Is a 24s battery dangerous?

Hi,
I plan to build a 24s (88.8v nominal voltage) Li-Ion battery for an electric paramotor
The peak voltage of a 24s battery is around 100v, which is pretty high

Anyone working with such batteries? Do you wear gloves or other protections? Is this voltage letal?

Yes, 60V is the usual boundary

https://en.m.wikipedia.org/wiki/Extra-low_voltage

https://www.sae.org/publications/technical-papers/content/2013-01-2877/

A single cell 3.6,v can be very dangerous.

I have build a 24S 16ah with four multistar.
I was very anxious when I was connected for the first time everything but I think that if you are very careful you will not have problems.
Anyway the first time I put on a double layer of nitril gloves by the way.
I think that the simplest way is to connect four turnigy 20ah as I consider lipos more sincerely than liions as you have less cells and you can monitoring every single cell with a good charger.
I have build my own all the connections as I needed to be sure that the work was made really good.
The 24S voltage anyway is very very dangerous and it is necessary to be conscious of that.

NexusG said:

Hi,
I plan to build a 24s (88.8v nominal voltage) Li-Ion battery for an electric paramotor

Click to expand...

Don't forget or leave out a truly good BMS or you will by 100% certainty run into troubble.

100% false that "a BMS" is required.

Definitely the case that all or most of the functionalities represented by that term need to be accounted for

but the degree those need to be automated very much depends on the use case and the owner.

And for each function there are literally hundreds of ways to implement.

BMSs are very often the cause of pack failure, and certainly blind reliance on just one layer of defense is the root cause of most.

An educated attentive owner is the most important protection.

I have read of, and seen old electricians checking AC mains voltage across two fingers on the same hand using a certain technique. If you use the wrong technique, the muscles in your hand contract, which is bad. Also, current running through your hand can "cook" the muscle tissue. Decades ago, I had a hot dog cooler that worked this way, until they were banned due to occasional electrocutions.

If you run current across two hands, it will pass through your heart, often causing it to stop beating, and sometimes cooking the flesh on your hands.

You are asking about DC, which is worse. AC has a cycle to it, and 60 times a second it passes across zero-volts. This means it is easier to pull someone off if an AC short. Not easy, but easier.

DC is constant flow, once flow is achieved. Dry human skin is considered safe to handle electricity up to 60V, and a fully-charged 14S pack is 58.8V

Of course, many members have gotten a jolt from 72V, and they are alive. If your skin is sweaty, that means it is covered in saltwater, which dramatically improves conductivity. Years ago, prisoners who were killed in the electric chair had their head shaved, and a cloth of salty water was placed on it.

The higher the volts, the farther a spark can jump through the air, so...the farther apart the contacts should be inside a switch. Also, the higher the amps, the faster the contacts should be.

With high-voltage DC, it is hard to break the arc that will be flowing through the air when you pull the contacts apart. DC switches for 100V are about four times larger than for 100V AC.

If you truly "need" 24S, I recommend two 12S sub-packs, that are connected in series to run.

spiros said:

I have build a 24S 16ah with four multistar.
I was very anxious when I was connected for the first time everything but I think that if you are very careful you will not have problems.
Anyway the first time I put on a double layer of nitril gloves by the way.
I think that the simplest way is to connect four turnigy 20ah as I consider lipos more sincerely than liions as you have less cells and you can monitoring every single cell with a good charger.
I have build my own all the connections as I needed to be sure that the work was made really good.
The 24S voltage anyway is very very dangerous and it is necessary to be conscious of that.

Click to expand...

Building a 24S battery from 6S lipo in series is a solution but it's not very convenient because you have to charge each battery separately
Moreover they are not long lasting, judging by my puffy lipo after 3 years

Of course a BMS would be appreciated but as john61ct says, it's also the cause of pack failure ! The battery will be jettisonable in case of catastrophic failure

One of the concerns I have is during the battery construction, where the cells and wires are exposed. I suppose gloves are mandatory during this step at this voltage level

spinningmagnets said:

I have read and seen old electricians checking AC mains voltage across two fingers on the same hand using a certain technique. If you use the wrong technique, the muscles in your hand contract, which is bad. Also, current running through your hand can "cook" the muscle tissue. Decades ago, I had a hot dog cooler that worked this way, until they were banned due to occasional electrocutions.

If you run current across two hands, it will pass through your heart, often causing it to stop beating, and sometimes cooking the flesh on your hands.

You are asking about DC, which is worse. AC has a cycle to it, and 60 times a second it passes across zero-volts. This means it is easier to pull someone off if an AC short. Not easy, but easier.

DC is constant flow. Dry human skin is considered safe to handle electricity up to 60V, and a fully-charged 14S pack is 58.8V

Of course, many members have gotten a jolt from 72V, and they are alive. If your skin is sweaty, that means its covered in saltwater, which dramatically improves conductivity. Years ago, prisoners who were killed in the electric chair had their head shaved, and a cloth of salty water was placed on it.

The higher the volts, the farther a spark can jump through the air, so...the farther apart the contacts should be inside a switch. Also, the higher the amps, the faster the contacts should be.

