Lead acid car battery related questions

yourminky

Joined May 31, 2019
28
Here are 2 photos of the actual capacitor showing the capacitance on the label. See the ballpoint pen next to it to compare size.

mvas

Joined Jun 19, 2017
538

Typically, used near the Car Audio Amplifier, far from the battery.
What purpose would it serve being attached directly to the battery?

Infinion

Joined Apr 2, 2016
9
The capacitor I have is a Monster Cable High Performance Stiffening Capacitor with 1 million micro Farad 20 volt capacitance (which is 1000 Farad). I found this photo of a similar capacitor on the internet, mine don't have the 3 digit display segment on top. It weighs several pounds.
View attachment 179744
I was wondering if you already had a capacitor in mind at the start of the thread... It's helpful to include as much info as possible but that's fine.
This is an electrolytic capacitor. Leakage will be lower than a supercap, jpanhalt mentioned the typical leakage referenced from electronics-tutorials.ws above. Adding to Jpanhalt about the 1,000,000 uF confusion, here is a chart showing the prefixes for the metric system.

you see, micro is 1/1,000,000th of a number. So 1,000,000 microfarads is exactly 1 Farad.

The "Monster Cable High Performance Stiffening Capacitor" is meant for big car amps that power subwoofers, and are placed between the battery and the amp to improve the response time of your system, because car batteries, although powerful, have slow response times to very fast demands for power like what your subwoofer and amp require.

These big amp capacitors are placed in parallel with the battery by design. The way they want you to use them though is to have the capacitor as close to your load (the amp, or in your case, the starter) as possible, because the more wire length (and impedance) between the power source and the load, the slower that power source will respond to demands for power. That's why I suggested in my post to decouple the battery from the starter with a resistor, to put more of the task of starting on the other power source. But a parallel connection right on top of the battery is fine too.

So go ahead and connect it, but follow the instructions for charging before you connect it, because a 1F capacitor will act as a short circuit and pull several hundred amps for a short period, this is also why that product you have taken a picture of has a "charging cable" in the manual with a small in-line resistor in it.

Capacitance Calculations (stopwatch and multimeter required):
If you want to calculate the true capacitance of your capacitor, do the following:

the voltage-current relationship of a capacitor as seen on Wikipedia is defined by
Solving for capacitance, we arrange the formula and it becomes
So we need to measure and record these variables.
1. With a power supply or battery, charge the capacitor to any voltage (put a light bulb in series to limit the inrush current) and then disconnect it from the power source. Record this voltage as your Vinitial voltage.
2. Get a resistor, like the one inside your charging/discharging cable that came with the "Monster Cable High Performance Stiffening Capacitor". Connect these cables between the charged capacitor's + and - terminals and start the timer on your stopwatch.
3. wait an arbitrary amount of time (1 second, 10 seconds, 100 seconds), then disconnect the cable from the capacitor terminals.
4. Measure the new capacitor voltage and record it as your Vfinal. If there is no difference between the numbers, increase your time and re-do the test.
5. Using Ohm's law V=IR, find the constant current that flowed during the test by taking the average voltage (Vinitial + Vfinal)/2 divided by your resistance. This constant current I will be recorded as i for the equation above.
6. Now just solve for your capacitance.
I am thinking about connecting the large capacitor in parallel to the car battery. Have anyone done this before? I know there will be a leakage current through the capacitor. Is that leakage current high enough that the car battery will be dead after sitting for a few days?
Find Leakage Current (multimeter or ammeter required)

This is even more straightforward to calculate.

1. Charge the capacitor up fully to the battery or power supply voltage, again use a lightbulb to slow down the massive inrush current that would result.
2. Once fully charged, disconnect one of the negative terminals and add in-line your multimeter with set to mA or uA, connecting the "COM" port to the battery minus terminal and the "A" port to the minus of the capacitor.
3. Whatever current reading you get is the constant leakage current of your capacitor at that voltage.

Like oz93666 says, your 1F electrolytic cap will take some strain off starting the engine. Some, but not all. Like I showed with my calculations in my post above, you need at least 100 of these to start a car by themselves. Better to use a supercapacitor module.

So to answer the original question in your thread of whether it is wise or not, just find the leakage current, then find how many Amp-hours your battery has, and see how many hours or days it would take the capacitor + your car's computer to drain the battery if the car was sitting. If you drive once a week then maybe it's not the best idea. If possible, please share as well.

oz93666

Joined Sep 7, 2010
737
Yes 1F is too small ... I suspect the relatively large size of this capacitor is because it is not technically a super capacitor , perhaps electrolytic , I think these have different characteristics , making them better suited to smoothing msec fluctuations in current demand for audio systems ...

Helping to start a car needs a more compact /Farad and cheaper , super cap ... ideally around 100F @16V but anything above about 15F will appreciably help.

Super caps have very low self discharge and in no way will flatten your battery.

yourminky

Joined May 31, 2019
28
I guess my 1 farad capacitor will have no effect on prolonging the car battery life.

