Hello Joe,
as Xikai Jiang wrote ESPResSo does not implement velocity rescaling thermostats (one of the reasons for that is that there are well known difficulties with this approach, if you don't now about that you should google for "Flying ice cube" effect.). The default thermostat in ESPResSo is a Langevin thermostat. DPD can also be used, but this is not the default approach and should only be used if the other features of DPD are needed. Langevin dynamics introduces a friction term into the equations of motion for the particles which is proportional to the velocity of the particle. The translational velocity you get is the one at which the electric force on your chain and the friction force on its particles are in balance. As such the stationary velocity you get depends on the friction you choose. This somewhat models the interaction between molecules and the solvent which in typical physical situations is present in reality. The DPD thermostat can be configured in such a way that it only has friction on the particle-particle interactions, so that the center of mass movement hast no dampening on it. In this case the translational velocity will diverge with time, because the external field keeps pumping energy into the system but no energy is dissipated.
Typical (coarse grained) simulations of charged polymers use the Langevin thermostat, or something more elaborate like a Lattice-Boltzmann fluid if hydrodynamic interactions should be explicitly modeled.
The choice of the thermostat is not a merely technical one, but should correspond to the physics of you system. Please read the relevant sections of the ESPResSo User's guide before you start simulations. If you want further help, you should describe more closely to what end you want to do your simulation and what physics you want to capture.
Cheers,
Florian