# What are the disadvantages of adding capacitors to car battery / alternator?

#### dante_clericuzzio

Joined Mar 28, 2016
246
I know a little about the capacitor will stabilize voltage flow and minimize the tiny fluctuation of voltage by the alternator. But i would like to know are there any drawbacks of adding capacitor for example electrolyte 4700uF to the battery (alternator)?

#### Papabravo

Joined Feb 24, 2006
19,874
I know a little about the capacitor will stabilize voltage flow and minimize the tiny fluctuation of voltage by the alternator. But i would like to know are there any drawbacks of adding capacitor for example electrolyte 4700uF to the battery (alternator)?
I don't think you would notice much effect in normal operation. The internal construction of a battery looks like a large number of parallel plates separated by an electrolyte. In the automotive environment, one of the big worries would be the "load dump" which happens when powered loads are disconnected from the battery while it is being charged by the alternator. Any capacitor in parallel with the battery would need to avoid an overvoltage failure during this time. I certainly would not risk the destruction of a $20,000 -$50,000 vehicle just to run the experiment.

#### ElectricSpidey

Joined Dec 2, 2017
2,281
IMHO placing a 4700uf capacitor at a car battery is pointless in regard to "stabilizing the voltage".

And JFTR voltage does not "flow".

Joined Jul 18, 2013
26,706
Is this in a automotive environment, if so there would be no advantage, if alternator used stand-alone, then maybe.

#### crutschow

Joined Mar 14, 2008
31,580
Do you have a noise problem you are trying to solve, or is this just an academic question?

#### dl324

Joined Mar 30, 2015
15,511
But i would like to know are there any drawbacks of adding capacitor for example electrolyte 4700uF to the battery (alternator)?
What are you trying to fix/improve?

#### dante_clericuzzio

Joined Mar 28, 2016
246
Do you have a noise problem you are trying to solve, or is this just an academic question?
Academic question for the sake of curiosity I am iniating the idea by looking the bad side of it first

#### dante_clericuzzio

Joined Mar 28, 2016
246
What are you trying to fix/improve?
Some people says it can stabilise the voltage fluctuation of the car alternator

#### ronsimpson

Joined Oct 7, 2019
2,627
The battery acts something like a capacitor. You would have to add enough capacitance to be equal to a car battery to make a difference.
I have worked on busses, trucks and train cars where the back end is a long way from the battery. In this case a capacitor in the back would make a difference, in the back.

#### LowQCab

Joined Nov 6, 2012
2,940
If You just want to quiet-down the usual noise,
a massive 2-Farad Capacitor, like used with extremely High-Power Car-Stereos,
will do a pretty good job, but only if it's connected with ~6-gauge Wire or larger,
and, You can't mount it under the Hood with all that Engine-Heat,
it has to be mounted inside the car.

Never use the car-body for a Ground connection,
and make sure that the body of the car is well connected to
the Battery-Negative, or the Engine-Block, or both,
with a heavy-Battery-cable and clean and tight connections.
.
.
.

#### MisterBill2

Joined Jan 23, 2018
14,280
The down sides of installing a 4700Mfd capacitor shunting a 12 volt battery in a normal automotive installation are the increased leakage because of the capacitor being in a hot location. The effective internal series resistance of a normal capacitor is much greater than the effective internal resistance of an automotive battery in good condition. That means that the capacitor will not help stabilize the voltage. But if the battery is an older second battery powering a high powered sound system then there may be a benefit. But that will really need to be a 4700 FARAD battery to benefit much.

#### dante_clericuzzio

Joined Mar 28, 2016
246
The battery acts something like a capacitor. You would have to add enough capacitance to be equal to a car battery to make a difference.
I have worked on busses, trucks and train cars where the back end is a long way from the battery. In this case a capacitor in the back would make a difference, in the back.
Very good -- i am looking for someone like you with real experience doing this...what is the capacitance value you are using to make it work at least i meant like for 12 volt car battery with alternator generating around 14.2 volt and fluctuate between 14.2 ~ 13.7 volts when most of the electronic are turned on especially the A/C.

#### dante_clericuzzio

Joined Mar 28, 2016
246
If You just want to quiet-down the usual noise,
a massive 2-Farad Capacitor, like used with extremely High-Power Car-Stereos,
will do a pretty good job, but only if it's connected with ~6-gauge Wire or larger,
and, You can't mount it under the Hood with all that Engine-Heat,
it has to be mounted inside the car.

Never use the car-body for a Ground connection,
and make sure that the body of the car is well connected to
the Battery-Negative, or the Engine-Block, or both,
with a heavy-Battery-cable and clean and tight connections.
.
.
.
Nice thank you for the good advice - I am taking it!

#### dante_clericuzzio

Joined Mar 28, 2016
246
The down sides of installing a 4700Mfd capacitor shunting a 12 volt battery in a normal automotive installation are the increased leakage because of the capacitor being in a hot location. The effective internal series resistance of a normal capacitor is much greater than the effective internal resistance of an automotive battery in good condition. That means that the capacitor will not help stabilize the voltage. But if the battery is an older second battery powering a high powered sound system then there may be a benefit. But that will really need to be a 4700 FARAD battery to benefit much.
Good insight ...thank you

#### MrSalts

Joined Apr 2, 2020
2,767
The AC is mostly powered mechanically (look at the serpentine belt on your engine pulleys) so, not much of an electric load.

Charging with clean DC is more likely to build sulfate bridges and shorten the life of a lead-acid battery. Note, I've never seen a battery charger with capacitors, just simple rectified DC from the transformer.

#### dante_clericuzzio

Joined Mar 28, 2016
246
The AC is mostly powered mechanically (look at the serpentine belt on your engine pulleys) so, not much of an electric load.

Charging with clean DC is more likely to build sulfate bridges and shorten the life of a lead-acid battery. Note, I've never seen a battery charger with capacitors, just simple rectified DC from the transformer.
Thank you for some of the realization.

#### MisterBill2

Joined Jan 23, 2018
14,280
Mr S makes a good point that I had not considered, about battery charging. But consider that the variation in battery system voltage is very well known, what sort of problem is there in having that variation? Any well designed 12 volt powered devices for automotive use can handle the variation quite well.
So I am wondering as to why the variation is a concern? All of the varied tape players that I have used in cars had enough motor speed control to function properly with those variations.
I did once ride in a vehicle that had a failed battery such that we always had to push start it. The headlights did dim and brighten with the engine speed. But driving with a failed battery is not close to normal operation.

Joined Jul 18, 2013
26,706
Some people says it can stabilise the voltage fluctuation of the car alternator
Modern regulators are pretty stable, any capacitor unless Really huge in value will be superfluous.
The voltage in a modern auto with motor running is ~14.5v

#### shortbus

Joined Sep 30, 2009
9,808
You guys are forgetting that after the engine starts and the battery is "topped off"(replace the charge used by the starter) the battery doesn't do anything. The higher alternator voltage makes the lower battery voltage moot.