With high-voltage DC, it is hard to break the arc that will be flowing through the air when you pull the contacts apart.

If you truly "need" 24S, I recommend two 12S sub-packs, that are connected in series to run.

Click to expand...

Sure, I think they touched the wire with the back of the hand
The skin acts as an electric insulator but if the voltage is high enough, the current is able to flow through it
But what is high enough ? From what you said, 60V is the thereshold of dangerosity in dry conditions
So actually, building a 24S battery would not be the best idea...

I can use lower voltage battery but very low kv motor have a better efficiency
Using two 12S sub-packs is slightly less convenient but 12S are widespread : 12S BMS and chargers are readily available

Years ago, the next performance upgrade for ebikes from 48V was 36V X 2 = 72V.

DIY packs and retail 72V packs are now more available, so dual-36V has become rare.

BMS failures are not uncommon and could lead to total loss of power. Personally I'd rather monitor the cells and let to pilot decide how to run them.

Grantmac said:

BMS failures are not uncommon and could lead to total loss of power. Personally I'd rather monitor the cells and let to pilot decide how to run them.

Click to expand...

Replacing the occasional bad BMS is less hassle than monitoring and balancing cells yourself, all of the time.

Human error is usually what kills BMSes, but it's also the reason to have one.

The problem is figuring how to protect the pack from BMS failure.

I'm not saying the functionality is not needed.

Just most reasonably priced implementations are crap quality, and that one purchase is not a complete solution.

NexusG said:

Building a 24S battery from 6S lipo in series is a solution but it's not very convenient because you have to charge each battery separately

Click to expand...

No, just gives the option.

Balance charging vs pack-voltage usage charging can be separate,

A precharge connector/system is an absolute must at that voltage. Otherwise you will destroy every battery connection you build.
You'll also want a controller with 4115 FETs. 100v will blow 4110 FETs.

Of course high voltage is dangerous. Finding a BMS for that voltage is dangerous.

I prefer high amps over high voltage, as it's a much less complex way to deliver a lot of watts. My favorite build was 80A / 46v ( aka 12S ), because i could just plug the battery into a 12S RC charger..

However, you will need a slightly oversized controller and beefy wiring on these high amp setups. and a precharge anyway.

spinningmagnets said:

Years ago, the next performance upgrade for ebikes from 48V was 36V X 2 = 72V.

DIY packs and retail 72V packs are now more available, so dual-36V has become rare.

Click to expand...

I will see if I can use lower voltage such as a 72V battery for my paramotor build,
I plan to use the T-motor U15L 43kv http://store-en.tmotor.com/goods.php?id=896
Lower voltage means lower rpm so I'll need a bigger prop to achieve the same thrust

Grantmac said:

let to pilot decide how to run them.

Click to expand...

How to ? I don't think I could play the role of a BMS by myself while operating the paramotor

john61ct said:

The problem is figuring how to protect the pack from BMS failure.

I'm not saying the functionality is not needed.

Just most reasonably priced implementations are crap quality, and that one purchase is not a complete solution.

Click to expand...

I agree ! Maybe another circuit monitoring the battery, playing an alarm or/and cutting connection between the bms and the battery in case of overheat, undervoltage ?
This adds complexity but for this use, security is essential
I don't care about the loss of a battery but I'm not planning to burn myself alive, a battery jettison system is mandatory

john61ct said:

NexusG said:

Building a 24S battery from 6S lipo in series is a solution but it's not very convenient because you have to charge each battery separately

Click to expand...

No, just gives the option.

Balance charging vs pack-voltage usage charging can be separate,

Click to expand...

You're right

neptronix said:

A precharge connector/system is an absolute must at that voltage. Otherwise you will destroy every battery connection you build.
You'll also want a controller with 4115 FETs. 100v will blow 4110 FETs.

Of course high voltage is dangerous. Finding a BMS for that voltage is dangerous.

I prefer high amps over high voltage, as it's a much less complex way to deliver a lot of watts. My favorite build was 80A / 46v ( aka 12S ), because i could just plug the battery into a 12S RC charger..

However, you will need a slightly oversized controller and beefy wiring on these high amp setups. and a precharge anyway.

Click to expand...

Yep, because of the capacitors in the ESC, there's a high current peak
You say it : more amps means more heat, less efficiency, bigger ESC
I suppose there's a compromise

There is a frequent argument about how to attain "X" amount of watts from high-voltage-low-amps, or....low-voltage-high-amps...

LFP made the argument that...if you plan ahead to use "enough" watts, then...you will spend very little time drawing max watts, so...

If you choose low volts and higher amps, the cost is fatter wire, but...as to the supposed "inefficiency" of running high amps through fat copper wire is that...when acceleration to max speed occurs in less than 3 seconds, he feels that the benefits outweigh the drawbacks.

One of the drawbacks mentioned was the high number of balance channels serving the battery pack. Meaning: a simple BMS can affordably use high-end components and still perform well. A 20S BMS is expensive, a 12S BMS using high-amps "might" be more reliable per the same BMS cost.

If I got some of this wrong, blame me, not LFP...

