Battery and Charger for Canon 7d

[bmpress]

Bob, Can you please illuminate the electronic circuitry in the camera and battery chip, regarding how it functions. My issue is whether or not to believe it when the 7d shows partial charge after the wall charger says the lion battery was fully charged. And if I use an accurate vom, what should the battery read on all pins?

[KeithB]

A VOM probably won't help since it does not load the battery. I suggest using it (with a spare!) and draining it completely a few times to try to sync things.

[Bob Atkins]

Keith is right that you can't determine state of charge of a Li battery from the terminal voltage. Li batteries have a very flat discharge curve so terminal voltage changes very little until you get to a low charge state.

My guess at what might be happening is that the Li battery may be nearing the end of it's life. The charger could be giving up at some point where it can't put any more charge into the battery and indicating an end-of-charge condition. However the camera may be sensing that the charge level isn't as much as you can expect from a new battery. That's just a guess. Canon don't publish any information on how they determine state of charge either in the camera or in the charger.

Li batteries last for anything from a few hundred recharge cycles to several thousand recharge cycles depending on how deeply they are discharged, the exact battery construction etc. They do "wear out" and the symptom is lowered capacity when "fully charged".

[bmpress]

I neglected to mention that the battery is a new one. And I had charged it with a cheap aftermarket charger that only has 2 pins rather than the 4 that the oem charger uses. And the battery does indeed work in my 7d, although the camera's battery indicator only shows a partial charge at the outset.

I was hoping that someone might have a schematic diagram of the oem charger and the oem battery so I could determine what is really going on here.

I suspect that the battery has two cells in series and the cheap chargers do not measure and account for any difference in charge levels between the two cells.

[Bob Atkins]

Is it a Canon battery or 3rd party? Some 3rd party batteries do not communicate with the 7D well (or at all). It's a "smart" system where the 7D actually reads (and writes) data stored in the battery. Also, 3rd party chargers  may not properly charge a genuine Canon battery and the genuine Canon chargers may not properly charge some 3rd party batteries (especially those without data storage). Just to complicate matters there are some 3rd party batteries that will charge on the Canon charger and which do have data storage built in.

I'm sure it is two cells in series because the nominal voltage of most Li cells is around 3.7v and the 7D batteries are nominally 7.4v.

I don't know how the state of charge of the battery is determined by the 7D. It's very unlikely to be a simple measurement of terminal voltage, though it could be reading terminal voltage as an initial calibration point. The actual state of charge during use could be a combination of voltage measurement and current integration with data stored back into the battery's memory relating to the calculated state of charge.

You could try measuring the terminal voltage of the "fully charged" new battery and comparing it with the terminal voltage of a known good fully charged battery. That might tell you something. I've also heard it said that some batteries don't reach full capacity until after a few charge/discharge cycles, but I don't know how true that is.

Just FYI, my fairly new 3rd party battery straight off my 3rd party charger shows a terminal voltage of 8.74v. A fairy well used Canon battery (which reads 2 bars out of 3 for battery condition) when fully charged on the Canon charger showed a terminal voltage of around 8.52v

[bmpress]

The battery is a 3rd party type and I only have an aftermarket charger up here in the NC mountains. And my vom battery just dropped dead so I can't measure anything until we get back to civilization. Fortunately, the camera works very well using this battery.

I surfed the web and learned more about these camera batteries. They have a chip in them which comprises both over-current and over-voltage protection with charge regulation. and some also use a temperature sensor to prevent high temperature damage to the battery. Apparently there are several IC manufacturers who make these chips by the millions for cameras, cell phones, etc. And the OEM Canon battery pack also has some kind of additional circuit that measures charge-used. But no information is public to show how it works.

There is also an issue of battery-life for these Lithium Ion cells. It seems that one can charge them to a higher voltage so you get more photos per charge, but...the cells don't last as long. So I believe that the non-OEM batteries will not last as long as the Canon type because of this phenomenon. And that is how they advertise higher capacity.

Bottom line is you get what you pay for. So keep more spare cheapies in your bag for protection.

[Bob Atkins]

Battery life is also influenced by discharge depth. Most rechargeable batteries (including Li cells) don't like deep discharge. In addition Li cells (unlike early NiCd batteries) don't show a memory effect. So for maximum life Li cells should be recharged after use, even if only partially discharged.