Infinion

Joined Apr 2, 2016
9
I guess my 1 farad capacitor will have no effect on prolonging the car battery life.

No worries, great thread. It was pretty interesting to get into, and we all generated some insightful content I think.

The capacitor might not prolong the car battery life, but it will keep your alternator happy and help you start your car as it continues to degrade with age. I still think you should still connect it and see how it goes just for the hell of it, though with calculations you would know for sure.

The best thing you can do for your 5-year old battery is to keep it above 11.8V... always, and keep each of the 6 cell's fluid levels above the plates with distilled / deionized water top-offs. Like other threads on allaboutcircuits go into, starter batteries aren't always constructed the best or give the greatest return on your investment, so there's only so much you can do before it's time to look for a new battery. Although you could probably save a bit of money in the long run with a hybrid setup (a cheap 100F+ supercap bank and a smaller lead-acid battery, or LiFePO4.)

mvas

Joined Jun 19, 2017
538
I would not pay $120.00 for capacitor that gives me an extra 20 milliseconds of cranking time. Yes, you read that correctly ... 20 milliseconds. That capacitor will not "make the alternator happy". That capacitor will have less than a 0.1% affect to the electrical system = a total waste of time and money. The "under the hood" temperature will most likely exceed the max rating of that capacitor and most LiFePO4's = early failure. If you want more cranking time, then buy the biggest battery ( by CCA rating ) that will fit in the tray. Infinion Joined Apr 2, 2016 9 I would not pay$120.00 for capacitor that gives me an extra 20 milliseconds of cranking time.
Yes, you read that correctly ... 20 milliseconds.
That capacitor will not "make the alternator happy".
That capacitor will have less than a 0.1% affect to the electrical system = a total waste of time and money.
The "under the hood" temperature will most likely exceed the max rating of that capacitor and most LiFePO4's = early failure.

If you want more cranking time, then buy the biggest battery ( by CCA rating ) that will fit in the tray.
I don't disagree that it's not a lot of capacity for starting. but he already has the capacitor. If he's not going to replace his 5yr battery or buy a larger 100F+ supercap module, it's the only thing besides his float charger that's going to help him for the upcoming winter concerns he mentioned, as caps work great in cold temperatures.

But you're right, buying a new overrated battery by CCA rating would be the best fool-proof solution.
I would, however, argue that the first 20 - 100 milliseconds is the most demanding region of starter operation (stall torque with zero back-emf). An old cold battery provides much less than its typical CCA, so a cap fulfills those instantaneous power needs.

Alternators have regulators and rectifiers in them, and capacitors smooth out AC ripple voltages. A large smoothing capacitor bypasses transients to the electrical system, so that's at least my basis behind why I said it would "make the alternator happy", but really, it's good for the whole electrical system. Lead-acid cells do a terrible job at clamping voltage spikes, and old batteries even moreso.

oz93666

Joined Sep 7, 2010
737
I would not pay $120.00 for capacitor that gives me an extra 20 milliseconds of cranking time. That's not the situation at all ...$50 worth of capacitor will relieve all the strain from the battery , it does not like supplying many 100's of Amps ... A capacitor will reduce that down to many 10's of Amps greatly extending battery life ....

With a decent capacitor you can also reduce your battery size to a quarter it's normal size , not only reducing battery costs to a quarter , but reducing weight , saving 15Kg in weight in your car will pay for battery over it's life in petrol savings ...

We've gone through the figures before , I forget on which thread.

mvas

Joined Jun 19, 2017
538
The Monster Stiffening Capacitor being discussed in this thread does cost about $120.00 ( new ) and it is only 1 Farad. It is rated at 83 amps maximum It only has a 1 year warranty Connecting this capacitor to an automotive battery is a worthless effort. That is the situation. A capacitor that is 50 times bigger and half price of this capacitor is a completely different story, but change no figures in this thread. oz93666 Joined Sep 7, 2010 737 The Monster Stiffening Capacitor being discussed in this thread does cost about$120.00 ( new ) and it is only 1 Farad.
It is rated at 83 amps maximum
It only has a 1 year warranty
Connecting this capacitor to an automotive battery is a worthless effort.
That is the situation.

A capacitor that is 50 times bigger and half price of this capacitor is a completely different story, but change no figures in this thread.
As the posts on this thread have covered , this 1F capacitor is old generation ...meant for audio use only (some people have a KW of audio power in their car , current drawn from batteryand alternator to power this fluctuates with sound level by the msec)....

Super caps for car batteries are priced around 1\$/F ... many different sizes available , from 10F to 33F @16V ...

Energy in 33F capacitor =1/2 c x v x v = 16.5 x 12. 6x 12.6= 2,620 J @ 12.6V

.................................................................16.5 x 10 x 10 = 1,600J so energy released during starting engine ( from 12.6 to 10 V) is about 1000J

You'd be surprised how quick a typical engine starts , only about 1sec work from the starter motor , drawing max 200A , quickly reducing to well below 100A .... Probably less than 1000J total .... one 33F could start an engine without any battery ... very easy to prove !