Manually balancing cells isn't an issue for a product that is only used occasionally, nobody is commuting via paramotor. It's no more difficult than making sure your BMS is functioning correctly.
Especially with larger format cells that don't use parallel strings you can trust they will not fail during use, a BMS can and will.

In use monitoring is as easy as setting current limitations in the speed control and using cell level alarms.

That's it I'm removing all 8,256 18650 cells and the 13 balance boards from my Tesla and replacing them with LiPo so I can monitor the balancing myself and be much safer from now on. These exploding BMS have got to be stopped.

Back to the original question, yeah, 24s is enough voltage to have bad things happen. Build so your risks are minimized as much as possible, especially in the event of a sporty landing. Fuses, cable shields, etc for starters. Don't fly with lipo duct taped to the paramotor, like we too often do with a test ride on a bike.

The most likely, is not really that you stop your heart. What is likely though, is that while assembling a large number of cells, you make an easy mistake and create a short. While this can be quite exciting when you mistakenly connect 6s positive to negative, vaporizing a plug in your hand. The plasma ball you are holding gets a ton bigger, and turns the metal in your fingers to gas a LOT faster and more energetically at 24s voltage.

So yeah, might be worth it to wear some mechanics gloves when connecting up cells into the 24s configuration. Ideally, don't have to break the pack up to charge it every time, so you don't run this risk every flight. Mostly this is so the first layer of skin you lose, is the glove. And of course, practice basic safety. Handle wires by the plug housings, and never grasp positive and negative bare wires with each hand. Never have both polarity wires bare at the same time. Cover one with a wire nut while working on the other end. With a terminal block involved, somehow cover one pole while you make your connections on the other, so you can't whoops it.

Virtually all of us here, at least us old timers, have made this mistake at some time, and received the KFF badge of shame. Kentucky fried fingers.

If there is any(!) way to get the power and speed needed from sub 60V DC I would never ever go to 100V DC.

At least in my Country anything above 60V DC needs special cables and systems (the orange stuff in electric cars). You are responsible to prevent anyone from touching that voltages.

From a technical side 100V DC gives you very significant arcs and will burn your typical contacts and will lower your options for capacities and MosFETS

dogman dan said:

Back to the original question, yeah, 24s is enough voltage to have bad things happen. Build so your risks are minimized as much as possible,...

Click to expand...

..which means building a 14s system.

You could build three separate batteryblocks that you connect in series to your para-flight system.
Kind of safer if properly designed.

st35326 said:

That's it I'm removing all 8,256 18650 cells and the 13 balance boards from my Tesla and replacing them with LiPo so I can monitor the balancing myself and be much safer from now on. These exploding BMS have got to be stopped.

Click to expand...

I'd guess that the BMS in your Tesla is a lot higher quality than what's sold to us eBike hobbyists, or put in battery packs.

A properly cycle graphed and matched set of Lipo packs might require a single balance or two throughout it's lifetime. I had a pack that was 5 years old and only balanced it 5 times, for prudence. I never looked at the numbers and saw that it was off.

However, most battery packs are not built this way. Not matched. And as they age, tiny differences turn into large and dangerous ones. Manual BMSing works, but it's a fine art you have to learn, lest you blow your house up.

st35326 said:

That's it I'm removing all 8,256 18650 cells and the 13 balance boards from my Tesla and replacing them with LiPo so I can monitor the balancing myself and be much safer from now on. These exploding BMS have got to be stopped.

Click to expand...

That's quite surprising, since lipo have a shorter lifespan and are more likely to ignite
It must have been expensive !

dogman dan said:

Back to the original question, yeah, 24s is enough voltage to have bad things happen. Build so your risks are minimized as much as possible, especially in the event of a sporty landing. Fuses, cable shields, etc for starters. Don't fly with lipo duct taped to the paramotor, like we too often do with a test ride on a bike.

The most likely, is not really that you stop your heart. What is likely though, is that while assembling a large number of cells, you make an easy mistake and create a short. While this can be quite exciting when you mistakenly connect 6s positive to negative, vaporizing a plug in your hand. The plasma ball you are holding gets a ton bigger, and turns the metal in your fingers to gas a LOT faster and more energetically at 24s voltage.

So yeah, might be worth it to wear some mechanics gloves when connecting up cells into the 24s configuration. Ideally, don't have to break the pack up to charge it every time, so you don't run this risk every flight. Mostly this is so the first layer of skin you lose, is the glove. And of course, practice basic safety. Handle wires by the plug housings, and never grasp positive and negative bare wires with each hand. Never have both polarity wires bare at the same time. Cover one with a wire nut while working on the other end. With a terminal block involved, somehow cover one pole while you make your connections on the other, so you can't whoops it.

Virtually all of us here, at least us old timers, have made this mistake at some time, and received the KFF badge of shame. Kentucky fried fingers.

Click to expand...

Yes indeed, most accidents seem to be caused by a short, rather than a direct contact with bare hands
The choice of good connectors is especially important, there are many case of accidents because of non-polarized connectors such as banana plugs
Thanks for your advices, I don't want to receive this badge !

I will rather design the powertrain to be supplied with 60V or 72V maximum