Charging a battery with a supercap

[James1986]

Hi guys,

Simple question here; I want to keep a 12v car battery (or any similar 12v Lead Acid battery) topped up at close to 100% charge with a supercap (to avoid sulfation, just like in real cars!). This one happens to be a 15v 58Farad capacitor (maxwell technologies) so what I want to know is:

How long will it take, assuming the cap is fully charged, before the voltage drops below the charging voltage of the battery (12v?) And how much energy will actually be transferred to the battery in this time?

What happens if the battery is fully charged to 100% when the supercap is also 100%; will the charge go anywhere?

With a 15v supercap, if empty, can it be charged with any voltage/current combination, or does it have to be more than 15v input and a specific current (which is less than the max current specified in the data sheet).

Thanks for your help!

 

[matk95]

im no expert but i recon that the battery will charge the capacitor upto its levle (12) then it will just sit there and do nouthing if you charge the cap up then conect it it might rais the voltage and damage the car electronics? anyway i don't think that your going to manage to have much off an efect on the batory

good look

 

[Hero999]

I don't see how the capacitor can do any good.

58F is still a tiny amount of power storage compared to the battery.

For example, suppose your battery is considered to be fully charged when the voltage is 14V and flat at 10V and it has a total capacity of 50Ah.

Now lets apply the same logic to the capacitor. If you charged it to 14V then connected a 1A current source it would take: t = 4 * 58 = 232s to reach 10V giving it an effective Ah of just 232/360 = 64.4mAh which is nothing compared to the battery.

Put it another way: how do you thing that adding a 64.4mAh battery in parrallel with a 50Ah battery could possibly prevent sulphation.

The only good thing a super capacitor would do is reduce the AC impedance which isn't much use unless you want to use it for a rail gun or similar pulsed power application.

 

[James1986]

I think you have misunderstood the point. The battery will not charge the capacitor... It is a one way system. The capacitor will keep the battery 'topped up'. Sulfation (I think its called) is the chemical errosion which occurs on one of the terminals (negative I think) due to repeated cycling of the battery. This is the main cause of car battery failure. The way that car batterys last so long in your car is that they are constantly topped back up to 100% by the alternator in the car. My application won't have an alternator, and thus will use the energy recaptured by the capacitor, to keep the battery topped up.

Anyone able to answer the original questions?

 

[Nigel Goodwin]

I think you have misunderstood the point. The battery will not charge the capacitor... It is a one way system. The capacitor will keep the battery 'topped up'. Sulfation (I think its called) is the chemical errosion which occurs on one of the terminals (negative I think) due to repeated cycling of the battery. This is the main cause of car battery failure. The way that car batterys last so long in your car is that they are constantly topped back up to 100% by the alternator in the car. My application won't have an alternator, and thus will use the energy recaptured by the capacitor, to keep the battery topped up.

Anyone able to answer the original questions?

It still makes no sense to me either - where is the capacitor 'recapturing' the energy from?.

 

[blueroomelectronics]

As previously said, the tiny supercap is no match for your much larger lead acid. How do you plan to top off the supercap?
Sort of like trying to fill your pool with an eyedropper.

 

[gitwel]

I have to side with HERO999 , the even if the SuperCAP was charged to 15 Vdc or any other voltage higher than the car battery. By its purposeful design incorporating a rather large internal resistance "ESR" compaired to the car battery, the charge inside of the superCAP just isn't capable of "maintaining the car battey". It would be gone in a flash and the superCAP would be charging off the battery.

It has something to do with the transfer of power, Joules fits into the equation somehow.

 

[Edd]

Use a battery instead of the capacitor. Oh, look at that you have one there already, how convenient.

 

[crutschow]

As noted, if you connect the cap directly to the battery it will rapidly discharge to the battery voltage value. The only way to use the cap to maintain the battery charge would be to have it run a constant current trickle charger circuit. If you maintained a trickle charge of 50mA (probably a minimum to minimize sulfation) the 58F cap would take about 38 hours to go from 15V to 13V (13V being a minimum float voltage for the battery).

 

[SMUGangsta]

Am i missing the point here?

The battery is not going to be used in a car, therefore no alternator. Therefore when current is drained from the battery it causes a chemical reaction causing domething to move from one set of plates to the other (kinda like electroplating?) im no good with chemistry.

Surely this battery - which I assume is connected to a load will drain - therefore needing recharged? (from the mains?) When it is recharged the energy/current whatever flows back into the battery - so would this not reverse the process? therefore removing the attached 'stuff' from the plate back into the solution or the other plate?

