Safely Using LiPo Batteries

Hello Tinker's. 

I had a concern or a Questions regarding these Batteries.

In the Video - 20:46 it is stated that such batteries should be charged though the Balancing leads

My Question is: if you placing that much Wattage though those tiny wires, won't that increase the chances of catching fire?

Maybe it's my ignorance, 4.2v x 4.0A = 16.8 Watts of power though each lead

ON the flip side, I'm happy and content to charge my phone with a 5v 2A rapid charger, and those are connected with 4 wires until it hits the PCB (so maybe I'm just ignorant of Wire sizes and tolerances)? 

@dazza The charge current does not flow through the balance leads, they just measure the voltage of each cell.

@dazza At about 19:53 you can see his left hand finger resting on the heavy charging wires while a bunch of voltage sensing balance wires are visible in the background.

I will have to disagree with you- in referring to the video

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

Is my Assumption correct?

I must also acknowledge that the Battery Chargers could be pushing the Wattage though initially until the cells are near at high percentage, example. 90%-99% charge, then they could Top-Up and Balance at the same time the remained of the Max Charge Capacity. <-- That's me Assuming

Knowing this information, we could make our own Charger.

@dazza 

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?

Where did the 4A figure come from?  Is that the maximum current the batteries can safely deliver? The wires don't split the wattage as such. Indeed unless very thin the wires would dissipate very little heat (wattage). The cells I assume are in series. They have all the resistance which increases with a counter voltage to the charger voltage as they "fill up".

@robotbuilder ahh the 4Amps came from the Video, this is what was selected for the Charge Rate

@robotbuilder The formula for the Max charge rate is in the Video or on the Website page (I'll check that later)

@dazza 

I should probably get up to speed with these modern batteries. I taught myself electronics before they existed. I did blow up a battery like a balloon using a 12volt car battery charger!!  I best watch Bill's video 🙂

My assumption with the NiCad battery in one of my current robots is that the electronics takes care of all the details. I just plug it into the wall socket until the indicator light says it is charged. I assume it automatically controls the rate of charging as well as turning it off when it is done. The circuit and battery come from a robot vacuum cleaner.

Another robot base uses a 1.3Ah 18v Lithium-Ion battery pack that comes from a hand held electric drill. You just plug it into the wall and when the green light comes on it is charged. I assume all the electronics are in this battery pack so I don't really need to care about any special circuits to charge or monitor the battery. It just goes dead instantly when it needs a charge which is probably why it came with two battery packs for a quick change.

I noticed some robotic vacuum charging circuits have a heat sensor inserted into the battery to prevent overheating.

I suspect building the electronics for charging and monitoring these batteries might be complicated or at least too expensive in parts to build yourself?

@dazza Sorry, you are being misled by the poor choice of words. Think of it this way, many cheap chargers for Lithium batteries do not have balancing circuits/leads at all. The charge current goes through the HEAVY Black and Red wires. Why do you think they are there?

Take my batteries as an example, I normally push 120A into 6 batteries in parallel, 180 if the sun is cooperating. They are actually rated for 50A each battery. I use 4/0 wires from the charger/inverter to the batteries. I have NO visible balancing wires, they are either not used or internal to the battery. Since I know there is an internal BMS, I assume the balance leads are internal. There is NO possibility those wires are 4/0, every set of balance leads I have ever seen are something close to 20GA from memory.

The word through is being used in a different way, it's not what you think. In this case it means 'with the aid of'

Posted by: @dazza

↑

I will have to disagree with you- in referring to the video

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.

@dazza 

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?

Hi @dazza,

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.

----------

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.

--------

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.

-------

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!!

-------

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

@dazza fyi @davee @will @robotbuilder I am WRONG. It is possible to charge with just the balance leads, albeit much slower of course. I found the following article that explains how both methods work. https://www.rchelicopterfun.com/lipo-battery-balancing.html

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.

@davee 

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.