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Charging 18650 Lithium Ion 3.7V cells

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Tony Smith 720/12/2012 14:45:34
812 forum posts
28 photos

Hi,

I use a pair of Trustfire Lithium cells in my transmitter, I'm not sure of the correct term for their chemistry, they're described as 3.7V but also say to charge to 4.2V. Link to the product here .. **LINK**

I want to charge them with my Turnigy A6 charger, do you reckon I should be using the LiPo program rather than "LiFe" or "LiLo", in view of that 4.2V suggested charge voltage?

Also, I was wondering if it is worth fitting and using a balance lead so I can use the balance charge program. I think, but I'm not 100% sure that the protection will mean that if one cell hits the maximum allowed charge then some current will still pass through the other cell. I'm not completely sure that's correct. What do you think?

Any other suggestions regarding the care and feeding of this sort of cell?

I'm a bit suspicious because when I stick the pack on charge it hits 8.4V pretty quick, then spends ages with the current gradually dropping. But then when I use the pack the voltage drops almost immediately to 7.4V and only very slowly from there. I makes me wonder whether charge above 7.4 actually does anything.

Thanks, Tony S

Brian Hammond20/12/2012 14:49:31
338 forum posts

If they say 3.7V then they are lipos

Steve Hargreaves - Moderator20/12/2012 15:03:56
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At 3.7volts nominal & 4.2V fully charged they certainly appear to be what we know as LiPos......

However I would proceed with extreme caution here.....a quick snip from the website..

"These fully protected cells simplify usage with their integrated PCB. Perfect for your single cell application these cells are not for pack building as doing so will defeat the PCB protection which is matched to the correct voltage on a single cell. If you are building a pack please choose one of our other cells and the appropriate PCB"


Two things here......1) It says don't use them as a pack....which is what you are doing in your transmitter. 2) these cells have an integrated PCB which will chop the power when the cell voltage drops to 2.5 Volts....this means your transmitter will switch off.......which might be inconvenient if you were flying at the time.....crook....admittedly they shouldn't get to 2.5V in normal use but you never know & to me its another PPOF.

IMHO I think using these cells in your transmitter is a very bad idea.....there are lots of other more proven methods around......this might make interesting reading for you

Ben B20/12/2012 15:42:37
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I wouldn't worry about the LVC (by the time the batts get to 2.5v you've got seconds of useful capacity left) but I would worry about using sprung contacts on a transmitter. I know people do and get away with it but I haven't got a single household device with such contacts (e.g. remotes, travel clocks etc etc) that occasionally don't get a bad contact and need rotating or remove and reinserting occasionally.

I seem to recall the lipo in my dx8 has a LVC device built in....

Tony Smith 720/12/2012 15:53:59
812 forum posts
28 photos

I've no problem with sprung contacts, if they're of suitable quality. They're used in much more safety critical applications than aeromodelling, for example in our (SOLAS approved) MOB floating light. I know aeromodellers hate them, but that feeling isn't really shared by other industries.

Having said that, the sprung battery holder in this particular transmitter was not good, mainly because the cells were too tight and didn't therefore bear properly on the sprung contacts. For that reason I have ditched the battery holder and gone for a soldered pack. Now it looks like that might not be the best idea, so maybe I'll be back to the battery holder. Using 6 NiMH should give a safe voltage range, or maybe even five - I really need to test the transmitter at these lower voltages.

Tony Smith 720/12/2012 15:56:46
812 forum posts
28 photos

My previous post got lost! I thought I'd said that I'm not bothered by a 2.5V cut-off, I set the low voltage alarm to 7.0V at the moment until I test the transmitter at lower voltages. I would be pretty sure it wouldn't work at 5.0V.

However I agree that it's a point of failure, and over charge protection isn't needed if I use a proper charger in any case. Maybe I should open them up and remove the protection circuit, and fit a balance lead while I'm at it.

Tony S

Steve Hargreaves - Moderator20/12/2012 16:03:59
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The Ultrafire cells won't fit a standard AA cell battery holder anyway.....they are too long.

Which Transmitter is it anyway Tony? Might LiFE batteries offer a good solution?

Lots here & an interesting discussion here

Tony Smith 720/12/2012 17:55:54
812 forum posts
28 photos

It's a Turnigy 9X. Using two 18650 cells is quite common with these, following the example from RC Model Reviews **LINK**

You don't put the cells into the AA battery holder, just fix then together into a "pack" and pop that in the battery compartment. I added a separate set of charging leads with an XT60 connector to match my charger so I can charge in or out of the transmitter.

We've wandered a bit off the original question of how best to charge these cells, but thinking it over they work perfectly well as they are, I think I'll just add a balance lead. The protection circuitry won't come into effect if the voltage doesn't go above 4.25V or below 2.5V so shouldn't interfere.

