https://www.powerstream.com/Wire_Size.htm

http://wiresizecalculator.net/calculators/advancedwiresize.htm

I'm glad everyone learned something. 

Strangely I was quoted, however I only repeated what was on the Video, and was asking for clarification (disagree all you want, I was just a repeating rabbit 😆 )

Looks like my Question about Wire Thickness vs Current/Voltage limits was answered. I assume I might find a table of some kind. 

All modern Batteries (lithium based) have balance leads, even if you don't see them

Now in the video it was charging at 4 AMPS, but this was conservative charging Amps vs what the Battery was rated for. 

I'm assuming (again) that the High Charge rate is done though the main power leads, and top-up with the Balance Cell leads, which involves a microprocessor of some kind. 

I think you both have the general picture now ... at least as far my limited understand, which may be flawed.

Namely, if you need fast (high current) charging capability, this is likely to use the main power connections for both charging and discharging... though of course there will always be exceptions with batteries with two pairs of high current connections for some reason or other.

The monitoring/balancing wires are usually much lower current capability, so they might be only used for voltage monitoring, or they may also play a part in the charging and discharging, especially if the BMS has a charge balancing capability. Obviously, if these wires are thin, which they typically are, then the current capacity will be limited, but even relatively thin wires, say 1 square mm, can still carry several amps. Assuming all of the cells of a battery  are of identical design and 'age', then the balancing should not exceed a few per cent of the total capacity, so the current flows should be modest compared to those required of the total battery capabilty.

My understanding regarding lithium-ion battteries, which include so called Li-Po s, is that the most important provision is one of cell monitoring ... in the event of cells being over charged or discharged, the entire chemistry of the cell can change, resulting in a very dangerous device.

Balancing is used to obtain the maximum capacity performance from a set of cells whch will all naturally have slightly different capacities, in spite of being identical design.  In the absence of balancing capability, the monitoring system will limit the battery to that of the weakest cell, as it is essential that no cell is over stressed.

Balancing is much more important in batteries which have cells in parallel, as well as in series.

There are two different balancing 'options' .. sometimes described as active and passive.

With the passive scheme, the higher performing cells (ie are exhibiting a slightly higher voltage) are discharged to match that of their immediate 'colleagues', so that they all work as a 'team'. My guess is this likely to be the scheme chosen for the batteries in the video.

The active scheme actually transfers charge from one cell to another as the battery is discharged, so that they are all kept at the same percentage of discharge of their individual capacities. This requires each cell to have an individual boost power supply so that it can accept the discharge voltage from another cell whch has charge to spare, and boost it to the voltage needed to charge the more depleted charge cell. This is obviously a considerable amount of electronics, although chip technology packaging can assist. Hence, I would normally expect this to be used in higher capacity batteries .... perhaps the battery pack for an electrically powered only car could be a candidate, where each cell will be much larger than the 18650s, etc. that we are generally more familiar with?

Hope this is clear, of interest and doesn't have to many errors.

Best wishes, Dave

Whilst writing this sermon, I see @robotbuilder introduces the suggestion the battery has built-in electronics.

How else can these packs be used with a simpler plug into the mains means of charging them?

From the sounds of it that would be essential for any LIPO battery pack.


https://youtu.be/M5MSSVN9eWw

Both means with and with out the big heavy cables. Basically the smaller batteries can get by with just the balance leads while my 7,200wH battery pack needs to use the 4/0AWG wires (1/2 inch). The 4/0 isn't needed for charging, 2AWG (1/4 inch) would suffice but the inverter can go as high as 300A.

re:

However, I would say that pushing 4A x 4.2 Volts is a lot of power for such wires, however I didn't consider that each wire would split the total wattage. 

In this case, it is a 3 Cell, therefore math: (4x4.2)/3 = Wattage per wire/line

Is my Assumption correct?

You are correct in saying the power transfer rate is Watts = Volts * Amps, as you show.

But that is not very helpful, when determining wire size requirements. It is better to consider voltage and currently separately, as they have different needs.

