# Modified Duracell Charger: CEF20 a.k.a "Mini Charger"



## PeAK (Jan 14, 2010)

This charger has been modified recently to fix issues with the unit not terminating predictably using AAA batteries. This results in the unit charging at 280mA regardless of whether AA or AAA are used. I mentioned that a *physically rewiring* the unit was possible but a simpler fix (post #8) was found.

The fix involved opening up the unit and cutting the leg of two diodes connected to the negative terminal of the AAA connectors. These diodes where used to generate a signal to the circuit to cut down the current. Here are *More details and photos* or scroll down to post #8.

The end result is a much more useable and predictable and faster charger (with AAA)...still without overheating the batteries. 

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Original Post: Duracells low current charger technology: EXCELLENT
Date: Jan 14/'10

Now that I've played with my first Duracell charger (CEF20) , ever, my initial impressions of this low cost charger are WOW! A year ago, I mistakenly lumped Duracell in with Energizer in terms of offering unimpressive batteries (save for Alkalines) and chargers. Lately, I've noticed their smart chargers occupying more and more of the aisle space in all sorts of stores.

see my earlier comments below:


PeAK said:


> I picked up this charger for $7...I think it is a steal:
> 
> 
> smart charger for one to two batteries
> ...




What is not in the specification is how *cool* the batteries are just before they finish charging. The charge current is about 300mA but I have other 250mA chargers that run much hotter. Some might claim that the charger is terminating early but early results indicate a very full charge and this has me suspect that it is using the *same technology* as the model CEF21 (a.k.a Duracell _Fuel Gauge_) which has a fond history on this forum as a very gentle/cool and yet full charging unit.

As you can see in the above link, it has an even fuller charge than the Maha C9000 without the need of a topping up charge. A) How does Duracell they do it ? *B) How do they do it at much less than the 0.5C charge rate ?* Maybe it's the result of a large multinational company (P&G) bringing the in-house expertise to bear.

If you want a reliable no-fuss charger (think grandparents and kids) that is easy on the batteries and wallet, pick up the Duracell mini Charger...it's a steal.

 *P.S.* I borrowed my brother's CEF23/"Mobile Charger"(Charge current=550mA) and noticed that some batteries got on the hot side. So for now, the comments about cool charging applies to the CEF21/"Fuel Gauge" (Charge current=400mA) and the CEF20/"mini charger"(Charge current=280mA).


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## Mr Happy (Jan 14, 2010)

*Re: Duracell's excellent low current charger technology*

One thing I'll note about the CEF21 is that while it appears to terminate the charge very reliably on AA cells at 400 mA, it frequently misses termination on AAA cells at 175 mA -- even on good cells like eneloops.

This is a shame, but it leads me to think that the C-rate recommendations for charging need to be modified with the battery size. Smaller cells like AAA need to be charged at a relatively higher rate, and larger cells like D need to be charged at a lower rate. For instance I don't think you should charge a 10 Ah D cell at a 1C rate of 10 A. Some data sheets suggest a maximum of 2 A.

Overall, I agree with you though. Duracell does seem to have its act together when it comes to chargers (except for the dismal failure to keep any good charger on the market for more than a year or two before replacing it).


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## PeAK (Jan 14, 2010)

*Re: Duracell's low current charger technology: EXCELLENT*



Mr Happy said:


> One thing I'll note about the CEF21 is that while it appears to terminate the charge very reliably on AA cells at 400 mA, it frequently misses termination on AAA cells at 175 mA -- even on good cells like eneloops


I would echo your comments on AAA batteries using the "mini charger". As in your case, I was using eneloops. The charging current in this case is 110mA. When the AAA eneloop batteries missed termination, did you notice them getting excessively hot ?


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## Mr Happy (Jan 14, 2010)

*Re: Duracell's excellent low current charger technology*



PeAK said:


> I would echo your comments on AAA batteries using the "mini charger". As in your case, I was using eneloops. The charging current in this case is 110mA. When the batteries AAA eneloop batteries missed termination, did you notice them getting excessively hot ?


No, they didn't seem to get warm at all, which was puzzling. Next time I do a test I will try to insert an ammeter in the circuit and find out what the current is. It is slightly possible the charger has reduced to a trickle charge and is doing a long top-off phase.


