Pairing a capacitor with a battery for quick charging? - Page 1
Forum section: electronics / beginners

author: boxvic
author_rank: Newbie
timestamp: April 30, 2014, 03:10:10 am
content: Hi all,I have a degree in Engineering Technology, and a few years experience as an electronics technician. I'm now following my real passion and working with industrial automation. I spend more time looking at ladder logic programs than circuit boards these days, and I'm starting to forget a lot of the stuff I learned years ago in school... but I've got a few projects at work that some of that forgotten info might be helpful for.In my plant we have an automated cart conveyance system. Automated sleds travel a preset course through the factory and pick up carts. They use gel cell VRLA batteries. Optimally they would all make it back to their charging station for several minutes to keep the battery going, and then get a full charge at lunches and shift changes. Obviously in a constant production environment that never happens though. We end up routinely changing out the batteries on the fly because sleds die in the middle of their route.I watched one of these sleds pull away from its charging station the other day after less than two minutes--gaining maybe 5% charge--and I was struck with an idea.Is it possible to use supercaps, that charge in a short time period, to slowly charge the batteries? I remember in school we built RC circuits and did all of the math, then actually changed out cap and resistor values to verify the formulas. I remember charging a cap in a few seconds and then discharging it through an LED over the course of a few minutes. In my head it seems like it would make sense, but again I don't remember enough to feel confident one way or the other.

author: Skimask
author_rank: Super Contributor
timestamp: April 30, 2014, 04:10:26 am
content: Quote from: boxvic on April 30, 2014, 03:10:10 amQuoteA good engineer would know why this, or any battery, can seem to gain that much "charge" in two minutes.

author: Mr. OBrien
author_rank: Contributor
timestamp: April 30, 2014, 04:32:08 am
content: Battery's store energy in a higher density than a capacitor. So If we assume that you are using a 12 volt battery at 100 Ah which contains 4,320.000 joules of energy and you have 5 super caps wired parallel that are 2.7 volts each at 350 farad which make for 13.5 volts and 38,880 joules of energy. so to replenish the 4,320,000 joules in the battery you would need to then series wire 112 banks of 5 paralleled 2.7 volt 350 farad super caps. which will take up 30 foot cubed of space to replace or supplement a battery that takes up a mere 2 or 3 foot cubed and i work on electronics so sorry if my math is poor on the spacial requirements but still this will take up a whole lot of space to work effectively. However yes it is possible.I am still learning so if their are things i missed I hope that other members correct me. Cheers!

author: ConKbot
author_rank: Super Contributor
timestamp: April 30, 2014, 10:57:22 am
content: Youre right that about the 4.3 MJ of energy in the battery, but the caps are in parallel so the voltage is still 2.7V in your example so only 6.3KJ of energy. But thats a pretty small cap bank. Maxwell does 500F 16V stacks (http://www.maxwell.com/products/ultracapacitors/products/16v-large-modules), which would be 64KJ of energy, or about 1.5 AH of charge @ 12v. Given lead acids mediocre charge efficiency, you could probably put 1AH back into a 12v battery, but you would need a DC-DC converter to suck all the charge out of the super cap and shove it into the battery. How big is the battery in carts in question? Given that you weren't cursing about changing them mid route, they aren't that big?But then again if they are running off of a lead acid, and are dead most the time, its going to hurt the battery, and the battery is going to lose capacity, which means its going to die even quicker. When you say is uses 'Gel cell VRLA' Which kind of battery do you mean? Gel cells are generally smaller batteries, (UPS size and smaller) and their surge current providing ability is poor compared to AGM and FLA/VRLA. AGM uses a liquid electrolyte in a glass matt instead of a gel, and has better peak current capabilities, and for an automation application like this, its what I'd use. VRLA generally indicates a flooded lead acid battery, and isnt considered 'non spillable' like the first two. They are cheaper than AGM, but perform similar, except they take a balance/equalizing charge better because the cells can gas. They do require maintenance though.My suggestion would be to shoe horn the biggest damn AGM battery you can in there, and if its significantly larger, upgrade chargers to match.

