I just bought a second battery for my Panasonic compact digital camera. It's a Lithium-ion battery. The charger that comes with the camera has a LED light on it. The light goes off when the battery has finished charging.

At the store where I bought the battery, the store clerk, who resembled a Radio Shack employee, warned me to be sure to charge my new battery for 12 to 14 hours.

He said that the charge that the battery currently contained (indicated by a 2/3 full meter when I plugged it into the camera, which the clerk said was a lie) was simply a "surface charge", whatever that means, and that the battery would need a full charge for best functionality. A small printed card that he slipped into the packaging repeated this same information.

Well, the little LED on the charger turned off after the battery spent only an hour in the charger. But the store clerk, and the printed card, both cautioned me that I should not pay attention to the little light on the charger; it would lie to me and the battery should remain in the charger for the full time period, regardless of the indicator light's state.

My argument is: The light indicates when the charger has stopped its function. In other words, once the light is out, leaving the battery in the charger is no different than leaving it sitting on the kitchen counter.

The store clerk says no, the battery is still being charged even when the light is out.

Who's right?

What they're telling you to do is to prime the battery by trickle charging it. The problem is that you can't and shouldn't trickle charge a Lithium based battery. Once a Lithium based battery is full, it just does a small topping up charge periodically. The priming was only necessary for Nickel based batteries. Once the charger light went off, they would generally just go into trickle charge mode. Lithium batteries tend to react badly if you overcharge them.

No idea why it is still generally suggested though.

So you're saying...

- The charger does in fact behave the way the guy said (trickle charging after topping off), but it's actually bad for the battery, not good?

- The guy would have been right if the battery type had been NiCd, but both the battery and charger aren't the right type for that behavior, and I was correct in assuming that the charger really does shut off when the light goes out?

No. I'm just bad at explaining things. I mean that the charger doesn't do trickle charging and only does topping up. Once the light is off you should be able to take the battery out and go use it.


Yes.

Thanks! :-)

It's interesting to note that I trust the opinion of someone on this BBS more than the opinion of someone who works at an actual battery store.

The Tandy vibe was strong in that place, I'm telling ya. Had we already determined that the store is a Tandy offshoot, or is that just an illusion on my part?

I find it very funny that my anecdote on radio shack employees seems to have become the default example

Generally speaking, the theoretical time to completely charge a lithium ion battery from empty to full is 1 hour, assuming a maximum safe charge rate of 1C. Note that some of the newer cells and chemistries (such as lithium iron phosphate) can be charged at higher rates, up to 5C or more in some cases.

Anyway, the actual time is always a bit more than that because of efficiency losses in both the charger and the cell itself, ie the internal resistance dissipates some of the energy as heat. In practice, 90 minutes seems to be pretty much spot on for a 1C charge from a modern lithium charger. Some are a little quicker, but not much.

If the battery is not fully discharged, you'd think it would take proportionally less time, but it doesn't affect it as much as you might think. The charging process for lithium cells is constant current for about half the total capacity, which is quite fast, then constant voltage for the rest. This can end up being something like 60-70% or so of the total time. So if the battery is half charged, it can still take nearly an hour to fully charge it from that point.

One other thing, a new lithium battery may not reach it's full capacity for several charge-discharge cycles. It won't take any longer to charge, but it will last longer in the device until it reaches it's peak. It can be very noticeable in very high current applications, like for example electric model aircraft I'm still impressed I can pull 40 amps at 20 odd volts from a battery the size of a pack of cigarettes for ten minutes, and have the thing only just warm enough to be sure that it's been used!

There you go, more waffle on the subject of lithium cells than you ever wanted!

pca

My observations totally concur with that.

(hi Patrick!)

My viewpoint

Most of the time, eg for phone cells, the recommendation is to stay at or below 0.7C - also, when you hit 4v, the last 200mV is done in constant voltage mode, so the charge current will taper toward zero and hence you're charging a lot more slowly towards the end. This tends to work out to be in the 2-2.5h range for a full charge.

Often the charger will determine "finished" when the current into the battery falls below C/20 (0.05C). Charging beyond that point - continuous trickle - is bad for the cell in the long run. Any Li-Ion charger should be stopping charge when the charging light goes out - no trickle.

As for the battery being 2/3rds or so full, that's how Li-Ions are shipped. Their shelf life is best when at about 2/3rd capacity so that's how they leave the factory. It's not a "surface charge" and is totally valid usable capacity.

Lastly, never charge below 0C or above 45C. Charge at 20C or above if possible, or lower the charge rate if you can't.

Is there a difference between charging a device while it is powered up or not?

I find that if I turn my phone off and plug it in overnight it lasts significantly longer than if I leave the phone turned on while plugged in. Is this something that generally happens with all devices (all phones/laptops etc)?

It depends. Some systems, where the external power brick is guaranteed to supply enough current to run the entire system AND charge the battery at max rate, disconnect the battery from the system when they connect it to the charger circuitry (eg: Rio Karma).

Other systems share input power between the system and the battery. There are 2 ways to do this:

1. All external power goes into the charger. System runs from battery only. If the system takes power during charge, it is in effect taking it direct from the charger (except when, say, there's a current transient so fast that the charger doesn't respond, in which case it comes from the battery). Obviously, charge time here is impacted by system load.

2. Have a priority setup where the system gets all its needs from the external supply, and any current "left over" between system needs and input current limit goes to the battery. Here, charge time *can* be impacted by system load, depending on what the max input current is and how much of this the battery charger is allocated. eg on an iPhone, the supplied brick is 5v/1A, and the battery charges at about 700mAish - so the system can take ~300mA without affecting the charge at all.

Neither of these should impact a full charge if the system is designed appropriately. If you are seeing what appears to be a lesser charge when the phone is not turned off during a charge cycle then either:

a) the charger is shutting off early and not retriggering (setup 1 above has issues of that sort, as the current drain never appears to stop and the charger's safety timeout can kick in, leaving the phone draining for a portion of the night)

or b) the battery metering system is getting confused and it really prefers a clean charge cycle with no drains in the middle.

Which phone?

Thanks Hugo.

I've seen this behaviour with varying degrees with my HTC Dream (G1), HTC Kaiser (TyTN II), Nokia E61, Treo 650 and Treo 600 definitely. Can't really remember about earlier phones of the non-smart type (6310i, T28, 8210, T10).

I wonder whether it'll happen with my Nexus One. I also wonder whether it will actually ever arrive, 25 days after I ordered it and its still 'on its way'.

Anyway, what I usually see when the phone is left on is that the charge will complete to 100% (or sometimes ~99% on the Treos or the horrible Windows Mobile phone) overnight. But the charge just seems to run down faster. As if there is less mAh per % of charge. I guess it must be down to imperfect metering.

Thinking about it now, although I have experimented I have not seen this trait with laptops - its clear by your explanation that sensibly they behave like the Rio Karma.

Ugh. Not exactly a great first impression.