Parallel packs

[Matt Carlton]

Where space is limited in terms of length (such as tank bays in IC conversions) is it possible to connect two lower capacity LiPo packs in parallel in order to use two smaller packs to make a shorter but wider pack?

If that is possible, is it better to treat them separately, or to charge/balance the two packs simultaneously? I imagine they would need to be balanced against each other?

[Dwain Dibley.]

It is possible, and I would treat them as one battery so all cells are balanced.

There are many balance boards on the market these days Matt.

D.D.

[Matt Carlton]

Thankyou 👍

[Roger Jones 3]

Posted by Matt Carlton on 10/01/2021 22:12:59:

Where space is limited in terms of length (such as tank bays in IC conversions) is it possible to connect two lower capacity LiPo packs in parallel in order to use two smaller packs to make a shorter but wider pack?

If that is possible, is it better to treat them separately, or to charge/balance the two packs simultaneously? I imagine they would need to be balanced against each other?

It's fine, but there are a few points to bear in mind.

They should be the same capacity, the same number of cells, and the same 'C' rating. Ideally the same make and 'model' too.

You can charge them in series or parallel. There are numerous balance boards available for parallel charging, rather less for series charging.

But in fact balancing is not necessary for every charge, I usually only bother once every ten or so charges and it's fine. (The standard charging protocol, which is constant current for most of the charge and constant voltage for the last bit, will tend to balance them anyway.) But you should definitely balance them now and again, charging and balancing each battery separately.

It's a good idea to buy a charger that has two entirely separate outlets and two entirely separate balance boards. Then you can do anything.

[MattyB]

Posted by Roger Jones 3 on 11/01/2021 08:02:47:

...They should be the same capacity, the same number of cells, and the same 'C' rating. Ideally the same make and 'model' too.

Packs have to be the same cell count to parallel charge, but not absolutely the same capacity - you can quite happily charge (say) a 3S 2200 and a 3S 3000 at the same time providing the starting voltages are nearly identical at the start (I don't parallel charge unless they are within 0.1V/Cell - always check each pack first individually!). Always connect the main connectors first (before the balance leads) as if you did connect two packs of slightly different charge statuses those connectors have the capability to take the current that will flow in that first second or two.

Re: C ratings, if they were anywhere near accurate I guess I'd agree, but since 99% are entirely made up I just check the IR (internal resistance) every now and again; provided they are within the same ballpark you can happily charge packs with different sticker C-ratings safely. IR is after all proportionate to the real world C rating of your pack (here is a calculator that gives you an idea of the real world performance of your packs, though be prepared not to like the results...!)

Posted by Roger Jones 3 on 11/01/2021 08:02:47:

...But in fact balancing is not necessary for every charge, I usually only bother once every ten or so charges and it's fine. (The standard charging protocol, which is constant current for most of the charge and constant voltage for the last bit, will tend to balance them anyway.) But you should definitely balance them now and again, charging and balancing each battery separately.

This is technically correct in terms of battery health, but I balance charge every time purely because it is safer - the charger knows the individual cell voltage of every pack, and if something goes wrong will stop the charge before a cell gets overcharged. Some people won't parallel charge for this exact reason (the charger is not monitoring the cell voltage of every cell, only the average of all the cell 1s, cell 2s, cell 3s etc), but providing you always check the voltages of all battery packs individually before you start and take all the usual precautions (don't charge unattended, charge on a hard surface, etc.) you will not have any issues. I must have 2000+ parallel charging cycles on batteries of all sorts of capacities and cell counts with no issues, but I do monitor my packs carefully by charging them individually every now and again and noting down the key parameters.

Edited By MattyB on 11/01/2021 10:01:47

[EarlyBird]

Posted by Matt Carlton on 10/01/2021 22:12:59:

Where space is limited in terms of length (such as tank bays in IC conversions) is it possible to connect two lower capacity LiPo packs in parallel in order to use two smaller packs to make a shorter but wider pack?

If that is possible, is it better to treat them separately, or to charge/balance the two packs simultaneously? I imagine they would need to be balanced against each other?

I use two 2200 3S in parallel here is a photo from my RF-4 build

I always balance charge them separately at the same time so they are at the same voltage when connected together, don't ask me why just seems a good idea.

