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Old Oct 14, 2006, 3:35 PM   #1
blr
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I've been looking at many chargers. It occurs to me that the excessive heating of the cells towards the end of the fast charge that is seen on several models (Maha 401 FS in fast mode, BC900 at high currents, etc.) is related not to overcharging but is a heat dissipation problem. On many compact chargers, the cells are sitting too close to ensure proper ventilation. This is obvious if you run only 2 cells in the end slotts. The end of charge temperature is much lower in this case. So, I decided to make a small experiment. I placed a metal heatsink on top of the cells, running 4 Sanyo industrial grade 2500 mAh cells on the BC900at 1A. At this current it is known that the cells are getting quite hot and the charger's thermal protection is being activated frequently. With the heatsink on, the cells got only warm to the touch, much cooler than usual, and no thermal protection was activated. Right now I don't have the equipment to measure cell temperatures, but the difference was quite obvious just by touching the cells. I'd say, they stayed around 45 Celsius max. by the end of charge.

If anyone has a suitable heatsink and a theromocouple or an IR thermomether at hand, please run proper tests. This could be quite useful. IMO it's better than adding a cooling fan, it's quieter and cheaper.
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Old Oct 16, 2006, 5:31 PM   #2
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blr, this seems very interesting, I was just wondering why the manufactures with there designers haven't seen this solution/situation?
Believe me I am not knocking you whatsoever, I sincerely hope you have discovered an obvious situation that has not been seen.
Sort of a , can't see the trees because of the forest situation.

Something like this would be right up "Silver Fox's" alley, he is the battery/charger Guru at this site!
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Old Oct 17, 2006, 8:46 AM   #3
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Changeling,this indeed looks like a typical design flaw. Besides this is not the only one of this charger. To me, the BC900 is a nice concept, i.e. a compact and affortable charger/analyzer for AA and AAA cells, but the implementation is not that good. Apart from the cell being too close consider also the following:

1. The power supply rated at 3 V, 4A. This means that the unit cannot be used in a car without adding a DC/DC transformer. Also if the PS fails after the warranty has expired, a replacement may be difficult to find.

2. As commented on candlepowerforums, the charger has a potentially fatal design flow that some of the components are working right at the edge of their specs. This most probably has caused the several reported meltdowns.

3. For a charger of this complexity, I'd like to see some better charge termination algorythms, such as 0dV (peak voltage detection), dT/dt, or at least -dV with better sensirivity

4. Discharged capacity is remembered and displayed only in test or refresh mode, i.e. it is not possible just to run a discharge cycle to determine capacity

5. The charger does not handle overdischarged cells.

6. Trickle charge current does not scale with the main charge current. 60 mA of trickle charge current IMO is a bit high, especially for NiCd cells, where it is approx c/10.



When you consider all this plus some unresponsive buttons and less than intuitive user interface the BC900 does not look so good.

To me it's OK for occasional testing and matching of cells, but for day-to-day operation I use other chargers.



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Old Oct 17, 2006, 2:16 PM   #4
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BLR I really wish you would send a message to the Silver Fox, he is well known on this forum and has really helped me, nice person.
I believe that with the 2 of you interrelating thoughts, could add a great deal to the understanding of this complex subject and the various chargers.
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Old Oct 17, 2006, 2:19 PM   #5
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BLR, forgot to mention it was the meltdown remarks that kept me away from the BC900. I went with the AccuManager 20 (haven't received yet).
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Old Oct 18, 2006, 5:42 AM   #6
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Hi Changeling


We are discussing these questions with Silver Fox and other knowledgable people on candlepowerforums (www.candlepowerforums.com) I have posted the same topic there and it has generated some discussion. Silver Fox has done a great job in testing many chargers and cells. I guess he is not responding here because he does not want to duplicate discussion from candlepowerforums.

If you have any specific questions about Ni-based batteries and charging please ask, I'll do my best to answer them here.



Best regards,

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Old Oct 19, 2006, 5:20 PM   #7
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Hello Blr,

Yes, I am ignoring commenting on this topic...

I happen to have a BC-900 charger that works as expected. Heat is not a problem with it. Having no experience with cells heating up, I see no reason to build a heat sink for it. It appears that others have had different experiences, so I am learning from them.

I am also waiting the Version 34 charger. It will be interesting to see what changes La Crosse has implemented in it.

Tom
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