Hello,
I own a Surfmaster Pi Pro , Pi Dual Field and a TDI SL , I try to answer Your question:
there are two aspects , one regards the battery itself and the other one the electronics.
Lets start form the latter.
By looking at the
schematic diagram I note that:
a) the MOSFET that drives the coil is connected directly to the battery, so the lower the voltage, the lower the flyback voltage in the coil, the lower the target response, THE LOWER THE SENSITIVITY to targets
b) The logic chips 5v power supply is driven by a
78L05 linear regulator that's able, by specs, to maintain 5V regulation only when input voltage is higher than 7 volts.
So, I can say that, for the electronics, to function properly, supply must be at least 7 volts or higher, that , for an 8 battery pack means 0.875 volts per cell.
Regarding the battery pack, by looking at a
typical discharge curve of a single alkaline cell at 100ma rate , I see that the curve is pretty linear until it crosses the 1 V line, after which I see a fast drop to zero. In particular, the residual battery capacity between 1 V and 0.7 V is about 100ma, that translates in about 30/40 minutes of endurance for the Surfmaster PI (is it worth it?).
Moreover, consider that, at 1 V per cell, Your battery pack is at 8 V, in other words 4.6 volts lower than a fully charged battery pack, that means a reduced detector sensitivity due to the lower flyback signal in the coil (again: is it worth it?).
In conclusion, I would not want to run my surfmaster at lower than 8 volts, but neither at 9 , or 10 , or 11 volts!
To bypass all of the above, You can build Yourself a rechargeable 10 cells (Yessss they fit!) battery pack using
Sanyo Eneloop NiMh rechargable cells that will stay well above 1.2 volts (or 12 volts for the 10 cells pack) for the full battery capacity that means at least 10 Hrs of detecting in the surfmaster PI .
If You recharge Your Eneloop battery pack after every day of use, You'll be sure to run Your Surfmaster always at maximum sensitivity, save money and be sure to find Your Surfmaster always charged after long storage since the Eneloops have a negligible self discharge rate !
FINAL NOTE: in the surfmaster circuit there is a timer
TLC555 chip that is connected directly to the battery power. This chip has a maximum operating supply of 15 Volts. A fully charged eneloop cell will be at around 1.42 volts, so the 10 cells battery pack will stay lower than the maximum limit of 15 volts that the 555 can tolerate.
NEVER try to build a 10 battery pack with alkaline batteries , You'll fry the Surf Pi.
Lucar