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Voltage only needs to be considered from the point of avoiding a flashover, short or shock. For voltages up to say  50 volts, a very thin 'intact' plastic shealth could be effective. The thickness of the conducting element is  irrelevant for any practical wire thickness.

In practice, the plastic sheath is given a rather thicker, 'reasonable' thickness for mechanical strength, resistance to abrasion, etc. which could puncture it. And mains cables are often double insulated to reduce risk of shock, again knowing that they will often be mechanically maltreated.

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For the conductor, the only factor to consider is the current. The thinner the conductor, the higher the resistance (per unit length). This resistance will cause the cable to become hotter when a current is passed through it. If the current is too high, the rise in temperature may be enough to melt the plastic of the insulation, allowing the wire to mechanically break through the insulation, with obvious catastrophic consequences.

In addition, if the wire is of significant length, then the resistance of the conductor will introduce a significant voltage drop, so that the voltage at the 'load' ... which in the battery charger case, is the battery being charged, will experience a lower voltage than at the 'source'. This can result in the voltage at the load becoming too low to function correctly.

......

In the video showing the charger and battery, the charging current was about 4A, and the very short wires meant that relatively thin wires would suffer little voltage drop, so they could be sufficient to carry 4A. Thus, there is not enough evidence to rule out the possibility of the thin wires providing the 'bulk' of this charging current, although I think it is unlikely for a number of reasons.

Balancing the cells implies some of the charging and/or discharging is individual to each cell, and clearly that cannot be accomplished through a single pair of high current wires when the 'battery' has more than 1 cell. Thus, at least some of the charging and/or discharging current flow will be through the balancing wires. A number of different schemes for balancing have been designed, but I don't know which ones have been deployed in these particular chargers.

Furthermore, the charger was controlling the charging according to the voltages on each of the individual cells, so that the balancing wiring is playing a major role in controlling the charging process, even if it only carrying a modest current.

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Whilst writing this sermon, I see @robotbuilder introduces the suggestion the battery has built-in electronics. 

Some lithium-ion (and LiPo is a form of Li-ion) batteries have indeed included electronics ... one 'extreme' example is a manufacturer of batteries with capacities suitable for direct replacement of lead-acid battery for a petrol/diesel car. In this case the battery was claimed to have sufficient electronics to make it a 'drop-in' replacement, in spite of the very different characteristics, such that it did not require the car to be modified.

Of course, extra electronics within the battery implies more cost, a source of heat generation, and so on, so it is not a universal approach. I am sorry, but I have not seen a 'tear down' on the particular batteries shown on the video, so I don't know if they have any built in electronics.

There may be some 'de facto' standards regarding built-in electronics, for batteries made for a particular market, but I am not aware of them. Clearly, any charging and balancing device needs to be designed to match them.

Some single cells include a simple device to limit the maximum current flow, as a crude protection against being short circuited.

Obviously, any 'tear down' would need to be undertaken extremely carefully -- I am not suggesting anyone does it!!

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Whilst it is possible to design and build simple chargers, the more demanding applications have resulted in complex schemes for the best performance, and in some cases that complexity has been the result of safety considerations, so take care.

Best wishes, Dave

Just watched Bill's video, must have missed it when it came out.  Lucky for your post :)  Now I know about the peculiarities of these LIPO batteries and where to refresh my understanding should I ever use one.  Maybe they come is a battery pack with all the electronics for charging them built in?

Lipo Charging 15:31

 "Charging though the Balancing leads is the preferred Method"

This is why I had the original Question.

However, I would say that pushing 4A x 4.2 Volts is a lot of power for such wires, however I didn't consider that each wire would split the total wattage. 

In this case, it is a 3 Cell, therefore math: (4x4.2)/3 = Wattage per wire/line

I disagree with you, the next part of the same video clearly shows the charging taking place with BOTH the power cord and the balance leads plugged into the charger.

There are videos showing how to charge them using the balance leads but they involve a dangerous hack in the knowledge that the outer balance leads connect to the same places internally as the power leads. I don't agree that this is the preferred method for charging.