You really don't NEED a BMS with cheap lipos

ESK8 Electronics
21

You know none of us were balance charging when Lipos were first in use right? The vast majority of our helis didn’t go down in flames (and when you do get a burning pack it’s under load when over discharged a lot of the time – think “hover boards”)

I have put 100’s of charges and hours through many many packs. I’ve been flying helis since I was 32 (16 years ago) Prob 14 years with lipos. I’m still using some of the same lipos for lesser duties, like powering FPV ground stations.

I have only ever had one pack swell in a big way (didn’t cause a fire) - and it was due to me trying to… add balance leads to a non balance leaded pack and messing up the soldering with a simple mistake. This was when balance chargers came to the market and everyone made out it was a must have feature. So I added balance leads to my older packs.

I’m willing to bet that more people here have damaged lipo packs by cutting the balance leads and adding BMS boards (when they are not experienced with working with high voltage and soldering) that just natural charging damage from inbalance.

I understand that lipos can cause a fire if overcharged. I understand that balanced cells are healthy. I don’t argue that.

However, charging with a power brick is low current (circa 2 Amps). In the high capacity packs we are using - one of my parallel setups is circa 24000 Mah - you are pretty much trickle charging the pack. It will equalise during this charge. It would take days for a duff pack to swell. You’d notice your charger had not completed before that.

I also fast charge at max amps (16amps on my RC charger) when I’m in a hurry - I do this in my garage. The risk is extremely minimal.

Saying that advising no balance charging is akin to advising not to wear a helmet is not really a fair comparison. Not wearing a helmet on a board going over 15 + mph is a known, high risk. You are inches from the ground and a crash will likely result in an uncontrollable fall (head impact to ground). But then many of us don’t wear a helmet cycling (and won’t unless forced) - this is NOT a high risk, it is a calculated risk. Most of survived our childhood without a helmet. Studies show you are statistically more likely to be hit by motorists when wearing a cycle helment as motorists see you as “safer” and give you less room. But still, people call you an idiot for not wearing a cycle helmet. You calculate the risk and take what you deem is acceptable. You shouldn’t base this decision on what others “insist” is true, or, try and 100% mitigate all risk.

If you rummage through your gadgets and take them apart, you will find that a LOT of them have multi cell lipos and do NOT have balancing circuitry in them. They “dumb” charge them to 4.2v/8.4v/12.6v but you still use them…

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PXSS - the moment you plug in the charger all the cell voltages start climbing. The lower cells lag behind but catch up as they charge. At the top of the charge, as the cells approach 4.2v each, the total pack voltage approaches (for 3s in this example) 12.6 volts. Lets say one cell is a little behind. You have 4.2, 4.2 and 4.0v. That’s 12.4, so the charger slowly trickles in power until it sees 12.6v. This will either happen as the cells go 4.3,4.3,4.1, for example, = 12.7 volts. The charger stops. The pack settles back to 4.2, 4.2, 4.1. This equals 12.5v so the charger puts a little more in. It keeps doing this until the pack settles at 12.6 v at rest. This will happen when all the cells settle at 4.2 volts, or you end up with something like 4.3, 4.3, 4.0. (this pack, I would bin)

It’s fine for a pack to be a little over 4.2v per cell. When you use a hobby balance charger with high current, it will take the cells way over 4.2 briefly, and wait for them to settle back to 4.2, and then deem the pack charged. It has to go over 4.2 to get the charge in (you need higher voltage to charge).

Even with a perfect balance charge, with the above example, I guarantee the pack would balance charge, and then settle back to 4.2, 4.2, 4.0 after 20 minutes of rest.

With “brick” charging you are effectively trickle charging. Your pack will hit max voltage, and the charger will back off, and only resume when it drops below full charge. If you have a dead cell, then yes, it will overcharge the remaining cells, but really, how often does this happen?

Incidentally, I’m talking about cells in series, any cells you have in parallel balance themselves though natural physics. That’s been discussed to death and proven on many RC forums.

1 Like
23

I see your point if you have long time experience with these kind of things and know how your batteries behave. BUT: There are a lot of people here who never had any contact with batteries before and ask questions like “help! why is my pack at 23V, it´s a 6s 22,2V?”. So i wouldn´t tell someone like that, that it´s ok to throw a brick on their battery and charge without anything else! An other point: You mentioned a 16000mah and a 24000mah pack, so lot´s of juice! People build beginner boards with 6s 5000mah and 20c batteries. With a big pack like yours you´re not stressing your pack and the single cells won´t really discharge differently and unbalance. With that small poor 6s 5ah, a fat ass at above 90kg (like mine) and the first hill you encounter, you throw a massive unbalance in that pack! Then people go home soon after the ESC cuts out here, scream at the battery manufacturer what a sht they build and charge their pack again. No time to let the battery rest a bit and you have huge unbalances! If you just use a brick in that case, you´re likely damage them!

So just be safe and only do experienced stuff if you now your parts and how they behave :slight_smile:

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What you are describing would never happen. I’m going to break it down a lot here so bare with me.

Same 3S scenario with your brick on what actually happens.