The main thing is that it seems the LiPo charge program is the correct one.

Thanks everyone, Tony S

Chris Bott - Moderator20/12/2012 20:35:39
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Tony, just for information really - the number 18650 purely describes the size of the can that the battery is in.

I think it means it's 18mm dia and 65mm long?

Anyway, I have some LiFe 18650 cells by A123 Systems.
Exactly the same packaging but a different chemistry, and requiring a different charging regime.
I also have some 18650 LiIon cells, different again.

So it is important that you know the chemistry to pick the right charger settings.

Personally I think the LiFe's are better suited for use in a Tx. Mainly because of the lower voltage, but also because they are more stable chemically. One possible drawback is that once the voltage starts to drop at all, they are pretty much flat, so it's quite hard to know what's left in them.

Tony Smith 721/12/2012 08:51:48
812 forum posts
28 photos

Cheers, I think you're correct as I can see lots of different cells called 18650 with different voltages. That's why I linked to the exact product, but annoyingly they don't mention the exact chemistry. I think "Lithium Ion" is a generic term?

John Cole21/12/2012 09:53:08
615 forum posts
24 photos

As the advert says, these are Li-Ion cells. They have both under- and over- voltage protection circuitry built into the cell and in this respect are differrnt from the standard 18650 Li-Ion cell.

Rigid Li-Ion cells come with different chemistries, depending on whether they are hard-carbon or graphite anode, and whether they are Cobalt or Cobalt- Nickel cathode (there are other variations). The differences are primarily reflecting the target application. Ultrafire are aiming at flashlight users for which voltage stability is required so they are probably not Cobalt-Nickel, and probably graphite anode. Their own website does not say.

A LiPo setting on your charger should give a constant-current charge to just over 4.1 volts then a constant-voltage charge to 4.2 (maybe 4.23) volts. That explains why they charge quickly initially.

The characteristics of these cells are quite different from those for which your Tx has been designed. Personally I would not use them in a Tx.

I prefer low-self-discharge NiMH.

Simon Chambers21/12/2012 10:28:46
789 forum posts
42 photos
Posted by John Cole on 21/12/2012 09:53:08:

The characteristics of these cells are quite different from those for which your Tx has been designed. Personally I would not use them in a Tx.

Depends on the transmitter. For example, the Aurora 9 is designed to take ordinary LiPo flight packs.

Si.

John Cole21/12/2012 11:20:02
615 forum posts
24 photos
Posted by Simon Chambers on 21/12/2012 10:28:46:
Posted by John Cole on 21/12/2012 09:53:08:

The characteristics of these cells are quite different from those for which your Tx has been designed. Personally I would not use them in a Tx.

Depends on the transmitter. For example, the Aurora 9 is designed to take ordinary LiPo flight packs.

Si.

And does it have sprung battery contacts like THIS Tx?

Tony Smith 721/12/2012 11:23:49
812 forum posts
28 photos

Posted by John Cole on 21/12/2012 09:53:08:

The characteristics of these cells are quite different from those for which your Tx has been designed. Personally I would not use them in a Tx.

I prefer low-self-discharge NiMH.

That's correct in a sense, in that the transmitter was designed for 8 separate AA cells in a battery holder.

However unless I was to fit a 35MHz module then there is nothing in the transmitter that needs more than 5V. The limiting factor is going to be how much voltage is dropped by the regulator feeding the main board, or those on any module that I fit. As long as that drop out is accommodated, the electronics are better off with the lower voltage, so no concern need arise regarding the voltage.

I'm not sure what other characteristics would be relevant, clearly current capacity is well within spec, charge retention will be better than NiMH (although I suppose you might say the set was designed for disposible cells).

John Cole21/12/2012 14:21:40
615 forum posts
24 photos

From what I have read, there are a lot of problems with UltraFire. See this for instance. Some reports of early LVC, with in your case immediate loss of Tx function.

If I wanted to go this way (which iI don't) I would use NON-protected 18650 cells, relying on my charger for over-charge protection and my Tx voltmeter to avoid under-charge problems. Though you won't destroy your cells until you take them below 1.0 volts per cell.

And I would buy Sony / Sanyo / Panasonic. Ultrafire are targetting the flashlight market.

Peter Beeney21/12/2012 15:00:44
1587 forum posts
59 photos

To answer the OP, I’d consider that these cells are designed to be charged in parallel from an unregulated supply, maybe a 9 volt source; and probably discharged in parallel, too. I think in the past most lithium-ion cells came with something similar, and if they are are used in any other device, such as a laptop or phone or many other toys, where the discharge current is totally predicable, then the on-board voltage and current control protect the battery, both in the charging and discharging mode. But it does limit the applications, and it wasn’t until this controlling mechanism was left off that modellers could treat them in an unrestricted way, such as using them as heavy discharge flight packs. This led to some complications to start with, bonfires being one of the more dramatic ones, but I suspect this is now generally under control, so to speak.

