Batteries in parallel - Page 1
Forum section: electronics / projects

author: Tioleco
author_rank: Contributor
timestamp: July 07, 2012, 07:49:54 pm
content: It is necessary to place resistors to equalize the power consumption of batteries when connected in parallel?

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 08:00:18 pm
content: this is a complex question that can only be answered after more information is provided, like what batteries are you putting in parallel ?

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2012, 08:31:11 pm
content: But in general, no. Resistance in a power circuit is not normally desirable.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 08:33:28 pm
content: Quote from: IanB on July 07, 2012, 08:31:11 pmtrue but if your alternative is starting a fire........ I have been told that putting lead acid batteries in series/parallel can be dangerous but i don't know hoe true that is. I think new batteries of the same type are ok but mixed models that have a different usage history if not new is a very bad idea.

author: Monkeh
author_rank: Super Contributor
timestamp: July 07, 2012, 08:40:04 pm
content: Quote from: Simon on July 07, 2012, 08:33:28 pmLead acids are used in both series and parallel configurations in staggering numbers in automotive, marine and UPS applications. But they are normally changed together, yes.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 08:44:56 pm
content: indeed i have a pack of 4 6V batteries, two in parallel and two in series to make a 12V pack with twice the amperage, but they are speced to be used in parallel. Often people asking about paralleling batteries are trying to build up a power bank with a bunch of used car batteries which I am told can result in disaster.

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2012, 08:45:44 pm
content: Quote from: Simon on July 07, 2012, 08:33:28 pmThis is always true when combining batteries together. Even new batteries that "should" be identical should be tested and checked for consistency before being combined.

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2012, 08:48:52 pm
content: Quote from: Simon on July 07, 2012, 08:44:56 pmNote that your "6 V" batteries actually consist of three 2 V cells wired in series. That's in fact the definition of a battery: a group of cells all joined together in series, parallel or both.But your observation about the dangers of used car batteries is sensible. Car batteries contain a lot of energy and can make spectacular fireworks or explosions.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 08:51:21 pm
content: Quote from: IanB on July 07, 2012, 08:48:52 pmIndeed and they are like that from new and identical cells which is why I only give partial credence to the hype on a certain other forum about putting batteries in series. I think paralleling old car batteries is where most fall foul

author: Monkeh
author_rank: Super Contributor
timestamp: July 07, 2012, 08:54:46 pm
content: Yeah, car batteries vary too much. Brand new ones of the same model, perhaps, but why would you do that? You can get more usable capacity out of purpose-designed SLAs at that money.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 08:57:12 pm
content: maybe for higher peak current output ? I guess it is just the fact that you can collect up second hand batteries for free. I have one out of my first car that I use as a portable power source.

author: G7PSK
author_rank: Super Contributor
timestamp: July 07, 2012, 09:02:37 pm
content: The only problem with parallel connection of car batteries or any other batteries is if one is not the same voltage, so if you are going to use a bunch of old car batteries you need to get them all to the same voltage level then when you charge them up again they will self balance. If you did that with a new and an old battery fully charged you would discharge the new one into the old one quite likely at very high current and cause violent gassing if you brought the new one down to the same level as the old one first you would be wasting a lot of capacity,I have done it with old plant just to get out of a tight corner and get it up and running again but if you leave the new battery in you will just waste it in the end.

author: madires
author_rank: Super Contributor
timestamp: July 07, 2012, 10:34:34 pm
content: Hi!Batteries got an internal resistance! If you want to connect multiple batteries in parallel you should use diodes for each battery to prevent any current flow between them.Cheersmadires

author: Tioleco
author_rank: Contributor
timestamp: July 07, 2012, 10:37:46 pm
content: Quote from: Simon on July 07, 2012, 08:00:18 pmFor digital camera, three sets of two 1.2V NiMH batteries in series.

