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Author Topic: LIPO in Parallel  (Read 1022 times)

Charlie

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LIPO in Parallel
« on: March 17, 2019, 01:50:35 PM »

Just wondering how to power a possible future project. Is it ok to connect 2x 5000mah 3s Batteries in Parallel? Iím looking to extend the run time.

Andyn

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Re: LIPO in Parallel
« Reply #1 on: March 17, 2019, 02:36:59 PM »

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Charlie

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Re: LIPO in Parallel
« Reply #2 on: March 17, 2019, 05:26:42 PM »

Hi Andy, how do you connect the low voltage alarm to the Lipo? I haven't seen these before. Do they plug into the balance lead?

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Re: LIPO in Parallel
« Reply #3 on: March 17, 2019, 08:57:09 PM »

They plug straight into the balance lead and when critical voltage level is reached they emit an extremely  loud beep. They can be left on for the duration of the run with no issues, just unplug them when storing batteries. I like to scotch tape them to the batteries for permanent  fixture.
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Martin [Admin]

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Re: LIPO in Parallel
« Reply #4 on: March 17, 2019, 09:27:38 PM »


If you do go parallel, only use matched pairs of batteries.
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malcolmfrary

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Re: LIPO in Parallel
« Reply #5 on: March 18, 2019, 09:45:51 AM »


If you do go parallel, only use matched pairs of batteries.


Not just matched according to the labels, they need to be in the same state of charge when they get connected.  If not, they will try to even themselves out very rapidly.  This is not good. 
Once they are connected, as long as they stay that way, they will remain in the same state of charge as each other.
For parallel cells, no balance needed, that's for series cells.  Effectively, its one big cell, the alarm connects as such, just looking at one cell.
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Martin [Admin]

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Re: LIPO in Parallel
« Reply #6 on: March 18, 2019, 12:47:23 PM »


For parallel cells, no balance needed, that's for series cells. 


Is that the right way round Malcolm?
I thought it was more important, with Any batteries in parallel that matched balanced, so they don't charge discharge into each other..... or have I misread your post?!  {:-{
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malcolmfrary

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Re: LIPO in Parallel
« Reply #7 on: March 18, 2019, 06:14:17 PM »

Is that the right way round Malcolm?
I thought it was more important, with Any batteries in parallel that matched balanced, so they don't charge discharge into each other..... or have I misread your post?!  {:-{




Just had a re-read of the OP, noting all of the words this time esecially the "3S" bit. :embarrassed:   A laptop has banks of parallel cells to get the capacity connected in a series stack to get the volts.  Paralleling two sticks of three still needs both sticks to start at the same voltage to begin with before they are connected.  The circulating current could easily exceed the C rating of one or the other (probably the one charging from its new friend).
An alarm plugged into each will check for any cell needing attention.  Recharging still means that both 3S sticks really need to get to the exact same end voltage.  I haven't seen it yet, but there might be a way to plug the balance leads together so that the whole can be charged together and regarded as one battery for convenient use, effectively turning it into one big battery.
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Colin Bishop

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Re: LIPO in Parallel
« Reply #8 on: March 18, 2019, 06:39:09 PM »

You do begin to wonder if this is progress. Time was when you just had two wires coming out of any sort of battery and you simply connected them to the load. Now there's all this stuff about balancing, minimum voltages, maximum charging rates and voltages and if you get it wrong there's a fair chance you could start a serious fire if not an explosion...

Colin
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malcolmfrary

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Re: LIPO in Parallel
« Reply #9 on: March 18, 2019, 06:55:57 PM »

You pack more and more energy into a smaller and smaller space, something has to give.  In this case, the result is that until the "next" generation of battery comes along where the safety problems are resolved, we need caution lest the energy comes out sideways and all at once, doing what chemical energy does, degrading itself down to heat.  Or heat and mess.
But lead acid had its own hazards, mostly from the contents themselves, being lead (toxic) and sulphuric acid (extremely corrosive) but also from the construction.  Plates that could warp under heavy current and plate to riser joints that could set the hydrogen/oxygem mixture off.  NiCad stuff was disinctly unfriendly, not sure what lurks inside NiMH batteries, but I bet it ain't health-giving.
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DaveM

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Re: LIPO in Parallel
« Reply #10 on: March 18, 2019, 07:09:08 PM »

What we need is an authorititive and easily-understood answer, it seems. Here's my attempt, for whatever it's worth -
Parallel batteries to me are anathema, for all the reasons Malcolm had given and a few more besides. I've personally never used them and I never will. What's the huge problem with bringing the model back to shore when it starts to slow down or beep and just swapping the battery for a fully-charged one? No chemical fallout or fire-risk there.
Works for me every time.

DM
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Colin Bishop

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Re: LIPO in Parallel
« Reply #11 on: March 18, 2019, 07:12:17 PM »

True Malcom but all in all, for general modelling purposes LiPos do seem to be a lot more volatile that the older technologies. A many people seem to be going for LiPo/ brushless setups when it is not really necessary. It just seems to be the fashion.

The Fairey Huntsman SLEC kit I built has the recommended small brushless motor and a 3S LiPo pack and indeed goes like the clappers like the original. However my much larger and heavier 48 inch Fishery Cruiser has a 'conventional' NiMH/brushed motor setup and powers along very impressively whist drawing just 2.5 amps. My latest model liner will also have a brushed motor setup with NiMH cells as the combination is more than adequate for the model. The motor/battery setup is a lot cheaper too.

