Author Topic: EEVblog #811 - How The Varta 15 Minute Battery Charger Works  (Read 43887 times)

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Offline Svuppe

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #75 on: November 01, 2015, 02:36:31 am »
That charger probably uses all the methods. -dv/dt is easy at 8 amps.
I found the datasheet of the charger:
http://www.produktinfo.conrad.com/datenblaetter/1100000-1199999/001172036-da-01-en-VARTA_LADER_LCD_ULTRA_FAST_4X_2400MAH_AA.pdf
According to that, the temperature is only used as a backup safety cutoff, when the temperature exceeds 75 degrees C. It doesn't mention dT/dt at all.

I am also curious as to how AAA batteries are handled. If the switchmode is a constant current source, then all those resistors in series with the AAA cells won't make a difference. I may have a look at that myself, as I have the same charger coming in the mail one of these days. I have had the old 15 minute charger, but it is about to give up now (screaming fan, that sometimes won't start until I smack it a couple of times). I didn't know Varta had made a new version until I saw the video.
 

Offline apis

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #76 on: November 01, 2015, 03:30:22 am »
That charger probably uses all the methods. -dv/dt is easy at 8 amps.
I found the datasheet of the charger:
http://www.produktinfo.conrad.com/datenblaetter/1100000-1199999/001172036-da-01-en-VARTA_LADER_LCD_ULTRA_FAST_4X_2400MAH_AA.pdf
According to that, the temperature is only used as a backup safety cutoff, when the temperature exceeds 75 degrees C. It doesn't mention dT/dt at all.

I am also curious as to how AAA batteries are handled. If the switchmode is a constant current source, then all those resistors in series with the AAA cells won't make a difference. I may have a look at that myself, as I have the same charger coming in the mail one of these days. I have had the old 15 minute charger, but it is about to give up now (screaming fan, that sometimes won't start until I smack it a couple of times). I didn't know Varta had made a new version until I saw the video.
Hmm, yeah, maybe dT/dt doesn't work when they use a fan (or maybe it would have a cost a cent extra) and, as HKJ pointed out, at 8 ampere detecting dV/dt should be easy. The battery manufacturers datasheets that Iv'e read usually recommend dT/dt though (and all the backups) as a more reliable method when quick-charging (at 0.8C). But I don't have practical experience, it's just what I've read in datasheets and guides.

I also don't get the purpose of the 1 omh resistors, I assume they are used when detecting if it's an AA or AAA battery inserted but I don't really see how. Maybe they measure voltage drop across to see if there is current flowing through the AAA slot or not? Limiting current to the specified 3 Amps for AAA would proabably be done by limiting the duty cycle like it does when charging more than 2 AAs.
 

Offline SeanB

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #77 on: November 01, 2015, 03:47:55 am »
The resistors on the AAA connectors are likely there for the discharge only, extra resistance so the AAA cell has a lower discharge current. On charge they add an extra voltage and power loss, so the unit likely reduces the constant current for the AAA cell charge, unless they allow you to mix cell sizes in charging. they have to reduce current for AAA as otherwise those resistors will have to dissipate 21W and will cook even with fan cooling.
 

Offline AF6LJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #78 on: November 01, 2015, 03:57:32 am »
Thanks Dave; the video answers some of the questions I had.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline apis

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #79 on: November 01, 2015, 04:04:37 am »
The resistors on the AAA connectors are likely there for the discharge only, extra resistance so the AAA cell has a lower discharge current. On charge they add an extra voltage and power loss, so the unit likely reduces the constant current for the AAA cell charge, unless they allow you to mix cell sizes in charging. they have to reduce current for AAA as otherwise those resistors will have to dissipate 21W and will cook even with fan cooling.
But when charging it's a constant current source so if you add resistance it will just increase the voltage and force the same current through the battery anyway, extra resistance will only waste energy/generate heat. On discharge the normal resistance looks like 3 ohm, so the extra 1/6 ohm (if I remember the number of resistors correctly) wouldn't make much of a difference: 0.38 A instead of 0.40 A. AAAs should be able to handle .4 A discharge anyway. Or am I missing something?
« Last Edit: November 01, 2015, 04:06:24 am by apis »
 

Offline bktemp

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #80 on: November 01, 2015, 04:25:34 am »
Maybe the 1/6 ohm resistor is only used to detect AAA batteries and then the multiplex ratio or the current is reduced?
 

Offline twice11

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #81 on: November 01, 2015, 04:28:59 am »
In the new video (you don't seem to have a thread for EEVblog #812 yet), you complain about the discrepancy for the battery voltage at 8A (or so) charge. I guess the voltage displayed by the charger is the no-load voltage in the short charging break, which is (of course) lower than the average voltage measured with the multimeter.
 