Charging them is series will most likely to by-pass the charging voltage control, but I’d at least check the voltage after charging to start with, to make sure they are being fully charged. I don’t think this battery control system is really designed this. This not a criticism in any way, but it does seem like landing outside the field to some extent. Changing the system needs some thought. To discharge in series and recharge in parallel, and in situ, can be done, but you have to be careful; especially if fitting a balance lead. Skyleader always used this system in their transmitters with nickel cells, but it does slightly complicate the circuitry.

I’ve noticed that some newer transmitters have a 2 cell lithium supply but I’d guess these have their own dedicated charger or could be charged with a standard lithium charger, with some restrictions on charging current perhaps. A situation where the balance lead has to be connected to verify the cell count before charging can begin must make lithium charging just about as simple as it gets!

If the transmitter takes 8 alkaline 1.5V primary cells then the voltage would be at least 12 volts to start with, but these will be a max of 8.4V. So any problems may be associated with low volts rather than high volts.

Good luck.

PB

Tony Smith 721/12/2012 15:12:00
812 forum posts
28 photos

Thanks Peter. Do you think the overcharge protection works by blocking incoming charge, or by allowing it to bypass the cell?

Tony S

Martin Harris21/12/2012 17:06:29
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Posted by Ben B on 20/12/2012 15:42:37:

I wouldn't worry about the LVC (by the time the batts get to 2.5v you've got seconds of useful capacity left) but I would worry about using sprung contacts on a transmitter. I know people do and get away with it but I haven't got a single household device with such contacts (e.g. remotes, travel clocks etc etc) that occasionally don't get a bad contact and need rotating or remove and reinserting occasionally.

I seem to recall the lipo in my dx8 has a LVC device built in....

I would be worrying if the overdischarge protection open circuits the output. It's all well and good assuming you wouldn't take the pack anywhere near 5V but if the cells go out of balance there's a distinct possibility that the pack could go open circuit if one cell dropped to 2.5V which it may well do very quickly from somewhere in the low to mid 3V range...

Edited By Martin Harris on 21/12/2012 17:07:23

Peter Beeney21/12/2012 20:05:59
1587 forum posts
59 photos

Tony, - The protection device will simply act like a voltage regulator. Regarding a single cell it just blocks the incoming charge, if you will, if it bypassed the cell it would just short the charging source out, no point in that. Also the same if there were a number in parallel being charged, this could be up to the point were the source current is at it’s maximum.

Two (or more) cells in series would need a different arrangement. The charging voltage has to be the fully charged voltage of the pack, but each cell is still regulated to 4.2 volts, and so it would require it’s own bespoke overall control circuit. It’s probably a miniature version of a standard balancing charger. The trick here is to keep the circuit for each cell insulated from each other, if you don’t a series pack will be shorted out. Something to be avoided.

I’ve not had anything to do with any of these ready made packs with the control circuits on, but I’d think each one is for it’s own particular application, i.e. a 4 cell pack device needs a 4 cell pack, not two 2 cell packs in series. This sort of jiggery pokery again might lead to complications.

It is possible to charge a series pack in parallel, though. If we assume a 4 cell pack the balance lead has 5 wires, with the two outer wires connected to the two power wires, neg and pos; and the inner three connected to the connection between each cell. We now need 4 chargers, and if we connect one to each pair of wires in single cell charge mode then each cell will charge as a single cell. But there is a proviso, each charger must have it’s own battery or power source, with no connections in between these, otherwise we would complete the series circuit. If the charging current is equal between cells the current flowing in the balance lead is zero, but at each end it’s the full charging current flowing, so this connection would need to be to the power wires. Actually, this is only just a balancing exercise really, starting at the bottom; and provided you can do the connections, you could do this to any sort of battery.

Really and truly, I’d tend to consider that the best idea might be to treat these as they are meant to be used. The reason modellers couldn’t used this type of regulated cell was because of the regulation! This is to protect the cell, and now it’s been removed and we treat them in an unlimited way I suppose we shouldn’t really be too upset if they do complain a bit.

Although it’s frequently ignored and mistreated, in all branches of electronics, as far as I’m aware, the power source is very important. When it disappears everything falls over. So it really needs to be as secure as possible.

Stayed charged……

PB

Tony Smith 730/12/2012 14:37:48
812 forum posts
28 photos

I just popped a balance lead onto the pack, although the two cells were very similar voltages, from memory 3.72V and 3.75V when I checked them. Now it's going through a balance charge and it looks as if something's amiss. The cells were pretty much neck and neck until around 4.10V, but then spent a long long time before finally completing, and during that time once cell was lagging behind. We'll see how it behaves now it's definitely balanced.

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