Capacitor bank in cars-need clarification: life expectancy

+start car from capacitor bank

Hello,

Car batteries have high internal resistance so they surely must put a big capacitor bank after it (in parallel).

To get enough capacitance they would have to use electrolytics, but electrolytics have low life expectancy.......usually about 2000 hours at 85C.

At say 55C they would have a life of 16000 hours but this is still only 1.8 years.

So howcome we don't always hear of cars needing replacement capacitor banks?

Re: Cpacpacitor bank in cars

Car batteries have high internal resistance

Click to expand...

Not particularly. It's rather low, in a milliohm order of magnitude.

so they surely must put a big capacitor bank after it

Click to expand...

Never heard about this.

Cpacpacitor bank in cars

Hello FvM,

If you disconnect your car battery then reconnect it you will see a small spark as you connect..thsi is due to the inrush into the large capacitor bank.

Lead acid batteries as i remember it have shocking internal impedances, it is very high internal impedance?

surely this is still the case?

Cpacpacitor bank in cars

In modern cars, some capacitors are possibly present, distributed in several electronic systems. But no capacitor banks, as you imagined it.

A today's battery has to supply a current of 300 to 900 A (depending on the capacity) at 7.5V and -18 °C in the EN cold start test. At normal enviroment temperatures, it can easily supply above 1000 A. I won't name this a high impedance...

Re: Cpacpacitor bank in cars

I have seen capacitors banks only in cars with very large audio power amplifiers.

Re: Cpacpacitor bank in cars

Hello!

There is no "capacitor bank" in a gasoline car.
A capacitor or capacitor bank may be used to improve car stereo power supply,
that's all.

Now for the battery impedance, or let's say the internal resistance. It's
extremely low as said above. Look at you battery cables: depending on the
car, they can be, say 1cm or more in diameter, right? That's because the
current that flows in it is extremely high. Some car starters need about
250 amps, and for trucks it's probably around 1000.

Now if you have 12 V only, how can you draw, say, 240 amps (240 in order to
have easy calculation)? This means that the total impedance of the circuit
(impedance of the battery + impedance of the starter engine) should be
lower than 12 / 240, or 0.05 ohms.

Now what's the repartition?
If your battery has a higher impedance than the starter, then the battery
will consume most of the power. Now if the starter has a higher impedance
than the battery, that's fine. but it can be shown that the best combination
is when the battery and the starter have equal impedances.
And if you want to be sure that your car starts in winter at -20 degrees,
then it's better to be optimized. It's like in any system, the best efficiency
is when both impedances are the same. In the above case, we have therefore
impedances of 0.025 ohm (25 milliohms).

Now back to the capacitor bank: suppose you have a capacitor bank in
parallel with your batteries. If the cables are connected, both battery and
capacitors have the same voltage, right? Therefore they cannot cause
sparks. If you see sparks, then there is another reason.

Dora.

eem2am said:

Hello FvM,

If you disconnect your car battery then reconnect it you will see a small spark as you connect..thsi is due to the inrush into the large capacitor bank.

Lead acid batteries as i remember it have shocking internal impedances, it is very high internal impedance?

surely this is still the case?

Click to expand...