When using the MX-4AA or MX-8AA battery sleds with your MixPre, it is important to use standard-size AA batteries. There are AA batteries on the market that are larger than standard size. When these larger batteries are placed in the battery sled, the sled can no longer lie flush against the back of the MixPre. When this occurs, the MixPre and battery sled usually make a proper electrical connection, and this is merely a cosmetic issue. However, some users have reported that the MixPre will not power up with these oversized AA batteries. Others have reported that the MixPre powers down when the sled is forcibly pushed onto the back of the MixPre, and it will not power up until the battery sled is reseated.
Sound Devices recommends removing nonstandard AA batteries and replacing them with a standard size AA batteries.