How to equate farads to battery capacity - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 12:55:19 pm
content: Do I understand correctly that having converted the joules in a capacitor it's Wh capacity will be expressed in j/3600 ?so for 9000F @ 16V = (0.5x9000x16x16)/3600 = 320W ?

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 01:08:54 pm
content: except 6x1500F capacitors in series is 250F of course so (0.5x250x16x16)/3600 = 8.8..Whpittiful

author: HKJ
author_rank: Super Contributor
timestamp: August 09, 2014, 01:51:46 pm
content: I had a bit of fun running two capacitors through my battery test, i.e. doing exactly the same discharges I have done one a few hundred batteries.300F:http://lygte-info.dk/review/batteries2012/Cooper%20300F%20supercap%20UK.html3000F:http://lygte-info.dk/review/batteries2012/Maxwell%203000F%20supercap%20UK.htmlAA/AAA index:http://lygte-info.dk/review/batteries2012/CommonAAIndividualTest%20UK.html

author: hkBattousai
author_rank: Regular Contributor
timestamp: August 09, 2014, 02:38:24 pm
content: Vbat: (V) Rated battery voltageCbat: (A-hr) Rated battery capacityQbat: (C) Amount of charge stored in the full batteryVcap: (V) Rated capacitor voltageCcap: (F) Rated capacitor capacityQcap: (C) Amount of charge stored in the full capacitorThe unit A (Ampere) is defined as amount of charge in C (Coulomb) transferred in 1 second. So, we can writeQbat= 3600 * Cbat.On the other hand, capacity of a capacitor is defined as the charge (in Coulomb) it holds per applied voltage (in Volt). Therefore,Ccap= Qcap/ Vcap. (*)If we think the battery as a capacitor, we substitute Qbatin (*).Cbat= Qbat/ Vbat= 3600 * Cbat/ Vbat.For example, a 12V and 60Ahr battery can be thought as a (3600 s/hr) * (60A-hr) / (12V) = 18kF capacitor.And of course, these are approximations. A battery and a capacitor will behave differently in practice.

author: qno
author_rank: Frequent Contributor
timestamp: August 09, 2014, 03:51:26 pm
content: You cannot compare capacitors and batteries as a replacement.Batteries have a reasonably flat discharge curve where the voltage stays above the battery empty voltage.A capacitor discharges with an e-curve.So after 20% of discharge the voltage has dropped below the battery end voltage.You need a converter to get the rest of the charge out of the capacitor.

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 04:01:04 pm
content: so basically all of those people selling supercaps on ebay and claiming how they are replacing traditional batteries are at best stretching the truth about some limited circumstances and at worse lying. I was thinking that as solar panels are getting ever cheaper (85p/W versus £2.00/W of 3 years ago) I might start adding a few panels but as I'm not able to sell the power from a homemade system and i'm hardly home during the day it may also be time to think about storage. I'm just a bit concerned about things like lead-acid batteries being very poor bang for buck. Obviously super caps are not the way to go.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2014, 04:31:23 pm
content: Quote from: Simon on August 09, 2014, 04:01:04 pmIf you're getting your technical knowledge from eBay listings, you might want to reconsider the assignment of blame here...just saying...Tim

author: IanB
author_rank: Super Contributor
timestamp: August 09, 2014, 05:05:41 pm
content: Quote from: Simon on August 09, 2014, 04:01:04 pmLead acid batteries are still the most economical storage technology for electricity. That's why they are the battery of choice in UPS's and in motor vehicles.If supercaps or lithium ion batteries had a better bang for the buck then product designers would start using those instead of lead acid for applications where cheap electricity storage is needed.

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 06:04:09 pm
content: Quote from: T3sl4co1l on August 09, 2014, 04:31:23 pmI just thought that as some guy had used supercaps to replace a car battery they might have decent capacity. The ebay listings made no specific claims and the whole idea of this thread was to work out the math of it. I had no ideas either way when i started the thread, or is this forum turning into something like avr freaks ?.........I think in my second post i concluded that the capacity is infact crap unless my math is wrong in which case I'd hope for someone to point that out as math is not my strong point!

