Internal Combustion Starter Battery (Circuit)

[methods]

Schindler Engineering Adventures. . . in Waking up @ 0420

Half of a cup of coffee in here.



A mentor of mine has gone thru 3pcs of expensive Lithium Starter pack (Automotive Starter Application) and he asked me to put together a minimal protection circuit.

Background
Typically you want to use a 3.3V chemistry for this. LiFePO4 or equ work out to a good match.

3.3V * 4cells = 12V + 1.2V = 13.2V Nominal

Typical maximum alternator output == 14.4V

14.4V / 4 = 3.6V
Which just happens to be the fully charged voltage of a LiFePO4
(big surprise there, since they were DESIGNED as lead replacement)

By Contrast - Typical Lipo Hobby Pack
3 * 3.6V = 10.8V
4 * 3.6V = 14.4V

As you can see, a 3.6V chemistry is not a good match. 3 cells is too low. 4 cells is too high. You wont be able to charge the 4 cell pack up enough for the big win. . . and the 3 cell pack will sag too hard IMHO.



Above is proof that I have started PLENTY of engines using a standard 3.6V hobby pack.

In that picture a 5.8L V8 was used on an unbalanced and previously D-Tested hobby pack to the point where one cell lit off in an impressive and hair removing fireball. ... so ... in case you were wondering. . . Y E S ... you can get quite the fireball if you go about it pig headed.



Doint it Wrong!
Dont be that guy.

What you want to do, is be *this* guy (shown below)



So...
I burned down my clock on this looking for a single picture (of the blown out Lipo) but lets rattle off the requirements real quick.

* Needs to prevent LVC failure under Starting Conditions
* Needs to prevent LVC failure under trickle Conditions
* Does not need to limit current
* Does not need to limit over-charge
* Needs to be very simple and minimally integrated
* Can not be a solid-state circuit (tho it could be)
* Must not parasitic self-kill
*

We could go on like that for a while.



At around 0445 (4:45AM) I drew the first go at it.



What that picture above calls out is the only complicated part of the circuit. In order for the BMS to control the contactor, you need a Bootstrap Voltage.

* Bootstrap Voltage must drop out @ LVC
* Bootstrap Voltage must be current limited
* Bootstrap Regulator must have a very low quiescent current



We all know by now that we can easily drive the low side coil-control mosfet with minimal current (microamps) so... not a big deal. We wont go over that any further.

One may ask about the logic of the bootstrapping
* Getting it to boot
* Getting it to let go

I included that logic in the whiteboard shot. In prose

* Current Limited regulator output appears BEFORE the key
* Turning the key OFF removes bootstrap voltage from primary contactor



In order for the circuit to work, sufficient time must be allowed between the ACC and Start functions. This time is apx 420mS. Turning the key . . probably provides enough time. . . but the failure mode is

* Turn key too fast
* Bootstrap voltage over-drawn
* Primary contactor never latches

You can deal with that 4 or 5 ways, but the easiest is to just turn the key a bit slower. A primary requirement is that the entire system install where the old battery was. If you look closely at my schematic, the only contacts are:

* Primary Chassis Ground
* Primary Positive Heavy Lug
* Any circuit post-key switch (ON a preference over ACC... to allow heaters and such to run without power on the Coil)

IIRC -
ON means you can run the Radio, heater, lights
ACC means you have voltage on the coil and are ready to fire.

anyway



The BMS circuitry is trivial and could be hacked from pre-existing modules. The most important aspects are::

* Wont kill itself if left on

So ...
Lets go thru that really quick

Current flows thru two paths
Current can flow thru Primary Contactor and thru Bootstrap Voltage
Primary Contactor opens upon LVC
Bootstrap Voltage terminates upon LVC

... So Primary Contactor coil current... standard BMS low side switch. Nothing to think about there.

for the Bootstrap regulator
* Enable Line, usually low side
* Pull its ground, if it is DC-DC Isolated
* High Side Switch (I never use these)
... Lots of options...

So
Low risk on that part of the circuit. Not worried about it at all.

The Battery will be removed from service in the event that any individual cell reaches the pre-determined (phone app set) LVC voltage.

..
For the Contactor, standard 500V 2000A gas filled DC Contactor wired in the correct direction. We would PREFER that it never blow open under load. . . BUT IT WILL. . . as you are most likely to shit out during a hard start.

So
In addition to the BMS we need a little bit of Hysteresis to stop the contactor from clackering. Most BMS's will do this, there are two primary methods

* Wait until voltage climbs to a minimum level
* Wait some fixed period of time

Obviously, we prefer the time delay. . . as you can still bounce hard with just the Voltage. Both would be best, and that is easy enough.

-methods

 

[methods]

Breaking DC at high current causes terminal wear on Contactors.

Typically,
Many of these contactors are rated for only ONE break. . . but. . . that is at max rating... so think something like 1000V or 2000V. At 12V and a few hundred amps, you can break full load many times.

Failure mode of the primary contactor (once points are destroyed) will be overheat to failure. For this reason, the primary contactor must not be co-located with the battery. It should be spaced off an inch or two, or have a good thermal firewall between its body and the body of the Battery and BMS.

The Battery itself. . . this can be as small as 5pcs of 18650

I recently bought 2pcs of "Car Starter" from Costco. Inside, not much battery. The whole thing could fit in the pocket of your sweatpants. So... not super worried about fussing over minimal Ah ratings. For this application, we need::

* Minimal Ah rating for desired runtime
* Minimal C rating for Starting 5 times back to back

The BMS needs to pick up the cell pack temperature to limit overheat failure. This is easy and common. A piggyback circuit can accomplish this. The cheapest and easiest is a "Thermal Switch".