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## PeAK (Jan 14, 2010)

*Re: Duracell's low current charger technology: EXCELLENT*



Mr Happy said:


> No, they didn't seem to get warm at all, which was puzzling. Next time I do a test I will try to insert an ammeter in the circuit and find out what the current is. It is slightly possible the charger has reduced to a trickle charge and is doing a long top-off phase.


My measurements on the AA charge circuit of the CEF20 indicate a repeating charging cycle of 5 seconds with the following breakdown:


3 seconds at 260mA
2 second interval where the current drops to 220mA. During this interval the voltage ramps down 10mV but the alternates between "no drop" and a increasing drop every 1/2 second.
Still, the current always exceeds 220mA until the LED turns green at which point the current trickles down to about 35mA. YMMV with a CEF21. My 200mA charger gets the batteries far warmer...WHY ?


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## mfm (Jan 14, 2010)

*Re: Duracell's low current charger technology: EXCELLENT*



PeAK said:


> Maybe it's the result of a large multinational company (P&G) bringing the in-house expertise to bear.


The in-house experts at Duracell already stated that charging at 1C with dT termination is the best. It's the economists, marketers and PHBs that decided to make slow cheap chargers.

As long as there are no measurements (of temperature, of voltage at termination, etc) there is just a lot of guessing and wishful thinking going on here.


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## PeAK (Jan 15, 2010)

*Re: Duracell's excellent low current charger technology*



Mr Happy said:


> One thing I'll note about the CEF21 is that while it appears to terminate the charge very reliably on AA cells at 400 mA, it frequently misses termination on AAA cells at 175 mA -- even on good cells like eneloops...*C-rate recommendations for charging need to be modified with the batterey size. Smaller cells like AAA need to be charged at a relatively higher rate*...



One thing that could be easily done is to modify the "fuel guage" charger to that it behaves similar to the "mobile charger" where the latter does not distinguish between AA and AAA and uses the same charge rate. *To achieve this, you would need to open up the unit and disconnect any connections to the bottom AAA terminals and rewire them to the corresponding AA terminals.*

I'm thinking of doing this modification for my "mini charger". The one tradeoff is that batteries may get a little warmer...but I'm for predictable termination.


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## PeAK (Jan 16, 2010)

PeAK said:


> One thing that could be easily done is to modify the "fuel guage" charger ....



The modification involves making two cuts to the one end of diodes D14 and D15. This is indicated by red lines in the photo below:





​ 
After the cuts are made, stand the diode vertically to ensure that cut end does not touch anything. What you should have is a circuit board that looks like the photo below:




​
Place the circuit board back in the shell and secure the screw back together. Congratulation, you now have a AAA mode that is 2.5x faster and better terminating.​


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## Mr Happy (Jan 16, 2010)

I have not disassembled a CEF21 yet. Can you explain the theory behind the above modification? It is not the form I expected the modification to take (which would have involved detaching the supply from AAA contacts and then connecting the AAA contacts to the AA contacts). Your modification is much simpler.

[Edit: OK, I have seen the updated post #1. But further info would still be appreciated ]


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## Mr Happy (Jan 16, 2010)

I assume RT1 is a temperature sensor? Curious how these often seem to have very weak thermal coupling to the batteries.


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## PeAK (Jan 16, 2010)

Mr Happy said:


> ...Can you explain the theory behind the above modification? It is not the form I expected the modification to take (which would have involved detaching the supply from AAA contacts and then connecting the AAA contacts to the AA contacts)



If you look at the first photo, you can see the negative battery terminal of the right bank AAA labelled B2NA. When the AAA battery is inserted on to the spring metal tab, it causes B2NA to short to terminal B2N (AA negative terminal). So the circuit is electrically the same for a AA battery as it is for an AAA battery. 

What is different is that D15 signals the charger IC so that the current is altered. When this signal is removed, the current remains at the AA level instead of the reduced AAA level. Note, there are no other connections to the respective B2NA and B1NA AAA terminals.

The configuration of the "fuel gauge charger"/CEF21 may not be the same but I have a feeling that it may use the same chip(s) with a more expensive/sophisticated circuit.

Good Luck with your unit


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## Mr Happy (Jan 16, 2010)

The CEF21 is considerably more complicated inside. With the double sided circuit board it I am finding it hard to follow the main circuit paths.

