Yes it will if completly isolated. But that is still a long time--look a fist sized block of magnesium is, say, 100 grams. With Mg27 that's gonna be 3.70 gram moles. And each gram-mole is about 6.02E+23 gram atoms.
Mow as Mg27 has a half life of 9 minutes (540 seconds) it's specific activity (Natural logaritm of 2 divided by half life in seconds) is 0.00128 s-1.
Take the number of atoms in the mass and multiply it by the specific activity and you'll get an ion (in this case a positive charge change as beta electrons exit the mass) . This will be about 2.86E21 positive ions per seconds. As a coloumb this is 467 Ci/s or 467 amps. So the beta decay would be a pretty good battery for a short while (BTW space capsules on long space voyages use radioactive materials as atomic batteries--they use long halflife materials--Pu241 is a fav with a half life in the billions of years).
Note this assumes that the loss of electrons is perfect loss. For that to practically occur the magnesium mass would have to maximize surface area--as a charged aluminum ion will tend to reabsorb the beta electron and cancel the charge. The energy will then be converted pysically, probably as heat.
So mass geometry will have an effect on the atomic battery.