 

[Edd]

Yes. The problem is the OP seems to think that a supercapacitor will magically keep the battery charged and allow perpetual motion - unless there is a recharging device, in which case the supercapacitor is not needed.

 

[blueroomelectronics]

Based on the OPs previous thread it's probably part of his regenerative braking charger for a bicycle.
Here's the rub IMO the added weight + friction of the dynamo will require more human effort that a non self recharging bike.

 

[Diver300]

If you maintained a trickle charge of 50mA (probably a minimum to minimize sulfation) the 58F cap would take about 38 hours to go from 15V to 13V (13V being a minimum float voltage for the battery).

I think it is 38 minutes.

58 * (15 - 13) / 0.05 = 2320 seconds = 38 ish minutes

 

[Hero999]

Where did you get that from?

If a 1F capacitor is charged/discharged by a 1A constant current source, it will take 1 second for the voltage to rise/fall by 1V.

In this case it's a 58F capacitor with a 1A current load and the voltage starts at 14V and falls to 10V giving a voltage drop of 4V. 4*58 *1 = 232s = 0.064h

I think you have misunderstood the point. The battery will not charge the capacitor... It is a one way system.

How is it a one way system?

The capacitor will keep the battery 'topped up'. Sulfation (I think its called) is the chemical errosion which occurs on one of the terminals (negative I think) due to repeated cycling of the battery. This is the main cause of car battery failure. The way that car batterys last so long in your car is that they are constantly topped back up to 100% by the alternator in the car. My application won't have an alternator, and thus will use the energy recaptured by the capacitor, to keep the battery topped up.

How does the capacitor recapture the energy?

The capacity of the battery is 775.86 times the capacity of the capacitor, how are you expecting the capacitor to make any difference?

 

[James1986]

Hi Guys,

Thanks for all your feedback.

I recently found a paper describing a system much like the one I am proposing (and is infact where I got the idea). Blueroom you are correct in thinking that this is in relation to my previous post, however, the friction you are talking about is beneficial in that it makes the bike slow down (which is what you want when you don't need to pedal downhill It also helps that the dynamo propells the bike )

One way systems can be designed by using diodes; I already have this set up.

I am not trying to build a perpetual motion device; all I want to do is extend the life of the battery. In NO way will the capacitor supercede the battery as the main power source for the load.

In particular circumstances high pulsed current will be sent to the battery. As you are probably aware, batteries are terrible at absorbing short bursts of energy, typically acquiring only 30% of the energy given (which is why you trickle charge for long periods of time). However capacitors are excellent, typically acquiring 90-100% of the burst (provided the wires are thick enough, and there are no heat losses blah blah blah).

So yes, whilst the pool (battery) will be only filled up momentarily by the rain (capacitor which has recaptured spent energy from another source); over time, this will extend the time needed before a full refil (recharge) is needed, and reduce the algae build up (sulfation).

Thanks to you guys who posted calculations, that should help me figure out my own sums!

I also apologise for being quite vague with my descriptions; I am trying to do the figuring out on my own, but I need the ideas and examples to help me understand it!

Crutschow - Do you have any links to anywhere where I can find an example trickle charging circuit? Then I can figure out how much energy I will be getting from my other source and see how long it will take to fill the cap up!

Nice one guys, thanks!

 

[blueroomelectronics]

James what you'll eventually discover is the kiss principal. The more bits you add to a simple charging circuit can actually reduce efficency.
IMO the weight gain on the bike plus the extra friction will require more human energy than a bike with no electronics.
Battery operated bikes reduce the effort only because it's not pedal power that charged the battery in the first place.

 

[James1986]

Agreed!

However, there isn't much extra weight when compared with existing e-bikes, and with clever circuit design one can keep efficiency relativley high!

Wouldn't you rather have an energy storage system for your e-bike that lasts 2+ years and then spend only £40 to have to replace the cheap lead acid battery as opposed to 6-12 months and pay £200 for a Li-ion?

 

[Edd]

Ah I see now, a smoothing/filtering capacitor.

 

[blueroomelectronics]

Supecaps have a high ESR. Good for trickle charging RTCC but will do nothing to keep a battery topped off.
If there was a simple solution to long cycle life batteries it would be common knowledge by now.
To understand batteries you really need to be a chemist.

 

[crutschow]

I think it is 38 minutes.

58 * (15 - 13) / 0.05 = 2320 seconds = 38 ish minutes

Correct. I confused my minutes with my hours.