Offline HKJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #82 on: November 01, 2015, 05:07:24 am »
It looks like Dave hit a trap for young players with the voltage.

My guess is that the charger will select lower current with AAA batteries and not only do it with pwm (This is the most common behaviour).

With the temperature I obvious gave to much credit to the charger, the maximum temperature is useful for finding old batteries and detecting low airflow.

Maybe it could be fun testing the charger to see how well it does.
 

Offline rr100

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #83 on: November 01, 2015, 05:20:48 am »
I have the charger.

Mixing AA and AAA is handled like having AAA across the board, 3000 or 1500 mA depinding on the number of cells.

Funny thing is that I bought it thinking to go through my huge pile of AAs throwing out what's really bad and keeping what's still decent. Now both the good and bad is that it shows Err on ALL the cells I knew are bad. You know, those that you take fresh off the charger to use with a remote control and they die next day or week. Incidentally the ones that are left are ALL low-self-discharge (and I think except for the Varta's some kind of rebranded Eneloops, be it from Panasonic or Fujitsu). And are much newer because at some point I switched from high capacity cells (2500-2700 mAh) to LSDs because the higher capacity ones were dying on me all the time.

Frankly I don't use so much AAs anymore and I don't like that you can't set the current but for the rest of the people in the house is just what's needed: just drop the batteries in and take them out when the fan cuts off. In most cases even a two minutes charge would get you through the day.
 

Offline rr100

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #84 on: November 01, 2015, 05:29:54 am »
One more thing: about the life of the fan - I think it is more or less a race to the bottom, to see what gives out first - the fan or the battery contacts. I wouldn't say they're particularly flimsy but unless I get much better at this (and quickly) I think we'll have a problem here.
 

Offline K6TR

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #85 on: November 01, 2015, 07:38:59 am »
Dave I was wondering while watching 2nd part: Why not to use FLIR to track current flow for discharge and charge, test modes ? :)
Wouldn't that help to reverse engineer the process? BTW that would also look freaking GOOD to watch how this thing heats up! :)

I would have done that, but the iProber is a more interesting tool I haven't shown much.

Thanks Dave....I hate it when I have to cut tracks to do troubleshooting. But sometimes its the only way. The IProber provides a very useful alternative.
 

Offline Ampere

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #86 on: November 01, 2015, 11:42:06 am »
Great video, Dave. I would like to see more reverse engineering videos in the future. It's always interesting to see how something was designed.

With regards to the Iprober... is there a way to do something similar with hobbyist-level equipment?
 

Offline oldway

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #87 on: November 01, 2015, 07:28:47 pm »
For me, the biggest flaw of this charger is that it can only charge the batteries with a single current value.
8A for an AA battery looks a little insane for me ! :scared:

It should allow to choose the charging current between 3 values, high, medium or low rate.

For 2 batteries:
High: 8A
Medium: 2A
Low: 0.4A

For 4 batteries, half of these values.
 

Offline HKJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #88 on: November 01, 2015, 07:33:20 pm »
Low: 0.4A

Bad idea, -dv/dt termination will not work reliable at this low charge current.
 

Offline oldway

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #89 on: November 01, 2015, 08:18:49 pm »
Poor answer: at 0.4A, only max V termination is enough...Is this difficult to implement ?
Fan is also not needed.
 

Offline HKJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #90 on: November 01, 2015, 08:31:44 pm »
Poor answer: at 0.4A, only max V termination is enough...Is this difficult to implement ?

If you want a full cell without over charge, it is impossible to do. Chargers with voltage termination either under charges the cells, misses termination or uses a secret* top-off charge. Top-off is over charging, but NiMH batteries can usual take it.

*With secret I mean that the charger is saying finished before it applies the top-off charge, i.e. the user may remove a under charged cell, believing it is full.
 

Offline alxpo

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #91 on: November 01, 2015, 11:01:00 pm »
What do you think regarding using of zero (┬▒opamp offset) burden voltage current shunts for measurements like 2nd part?
Like this, but sure with powerful opamps and lower resistors.

From EDN DI
« Last Edit: November 01, 2015, 11:03:31 pm by alxpo »
 

Offline oldway

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #92 on: November 01, 2015, 11:06:42 pm »
All the low cost, low current AA and AAA battery chargers work on the principle of max V termination.
Very little over charging give a great over voltage, for this reason, ever if there is an over charging, it is very reduced and not harmfull for the battery.

Wat harmfull is for the battery, that's to charge it with current as high as 4C... :--
It's out of specifications of (most or all ?) the battery manufacturers.

