Thieves put themselves in danger by breaking into two mobile phone transmitter stations in County Durham to steal batteries, police said.
The break-ins were at isolated locations on the outskirts of Durham and police believe they were linked.
More than 30 back-up batteries were stolen from the two stations. They are valued at about £400 each and are a similar size to car batteries.
Durham Police said the people who broke in risked electrocuting themselves.
A station owned by Orange in a farmer's field in West Rainton was broken into. It is thought it may have happened in the past few weeks but has only just been reported.
A T-Mobile-owned station between Kelloe and Trimdon Station was broken into overnight between 19 and 20 August.
Sgt Jim Cowell said: "These transmitter stations are deliberately housed in remote locations and have considerable security because of their potential danger.
"Whoever did this could have electrocuted themselves or put the engineers who discovered the damage at risk.
"The batteries they stole are very specialised and we are hoping someone might hear of them being offered for sale."