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 06:09:37 pm
content: Quote from: IanB on August 09, 2014, 05:05:41 pmI have been reading ebay listings claiming just 700 cycles of 70% discharge so obviously they need carefully looking after and also don't have all of the capacity they claim (presumably that whopping 110Ah over 20 hours is actually 55Ah). The other problem i will have with a lead acid battery is that my grid tie inverter may not like it too much, ideally I need 14ish volts for the inverter to work as it has built in MPPT, I probably need a grid tie inverter with no MPPT which really is meant for panels and not batteries, but that's another cost and i may not find one but I'm yet to look.

author: robrenz
author_rank: Super Contributor
timestamp: August 09, 2014, 06:10:59 pm
content: I think the car starter situationmaybe valid as it is very high peak amps for (hopefully) a very short duration.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: August 09, 2014, 06:14:55 pm
content: What about getting the energy out of those supercapacitors? Lead acid battery is easy because it's empty around the 12V mark. For capacitor the relationship of charge and voltage is linear. Should also consider the total system cost, not just a single component.Quote from: Simon on August 09, 2014, 06:04:09 pmFunny I was about to write about that same guy. Actually he just used the capacitors because they can supply lot of amps for a brief moment and have low internal resistance. Doesn't have much to do with the fact that they also hold lot of energy. Car lead acid batteries have to be so big and designed as starter batteries so when they are charged they can supply the big amps. Not a lot of energy is actually required to start a car.Did you see the video where he put cheap remote controlled plane/car lithium polymer pack in series with some super capacitors. In something like weeks time he had failed cells on that pack because he didn't bother to balance the pack, just let that car alternator do it's thing and hope for the best. Those super capacitors would also need some balancing and over voltage protection so they could have a chance to work like specified in the datasheets.Free energy people on youtube rarely care if components are used properly and work as long as they could. They just put something together and when it stops working they tinker with it after they have tinkered with it several time for the last month the can just say it has been "working continuously" for that same month.

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 06:15:28 pm
content: Quote from: robrenz on August 09, 2014, 06:10:59 pmindeed and that is why i asked the original question so that I could verify.

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 06:19:45 pm
content: Quote from: Legit-Design on August 09, 2014, 06:14:55 pmErr as far as i know he had nothing to do with the "free electricity" cult and was simply pointing out that as we have already discussed only a small amount of power is actually needed to start a car and supercaps can hold enough.Yes realistically a supercap would need a boost SMPS attached to boost the voltage as it discharges to help make better use of the available power.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: August 09, 2014, 06:25:23 pm
content: Quote from: Simon on August 09, 2014, 06:19:45 pmhttp://laserhacker.com/this is the guyALTERNATIVE PROJECTS, INVENTIONS, EXPERIMENTS AND DIY PROJECTSCrystal cells, atmospheric energy.http://laserhacker.com/?p=110his smaller boost pack with the failed battery.When I first saw his video of the bigger boost pack/capacitor battery replacement I thought "wow this guy must really know what he is doing" turns out he was just ignorant.

author: eevbstedt
author_rank: Contributor
timestamp: August 09, 2014, 06:31:28 pm
content: Was just about to post these and Legit-Design kinda beat me to it.Replacing My Car Battery with Capacitors! 12V BoostPack Update- lasersaber Apr 8, 20132V BoostPack Starts Freezing Cold Engine - Capacitors Replacing Car Batteries- lasersaber Nov 13, 2013

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 06:35:55 pm
content: I can't even remember the particular video or guy i'm referring to and i watched it well over a year ago and in the particular video i saw he made no reference to anything weird and i couldn't care less what his other views are, if you have such a gripe about the guy go start another thread and have your little whinge

author: IanB
author_rank: Super Contributor
timestamp: August 09, 2014, 07:00:48 pm
content: Quote from: Simon on August 09, 2014, 12:55:19 pmYes, 1 Wh = 3600 Ws = 3600 JQuote from: Simon on August 09, 2014, 01:08:54 pmWhere energy storage is concerned you can multiply one capacitor by the number of capacitors to simplify the calculations. It will give the same answer.If we take a single 1500 F capacitor charged to 2.5 V, the total energy stored will beE = ½CV2= 0.5 x 1500 x 2.52= 4700 J = 1.3 WhTherefore putting six such capacitors in series would yield six times that, so,E = 6 x 1.3 Wh = 7.8 WhThis is the number you got.A better way of measuring capacitor energy storage is to consider the minimum and maximum usable voltages.Let's say the minimum usable voltage on a 2.5 V capacitor is 1.5 V (below that we need an impractical boost circuit to get the energy out). So the practical energy stored is:E = 0.5 x 1500 x (2.52- 1.52) = 3000 J = 0.83 WhWith six capacitors that would give:E = 6 x 0.83 = 5 WhWhen related to a 12 V battery, that would be equivalent to 5/12 = 0.4 Ah.

