

Current is measured across a resistor. The resistor is in the positive output. The power supply can deal with this independently, but now put a computer in the mess.
 
The real problem is probably too many people had the customer service reps beating their heads against the wall trying to figure out what was happening. After having them sent back and checked they either found something cooked due to poor installation or no problems are all besides a poor install.
 

LowQCab

Joined Nov 6, 2012
1,308
The Engine-Block in my Truck is the "Star-Grounding-Point" for every major Power consumer.

I have 4-10,000uf Capacitors stashed where the Passenger-side Air-Bag used to be,
for Filtering major Power-Distribution-Nodes.
This is partly because my 2-Batteries are in Plastic-Battery-Boxes in the Bed of the Truck.

I also have 2-2ga. Ground Cables straight to the Engine Block, my Starter is soooo happy.

4- Cooling-Fans, the AC-Blower-Motor and Compressor-Clutch,
the Windshield-Wipers, and the Anti-Lock-Brake-Unit,
all run off of individual 100-Amp, 3-Phase-Rectifiers, With Capacitor smoothing,
directly from the Alternator-Windings,
with no Battery connections.
They don't work unless the Engine is running.

The only problem I've had is that, when the Windshield-Wipers are set to "Intermittent",
and I shut-down the Engine,
the Capacitors will make the Wipers jump half-way across the Windshield and stop.
(I'll fix it sooner or later) (probably never)

The 1000-Watt, 120V, Inverter is mounted about 2-feet from the Batteries inside my Tool-Box
to keep that high-Current run really short,
and keep the Inverter happy in a convenient, cool, and dry spot.
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Ian0

Joined Aug 7, 2020
3,754
The Engine-Block in my Truck is the "Star-Grounding-Point" for every major Power consumer.

I have 4-10,000uf Capacitors stashed where the Passenger-side Air-Bag used to be,
for Filtering major Power-Distribution-Nodes.
This is partly because my 2-Batteries are in Plastic-Battery-Boxes in the Bed of the Truck.

I also have 2-2ga. Ground Cables straight to the Engine Block, my Starter is soooo happy.

4- Cooling-Fans, the AC-Blower-Motor and Compressor-Clutch,
the Windshield-Wipers, and the Anti-Lock-Brake-Unit,
all run off of individual 100-Amp, 3-Phase-Rectifiers, With Capacitor smoothing,
directly from the Alternator-Windings,
with no Battery connections.
They don't work unless the Engine is running.

The only problem I've had is that, when the Windshield-Wipers are set to "Intermittent",
and I shut-down the Engine,
the Capacitors will make the Wipers jump half-way across the Windshield and stop.
(I'll fix it sooner or later) (probably never)

The 1000-Watt, 120V, Inverter is mounted about 2-feet from the Batteries inside my Tool-Box
to keep that high-Current run really short,
and keep the Inverter happy in a convenient, cool, and dry spot.
.
.
.
Is that to power your 1,000,000 Watt audio system for your drum-and-bass music?
 
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LowQCab

Joined Nov 6, 2012
1,308
Nah, it's only around ~400-Watts total, it's not a Circus-Wagon.
I'm more into truly High-Fidelity, think "Rush-Tom-Sawyer".
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nsaspook

Joined Aug 27, 2009
8,898
We need to be careful on how we characterized a battery with a capacitor. First neither stores charge, they both store energy but in different ways.

A typical car battery might have a high 'equivalent' (the capacitance required to store x kJ of energy at x V) capacitance for its stored electrochemical energy but the actual electrical capacitance (separate conductors separated by a dielectric) is not very high. So it's not a capacitive spike filter. It's a low impedance voltage source that stabilized changes in voltage by the ability to source or sink high levels of current from the electrochemical reactions keeping the cell redox in equilibrium.

A D-cell battery has a capacitance of less than 20 microfarads. It has a 'equivalent' capacitance of ~300 Farads if we take a similar sized supercapacitor.

https://www.sciencenewsforstudents.org/article/explainer-batteries-capacitors
Energy can be stored in a variety of ways. When you pull back on a slingshot, energy from your muscles is stored in its elastic bands. When you wind up a toy, energy gets stored in its spring. Water held behind a dam is, in a sense, stored energy. As that water flows downhill, it can power a water wheel. Or, it can move through a turbine to generate electricity.

When it comes to circuits and electronic devices, energy is typically stored in one of two places. The first, a battery, stores energy in chemicals. Capacitors are a less common (and probably less familiar) alternative. They store energy in an electric field.
 

LowQCab

Joined Nov 6, 2012
1,308
We need to be careful on how we characterized a battery with a capacitor. First neither stores charge, they both store energy but in different ways.

A typical car battery might have a high 'equivalent' (the capacitance required to store x kJ of energy at x V) capacitance for its stored electrochemical energy but the actual electrical capacitance (separate conductors separated by a dielectric) is not very high. So it's not a capacitive spike filter. It's a low impedance voltage source that stabilized changes in voltage by the ability to source or sink high levels of current from the electrochemical reactions keeping the cell redox in equilibrium.

A D-cell battery has a capacitance of less than 20 microfarads. It has a 'equivalent' capacitance of ~300 Farads if we take a similar sized supercapacitor.

https://www.sciencenewsforstudents.org/article/explainer-batteries-capacitors
You're absolutely right,
I should have said "acts-like" a huge Capacitor.
Never the less, the end result is very similar to having a huge Cap there.

Wow, I decided to see if I could find any info on SLA-Battery-Capacitance .....
One study found the "equivalent" Capacitance to be around ~750-Farads !!!!!
This was on a 65Ah, 12V, Car-Battery.
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Battery Model .PNG
 
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