Is it safe to put a "charged" battery back into the charger?

[PhotographyFirst]

It seems with my Canon 6D and previous bodies that used the larger LPE6 batteries, I could fully charge the battery to a green light, pull it out, put it in several days later, and it would take only a few seconds before showing a green light again.

I've noticed something about my Canon T1i. If I charge the battery in the charger until the green light comes on, let the battery sit overnight, and then put the battery back into the charger, it then takes a long time for the green light to come back on. Almost as if it is charging completely over again. This happens with all 5 of my batteries for the t1i.

Is this somehow overcharging the battery or should it automatically know when it is charged to the max? Are the batteries somehow not being fully charged the first time around? Is the heat buildup maybe causing a premature "full charge" the first time around?

I thought all cameras had automatic stops for the charging, but is this not the case in my situation? Am I damaging something by putting the batteries back in? Are the Rebel t1i (500D) batteries too "dumb" and simple to know when they are overcharged?

I would like to know if this is OK, as I want my batteries to be fully charged before heading out on my backcountry hiking trips.

Thanks!

 

[Mt Spokane Photography]

This is not normal, and potentially dangerous! There could be a charger issue. Since it happens on all your batteries, its very suspicious. I assume they are all genuine Canon batteries. A defective battery could have damaged the electronics in the charger.

Bad chargers can overcharge batteries, which can damage them, or even cause them to enter thermal runaway, which could start a fire.

I'd try a replacement charger. If the batteries are third party batteries, try a Canon battery.

 

[PhotographyFirst]

This is not normal, and potentially dangerous! There could be a charger issue. Since it happens on all your batteries, its very suspicious. I assume they are all genuine Canon batteries. A defective battery could have damaged the electronics in the charger.

Bad chargers can overcharge batteries, which can damage them, or even cause them to enter thermal runaway, which could start a fire.

I'd try a replacement charger. If the batteries are third party batteries, try a Canon battery.

Thanks for the reply. It's a mix of Canon and STK batteries. I was going to get another charger anyway so I can charge the set faster, so maybe I will see if there is any difference between the two.

 

[Mt Spokane Photography]

From Walt Kester and Joe Buxton, Analog Devices--Li-Ion Charging: Li-Ion batteries commonly require a constant current, constant voltage (CCCV) type of charging algorithm. In other words, a Li-Ion battery should be charged at a set current level (typically from 1 to 1.5 amperes) until it reaches its final voltage. At this point, the charger circuitry should switch over to constant voltage mode, and provide the current necessary to hold the battery at this final voltage (typically 4.2 V per cell).Thus, the charger must be capable of providing stable control loops for maintaining either current or voltage at a constant value, depending on the state of the battery.

The main challenge in charging a Li-Ion battery is to realize the battery's full capacity without overcharging it, which could result in catastrophic failure. There is little room for error, only ±1%. Overcharging by more than +1% could result in battery failure, but undercharging by more than 1% results in reduced capacity. For example, undercharging a Li-Ion battery by only 100 mV (-2.4% for a 4.2-V Li-Ion cell) results in about a 10% loss in capacity. Since the room for error is so small, high accuracy is required of the charging-control circuitry. To achieve this accuracy, the controller must have a precision voltage reference, a low-offset high-gain feedback amplifier, and an accurately matched resistance divider. The combined errors of all these components must result in an overall error less than ±1%.

 

[PhotographyFirst]

From Walt Kester and Joe Buxton, Analog Devices--Li-Ion Charging: Li-Ion batteries commonly require a constant current, constant voltage (CCCV) type of charging algorithm. In other words, a Li-Ion battery should be charged at a set current level (typically from 1 to 1.5 amperes) until it reaches its final voltage. At this point, the charger circuitry should switch over to constant voltage mode, and provide the current necessary to hold the battery at this final voltage (typically 4.2 V per cell).Thus, the charger must be capable of providing stable control loops for maintaining either current or voltage at a constant value, depending on the state of the battery.

The main challenge in charging a Li-Ion battery is to realize the battery's full capacity without overcharging it, which could result in catastrophic failure. There is little room for error, only ±1%. Overcharging by more than +1% could result in battery failure, but undercharging by more than 1% results in reduced capacity. For example, undercharging a Li-Ion battery by only 100 mV (-2.4% for a 4.2-V Li-Ion cell) results in about a 10% loss in capacity. Since the room for error is so small, high accuracy is required of the charging-control circuitry. To achieve this accuracy, the controller must have a precision voltage reference, a low-offset high-gain feedback amplifier, and an accurately matched resistance divider. The combined errors of all these components must result in an overall error less than ±1%.

It's amazing how many Li-Ion devices are out there with very few failures to speak of. Ignoring the exploding hoverboards of course.

Maybe that's why most of my devices have cells made in Japan and not somewhere else where they are more likely to mess them up?

 

[tpatana]

Do the batteries get hot on the second charge? If yes, that might be dangerous. In general, Li-Ion chargers are really good in preventing problems, so in most cases I'd say not to worry. But naturally something can break, and cause problems or even accidents/fires.

One time I saw soft-cell battery turn into round balloon. Luckily didn't go more than than. User error in that case.

 

[9VIII]

It really is amazing that more of our current devices don't blow up.

I did a bit of research into RC cars a few years ago and they recommend that Lithium Polymer batteries never be charged unattended, because they melt down in a blaze of acid flames when they go.
Common practice is to put them in a fireproof bag when charging.
Of course those are often half pound batteries being stressed to destruction, so it's not the same application, but Lithium based cells are generally less stable, especially compared to NiMH (but NiMH specifically sucks for RC, they just drop voltage way too fast).
Good batteries have circuits that artificially shut off the battery before anything serious happens though, thus the you don't hear about more than a few cell phones exploding every year.