  In wireless networks, consisting of battery-powered devices, energy is a
costly resource and most of it is spent on transmitting and receiving messages.
Broadcast is a problem where a message needs to be transmitted from one node to
all other nodes of the network. We study algorithms that can work under limited
energy measured as the maximum number of transmissions by a single station. The
goal of the paper is to study tradeoffs between time and energy complexity of
broadcast problem in multi-hop radio networks. We consider a model where the
topology of the network is unknown and if two neighbors of a station are
transmitting in the same discrete time slot, then the signals collide and the
receiver cannot distinguish the collided signals from silence.
  We observe that existing, time efficient, algorithms are not optimized with
respect to energy expenditure. We then propose and analyse two new randomized
energy-efficient algorithms. Our first algorithm works in time
$O((D+\varphi)\cdot n^{1/\varphi}\cdot \varphi)$ with high probability and uses
$O(\varphi)$ energy per station for any $\varphi \leq \log n/(2\log\log n)$ for
any graph with $n$ nodes and diameter $D$. Our second algorithm works in time
$O((D+\log n)\log n)$ with high probability and uses $O(\log n/\log\log n)$
energy.
  We prove that our algorithms are almost time-optimal for given energy limits
for graphs with constant diameters by constructing lower bound on time of
$\Omega(n^{1/\varphi} \cdot \varphi)$. The lower bound shows also that any
algorithm working in polylogaritmic time in $n$ for all graphs needs energy
$\Omega(\log n/\log\log n)$.
