Weird Battery phenomena

To make a long story short, the AGM batteries are an evolution of lead acid batteries with multiple thinner plates of leads. This provides more surface for the chemical reaction, and hence more cranking power.

The downside is that thinner plates means these batteries are more fragile and need more care to reach a similar lifespan.

 

As long as the battery can supply enough current (amps) to crank the engine, the voltage drop is not relevant. Useful life of a battery is based on the number of charge/discharge cycles. This assumes it it not allowed to either completely discharge or freeze. I have used Interstate batteries in several cars for years, and they have lasted 6 to 10 years with a Battery Tender attached,

 

This is true for our vintage cars but do not apply to the 458: there must be enough voltage so as to properly feed all the electronic modules and sensors of the car. Most sensors are using a 5v regulator so can cope with low voltage, but other sensors are not so tolerant and will throw tons of errors to the ECU.

I am not a fan of fancy batteries (AGM, spiral wound,... ), but on a modern car these are the way to go.

 

Cranking voltage [+50] to CU's are critical for healthy communication on the can bus..as indicated if the CU works off a 5v reference it will have no problem communicating while cranking even with a marginal healthy battery however if that same CU is 12v dependant then with that same marginal battery you may get cranking voltage to momentary drop below 10v which in most cases CU's check out...this is when error indications' take place.
In most cases but not all a restart clears all error displays since the battery has just received a charge from the alternator and voltage is slightly up..
Battery's from my experience can be perfectly good one day and fail the next, some even start the vehicle just fine but have a marginal CCA.
It is also my observation that F cars are a bit sensitive to these voltage variations...
Just a additional technical suggestion which I don't see mentioned we must also be sensitive to B+ Battery cable and battery ground voltage drops while cranking.
A excessive voltage drop in either cable could give a similar symptom.

 

Just to clarify... The ECUs are deriving 5 volts from 12 volts via some kind of regulator. It doesn't mean that the battery voltage can drop to, say, 5.5 volts and still allow the ECUs to function. Integrated circuit voltage regulators can generally handle a wide range of input voltages (e.g. some can handle 8 ~ 25 volts). Basic zener diode regulators may go lower, but there are current limitations.

 

Agreed; my vintage V-12 doesn't contain a single ECU. I'm perplexed that Ferrari would design such a complex electrical system that a slight drop in battery voltage during starting would cause the whole electrical system to shut down. If the ECUs need a constant 5 volts to function, there are voltage regulators that can store energy to deal with a momentary drop below 5 volts.

 

A car battery is 6 x 2v lead acid batteries in serie. A good agm cell has an internal resistance of 2milli-ohms. So 12 mohms in total. If you need 200Amps to cranck the engine, you will experience a 2.4v voltage drop. A regulat battery has an higher internal resistance so higher voltage drop.

Also in order to clear the dendrite buildup in the battery, AGM are overcharged (1v more), and spiral wound AGM (optima) even more.

So an AGM has higher resting voltage and less voltage drop, which result in more voltage when cranking.

 

Voltage is an expression of how much the electrons want to flow from point A to point B, sort of like the pressure in any water system.

When you turn the starter, the electrons see an open path and want to flow based on the load demands of the starter. It's like turning on a faucet.

However, if you have two different batteries - which each have different internal resistances of their own - that can be thought of like having two different size garden hoses. The larger hose (i.e. less restriction) can provide more pressure (higher voltage) when called upon, because the hose itself isn't as much of a limiting factor in the system - thus more flow will occur. When your battery has a better design and less internal resistance (kinda like having a better flowing garden hose), then by its very design there will be less internal restriction occurring when flow is needed.

More electrons flowing means greater current, which - due to Ohm's law - means a higher voltage level will be present in the system (since V = I * R). More restriction (in the form of the battery not being able to flow as many electrons due to internal resistance) means fewer electrons (lower current) will occur; this lower "I" [current] value will manifest itself as a lower voltage in the system, aka a higher "voltage drop" when measured.

Ray