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 07:02:41 pm
content: thank you

author: Simon
author_rank: Unknown
timestamp: August 09, 2014, 07:07:42 pm
content: Just checked my inverter, it's 14-28V so looks like I'm buggered unless I switch to 24V

author: eneuro
author_rank: Super Contributor
timestamp: August 10, 2014, 07:36:20 am
content: Quote from: Legit-Design on August 09, 2014, 06:14:55 pmMaybe do you know how much Wh or kJ is needed to start car with diesel 1.4 HDI engine?Last time I've made experiment while 7 years old Varta battery had too small current to start car even in the summer, so I've put it with another one old car battery in pararell-charged them and balanced them before connection using old 12V 55W light bulbIt wasn't supprise that car started on those 2 batteries.Of ocurse it is too small space close to engine to put there 2-3 batteries, but maybe adding a few supercapacitors could do the trick?However, it could be better put 2-3 batteries in pararell, while we have higher total capacity.I've successfully started to parrarel them and use them with external 12V->230VAC power inverter as DIY UPS where like in the car those batteries are charged all the time to 13.5V-13.8V under load and there is no switching between failed grid 230VAC and output inverter 230VAC, while power inverter deliver all the time needed powerSo simply but it works and all I needed is a custom battery charger.BTW: It is easy make overvoltage protection circuit for 2.5Vmax supercapacitors using TL431I

author: Simon
author_rank: Unknown
timestamp: August 10, 2014, 08:23:31 am
content: putting super capacitors in parallel with a car battery is a fair compromise to help start in cold weather.

author: G7PSK
author_rank: Super Contributor
timestamp: August 10, 2014, 09:23:07 am
content: Lead acid is best for power storage still if weight is not a consideration. I installed a 900 amp hour 24 volt pack 15 years ago in a wind turbine set up and it is still going strong, had one acid change with a wash out 2 years ago but that is all other than the occasional top up with distilled water.

author: Simon
author_rank: Unknown
timestamp: August 10, 2014, 09:27:03 am
content: Well i can get 85Ah leisure batteries for £47 so won't break the bank to get two for 24V and I'd have 1KW available on a 50% discharge. I'd need to make me a step-up converter with built in MPPT control to charge the 24V pack from the 17.5V panels and then I'd have 24V to go into my GTI that will give me power back whenever I flick the switch. Naturally I'll need a voltage cutout circuit to stop the batteries discharging past 50%

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: August 10, 2014, 09:58:02 am
content: Quote from: Simon on August 09, 2014, 01:08:54 pmThe energy stored in a cap is a function of V squared, that's why if you halve V you'll store just 1/4th the energy etc.With the caps connected in series you're charging each only up to V/6 thus you're storing only 1/36th (1/36 === 1/6 squared) the energy you'd be storing in these exact same caps charged up to V each.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2014, 09:59:59 am
content: I've replaced my car battery with maxwel boostcaps. (but ive not made a video of it so that must be some other guy)6x 2600F 2.5V maxwel boostcaps = 15V 433F cap in car.Starter motor is like ~200A for 1-2 seconds. (its less once the engine is turning over)So 200A/433F = 0.46 volts drop per second worst case (its less than this in reality, but peak is ~200A)A car starter motor will turn the car engine over fast enough to start until about 8V soAssuming a cap that the car has charged up to full ~14V, then that's 6V of usable voltage range.6V / 0.46 = 13 seconds of continuous cranking before the caps are too low to start the car.If i leave my ignition on with engine off i've only got ~4min before the cap is too flat to start.Mainly due to the electric radiator fan which chews up lots of power.

