i was using 4 Sanyo HR-3U 2700 NI-MH and suddenly was going to recharge them, and one battery started diplaying null on my BC-700 . i though the battery was fault, but after reading it probably debeloped a dead short (the BC-700 will reject a good cell that has been completely discharged to zero volt), i just used another standard charger for about 1 minute, then placed the battery on the BC-700 and voila, its working again.
In case one of your NiMH/NiCd cells has developed a dead short, its terminal voltage will remain at zero volt no matter how long it is being recharged. That is why the BC-700/900 displays 'NULL" in this case. Unfortunately this feature is not fool-proof, as it will reject a good cell that has been completely discharged to zero volt.
The Maha C9000 handles completely discharged cells better. It starts charging at a lower current (125mA) until the cell voltage exceeds 1V. On the other hand, it will reject old cells with higher impedence - it won't even discharge/recondition those.
One feature of the BC-900/700 is its ability to charge cells having such high impedance that they would always be rejected as faulty by many other chargers. Maybe the "null" feature is functionally linked to the BC-900/700's ability to charge severely degraded cells. Try as I might, I can't think of a good reason to include either feature in the BC-900/700 design.
The Lacrosse 9009 is newest among several other chargers that I have. It showed null on some AAA cells that I know work ok, but not to full capacity. My tester is a lamp light connected to a battery holder. Foolproof in my mind, and allows a voltage test while loaded down. Of all the chargers that I have, my SunPak will recharge all my batteries where the others reject them. I'm hoping my Lacrose impresses me othewise.