I've had the Sensation M901 since it was new. Now that it's been several years, the sealed lead acid batteries in the Dodd battery kit for the M901 aren't holding a charge anymore. Without the charger attached, they only play for a couple seconds before the amp shut down. Also, I might have been hearing more grain in the high frequencies - perhaps from the charger?
So I bought two of these Yuasa brand 7 amp-hour sealed lead acid batteries for $19 each with free shipping ($38 total):http://www.amazon.com/gp/product/B002MA6FYG/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1
Their specs claim an internal resistance of 25 milliohms, half that of the PowerSonic batteries that came stock with the Dodd kit (50 milliohms). Note they have the same F1 terminals as the Dodd kit so the existing wires/connectors are plug and play. I put the batteries end-to-end with the terminal sides facing each other. Just make sure to connect things in the same order (short cable connecting red terminal to black terminal, longer cable keeping the red/black color of the wire consistent with the terminal).
It may be my imagination, but the sound with the new batteries might have more bass and be a bit smoother. Not sure if it's the lower internal resistance or the freshness of the batteries, or both. But for $38 total plus 5 minutes installation I'm a happy camper! Another bonus is 7 amp-hours of capacity, versus the original PowerSonics with just 5 amp-hours.
If your Dodd battery kit is more than a couple years old and you use your amp regularly, I recommend trying some fresh new Yuasa batteries.