author: Simon
author_rank: Unknown
timestamp: August 10, 2014, 10:01:01 am
content: whether you put the capacitors in series or parallel the power stored will be the same

author: penfold
author_rank: Frequent Contributor
timestamp: August 10, 2014, 10:15:40 am
content: QuoteThat's not entirely true... if you adapt the voltages to displace the same charge in each capacitor based on whether it is series or parallel, then yes you can store the same amount of energy in the system. (note that you can store energy not power)There's been a fair amount of research done into the utilisation of surplus electricity and if you take into account the system cost, the efficiency of storage (power conversion into the capacitor and ditto outwards) then you're better off dumping the excess energy into a well insulated hot water tank.But if you're insistent on storing the energy as electricity then you're probably best noting that for static applications a lead acid battery really cannot be beaten for bulk storage, and if you manage them well and use proper deep cycle batteries then you shouldn't see any problems due to cyclic effects

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: August 10, 2014, 10:23:54 am
content: Quote from: penfold on August 10, 2014, 10:15:40 amExactly. With 6 caps in series you'd need to supply 6*V to store the same energy as with 6 in parallel charged to V.Quote from: penfold on August 10, 2014, 10:15:40 amYou beat me to it. @Simon: power is a measure of how fast the energy flows from a place to another.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: August 10, 2014, 10:47:19 am
content: Quote from: Psi on August 10, 2014, 09:59:59 amHow do you handle recharging those capacitors on day to day basis? How long they actually hold enough charge to start the car when not in use? What about over voltage conditions, from what I understand car electrical system can have +80VDC peaks on a load dump from the alternator, normally a lead acid battery will eat that away. From what I understand they don't self balance either, where lead acid battery can just boil other cells until the cell with low voltage catches up.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2014, 11:49:10 am
content: Quote from: Legit-Design on August 10, 2014, 10:47:19 amI just let the normal car alternator do it.It gets a bit annoyed if the caps are below 10V because they don't "come up" to normal voltage right away like a battery would.The alternator ends up dumping its max current into the caps which will stall the car if the engine is at idle.So when starting the car from caps below 10V i need to give the engine a little gas to keep the RPM up for a few sec until it charges the caps above ~10V.Quote from: Legit-Design on August 10, 2014, 10:47:19 amDepends on the load when the car is off.If you have a car alarm and other accessories maybe 3-7 days before you can't start the car.If you have zero current with the ignition off then they will last months. I have some boost caps that have been sitting in my room for ages. From memory they went from 2.5V down to 2V in like 4 months.Quote from: Legit-Design on August 10, 2014, 10:47:19 amI've not noticed any issue yet, they have been in my car for a yearCaps have much lower internal resistance than a battery so should be even better at absorbing spikes.Quote from: Legit-Design on August 10, 2014, 10:47:19 amYeah, i was worried about this too.I thought i might need some mosfets across each cap to connect a resistor in parallel, or maybe a zener, so the voltage was drained away above the caps max volts. However i kept an eye on the voltage of each cap and they seem to stay more or less balanced.The higher the voltage you charge them too the faster the lost from leakage. So i guess they keep themselves balanced because the highest cap loses it's charge faster when the car is sitting around doing nothing.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: August 10, 2014, 02:05:10 pm
content: 1st off I am not an engineer , I suck at the math side of all this .But on the other end I have been in the auto repair & solar for about 35 + yrs.To me this is a classic reinventing the wheel - attempting to replace lead acid batteries with caps with engine starting as the example .In real world normal car use the cap will just not hold up / or compare to the battery .And the main reason is time , a bank of capacitors do not have the amp hours as compared to batteries capacity .There are very few situations where the advantages of caps over batteries win and most of those would be special applications , like save the weight at a drag strip and just starting the engine once per use .In normal car use , if you had to run a bunch of errands short runs and lots of starts .Then when is comes to solar , there is very little in comparing the starting battery to the correct choice for deep cycle battery , the draw is not as high in solar but in both applications what gives batteries much more use is time , comparing the high draw over time , then with batteries , there is a recovery [ with out a charging source ] that caps just do not have .Lithium batteries are close to over coming the need for MASS to supply power , but they still have too many issues to replace lead acid for either engine starting or solar .

