|1550 forum posts|
Simple, because at the time it does not realise the cell voltage is over 4.2v due to a poor connection.
|Richard Clark 2||09/07/2020 07:23:20|
|422 forum posts||
You are of course correct. I was wrong.
But I'm still mostly not going to bother. Because as soon as you use it, completing the circuit with the ESC/motor each 'high' cells charges any adjacent low one so it self corrects.
Which is probably why yours measured in near perfect balance after flying it.
|Chris Bott - Moderator||09/07/2020 07:59:57|
6839 forum posts
I'm sorry Richard but that's incorrect again. As soon as the ESC is connected, ALL cells see the same DISCHARGE current which is in the opposite direction to a charge current so all will start going down.
While I agree that most lipos will stay in balance for a number of unbalanced charges and some will for many charges, any that are out of balance are likely to have that imbalance become slightly greater at each charge cycle.
This is why balance charging was invented in the first place. Early lipos had no balance lead, early chargers had no balance facility and people had fires!
I've heard folk complain that balance charging takes too long. Well, if that happens, it's telling you that your packs are being pushed out of balance by your charge cycles. I balance charge every time. It keeps my packs in balance. Very little time is spent in the balance part of the charge so my charges take only as an unbalanced charge.
For anyone to suggest that balance charging is unsafe and that it's safer to charge without balancing (because of one event that was in effect a fault situation) really is dangerous talk and is doing a disservice to beginners reading this forum.
Of course anyone can make their own choices. But balance leads are there for a reason and I will always use them. I also charge in a safe place and carefully monitor my charges too and I use a separate device to check voltages before and after charges and flights.
|Nigel R||09/07/2020 08:43:31|
3987 forum posts
"trying non-balanced charging over a long period as I have done?"
I have seen results of this in a professional context. Unbalanced charging does not end with the lithium battery packs magically returning to a state of balanced cells. Quite the opposite, in fact.
" As will the charging current for all the cells (though it won't be the same for each) "
I can assure you they very much will be the same on each cell, a series connection has no other possible outcome.
"Thus the cells are limited by the worst one 'naturally'."
They are not.
"the worst one (which has the highest internal resistance) can't go over voltage if you have entered the number of cells correctly or less reliably (so check), the charger has detected the number."
Chargers (generally, perhaps there are exceptions, but it would be necessity slow the charge process) do not apply the 4.2V target to individual cell readings during the initial fast charge. The high current causes the reading at the cell terminal to be only approximately indicative of a cells state of charge. Age and condition of cell as well as charge current make this worse. It is not something easily compensated for. The charger will not know the age or condition. It will not know how much effect applying charge has.
If the charger misdetects the cell count, then something is badly wrong with your pack, and it needs special attention (the bin, would be my first suggestion, although I have seen suggestions to save packs by stuffing a constant charge in for a few minutes to try and "rescue" a dead cell).
"It's the excessive heat when using and charging the Mojo at the same time is the problem, though Chord say it is fine to do. "
Again, being blunt, Chord are wrong. I would imagine the reason it gets hot is because they do not have a proper mechanism to manage the selection of power input, nor a proper charger. A simple scheme of permanently charging whilst running from a battery just doesn't work on lipo (unless your design goal is to degrade the battery very quickly in order to mandate a frequent replacement cycle).
Laptops (for example) have more sophisticated power management and are quite capable of running from mains whilst the battery is connected, and neither drawing power from the battery nor charging it.
"completing the circuit with the ESC/motor each 'high' cells charges any adjacent low one so it self corrects"
You are, incorrect again. It is impossible for this to happen with a series connection.
I would make the polite request that you stop adding more misinformation to this thread, as your posts seem to demonstrate an incomplete understanding of batteries and electronics.
Cell lifespan effects coupled with high fast charge current will produce a high voltage reading which is at best loosely indicative of the state of charge of a particular cell. The charger does not know what the age effects are and cannot compensate. At best, it can set a maximum limit, but that limit must by necessity be over the 4.20 target if the fast charge is to be effective; when charge current is cut to zero, the cell voltage will gradually drift down over the course of several minutes. Only after that time has passed, with no charging being done, can you get a more accurate reading on each cell.
Chris Bott: "I've heard folk complain that balance charging takes too long. Well, if that happens, it's telling you that your packs are being pushed out of balance by your charge cycles. I balance charge every time. It keeps my packs in balance"
Could not agree more.
Balance every time, do that, and the balance will be quick.
If a particular pack starts taking a long time to balance, that's an early warning on the health of that pack.
|Tim Kearsley||09/07/2020 08:54:59|
687 forum posts
But this doesn't make logical sense. If the charger is displaying a Voltage of, say, 4.35V then it "knows" that the offending cell is over-Voltage - how else can it be displaying it? I can understand the similar scenario where some high resistance connection results in the charger "seeing" a falsely low Voltage, but then it would display that false reading wouldn't it?
|Nigel R||09/07/2020 09:03:29|
3987 forum posts
As mentioned above, for good reason I doubt many chargers actually use the individual cell voltages to limit the fast charge.
They do, however, all appear to display what they sense.
