What is a safe current rating for typical balance leads

[Tony Smith 7]

Hi,

While I'm doing some other electrical bits I thought it time to make up some better leads and adapters for the charger. One idea that I had was to use a single assembly that both charges and balances through the balance connector by taking the outer connections to the charger output ports as well as the balance connector. I can't see why that wouldn't work, but was wondering what sort of current I would need to limit myself to. Is there any generally accepted rating for balance wiring, one or two A would do me?

On a related note, I wonder whether I could make a single harness with both 2S and 3S connectors that I could leave permanently connected to the charger. I'm not completely convinced that nothing would be shorted out by such and arrangement, I'd have to draw it out. But if the current rating for balance leads is too low, it's all academic.

Tony S

[Chris Bott - Moderator]

I'm sure they will be fine at one or two amps Tony. I've charged individual A123 cells through them at 5A with no problems at all.

[Tony Smith 7]

Cheers, just need to find the bits and I get get set up. I've also found Tim Mackay's authoritative post on parallel charging. His diagrams and my own sketch both confirm that I can't make a dual purpose 2S and 3S harness - doing so would short out one cell of the 3S. Never mind.

[Peter Beeney]

Tony, - As I’m not very familiar with LiPo charging I’m a bit intrigued by your posts. Are you contemplating charging in parallel and series at the same time? It can be done, but you’d have to be careful. With regard to the current rating of the wire, as Chris says, I too would be quite happy with 5 amps, but you can bear in mind that if the current flow rates from parallel chargers are equal the current flowing in the balance leads is zero anyway. It would only be any difference in the individual charging rate that would be flowing, but if one parallel charger failed then the full current would flow. So the lead size has always got to be big enough to accommodate the maximum current anyway.

Using larger capacity cells may warrant using larger cross-sectional area wire anyway.

Also thinking about the 2S and 3S parallel lead, I’m not entirely sure how this would short anything out? It must surely depend on what, and how, you connect to it? Again, you mention parallel charging here, this would easily be possible, but only with packs of the same cell count. If you wanted to charge a 2S and a 3S in parallel, I reckon this would require a specialist charger, so I don’t think it’s ever going to be even contemplated.

Interesting speculations here………

PB

Edited By Peter Beeney on 29/12/2012 14:26:12

Edited By Peter Beeney on 29/12/2012 14:30:07

[Chris Bott - Moderator]

Peter I'm sure you know all this, the usual layout to balance charge lipos includes a balance connector which connects individually to both ends of the battery and every cell junction in between.

But this isn't enough for most chargers, they also need the fat wires plugging in too. Very necessary when higher charge currents are being used. I think it also helps the charger know how many cells are plugged in, an extra safety measure if you like.

What I read Tony's post to mean is that he wants to make a harness up that plugs into all the necessary charger ports, but then only needs to connect to the battery using the balance connector.

I don't see any issues with this, but I do hope others will chip in if a problem has been missed.

[Tim Mackey]

I use a similar harness to charge indivuidual smallish capacity 2s batteries when they are buried in a foamy model such as several of my slopers. The actual main power leads of the battery were removed as they are simply too bulky - and power delivery and charging is done only through the balance plug. The harness has main power leads to plug into the charger, and a balance connector, whilst the battery end has just a balance "socket" which also doubles up as a "switch".

I have charged through this method at up to 2A with no signs of a problem.

I did a post about this some years back, so I will have a trawl now and see if I can find it

Cant find it... but heres a link to a similar topic

Edited By Tim Mackey on 29/12/2012 15:50:49

[Peter Beeney]

Yes indeed, Chris, exactly so! … So then why not mount a 2S and a 3S connector on a piece of PCB, with a neg and pos heavy duty power lead, extend the balance wires to a suitable connector to link to the charger and then it would be possible to charge a 2S or 3S pack at will… But not both together, unless you made special provision for this. I believe these type of multi connection units are made commercially, are they not? This arrangement could be left permanently connected to the charger, or not, at the user’s discretion.

I’d consider the charger will always charge the pack in series, so you would always require the power leads to be connected. So the balance currents are only ever going to be minimal, just milliamps. Even if you charged each cell in parallel the end of pack connections carry the full current, so you’d require the power leads to be connected but you still wouldn’t require much of a balance lead, it carries little or no current, until something goes wrong, that is.