There are four resistors that may perform the same function as the diodes you identified, but I am not sure.

I am going to post photos, for entertainment if nothing else.


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## Mr Happy (Jan 16, 2010)

I've started a new thread for the CEF21. See here.


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## PeAK (Jan 17, 2010)

PeAK said:


> *...**B) How do they do it at much less than the 0.5C charge rate ?*...



I'm running an overnight charge on the infamous Energizer 2500 mah batteries using the "mini charger". By all rights, it should not terminate the charge (See Update below in blue)




​Part of the problem with the known leakage problems is that the battery chemistry is subjected to damaging electrical condition below 1.0V. The batteries have not been in use and came out of storage measuring about 0.86 volts...not good. It's going to be interesting to see if the green light comes on in 13.4 hours at a 0.11C/280mA charge rate... *smart trickle charge*, anyone ?

==========================
=======*10 hours later*=========
=========================
​ Almost to the minute, the batteries that went into the charger at 11pm ended the charge at 9am with the Green LED coming on and continuing on with a trickle charge.
The 280mA charge rate corresponds to 40% more current than the 0.1C forming charge rate (i.e. 0.14C). If you do the math on the total charge, this corresponds to 10hours * 0.14C =1.4C ...just a bit shy of the 1.6C (i.e. 16 hours * 0.1 C) put in by a forming charge. This "math" also holds true for the AAA charge rate of 110 mA if you assume 1C=800mah.

Not specified in description is whether on not this unit has a time out functionality built in to the unit. So these batteries may have terminated on that condition. To test, we will run a sanity check that takes out a small of charge and test the overcharge ratio.

Update:
When the supposed charged pair of crap batteries (Energizer 2500) that were discharged slightly of 33mah, each was put on the charger and observed to not terminate. The conclusion is the overnight charge stopped due to a *built in 10 hour timer*. Providing the batteries are run right down (i.e. in your kids toys) as a discharge part of a refresh cycle and the charger's charge current would almost approach a modified forming charge with slightly higher current but for less time....maybe it's all by intention from the design boys at Duracell


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## PeAK (Jan 21, 2010)

*Modified Duracell Charger: CEF20-m*

So here it is the final state of CPF influenced "Mini Charger" now christened the "Modified Mini Charger" or CEF20-m. Here's a list of changes:



Charge rate for AAA increased to a more *CPF friendly* 0.35C (280mA)
Removed lid as is general *CPF practice*. Now used as a battery stand
Updated packaging...look for the words "Modified"
 Note: :devil::devil::devil:
_ If you do not see in a store near you, be extremely patient, as supply is extremely limited_:devil: :devil: :devil:

Below is a picture of the "new" unit with the updated packaging:



​


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## Mr Happy (Jan 21, 2010)

Personally, I wouldn't remove the lid on a slow, cool charger. Keeping the lid on may help the temperature sensor detect a temperature rise when terminating the charge.


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## PeAK (Jan 21, 2010)

Mr Happy said:


> Personally, I wouldn't remove the lid on a slow, cool charger. Keeping the lid on may help the temperature sensor detect a temperature rise when terminating the charge.



Good point...we'll introduced that feature back in on the 3rd generation CEF20-m*L* and an abestos lined lidded version in the 4th generation CEF20-m*ld* **.

** The 4th generation unit was a result of feedback from people using their chargers in harsh outdoor environments such as "*l*oading *d*ocks" in Artic temperatures ​


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## TakeTheActive (Jan 21, 2010)

Mr Happy said:


> ...*Keeping the lid on may help the temperature sensor detect a temperature rise when terminating the charge*.


According to the Duracell Spec Sheets, both the CEF20 Mini Charger and the CEF23 Mobile Charger are capable of terminating on Max Temp. So, I agree - leave the cover on to allow it to detect the temperature as designed.

Similar situation with my La Crosse BC-900 - I raise it up with two old-fashioned, 6-sided, wooden pencils for airflow (no fans). If the cells are going to get hot, I want the built-in safety to work as designed. I use the BC-900 for my *CRAP* (mostly due to High Internal Resistance) cells, so heating occurs more often than when charging vibrant, healthy cells.

NOTE: This doesn't include the current BC-9009 microprocessor and/or MOSFET problem. Pencils and/or fans aren't going to help that.


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