author: Legit-Design
author_rank: Frequent Contributor
timestamp: August 10, 2014, 03:12:32 pm
content: Quote from: KD0CAC John on August 10, 2014, 02:05:10 pmWhat about cost of replacing that starter battery? Second hand big ultra/super capacitors could be had for maybe twice what it costs to replace a starter battery. To me the cost or replacement might be a major point when deciding something like this. As a bonus no need to worry if the capacitors fall to zero as they are capacitors and can just be charged up again and are good to go. It's easy to ruin a lead acid battery by not charging it, also might freeze in the winter if not fully topped up. Living some place where temperatures might fall below -30 degrees Celsius, a capacitor would not mind if it was stored at that temperature fully depleted. Also with big capacitors a smaller lead acid battery would suffice, to keep cars systems like central locking functioning correctly. Big starter batteries are pain to carry around, some don't have a handle which makes it even worse.But replacing starter battery with capacitors mostly wins major tinkerer points, most likely won't become mainstream. For 99.9% of people with cars it makes sense to just use what has worked before.Quote from: Psi on August 10, 2014, 11:49:10 amthank you for taking the time to answer

author: Rufus
author_rank: Super Contributor
timestamp: August 10, 2014, 03:36:35 pm
content: Quote from: Simon on August 10, 2014, 09:27:03 amAnd £94 would buy around 830 kWh of mains electricity so you would need to get 830 cycles out of the batteries to break even on just battery cost.

author: eneuro
author_rank: Super Contributor
timestamp: August 10, 2014, 07:58:25 pm
content: Quote from: Psi on August 10, 2014, 11:49:10 amI think you might need overvoltage protection to those supercapacitors 2.5Vmax in series charged by alternator and it could be better to do not charge them to its 2.5V maximum ratings as I remember from its datasheet its lifespan shortens then?Hopefully, 14.5V from alternator ( I see sometimes 14.2V-14.3V without load) divided by 6 gives around 2.40V per supercapacitor, so I think it is good idea set this limit to something around it?For diesel engine factory starter battery ratings for VARTA L4 was 640ASo, it could be perfect to have in parrarel 3 such 6 in series, so 18 such supercapacitors needed, but it looks it could be possible pack them in more compact format to be able to put in battery plastic shield box for replacement.However, additional standard backup battery pack is a must, but one can make a DIY and put them away from engine, while if they are not needed do start a car, those batteries could charge those capacitors with much smaller current in the emergency, so longer distance is not a problemIt has another advantage-lead-acid battery does not like high temperatures which shortens its life if we look into its datasheets, so putting it close to engine is the worst place possible, but in "classic" cars it is a must while there is no other way to get hundreds of Amps needed to start those heavy engines, I guess.It looks like it might work, while a few old car abtteries could be put in pararell somewhere back in the car to charge those supercapacitors if neededI've developed 2.5V+ overvoltage protection circuit and this prototype for 14.5Vmax works fine.The only thing is faster mosfet driver with well designed deadtime needed, while when voltage is close to its set limit, there is many switches off/on with higher frequency and bad slow mosfet driver will not work while mosfet VGS will be below 10V and working in resistor mode andeven at small 55W-100W without good heatsink.It should be easy make this circuit 1 inch2 (2.5cm*2.5cm ) on double layer PCB in compact form to put together with controlled supercapacitor

author: Kevman
author_rank: Regular Contributor
timestamp: August 11, 2014, 03:42:24 pm
content: Quote from: Psi on August 10, 2014, 09:59:59 amMay I ask why? I saw the laser guy's video and thought "cool, I wonder if I should do that." Then I realized that I could think of no benefit, especially when compared to the downsides- if your car takes a little more time than usual to start you'd be screwed, can't listen to the radio (or do anything) with the engine off, if you left any of the lights on you'd be screwed, etc.I can really seeing this helping on those jump starter things, where the weight is a concern, though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2014, 08:08:37 pm
content: Quote from: Legit-Design on August 10, 2014, 10:47:19 amA load dump condition will simply be absorbed. The 80V (or so) excursion has no less than 0.5 ohms ESR, so peak currents are less than starting current, and for a shorter duration (< 200ms). In other words, it's more than sufficient to largely short it out, as is of course true of the conventional battery.Ironically, this spec does not matter, because a load dump occurs when the alternator delivers power into a suddenly open battery. It's a system spec, specifically excluding the battery.