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2012, 11:00:48 pm
content: Quote from: madires on July 07, 2012, 10:34:34 pmIt's not normal or useful to do that. Where did you get that idea?

author: madires
author_rank: Super Contributor
timestamp: July 07, 2012, 11:29:44 pm
content: Hi!Quote from: IanB on July 07, 2012, 11:00:48 pmBased on the battery type and capacity large currents may flow between directly paralleled batteries. And if one battery fails and its internal resistance decreases it will sink all current and overheat. That's nothing you would try with car batteries.Cheersmadires

author: IanB
author_rank: Super Contributor
timestamp: July 08, 2012, 12:50:08 am
content: Quote from: madires on July 07, 2012, 11:29:44 pmIn typical UPS applications you will find lead acid batteries connected in parallel with thick heavy bus bars. As long as you ensure only balanced and equally charged batteries are placed in parallel they will maintain their balance during charge and discharge. Any failing cells in a battery will manifest as lower voltage and longer charging.One thing to mention though: although some batteries can be connected in parallel and charged that way (e.g. lithium ion and lead acid), other types like NiCd and NiMH cannot do this. If you try to charge NiMH cells in parallel with conventional chargers you will rapidly destroy them.

author: digsys
author_rank: Unknown
timestamp: July 08, 2012, 06:44:19 am
content: If the batteries are of equal specs and PREFERRABLY from the same batch, they work WELL in parrallel.Just before you do the connections, make sure they are approx the same level, don't have to be exact.Two good things about parrallel - 1/ They "look after each other" to a great extent. ie "Stronger" oneswill keep "weaker" ones charged up. They remain weaker, but less "work" is expected of them. IF yourun a string as well, though, it does reduce the total capacity. 2/ Batteries MOSTLY go high impedancewhen they grow old, so the pack doesn't suffer from high energy discharge. IF you want that scenarioreduced, then put fuses/fusable links on each battery. A bit messy, but works fine. We don't usuallybother, just rely on many sensors and the BM.ALL Solar EV vehicles run up to 7-12 Lithium cells in parrallel, we do it for redunancy.I've made up dozens of packs. I've also made up several Lead acid packs for EVs, 99.9% FINE.But DEFINITELY NEVER MIX them !!

author: madires
author_rank: Super Contributor
timestamp: July 08, 2012, 11:14:09 am
content: Hi!Quote from: IanB on July 08, 2012, 12:50:08 amSmall UPSs got the batteries connected in series, because a lower current is simpler to handle (thinner wire, cheaper MOSFETs, cheaper transformer). Large UPSs got several of those series packs paralleled. But you have to make sure that a broken pack doesn't sink the whole ship. I'll write more about the latter in another reply.Cheersmadires

author: madires
author_rank: Super Contributor
timestamp: July 08, 2012, 11:48:36 am
content: Hi!Quote from: digsys on July 08, 2012, 06:44:19 amIt works mostly the time when using the exact same battery. But I'm worried about the probabilties someting may go wrong. Three 9s aren't safe enough when dealing with high currents. It's not about a failing battery pack, it's about what can happen when a battery fails. Overheating, explosion or batteries on fire aren't fun. It's a real danger. I've seen some not so nice results of those "works 99.9% fine" solutions :-(Cheermadires

author: digsys
author_rank: Unknown
timestamp: July 08, 2012, 12:10:09 pm
content: QuoteI quote 99.9% because NO-ONE can quote 100%, NO such thing. And with the advent of EVs, that number will drop incrementally. A Flat AA battery isnot 100% safe !! Over 2 dozens Solar EVs have paralleled Lithium batteries for up to 25 yrs, and we still do it. There's been VERY few accidents, andof those, the EXACT mechanism is arguable. I've assisted in "autopsies" !! The BM with it's various Temp sensors etc warn of any dangers.If anyone had to guarantee 100%, then you can kiss EVs and everything else adios. Do you know what the acceptable death rate for the motor car is?before notifications / recalls are made? You'll be surprised. Paralleling batteries has one of the lowest % risks. BEFORE we finalise ANY design, there'sone thing we ALWAYS consider. Will the EXTRA safety components actually DECREASE MTBF ! I work with this stuff.