Your brick charger does not charge to 4.2v per cell to be exact they usually have a tolerance of 0.05V per cell and most of these usually overcharge. So lets assume your brick actually charges to 12.7V. Your charger is also not capable of charging extremely small currents. Most bricks shut off at 5-10% of the current rating. This is because if you didn’t have a shut off, your cell would be on charge for an infinite amount of time and they would blow. So another assumption is that your laptop charger shuts off at 0.1A if its a really good one. So your charger will over charge to 12.7V at 0.1A. At no load this could absolutely mean exactly 12.6v for the pack but you already overcharged them for however many hoirs they were at 12.7.

You have a cell that is 0.2v behind all others while charging. This suggests that it’s internal resistance is lower than the others. If you have a decent hobby charger, it actually tells you the internal resistance of each cell in a pack. This is important because internal resistance is really the measurement of health. The lower, the better and no two cells will ever have the same. If you buy from a good lipo brand (Thunderpower) you can call them and ask them to send you cells that are closely matched. You have no control over this if you buy from hobbyking.

So lower internal resistance is good, high is bad. This is because when you apply a current through the batteries, your voltage sag or rise (discharge and charge respectively) is equal to current x internal resistance. The heat generated is current ^2 x internal resistance. So for a higher internal resistance, you get more voltage sag and more heat.

Back to your example, the cell that is 0.2V lower while charging is actually in better health than the other two and not the other way around. So you stick them in the charger and lets say it stops at 12.7 and your cells settle at 4.0, 4.2, 4.2 after a while. IF (thats a big if) this total voltage of 12.4V is under the charge trigger, then the brick will start charging again, otherwise it will leave them there. So they start charging again, now only at 0.1A, the cell with lower ir will increase voltage slower than the other two because of v=i*r. So your battery reaches 12.7 and the current dips below 0.1A so your charger stops. Your battery now settles at 12.6, your cell voltages will NEVER (physically impossible) be better than 4.133 4.266 and 4.266 (This would be the scenario where your cells have perfectly matched internal resistance but one was simply at a different point in the charge cycle which is unrealistic).

Now if you have quality cells and they have a .01V spread more than likely they also have a good spread of internal resistance values and they will not go out of balance over night. This happens over tens of cycles and if you like me have lots of batteries, you really don’t cycle them all that much. The only cells I’ve cycled more than a hundred times are cells that are a few years old and belong to specific aircraft. The problem arises when you don’t have quality cells and your voltage spread is larger as these could have large varying internal resistances which would go out of whack in a matter of a few cycles.

But even then, if you have good cells and you never check them. They will go out of balance and your board is now a serious hazard under your feet.

As far as balance chargers charging above 4.2V. You’re wrong. A good balance charger will charge to whatever voltage you tell it to and the batteries should settle at a voltage lower than this. If you are doing it another way then you’re doing it wrong. I don’t have any more time to explain but hopefully later today I can explain this to you

I hope you find this informative.

2 Likes
25

I have an Astro 109 charger from back in the days, which I still use. It’s not a balance charger- they didnt exist then. It “pulses” in charge towards the end, pushing the cells way over 4.2, then waits a second or so for them to settle/rest, checks the voltage, and then repeats, until the cells stay at 4.2 for at least “x” seconds (around 10 I believe). I use thsi charger as my skateboard “fast charger”. Whereas my modern balance charger (which I use indoors) reduces the current as the cells approach full.

I’m currently charging my board with the “brick”. I’ll post what the cells do as the charge progresses today.

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Checked it.

Pack 1: 4.18 4.18 4.18 4.17 4.18 4.19 Pack 2: 4.12 4.11 4.21 4.21 4.21 4.22

Both packs are 25.08 each in total (charged in parallel), So that’s obviously the limitation of my brick charger. (i.e. a 1.2volt deffieciency)

Pack two has two cells that are 0.08 and 0.09 volts out (from 4.2v) and the other cells have been overcharged by 0.01v.

I can live with that, and that’s the point. I’ve used the packs loads. lots of 15 mile runs last year, never balance charged. Simply plugged into the brick charger.

As a final test, I’ll now balance charge them and see how they fair, particularly pack 2. Though as I say, I’m happy to live with 0.09v difference on a couple cells.

p.s. PSXX I did indeed find your post informative.

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Ok, balance charged both packs (although this thread wasn’t really meant to be about balance charging, more about BMSs).

I left the packs overnight to settle back to rest.

Pack 1: (pretty much the same, just dropped 0.01 here and there at rest

Brick charger – Balance charged

4.18 – 4.17 4.18 – 4.18 4.18 – 4.17 4.18 – 4.18 4.17 – 4.16 4.18 – 4.16 4.19 – 4.17

Pack 2: (again, pretty much the same, dropped 0.01 here and there, and first two lower cells have gained 0.02v each)

Brick Charger – Balance Charger

4.12 – 4.14 4.11 – 4.13 4.21 – 4.17 4.21 – 4.20 4.21 – 4.19 4.22 – 4.20

So all in all, the balance charging made sod all difference…

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I haven’t heard of that charger, but I’ve also only been seriously involved in RC for about 4-5 years. I have 4 iCharger 308 Duos, 1 iCharger 4010 Duo, A hobbyking Reaktor and some SkyRC B6s.