Cheers

Steve

[John Lee]

If you search you may be able to find a suitable parallel pack already made up, eg

[Lima Hotel Foxtrot]

Posted by Roger Jones 3 on 11/01/2021 08:02:47:

But in fact balancing is not necessary for every charge,

No, no, no, no, no. Wrong. Unsafe. Irresponsible.

[Lima Hotel Foxtrot]

Re parallel charging: Given that you would be using two totally separate batteries, I would charge them as two separate batteries and just not do it. I know that in theory parallel balance charging is fine, but the charger is treating the two separate bundles of cells as one bundle of cells so I can't see how errors won't creep in over time.

I see it as a fire safety and battery health issue, and even though you can do it does not mean you should do it.

I have A LOT of lipos of different charge cycles and cell health, and never parallel charge them.

[EarlyBird]

LHF I agree with you. I see no advantage in not always balance charging.

[MattyB]

Posted by John Lee on 11/01/2021 10:59:31:

If you search you may be able to find a suitable parallel pack already made up, eg

There are only minimal advantages in those kind of packs 0 they are slightly smaller for a given capacity, but IMO that's about it. You can't charge, balance or use them separately, and heat dissipation is worse (as there are more cells "stuck" in the middle). Two packs wired in parallel is generally a better bet unless this is really the only way you can get sufficient capacity in a given space.

[Matt Carlton]

Two packs in parallel seems a better option from a variety of points of view. Not least that they can be more flexibly located in the fuselage.

I do need a new charger now though as my single output one is not going to be good enough.

This one looks interesting;

Sky RC charger

[Nigel R]

Posted by Lima Hotel Foxtrot on 11/01/2021 11:17:08:

the charger is treating the two separate bundles of cells as one bundle of cells so I can't see how errors won't creep in over time.

Errors won't creep in during charge, because it is being treated as one bundle of cells.

Voltage can and will drift after charge has finished, but that does not affect the charge/balance process.

Balancing is always a slow and steady phase of the process, cells will be quite relaxed by then, there will be little drift afterward.

Any errors from charging quickly become moot during normal use in an airframe anyway.

As MattyB's post, make sure they are within 0.1V before connecting them together to limit the rate of charge transfer between packs.

[Nigel R]

Posted by Matt Carlton on 11/01/2021 15:25:52:

Two packs in parallel seems a better option from a variety of points of view. Not least that they can be more flexibly located in the fuselage.

I do need a new charger now though as my single output one is not going to be good enough.

This one looks interesting;

Sky RC charger

You only need a parallel board which costs under £10.

I only do parallel charging through a board with a 20A charger, so a set of 4 to 8 packs is done within the hour.

Charging them one at a time in sequence would be going right around the houses to put it mildly.

[MattyB]

Posted by Nigel R on 11/01/2021 16:48:46:

Posted by Matt Carlton on 11/01/2021 15:25:52:

Two packs in parallel seems a better option from a variety of points of view. Not least that they can be more flexibly located in the fuselage.

I do need a new charger now though as my single output one is not going to be good enough.

This one looks interesting;

Sky RC charger

You only need a parallel board which costs under £10.

I only do parallel charging through a board with a 20A charger, so a set of 4 to 8 packs is done within the hour.

Charging them one at a time in sequence would be going right around the houses to put it mildly.

Agreed. The other factor is that charging one at a time will either take a lot longer or you will have to push your packs much harder to get them charged in the same time (4-5C). This will build up more heat inside them and decrease cycle life and increase IR over time, as well as meaning they come off less well balanced. It's only my opinion, but I personally believe charging 5 packs at 1C in parallel (having checked their starting voltages carfeully first on the charger) is no less safe than 5x 5C charges of individual batteries. YMMV.

PS - A funny story... I remember having a discussion with a guy who told me at a club field that I was charging 5 packs "unsafely" in parallel at 1C (in an ammo box under a fire blanket, attended). After this discussion he to proceeded to connect a single 3S 2200 and charge it straight on the bench at ~7C (15A), AND walked back out to the pits whil he did it! Errrr... 

Edited By MattyB on 11/01/2021 17:23:04

[Matt Carlton]

If using a parallel board, I assume the packs must be the same. Therefore, if I have a charger with a single output, I can charge multiple "identical" packs, so if I want to charge, say, 2 x 3S 1600 though the board at 1C, the output would need to be 3.2A. However, if I also want to charge a 3S 2700 at the same time, also at 1C (2.7A) then I would need a second output from the charger. That must also be the case in order to charge a 3S pack and a 4S pack at the same time.

