Multiple caps on one supply and in series

[morten]

For a batterybackup for a burglar alarm I would like to use supercaps. The alarm runs 12 V and the supercaps are around 5 V max. I would like to ask if some one knows a simple way to konnect 2 or 3 supercaps so that they get charged (each only with nominal V) by the powersupply from the alarm (12 V), and so that if the power fails, they are connected in serie and give the combined V to the alarm?

I am new to this

Morten

 

[tcmtech]

Easy. Use a zener diode with a few tens of ohms resistor in series across each capacitor.

 

[Grossel]

There is nothing wrong about the idea about connecting several caps in series to obtain higher overall voltage tolerance.

However, bear in mind that the total capacitance (in farad) will be much lower compared to one cap. If using three caps in series, the resulting capacitance will be approximate one third, given that all three caps have equal capacitanse.

Also, electrolyt caps doesn't like reverse voltage. I haven't read up on how much reverse voltage a cap can withstand, but just to be on the safe side, you might put a diode in reverse in parallell to each cap.

 

[Nigel Goodwin]

Supercaps aren't really particularly 'super' - use batteries, either lead acid or NiMh (much more 'bang for the buck' and greater performance) - but almost all burglar alarms use lead acid (the new one we've just had fitted at work has two, an 18AH one, and one slightly smaller - apparently modern regulations require it).

 

[WTP Pepper]

The problems with caps/super caps is their discharge characteristics. A lead acid battery will holds its voltage under discharge until it runs flat. This will under normal cicumstances keep the alarm panel alive during power outages.

A cap will instantly start discharging the moment the main supply is removed at a negative exponential rate. The supply could be useless within minutes.

Super caps are best for very low current drain apps like CMOS clock backup on a PC motherboard and not for anything that draws mAmps/100mA.

 

[tcmtech]

Super caps are best for very low current drain apps like CMOS clock backup on a PC motherboard and not for anything that draws mAmps/100mA.

Hmm. This guy shows otherwise. Six 350 Farand super caps (in series) from Digikey for $11 each.
Starts car without problems.

https://www.youtube.com/watch?v=z3x_kYq3mHM


BTW Seven 2700 farad supercaps in series can start a diesel semi truck!
So yes super caps can provide a lot of amps for short term not just low current loads.

 

[morten]

Thaks all. I have kind of lost interest in supercaps. Will use a batteri instead of. You have been very helpfull.

Morten

 

[WTP Pepper]

An interesting video. I was assuming (wrongly) we were talking about the 1F caps readily available.

I note that his car started on 10V (it also looks like a pretty basic car so may not have an alarm to drain it). On my last car, I had a cell go down in the battery. It would readily crank the engine but was insufficient to power the injectors and EHT system so wouldn't start.

If this was commercially credible, it would have been done on a mass scale even if just down to the environmental issues of not using lead.

With the standby current of my car ~ 12mA, it wouldn't start after 3 hours. With my 60Ah battery, it will be half flat in 100 ish days and probably still start. OK I am not taking into account self discharge, but on capacitors that size it will be a fair percentage of the lifetime to consider in the sums.

 

[Nigel Goodwin]

If this was commercially credible, it would have been done on a mass scale even if just down to the environmental issues of not using lead.

Ignoring the long term discharge problems, just compare the discharge curves of a capacitor and a battery, soon shows why capacitors aren't much use for replacing batteries.

 

[WTP Pepper]

Ignoring the long term discharge problems, just compare the discharge curves of a capacitor and a battery, soon shows why capacitors aren't much use for replacing batteries.

Agree. That's what I was trying to cover in #5

 

[tcmtech]

I wasn't trying to say that ultra caps should replace batteries. I was showing that the new ultra and super caps are more than capable of producing high currents while keeping their voltage up for short bursts.

Which BTW if you think about it a 8 pack of C alkaline batteries used to charge up a bank of 6 super caps like the guy in the video is using would make for a very effective and compact jump starter system.

As far as cost wise a big lead acid battery is still far far cheaper.

 

[Norator]

I x dT/dV = C so with I = 150A, dV = 3v and dT = 3 seconds, C = 150F, at least on paper.

 

[tvtech]

Ignoring the long term discharge problems, just compare the discharge curves of a capacitor and a battery, soon shows why capacitors aren't much use for replacing batteries.

Exactly

I also looked at them with my little project.....No go.

A battery is a battery. And a cap is a cap. Forever.

Batteries have the ability to provide high current for a short while....and then recover and continue providing lesser power for a long period of time.
A Cap always gives it's all when you want it. After that it has no more to give. Absolutely no staying power whatever or capacity.

The Supercap thing interests me though. Many people around the World (especially Manufacturers) claim eventually it will replace the conventional battery.

I really think not. Until a Cap has reserve capacity.....and where will it get that from.....a Cap is a Cap. No matter how many Amps it can do for a split second or even a few Seconds...after that it has nothing left. It is a just a Cap after all.

Picture this: A huge vehicle with this massive engine that has it's starter motor powered by this amazing new Technology called the SuperCap. Before the same vehicle used normal Lead Acid batteries. Call it 500kg battery weight. And it always started.

Enter SuperCap. Weighs nothing. Board is very small and light. People are amazed this little board weighing next to nothing can do the job of the 500kg replacement.

Operator pushes start button. Split second of hectic energy given to the main motor via the starter motor. Motor does not start....

No such thing as a second chance. Cap bank has utilized all it's energy...no reserve.

I hope the Companies promoting/manufacturing SuperCaps are responsible and don't try and fool people. I shall be watching closely.

Regards,
tvtech

 

[tvtech]

I x dT/dV = C so with I = 150A, dV = 3v and dT = 3 seconds, C = 150F, at least on paper.

Hows AAC lately? Slow tonight?

 

[Grossel]

Operator pushes start button. Split second of hectic energy given to the main motor via the starter motor. Motor does not start....

No such thing as a second chance. Cap bank has utilized all it's energy...no reserve.

The driver would have a little portable generator so he can charge the cap - by hand. Second chance

 

[tcmtech]

Going on the automotive ultracapacitor concept there is now no reason that vehicles need to have so much parasitic drain. If you figure the average cheapo cell phone can sit for days without its battery going dead (my Sonim can go over a month!) there is no reasons that a vehicles systems couldn't be made as efficient in off mode as well.

The point is given a good set of ultracaps to handle the high starting current loads and a small multi Ah battery there should be no reason a vehicle can't be started and work that way.

 

[Nigel Goodwin]

Going on the automotive ultracapacitor concept there is now no reason that vehicles need to have so much parasitic drain. If you figure the average cheapo cell phone can sit for days without its battery going dead (my Sonim can go over a month!) there is no reasons that a vehicles systems couldn't be made as efficient in off mode as well.

The point is given a good set of ultracaps to handle the high starting current loads and a small multi Ah battery there should be no reason a vehicle can't be started and work that way.

But why would anyone want to? - what use is a greatly inferior and more expensive option?.

 

[WTP Pepper]

The driver would have a little portable generator so he can charge the cap - by hand. Second chance

An ideal solution. Something you would be happy to do when it's freezing cold or peeing down with rain.

 

[Norator]

"Hows ABC lately? Slow tonight?

Anyway, you really need to maybe focus more on the less impressive practical stuff and sense.
Rather than your Formulas that make none. Your assumed cleverness is not always a smart thing to show in public.

Keep the formula stored for a relevant/pertinant exam question."

Yes sir, I'll try to do better in the future.
What's ABC?

 

[tvtech]

LOL. I meant AAC.

Regards,
tvtech