All of my iChargers will charge batteries within 5-6mV of each other when balancing. The reaktor has a 10mV ish range while the B6 has a 20mV range. I suspect your balance charger isn’t that great.

I’ve charged 8S batteries that are way out of balance and brought them back within 5mV

29

I can’t find what about lithium cells makes them not capable of trickle charging at the end of a charge…if it sits on a 4.2 for more than hours. I read about how ideally there’s four charging stages starting w a trickle, and then cc, then cv, and then it shuts off. Is it due to using a switching charger or truly the chemistry and if the chemistry why? I think it’s the chemistry as I’ve seen linear power supplies that use a timed cut off when at full charge Edit Seems “floating” lithium isn’t a problem and it’s just the high voltage the cell is at from this article

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I can lead you to the proper articles as of why. Not right now though as it’s date night. If I dont post by the AM, remind me.

@Hummie I have a few minutes so I’ll try to explain it really quick. It has to do with the chemical reaction. Charging the battery is an exothermic reaction so this means that heat is released during charging, not only that but you also have electrical heating effects on the battery, that is the heating due to I^2*R. The chemical reaction on lithium batteries is also not 100% reversible which is why you lose capacity over time. The non reversible part of the reaction creates some kind of oxide within the cell, (I don’t remember exactly what it is but it is not usable) this oxide causes the internal resistance of the cell to increase which increases heat generated, the reaction produces more oxides at voltage states and high temperatures. More oxides -> more heat -> more oxides. This is what battery engineers call thermal runaway. The more time your batteries spend at 4.2V, the more oxides you generate, that is why a lot of RC guys only charge their batteries before they fly and dont leave them stored at full charge.

So you have the whole thermal runaway issue. Above that, the oxides occupy more space within the cell than the electrolyte. LiPo pouches have a constant volume. PV = nRT. That means that if temperature rises within a cell, you have to increase the volume or pressure. You have a shrinking volume since the oxides occupy space and your temperature is rising. The only other variable is pressure (n is the number of molecules and R is a specific constant). So LiPos puff and eventually blow up. I’m not sure which of the chemicals ignites with oxygen but yeah. LiPo fire yayyy.

Li-Ions have the vents to release gases and delay the explosion but eventually they also catch fire.

I think batteryuniversity.com has some of this info but most of it is explained in scholar articles.

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some articles tell that its a sure fire if you were to trickle charge lithium. All regulated chargers will shut off, but some after even an hour or two. If the only reason that floating the cells on a charge of 4.2 is bad is the oxides or plating or whatever is happening at that high voltage then it wouldn’t be fire worthy no? I charge to 4.2 and leave my cells around and while they may sink to 4.15 or something quickly, even if I charged them to sit at 4.2 it’s not asking for a fire. Isn’t there some other reason articles scream fire if you try to float lithium? They dont even state that it’s a high voltage. I’d be happy to float at 4.1.

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is the title means i can use laptop style charge to charge my lipo direct without BMS? i have charge spec 33.6v 3A.

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Benwong, that’s exactly what I am saying, and it’s exactly what I do with 4 boards in total.

Your numbers suggest 8s lipo pack. 2 to 3 amps is common for brick chargers.

All you have to do is just keep an eye on your packs once in a while.

With balance charging (or a BMS board), you are going to find out of a cell has dropped significantly when you do the charge, because the charger will come up with an error, or take forever. Without balance charging you won’t get the warning. It’s not a common problem, but it can happen.

For me, I would know if I has a cell problem because the boards performance and range would drop significantly.

I “brick” charge mine most of the time, and once in a while I balance charger them as a kind of “service”. To keep an eye on them.

If you dont have a balance charger to check them once in a while then a simple lipo checker/alarm like this: http://www.ebay.co.uk/itm/RC-Lipo-Battery-Low-Voltage-Tester-1S-8S-Buzzer-Alarm-Checker-Test-LED-Indicator-/252401918078?var=&hash=item3ac453a07e:m:md6n6fjcqhfTOJGZxs7q_gA is a great idea - then you can keep an eye on things, when the board is in use, and when it is charging. It just plugs into your battery balance plug.

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thank for your advise~~ finally i understand the “brick” that you all mention~~ :joy::joy: here is the “brick”, actually not that big and heavy~~ haha.

i am using this for changing… withBMS.

:blush::blush:

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Indeed :slight_smile:

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http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

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“Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible.”

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No one is suggesting a trickle charge PXSS. I said it is “like trickle charging” in as much as it is low current.

The brick chargers won’t trickle charge. They charge to 4.2v per cell then stop, only resuming if there is a significant drop.

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I think he’s responding to my questions about the safety of “float” charging (having the cell constantly exposed to a certain voltage).

you can “float” charge to 4 volts with lithium and be fine. It’s just the danger of going over 4.2 thats the danger.

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The plating is dangerous @hummie. It increases the pressure inside the cell and the internal resistance goes up. At a certain point, your cells go into thermal runaway and there isnt a way to know when this will happen without closely monitoring temps and internal resistance