Edited By Nigel R on 09/07/2020 09:04:57
|204 forum posts|
As a novice the dangers of lipo batteries were emphasised to me and the advice given was :-
Obviously having been made aware of the danger I complied with my instructor's and manufacturer's advice.
I am stunned to learn that lipos are used in consumer electronics and therefore it is easy to assume that in our situation they must be completely safe.
I think not!
I will continue as I have always done which to me is the safest way.
I now know that when the balance phase takes a long time it is an indication that the internal resistance of cells in the pack is increasing. I have observed that there is also a loss of power which can result in the loss of the model. When the balance charge time increase I buy a new lipo before I am forced to repair or buy a new model.
|Richard Clark 2||09/07/2020 09:48:27|
|422 forum posts|
Chis and Nigel,
All I can say is this:
I have used lipos in model planes since day one, right from the time the Israeli military pure metallic lithium cells could be obtained if you were in the right sort of work (which I was). Long before balancing was thought of, and have never had a fire except when I deliberately destroy them before getting rid of them, which I do as a matter of course. Because even a nominally 'discharged' one can catch fire if accidentally shorted.
So I DO bear the dangers in mind.
It was a balance charger that caused the problem. A poorly designed one. I mostly use a Thunder Power 1430C which will charge their own batteries at up to 12C rate and 14S. (They don't recommend you charge other makes of battery at such a high rate)
You have to enter the number of cells, the capacity, the desired charge rate, and a time limit (all this can be stored per battery if you wish).Balancing is an option.
Normal charge end is 4.2 volts by default. This 4.2 value can be altered but if you alter it is not stored. as 'routine'. This is deliberate of course.
Connect the battery and press 'Check'. If all is ok it starts. Every ten seconds or so the charge stops and. everything, balance included, is rechecked automatically. This only increases the charge time by about 8% and there isn't an option to remove it.
If every charger was as well designed as this I would unhesitatingly balance every time. But as many customers appear to be mostly interested in low prices they aren't.
(PS: Many laptops used to have a facility that even when permanently plugged in you could lower the 'stop charging percentage. So the battery would not charge beyond, say, 70% full. Unfortunately this is not so common today and keeping it 100% full all the time greatly reduces the service life.)
Edited By Richard Clark 2 on 09/07/2020 09:52:16
Edited By Richard Clark 2 on 09/07/2020 10:05:12
Edited By Richard Clark 2 on 09/07/2020 10:07:21
|Tim Kearsley||09/07/2020 10:31:17|
687 forum posts
If you are referring to the charger I used when I saw the high cell Voltage, it is a Graupner Ultra Duo Plus 60, which was not a cheap charger when I bought it (somewhere around £280) and I don't think is "poorly designed".
As for charging at 12C, well I wouldn't do it even if the manufacturer said it was possible. I'm never in a hurry as I prepare batteries the night before a flying session, so I charge at 1C.
|Nigel R||09/07/2020 10:40:11|
3987 forum posts
The manual for your charger states:
"The balancer can also further ensure that you’ve set the cell count correctly, and as a result
Emphasis is as per the manual.
Edited By Nigel R on 09/07/2020 10:40:42
|1550 forum posts|
Two possibilities. It could be I moved the battery thereby restoring a good connection before I looked at the charger, or it is possible that the poor connection corrected itself just before I looked at the charger.
Either way, I believe a poor connection in the balance lead could result in overcharging on a balanced charge. Far easier, and it seems, safer to normally do a standard charge, and only if the battery shows more than 0.1 or 0.2v difference between cells to finish off with a balance charge.
Pity really, because my chargers do quite a fast balanced charge.
Out of interest, my charger will not charge unless the balance lead is connected, and it always does a cell count first to check.
Edited By Andy48 on 09/07/2020 11:11:23
|Richard Clark 2||09/07/2020 11:13:39|
|422 forum posts||
Believe it or not mine came with a manual. Which I read at the time and still occasionally look at.
|Richard Clark 2||09/07/2020 11:31:51|
|422 forum posts||
It didn't pick up your problem though. The difficulty with some chargers is that they are designed and made in a country where the government can tell them to stop that toy plane stuff and start making plastic garden buckets from next week. It may well be good for the 'people' but it doesn't help us much as the manufacturer has no 'commitment' to what they are doing at any given time.
I wouldn't charge at 12C either, though I tried it once 'experimentally'. Usually I charge at 2C, both at home and on the field..
737 forum posts
I must admit to being a bit surprised at your reference to 0.1 or 0.2v difference. I have just put a couple on charge and the 3s battery has a difference of 0.029v between highest and lowest cell, and for the 10s the difference is .026v. Balancing starts as soon as the charger starts and I would expect cells to be closer at the end.
If any battery shows as much as 0.1v at the start of the charge I would assume a faulty connection or a suspect batery and act accordingly.
Your system obviously works for you, but I don't believe my chargers measure cell volts while balance current is flowing, as a modern battery management IC should take care of that, so I will continue balance charging. each to his own I suppose.
|1550 forum posts|
Sorry, I meant 0.01v
However, this also begs a question as to how closely cells have to be balanced? I've checked many new batteries and found even the best have some variance in the IR.
Edited By Andy48 on 09/07/2020 14:44:24
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