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Old Nov 26, 2006, 10:06 AM   #11
Junior Member
Join Date: Nov 2006
Posts: 1

Bonjour !

I am a French battery user and I fell upon your interesting discussion.

I am, like you, dissapointed by "high performance" NiMH AA batteries from Sanyo. I bought recently Sanyo's NiMH 2700 mAh AAs to power a Pentax Istd L SLR camera. They seemed good when used immediately but the camera shows the "low batt" icon flashing after two or three weeks without any use. OK, but now read the following :

I use an old portable CD player to test the capacity of my AA NiMH batteries. With fully charged 2700 mAh AAs from Sanyo, the player operates during 6 and a half hours. Now, if I charge these batteries again, put them into my Pentax and make photos until the "low bat" icon flashes, and then install them into my CD player, it runs OK for 4 and a half hours !

This means that several equipments (not only my Pentax) have a very high threshold to detect low batteries and cancel the power supply very early. Many of us think then that the batteriesare completely flat, but this is not true. In the case of my Pentax, The voltage (with no load) is approx 1.29 Volts per cell when the camera refuses to work. A problem is that we recharge them whereas they are only half-discharged. There is an endless discussion to know if it is bad or not to do this .... Maybe it is not too good to do it too often...

Excuse my bad English

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Old Nov 26, 2006, 12:59 PM   #12
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Join Date: May 2004
Posts: 339


Welcome to the forums and thank you for the input.

The high cut-off voltage is indeed a problem for many electronic devices of today using multicell batteries. The engineers do it for one main reason, namely to avoid cell reversal. Take your camera fo example. It uses 4 AA cells. In the ideal case all of them will have identical capacities, and will discharge evenly. In real life, however, the things are not so ideal. There is always a cell or two in a pack that are weaker than the others. If the device was made to stop working when all of the cells were at 1 Volt (this means 4 Volts for a 4 cell pack), it may happen that the weakest cell will overdischarge. Doing so the cell may leak and destroy your camera. The only solution is to increase the voltage, at which the camera stops working to, say 1.15 Volts per cell or 4.6 Volts for the whole pack. This ensures that even if you insert 3 fully charged cell and one fully discharged by mistake, the camera will cut-off before anything dangerous happens. Unfortunately, it comes at a price. The cells never recieve a full discharge. The strongest ones may never be discharged below 60-70 %. Slowly they develop what is called a voltage depression, usually caled a memory effect. This leads to a decrease of the voltage of the cell during discharge. Consequently, the camera starts to turn off for low battery even earlier.

The solution is to have a charger that can charge and discharge all the cells separately. Run a discharge cycle on every 10-15 normal cycles and things should be fine.
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