Varying Voltages.

[Peter Beeney]

This thread might just be of some interest to anyone that does a bit of indoor flying and it also comes about partly as a result of fly boy 3’s thread - Lipo fire,@13/01/2014 21:54:41, which contains some info on lithium polymer cells.

We’ve been using quite a lot of small single cells recently, amongst them the standard 240mAh and the larger 380mAh, both as used in the Hubsan x4 small quad. I have 3 charge leads, as supplied in the boxes, they plug into a USB port. I’ve checked two batteries after being charged by two different leads, the third one is being borrowed at the moment, and the fully charged voltage reading on the two cells showed 4.396 and 4.391 volts respectively. It should be 4.2 ± 0.05 volts, so that’s around 146 - 196 millivolts over the top. I’d already bought some small inline plugs and sockets because I wanted a bit more flexibility when charging; if I forget the laptop usually there are others around but that might not always be the case; and this now enables me to also be able to plug into my field charger so that I can control the charging current rate and see what is going on. The reason I’ve gone for a multi-connection lead is so that we now have a ‘and/or’ choice, however in the light of this we probably won’t be using the USB regulator lead too often…

It will be interesting to see what the charged battery voltage reading is on the third lead, but I suspect it will be the same, and so in which case I’d have to say this was not an accidental situation, because accurate voltage calibration should be very easy to do. I don’t know how much effect, or indeed if any at all, constant overcharging has on these cells, but I’m sure it doesn’t do them much good, at least; and I very much suspect the manufacturer won’t be losing too much sleep if a few cells have to be purchased now and again and again and again; and they are not cheap! We have found that in general these small cells are sometimes not the best at living to a ripe old age anyway, they can soon run out of revolutions. Although this could be down to a variety of reasons, perhaps, it’s quite difficult to treat these with velvet gloves, with the best will in the world they do get a bit of stick occasionally. In fact, it’s most of the time when they’re used by a bunch of exuberant schoolboys!

The E-flite chargers appear to be much better, the Celectra, single cell voltage like the others, but with variable current output; the one cell 70mA output for Mini Vapor cells etc., and the 0.1A USB lead all seem to be within a gnat’s whisker of 4.2 battery volts after charging. The USB lead is for charging the 50mAh Ares cell for the little Ares 3 channel Tiger Moth. 4 spare cells are £20! This possibly tends to make up for the relatively inexpensive models. The individual transmitters with many of the planes have a charging port for their respect receivers, as does this Tiggie, but I’ve not checked these, and we don’t use them anyway. No point in caning the tx batts too much. We also use a number of the small Speedmaster V911 helicopters, with both the old and new style batteries, and again the charged cell voltage would appear to be more or less within limits.

Generally, then, these little chargers seem to be on the ball, the problem being that if they are seriously under or over charging cells it would largely go unnoticed. Many modellers will most likely have a multimeter but even then do they get checked?

I don’t think the overcharge discrepancy here is that serious, I suspect it has to go quite a way before the cell actually becomes incandescent with rage; I’ve never really tried this but perhaps when someone has a lipo that’s become permanently tired I’ll have a dabble at lighting it’s fire. But I will note any results; gradually take it up in voltage and current until it pops. I have occasionally checked the voltage of the fully charged larger size packs, and they are invariably very slightly under 4.2V/cell. So I suspect that the chargers might err very cautiously on the down side, playing for safety perhaps. But that’s only my two chargers, so it could be just coincidence.

Another slight irritation to overcome is the plethora of different plugs and sockets that are used with all these cells. In some cases it’s possible to buy some spare leads and modify (vandalise) these according to needs, but that’s not always possible. Again, I very much suspect that it’s simply just another manufacturer styled ‘organised chaos’ ploy.

PB

[John Privett]

I've just had a quick browse through some LiPo datasheets online. Most state normal charge voltage (as you'd expect) as 4.2v, and maximum permitted 4.23 or 4.25v.

Some of the data sheets go on to specify "Safety tests" and one of these tests is an overcharge test. In this the datasheets state that the battery is charged in the normal manner, but using a higher voltage. In one instance this was 4.6v, in another 4.8v, and in a third 5v. In the first case the acceptance criteria was "No deformation or leakage", in the other two it was "No explosion or fire".

So, is your overcharge on the USB leads to almost 4.4v significant? The datasheets would tend to suggest that (at least as a "one-off" ) you're not going to burn the place down - which matches your experience. But as for reducing the life of the batteries? I suspect it will, but who knows!