You could probably shoehorn a V8 into a Ford Fiesta if you really tried - but why bother if you only do normal motoring.

As always, it is just horses for courses really.

Colin
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Colin Bishop

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Re: LIPO in Parallel
« Reply #12 on: March 18, 2019, 07:17:05 PM »

Dave, LiPos aside, there is an option for scale models in having a completely separate setup fot twin screw scale models where each motor has its own battery and ESC so there is no power crossover between the two propellers. I do agree with you that coupling battery packs together in any sort of configuration, parallel or series is inviting trouble. Why do things the hard way?


Colin
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JimG

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Re: LIPO in Parallel
« Reply #13 on: March 18, 2019, 09:25:01 PM »

Why is there always the same scaremongering when anyone suggests connecting two packs in parallel. This has been done for years from lead acid then NiCad, then NiMh now LiPo. The idea that paralleling two packs will give an incredibly high current as one charges the other was debunked years ago on RC Universe by Red Scholfield from one of the battery manufacturers. As long as they are reasonably close in charge the current will be well within the limits of the wires and cells. The surge current on connecting to a brushless esc is much greater as you are charging capacitors at 0V, a near short circuit. This is why I have fitted anti spark connectors to my 6S packs, I have seen damage to the connectors in the past through sparking.
For charging two packs in parallel there are parallel charging boards to connect to a LiPo charger. These parallel both the power connector and the balance leads to allow for a balance charge. Some allow up to around 6 packs to be parallel charged.
I have often used 2 packs in parallel both in planes and boats without any problems and will do so again. While I do make sure the capacity of both packs is the same I don't bother with checking the charge state of each, just using packs charged on the same or previous day. Since each are well used packs even though they may start off close they will discharge with sitting at different rates so will not be identical when connected.( I have seen a post on RCU where someone said that they discharge unused packs for storage at the field by connecting them in parallel with used packs bringing both to a half charged state. :o )
Jim
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Charlie

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Re: LIPO in Parallel
« Reply #14 on: March 19, 2019, 12:32:30 PM »

Thanks for all of the replies.

JimG

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Re: LIPO in Parallel
« Reply #15 on: March 19, 2019, 12:39:15 PM »

Just as an addition to the so called need to make sure that the batteries are identical I decided to do a simple experiment. I took two 3S 8000mAh packs which are the same make but were at vastly differrent states of charge, , one was about 95% and the other was 3%. (it had been sitting around for over a year without charging.) These were connected in parallel with a Wattmeter in series with one pack to measure the current.Immediately after connecting it showed a current of 20A which rapidly dropped, after a couple of minutes to around 8A (normal charging current at a 1C rate).. At no time did the wires or connectors become more than slightly warm to the touch. At the end of the check there had been around 1.25Ah transferred and the low pack was at 25% and the high pack at 73% after being left to to sit for a few minutes. It therefore seems after this quick check that there is no problems with connecting two identical capacity packs in parallel even if their charge state is widely different.The myth is BUSTED.

Jim
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Charlie

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Re: LIPO in Parallel
« Reply #16 on: March 19, 2019, 02:09:42 PM »

Thanks Jim, good to know

DaveM

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Re: LIPO in Parallel
« Reply #17 on: March 19, 2019, 02:20:43 PM »

Shows that no-one's too old to learn, dunnit? I was interested in your mention of anti-spark connectors. Is this the type you've fitted? https://hobbyking.com/en_us/xt90-s-anti-spark-connector-2pairs-bag.html
DM
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Martin [Admin]

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Re: LIPO in Parallel
« Reply #18 on: March 19, 2019, 02:49:07 PM »


Why is there always the same scaremongering when anyone suggests connecting two packs in parallel. .......

Jim

That fair enough Jim and thanks for actually doing a MythBusters for us!    :-))

I think the alarm bells are due to the videos we've all seen on Utube of damaged or overstressed LiPo batteries causing you not to have a nice day anymore..... and are running scared....  O0

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malcolmfrary

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Re: LIPO in Parallel
« Reply #19 on: March 20, 2019, 09:56:08 AM »

Did the low charge battery come back up to a full charge with the expected performance after it was connected to a charger after the test?  If it wasn't a good battery, the results become suspect.
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JimG

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Re: LIPO in Parallel
« Reply #20 on: March 20, 2019, 12:52:38 PM »

Shows that no-one's too old to learn, dunnit? I was interested in your mention of anti-spark connectors. Is this the type you've fitted? https://hobbyking.com/en_us/xt90-s-anti-spark-connector-2pairs-bag.html
DM
Thats the connectors , completely stops the loud spark I always used to get on connecting a 6S pack. Only thing to remember is to make sure you push them fully home or the resistor  stays in circuit.
Jim
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JimG

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Re: LIPO in Parallel
« Reply #21 on: March 20, 2019, 12:55:45 PM »

Did the low charge battery come back up to a full charge with the expected performance after it was connected to a charger after the test?  If it wasn't a good battery, the results become suspect.
Still haven't given a charge to them yet. The low pack will only be getting a storage charge as it is not being used at the moment, the other will be full charged  as it is in regular use. Both however are not heavily used so will still give good performance even when near the end of their life.
Jim
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