It makes sense to have a more capable charger having decided to move over to electric flight, rather than struggle on with an 80W 7A charger

Edited By Matt Carlton on 11/01/2021 17:46:20

[Geoff S]

Posted by MattyB on 11/01/2021 09:54:38:

Re: C ratings, if they were anywhere near accurate I guess I'd agree, but since 99% are entirely made up I just check the IR (internal resistance) every now and again; provided they are within the same ballpark you can happily charge packs with different sticker C-ratings safely. IR is after all proportionate to the real world C rating of your pack (here is a calculator that gives you an idea of the real world performance of your packs, though be prepared not to like the results...!)

How true.

I've just bought a couple of 6S 4.5 AH LiPos from an eBay seller. They claim to be 60C - ie 250 amps. More than adequate for my Sopwith Pup whiich only dras 50 amps flat out, which will be rare and for very short bursts. My main criterion was the size to make sure they fitted the space available.

They're fitted with X90 connectors so I needed to buy some bfore I checked the IR with my Wayne Giles meter or my iCharger. The actual C rating of each cell is 12 to 15 and the IR is around 5 milliohms/cell. So the actual maximum recommended current is about 50 amps but still sufficient for my needs.

As long as they last well, I'm happy. They were relatively cheap (£87 for the pair) so fingers crossed. I just wish I could use them.

Claimed 'C' ratings are, in my experience, totally fictitious.

Geoff

[MattyB]

Posted by Matt Carlton on 11/01/2021 17:45:28:

If using a parallel board, I assume the packs must be the same. Therefore, if I have a charger with a single output, I can charge multiple "identical" packs, so if I want to charge, say, 2 x 3S 1600 though the board at 1C, the output would need to be 3.2A. However, if I also want to charge a 3S 2700 at the same time, also at 1C (2.7A) then I would need a second output from the charger.

No, providing the starting voltages and IR are close you can charge packs of different capacities but the same cell count in parallel. Because we are charging in parallel the cell voltages will rise at the same rate - remember when  all the charger is effectively seeing is a single pack. My only caveat would be don't charge small packs (anything less than say a 1000-1300mah) in parallel with bigger ones, as there is more likely to be a large disparity in IR between those.

Posted by Matt Carlton on 11/01/2021 17:45:28:

...That must also be the case in order to charge a 3S pack and a 4S pack at the same time.

Yes, that bit is correct unless series (serial) charging. A small number of people do that, but I personally prefer to parallel charge. This is because you are working with lower voltages if parallel charging, and most chargers tend to be limited to 6S or at max 8S. You could only charge 2x 3S or 4S packs in series vs a larger number if parallel charging.

Posted by Matt Carlton on 11/01/2021 17:45:28:

It makes sense to have a more capable charger having decided to move over to electric flight, rather than struggle on with an 80W 7A charger

Yes, definitely. I do not recommend anyone starting out in electric have less than a 300W charging setup, but really the bigger the better - it gives you so many more options and room to grow into larger packs and models. Nobody every wished they had bought a less capable, lower wattage charging setup! The best power supplies for the bigger DC chargers are are from servers; search in this forum and RCGroups for lots of threads on converting them.

Edited By MattyB on 12/01/2021 12:27:58

[MattyB]

Matt, one final thought... This article on charging is well worth a read, it explains what the charger is doing during the different phases of charging a lithium ion pack such as our lipos.

[Nigel R]

Battery university is a gold mine of information.

[Matt Carlton]

Generally, is it preferable to have a pocketful of charged packs for a days flying, or to try to charge at the field? Many of the higher powered chargers seem to need a DC input higher than 12V, so, if I get a charger which can use a combined AC/DC input, can I simply plug the charger into a 240V inverter, plugged into the cigar lighter socket of my car? I assume yes, providing that the wattage is within the limit of the inverter. Might need to leave the car engine running though!

I know that 16v lead acid batteries are available but they're really expensive and heavy to lug around.

[Alan Gorham_]

Many of the higher power chargers can charge higher voltage (5s - 6s) packs from a 12V dc source due to the design of their internal power supply. It is not necessary to use a 16V dc source.

Any lead acid battery that you choose for field charging will by it's very nature be heavy and it will also have a limited life, so you will have to factor in replacement costs.

I used a 12V 100Ah leisure battery as my field charging source for a few years.