Edited By John Privett on 26/01/2014 14:24:55

[crispin church]

how do you know the meter you are using is right ?

all your cells could be less or more

as long as they are not hot and swelling up and they work

you would be better of with a multi lipo charger with lcd read out at least you will see during charging what they are according to the built in meter says and the auto cut off when charged

[Dave Bran]

Well, firstly, I have never ever trusted a USB "charger lead", I don't have to use them so I don't!.

Yes, the plethora of plugs and sockets is a pain. The type used on the Hubsan Quad is used on some RC micro cars, TRAXAAS is one from memory, but you have to be REALLY careful as they use reversed polarity!!!

I have sleeved mine when making up leads so the heatshrink colour is "right".

Beware of the super multi leads widely sold, I have never used them without electrical check first and have found that some (from Giant Cod) have been reverse polarity (not just the ones like TX Co-Axials which are supposed to be). Had one supplied and it was dead short................what would have happened if I had not checked first and connected a big LiPO?? It would probably have welded itself connected and "run away".

Also, it is only too easy when charging a lot of differing batteries after a session inc a small batteries to "lose" one still connected amongst the bulky loom and connect something else, esp if you are using several chargers at once.

[Peter Beeney]

I’m not concerned too much from the danger angle of this amount of overcharge, in fact if all these Hubsan charge leads are the same then there might be a number of cells around charged to these levels. But I’ve have thought it would be just as easy to make the cut-off point 4.2 volts as 4.4 volts.

I did post in the Lipo fire thread about a 3S being charged to 14.76 volts, that’s nominally 4.92 volts per cell, and so far this doesn’t have appeared to have affected it at all, but it’s difficult to say about the longer term; however, it’s by no means new anyway, so even that might be difficult to assess, if it suddenly gives up it might simply be worn out. This in fact is one little beef I have, I bought a large handful of the miniature cells for one of the very popular indoor slow flyers, many of which very quickly expired, but I soon established that it was difficult to get them even checked out, let alone replaced. They were being charged correctly, one reason I was told that the storage voltage was too high, but at that time they were being used twice a week so when does storage become ‘storage’… seems to me this might occasionally be a nicely convenient get-out clause…

Way back I was given 4 of the very first lithium cells to play with. 450mAh, as I remember, from the US of A, single cells, the connections were just aluminium tags, the first obstacle to overcome. We couldn’t get a charger, so I used my bench supply. No discharge unit either, but I did have some good resistance units. I charged one up to 6 volts with no apparent damage, but a couple of discharges later revealed that it was shot! So I tried to set fire to it with a match, which didn’t succeed, but my gas torch did. But then it just burnt relatively quietly, nothing like the videos, so there may be a difference with a lipo included in a fire, as opposed to one that is self igniting by overcharging. But these were the very early variety, maybe the modern ones are different.

The voltmeter that I use is a Fluke 8060A, which is now quite old. At the time when I bought it though, it was one of only a very few portable handheld meters that were accepted within the spec. for testing and commissioning some brand new designs of specialist industrial switch mode power supply units, amongst other things. It’s a sub-standard jobby, which means it was calibrated against a standard voltmeter, that’s top man. But I have to admit, it is now out of calibration, because that’s very expensive, so that makes it just a ordinary meter, but I’m satisfied that as far as me and modelling lipos are concerned it’s perfectly adequate. The very reason that I use home made charge leads is exactly so that I can use my Schulze Chameleon 8-936g charger/discharger, this is multi cell, up to 14S lithium, with 3 simultaneous output ports, at least for one cell, plus a graphical current against time readout trace, in the past this has been extremely useful in identifying battery faults. I also use stackable leads, this simply means I can easily couple in my voltmeter to get a precise grip on the voltage of whatever battery is connected.

And finally, I do very carefully check out everything before I start to use it; it’s a bit second nature, really, in a past life a misconnection would have resulted in a mini lightning strike; and sometimes an extremely nasty shock, or worse. Fortunately I never managed either, but I think was only down to the fact that I was a bit scared of it… I use plugs and sockets, in an effort to make the connections a bit polarised, that means they only connect one way, when a number of grubby little mitts can get into something ad lib, it needs to be as foolproof as possible. A bit of a tired old cliché perhaps, but my motto has always been Safety First.

So it’s beginning to look as though the next experiment might be entitled Operation Visible Volts, an attempt to self-ignite a lipo; we’ll try and get a notion on just exactly what lengths we have to go to to achieve this, or indeed only very little, perhaps.

PB