... A thermal switch is a Normally Closed (NC) switch that Opens above a set temperature. For our application, this would probably be something like 85C. 60C would be cutting it too close, but that may be something to experiment with.

You would just wire the thermal switch inline with the primary contactor coil, right above the N-Channel mosfet (low side switch).

OR

Run a thermistor to the BMS

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[methods]

To deal with a Dead Battery.



You will manage that with something like what is posted above. They cost about $50 bones at Costco. You charge them up via USB from a ciggy-charger. They hold their voltage for 6mo or a year. They display state of charge.

* Works as a mobile power pack - can charge anything USB or even an Apple Laptop (via USB-C)
* Works as a flashlight
* Works as a Jump Starter

So - KISS
We dont have to solve the dead battery issue. You will treat dead battery exactly as you treat it with Lead Acid. All the same problems... like you might see with a Prius 2nd Gen.

...

In Prius 2nd Gen
If your battery dies
You have to

* Use the key on one of the front doors
* Climb into the hatch back
* Somehow lift the spare tire cover, and get the side panel to the battery off
* Somehow get jumpers down and inside of there

If your car was packed full of shit, hosed.
Anyway...

All modern cars allow you to enter with a mechanical Key. EV's do it different, but then.. they have a REALLY BIG battery that (even dead) could function the locks for 10 years.

and...
Nobody needs a starter battery on an EV.

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[methods]

To grossly over-simplify, and eliminate the BMS... one could::

* Take an off the shelf 4S LiFe Battery
* Attach to that a low end balancing unit that bleeds highest (AstroBlinky)

That covers balance of the battery, and the battery will "rest full" nearly all the time, so... that will work.

LVC function (well after Tzero) will then be a matter of LVC x 4, so @ Pack Voltage. This can be accomplished with a VERY simple circuit.

2.5V x 4 = 10V

You will want to stop a trickle discharge at or before 10V.
You will have to allow some sag below this during Starting (short Circuit)

I would set a comparator circuit to 10V, and have a capacitor on the output (time constant) that takes 5 seconds to glitch. 5 seconds is plenty of time to start a car and you really should not crank it longer than that.

I DO
ALL THE TIME...

But usually it is just lame wear and tear on the components. Makes much more sense to crank for a few seconds, walk away. Crank for a few seconds, walk away.

...

Alternately
One could have a simple Current Peak Detector... but... why make it complicated?

There are probably 6 dozen ways to manage false-dropout under starting conditions.

* Sag under start, recovers significantly
* Sag under leach, does not recover

So...
You can avoid this completely by just placing a timer on the circuit. WIth Voltage... you can detect

* Alternator Running
* Key ON

With either of those you can set a timer such that the battery turns itself off after some period of inactivity. This is frowned upon for drive reliabilty... but you can do it.

ON THAT NOTE


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[methods]

ON THAT NOTE

... of RELIABILITY

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[methods]

The most critical aspect of the system is NOT preventing over-discharge of the lithium pack. The most critical aspect of the system is maintaining UP TIME while the operator is in motion.

The circuit MUST NOT, MUST NOT, MUST NOT... under any circumstances... false trigger and remove the battery from the system.

... While driving
The Alternator will continue to provide the power necessary to motor

In a compound failure situation (which happens often)
Where the Alternator has failed (they do, about every 5 years)
In that case you are driving on nothing but battery and it is the batteries job to give you everything it has until it is destroyed.

...

Just like an Engine gives you everything it has until it is destroyed... so remember that.

-methods

 

[methods]

Autozone sells counterfeit car parts.
These parts are known to fail

* In 1 week
* In 1 month

Dont tell me otherwise, I have gone to the return counter plenty of times. So... It has to be assumed... that on any vehicle.... that the Alternator can and will go out at any time.

SO
For the Reliability calculation, we have to assume "Alternator down, motoring on battery alone"

... No warning lights
Car is responsible for showing a warning light

... Just... Keep power to the Coil and Injectors as long as you can. LVC out once no more can be given.

No false triggers!

-methods

 

[methods]

Things to consider

* Very high temperature - 125F day, inside of an Engine Compartment

* Very high and continuous Vibration - inside an engine compartment

* Very high chance of Liquid Water Ingress - 120mph wind, pressure washer status

* Very high chance of Solvants, Oils, Greases, Solvant Fumes... so... has to be totally legit

* Very high chance of Fire, so ... UL rated (or just Steel) components. You do not have to contain the fire.

* Very high chance of short-circuit ... People drop wrenches across the terminals all the time. I would hide the positive terminal behind the contactor, and expose only that. Lithium can dump a lot more power than lead can, as it does not sag as hard under extreme load. SIGNIFICANT power/heat can be produced. Many, many HP.

* Very high chance of severe Shock - people drop batteries all the time. People get into bumper-cars all day every day

* Very high chance of extreme cold - well below zero - it WILL go down below freezing, so think about that. Think about a battery warming button as well

Note: You can warm up a Lithium Battery by just running it against itself*

-methods

 

[99t4]

To deal with a Dead Battery.

You will manage that with something like what is posted above. They cost about $50 bones at Costco. You charge them up via USB from a ciggy-charger. They hold their voltage for 6mo or a year. They display state of charge.

* Works as a mobile power pack - can charge anything USB or even an Apple Laptop (via USB-C) OK.
* Works as a flashlight OK.
* Works as a Jump Starter NOT. At least, not in my (real world) experience. Not even close.