author: fcb
author_rank: Super Contributor
timestamp: April 30, 2014, 11:17:12 am
content: An interesting problem. You need to provide more data before it can be analysed:What make/model/capacity is the battery?What is the typical drain on this - i.e. how long does it last under normal usage (e.g. 1hr - 10hrs)?What is the duty-cycle? I.e. how long does it sit on the charger for vs. idle vs. active.What is the charging regime currently? i.e. how fast is the current charger.How much power is available at the charging station? (single phase 240V 13A, more??, etc...)Do you have a budget? What is the true cost of the flat carts - is it actually worth solving the problem?My initial thoughts is that you could replace the battery with something like NiMh or Li-ion cells, these can be charged far harder than LA's, but they are expensive and will have a fairly short life if you are constantly charging/discharing them.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 30, 2014, 11:23:11 am
content: If you had supercaps on the cart, which contained an amount of energy comparable to the amount which can be stored in the battery they're supposed to be charging, then why have batteries at all?

author: Rerouter
author_rank: Super Contributor
timestamp: April 30, 2014, 12:30:30 pm
content: The other factor you could look at instead is to use a smaller capacity super-cap array but with massively higher charge rates,the charging station becomes a high current trickle charger into a super cap array about 5 times bigger than in the vehicles, the vehicle reverses up and makes contact, the system detects this and after a short delay connects one to the other and charges it up at a few hundred amps in a few seconds, then it runs off and does its next run,the main limitations would be the connections between cart and charger being happy to handle that current, and a smart enough power supply on the carts so you can extract more of the energy out of them as the voltage approaches 0V,

author: max_torque
author_rank: Super Contributor
timestamp: May 01, 2014, 11:17:06 am
content: Two major issues:1) COSTand2) Batteries tend to have a small slope of voltage with charge, capacitors a very steep one, so you can't just put them in parallel without some form of (expensive, see point 1) DC-DC between them.........(much better would be to move to a modern battery chemisty, but i suspect the cost involved (probably requiring additional battery management systems) would make that step, er, expensive (see point 1 again)......!!

author: digsys
author_rank: Unknown
timestamp: May 01, 2014, 12:30:20 pm
content: Using a capacitor of a size that would be beneficial is BLOODY dangerous !! We use them in EVs with LiIon packs, mainly forESR - peak charge / discharge buffering, but let me tell you - I fear the caps FAR more than 5KW+Hr Lithium packs !! :-)

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2014, 01:07:16 pm
content: All I have on the UPS battery is a 150000uF capacitor bought as scratch and dent from Toploser, and another 18000uF made from a whole lot of separate used Nichicons. Helps with the discharge as the ESR of the battery increases as it discharges. Added about 10 minutes extra runtime till low voltage cut out.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 01, 2014, 08:10:32 pm
content: Cool idea, but...Wouldn't an easier solution be just to buy some extra sleds so they can be "round robin-ed"* in the in the charing dock?That is safer, supercaps are dangerous. Is it also faster, you can buy one right now, and you don't need to modify the (warranty of the) sleds.On the other hand. Accelerating anything takes a whole lot more energy than driving. Maybe you can just put in a boost cap?http://en.wikipedia.org/wiki/Round-robin_scheduling

author: rexxar
author_rank: Frequent Contributor
timestamp: May 01, 2014, 08:17:05 pm
content: Why not just replace the batteries with a larger or denser type, or even parallel an extra one or two-- double the capacity, and it'll last through the shift.As an aside-- how well do lead batteries stand up to rapid cycling with frequent tiny charges like this? Seems like this would be hell for any battery.

author: IanB
author_rank: Super Contributor
timestamp: May 01, 2014, 08:34:36 pm
content: Quote from: boxvic on April 30, 2014, 03:10:10 amWhy words like "optimally" and "obviously"?What you have here is a poorly designed system that does not meet its purpose. The way to improve it is to look at the system design as a whole. Are there enough sleds? Are there enough charging stations? Given the average number, distance and payload of sled journeys per hour, is the average charge rate available sufficient to handle this?One way to prevent sleds dying mid-journey is to put intelligence in the charging circuit. Make it so that sleds will not leave a charging station unless they are at least 50% charged (or something). If a sled won't start a journey with insufficient charge, it will not conk out half way.Having done this, you may now look at ways to increase the system capacity. Batteries with faster charging rates are certainly one option. More sleds and more charging bays are others.Even if it were possible to find a magic supercap that could work, it wouldn't charge any quicker than the existing battery unless the charger were able to supply more power. Can the existing chargers actually charge faster, or are they already charging as fast as they can? If the chargers are the limiting factor, then faster batteries or bigger batteries won't help.