author: SeanB
author_rank: Super Contributor
timestamp: July 08, 2012, 12:29:43 pm
content: $25 - Ford Pinto.

author: madires
author_rank: Super Contributor
timestamp: July 08, 2012, 01:39:15 pm
content: Hi!Quote from: digsys on July 08, 2012, 12:10:09 pmNobody can give you 100%, that's for sure. But some marketing departments try to :-) How do you deal with batteries off specs? Do you try to single them out before putting into an EV?Cheersmadires

author: digsys
author_rank: Unknown
timestamp: July 08, 2012, 02:00:01 pm
content: QuoteIn the case of the Solar EV racing cars, because we push our cells to the absolute limit, our matching method is very strict.We are supplied with pre-matched cells, 1-2 passes, and test data (mainly Internal impedance curves). We check, sort andmake up "modules", usually ~5-7 in parallel (the current car will have 21 in parallel !!) Then we run 3 conditioning cycles,both TOP and BOTTOM matching, then check the BM graphs. All "modules" need to be MAX +-25mV from median, usually 10mV.If so, we know ALL single cells in ALL "modules" are tightly matched. We rarely ever find any that drift after that, even years later.In the EV packs, we're no where near as strict. Basically, IF you have more than 1 set of parallel "modules" your BM simplyneeds to do an accurate voltage tracking, at all current levels. If they are within +-10-25mV, then you can be CERTAIN thatALL the individual cells are still matched. There is also one Temperature sensor per "module", BUT THAT can depend on air flow,air eddy currents etc, so we don't place as MUCH weight on it.For a Single parallel bank, we sense the current for each cell / battery. OFTEN it's just the bridge cable, so no complex wiringis needed. 1-2mR x100 fixed gain amp, is plenty to work with.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 09, 2012, 05:17:01 am
content: Many people worry about placing a discharged secondary battery in parallel with a charged battery,because they expect a large discharge current from the charged one into the "flat" one.Intuitively,this sounds obvious,because if you place a charged capacitor in parallel with a discharged cap,you,will get a high current flow into the discharged one.But Lead Acid Secondary batteries are NOT capacitors.Real batteries are often drawn as a "perfect" battery in series with an internal resistance.Only a small part of this is real resistance,the rest is the equivalent resistance of the chemical process which must take place to produce electricity at the output terminals,& conversely to store electricity in chemical form upon charging.As a battery discharges,this internal resistance becomes larger,until you cannot draw enough useful current from the battery.The point is,the internal resistance acts like a real resistance,so that when you place a source of charging current across the battery terminals,the current flow into the 'flat"battery is limited,until the battery has charged sufficiently for the internal resistance to have reduced.If this was not the case,the practice of "jump starting" cars would not be practical,or safe,& the small battery chargers which are commonly available would blow up when they were first connected & switched on.

author: mc
author_rank: Regular Contributor
timestamp: July 09, 2012, 04:00:44 pm
content: Ford transits from 2000-2006 had the option of paralled batteries to boost capacity, and were joined directly together with cables. The only thing fused was the main battery earth lead (which used to go pop when idiots tried starting them in gear!)From 2006 on, Ford added a battery saver relay to the two battery option, which directly connects the batteries together as soon as you turn the ignition on, and disconnects them either when you turn the ignition of, or if the battery voltage drops below a preset level. When disconnected, all the ignition controlled stuff runs of one battery, and 'saves' the other battery for starting, hence the name battery saver relay.I've seen both systems have one dead battery, and still function fine. You sometimes get excess gassing, but there's usually noticeable starting problems by the time the batteries get that bad.The biggest issue with the early system, is if you get one dead/shorted cell, that battery will pull the other one down, however it's not an instant process. Yes, the dead battery will gas due to the five good cells being overcharged, but the fumes are (should be) vented to the outside. It's no worse than driving a single battery system with the same problem, other than the good battery will continue the process for a bit longer.