It is also an insane loss of energy (It even need a fan to cool the batteries  :-DD)

@alxpo: There already exist an implemented battery voltage monitoring, as individual voltage of each cell can be displayed.
It is not necessary to add nothing as hardware.

But Dave has discovered that the displayed voltage is lower than the measured voltage...we should investigate why this happen...?
« Last Edit: November 01, 2015, 11:15:45 pm by oldway »
 

Offline HKJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #93 on: November 01, 2015, 11:16:31 pm »
All the low cost, low current AA and AAA battery chargers work on the principle of max V termination.
Very little over charging give a great over voltage, for this reason, ever if there is an over charging, it is very reduced and not harmfull for the battery.

No (Many low cost do never termination or only uses a timer) and it is as I wrote before.
I will suggest you go to my website and check how different chargers works: http://www.lygte-info.dk/info/indexBatteriesAndChargers%20UK.html
 

Offline KD0CAC John

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #94 on: November 01, 2015, 11:49:21 pm »
HKJ ,

I see you have looked into a lot of these battery chargers etc.
After watching Dave's video , I looked / Googled around looking for one of these and only found the Varda on eBay from Australia & 220v .
With your testing etc. , with similar functions as the charger in the video [ and batteries ] what would you recommend for US .   
 

Offline rr100

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #95 on: November 01, 2015, 11:51:55 pm »
It is also an insane loss of energy (It even need a fan to cool the batteries  :-DD)

The batteries need to be cooled anyway due to the way chemistry works, by probably roughly the same amount (energy-wise), except that because things happen much quicker we can't rely on the breeze to take out the excess heat. Yea, sure running the fan is extra energy expense but it is really next to nothing. And in any case a compromise we're willing to take, even at much larger scale with our servers, desktops, laptops, even Surface Pro tablets and Intel's compute sticks have fans, cars, A/C units and much more.

As total energy goes probably the regular slow overnight charger that is just an old school transformer, a diode or a bridge plus some resistor(s) as a crude constant current source would eat more energy to deliver the same charge to the same batteries.
 

Offline HKJ

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #96 on: November 02, 2015, 12:10:36 am »
HKJ ,

I see you have looked into a lot of these battery chargers etc.
After watching Dave's video , I looked / Googled around looking for one of these and only found the Varda on eBay from Australia & 220v .
With your testing etc. , with similar functions as the charger in the video [ and batteries ] what would you recommend for US .

For charging only the Panasonic BQ-CC16 and BQ-CC17 works very well.
For analyzing chargers Opus works good (They are very good at terminating correctly).

The old classic analyzing charger is Powerex MH-C9000, it uses voltage termination and terminates before the batteries are full, then it supplements with two hours top-off charge after it has reported ready.

The project called Ultrasmartcharger is probably also a very good charger, but I have not looked at it (It is open source).
 

Offline oldway

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #97 on: November 02, 2015, 12:55:00 am »
It is also an insane loss of energy (It even need a fan to cool the batteries  :-DD)

The batteries need to be cooled anyway due to the way chemistry works, by probably roughly the same amount (energy-wise), except that because things happen much quicker we can't rely on the breeze to take out the excess heat. Yea, sure running the fan is extra energy expense but it is really next to nothing. And in any case a compromise we're willing to take, even at much larger scale with our servers, desktops, laptops, even Surface Pro tablets and Intel's compute sticks have fans, cars, A/C units and much more.

As total energy goes probably the regular slow overnight charger that is just an old school transformer, a diode or a bridge plus some resistor(s) as a crude constant current source would eat more energy to deliver the same charge to the same batteries.
The energy losses are coming from the Joule effect in the internal resistance of the battery.
These losses are proportional to the square of the charging current.
8A is 20 x 0.4A, so the joule losses/s (W) are 400 x greater with a charging current of 8A instead of 0.4A.

For the same charge (quantity of Coulombs), time is only divided by 20 so it is still a huge waste of energy  :--
(twenty times more energy lost in joule effect in the internal resistance of the battery)
« Last Edit: November 02, 2015, 01:18:20 am by oldway »
 

Offline rr100

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #98 on: November 02, 2015, 02:23:10 am »
You are thinking about the internal resistance of a battery like a normal resistor just in series with the battery. This is not a correct model and you can easily test it just by injecting currents and plotting the voltage across the battery versus current (at the very same charge level, preferably take all the measurements within a few seconds on the same battery), it won't be a linear dependency.
 

Offline oldway

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Re: EEVblog #811 - How The Varta 15 Minute Battery Charger Works
« Reply #99 on: November 02, 2015, 04:17:04 am »
Nobody said that internal resistance of a battery is a fixed value...It varies with temperature, concentration of electrolyte (and thus stade of charge), and so on...
 


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