I did raise the question of the balance connection being used as a ‘cell count’ a while back, only to find that some chargers already do this. Excellent; I think this is a really major safety feature. It means that the the charge won’t fire up until correctly programmed. Voltage discrimination might work on low cell counts, but how does it decide if say 15 volts is a partly charged 4S, or a flat 5S? If that leads to the 4S being charged as a 5S it might have repercussions.

Maybe what I’d consider is that it comes down is the fact that if you are happy the balance lead wires are capable of carrying the full charge current then there is no need to provide or connect the power wires. If not, then the power wires need to be connected.

PB

[Chris Bott - Moderator]

Peter the charger always needs the power wires to be connected, but they could be connected to the balance wires on the extension unit. But herein lies the problem with trying to make a 3S and 2S unit in one if you want to use only the balance connector on the battery.

When charging 3S or 2S the negative power wire is always connected to the same balance wire, at the battery.

However the positive power wire is connected two steps up for a 2S or 3 steps up for a 3S, so not to the same balance wire in each case. Tony had already spotted this issue.

[Chris Bott - Moderator]

I guess Tony's adapter would look a bit like this?

The problem being that the fat red wire needs connecting to two different places.
A switch could fix this.

[Tony Smith 7]

Posted by Chris Bott - Moderator on 29/12/2012 16:59:02:

However the positive power wire is connected two steps up for a 2S or 3 steps up for a 3S, so not to the same balance wire in each case. Tony had already spotted this issue.

Yes indeed, trying to make a dual-purpose 2S/3S harness would short out the third cell on a 3S. That's why it's good to sketch connections out before trying. Shame because at the moment I only charge smallish 2S and 3S packs, so it would have been nice to have a set of permanent connections on the charger.

[Peter Beeney]

Yes, I’m quite happy with that, you can certainly use the power lead or the balance lead to charge the cells, depending on the charging current, but I think you would have to make a variable connection, otherwise you’d have an open, rather than a short circuit surely? The balance wires are common at the battery, it’s within the charger/balancer that the difference lies.

You can connect a 2S to a 3S, but, as I said, you would need a specialist charger to charge it, but if you connect the two negatives and the two positives together then you certainly don’t need the balance lead to get into trouble. The 3S would instantly discharge into the 2S, the current would be proportional to the voltage difference and the combined resistance. It could be quite high. Both cells of the 2S would be shorted by the 3S; and you would definitely would not be able to use this in this condition.

When I watch the electric bods charging at the field they just use a balance lead lead which seems to be a minimum of effort. I’m not sure it could be made any easier. This whole system needs to be made fail-safe, in the sense that it needs no operator input to work correctly. Any incorrect setting should result in a simple shutdown of the system. It seems to me that many of the malfunctions we see are the result of operator error. I always try and stick to the old adage, KISS, Keep It Strictly Simple!


I shall hopefully be talking to my electric friend tomorrow, weather permitting, but he very seldom balances his packs anyway, his original charger didn’t have that facility. But I will look at his system, and maybe find out how it’s connected.

I’m sure that it must be the same as all the LiPo chargers, so then for one thing I’d be interested in the charge rates, he charges packs as fast as possible, and so I can then relate that to the balance lead size. From the little drawing, I’d say it’s only the two outer wires that are carrying all the current in both cases, the other wires are just the normal small balancing currents.

PB

[Chris Bott - Moderator]

The point Peter is that many (most?) LiPo chargers won't allow a balance charge without both balance and power cables attached.

Tony wants to do this using just the balance connector.

[Tony Smith 7]

Posted by Chris Bott - Moderator on 30/12/2012 10:24:39:

The point Peter is that many (most?) LiPo chargers won't allow a balance charge without both balance and power cables attached.

Tony wants to do this using just the balance connector.

Correct, but I'm going off the idea now because I have both 2S and 3S batteries, so if I need to connect the correct harness each time then there's even more to plug in than normal. I had this idea (cloud cuckoo land) that I could leave it connected with a 2S and a 3S connector permanently available, with just one connection to make each time.

I might make something just for 2S for the transmitter, so I can charge without taking the pack out, and I don't have to have a great XT60 connector cluttering up the battery compartment. I need to make some charging adapters anyway.

[Tim Mackey]

Combi charge lead

Edited By Tim Mackey on 30/12/2012 16:43:51

[Sparks]

To answer the original question, the current rating of the JST XH series of connectors typically found on the end of LiPo balance leads is 3A DC.

Sparks

 

 

 

 

Edited By Sparks on 30/12/2012 16:54:23