## 1 Introduction  
Non-road mobile construction machinery powered by a traditional internal combustion engine has problems such as low efficiency, bad emission and high pollution, which have a great impact on source utilization and global climate. Due to low noise, zero emission, pollution-free characteristics, pure electric drive construction machinery becomes a new development direction of construction machinery [1; 2; 3; 4; 5; 6; 7; 8]. Electrification of the forklift is developed early and is relatively mature in the field of construction machinery. However, currently, the pure electric forklift on the market uses the low voltage system basically and the rated load is under 5-ton, which are called small type forklifts. There is a lot of gravitational potential energy that can be recovered for all the forklifts. Their lifting system frequently reciprocates up and down and converts the potential energy into heat energy, causing the high temperature of the hydraulic system and the short life of hydraulic components [9]. In view of the above problems, a potential energy regeneration system (PERS) can be put forward to regenerate the potential energy.  
[PERSON] et al. tested a forklift with a load of 1 ton. The test results showed that the lifting system consumed 3361 kJ, the walking system consumed 4110 kJ and the steering system consumed 907 kJ, that counted for 40.1%, 49.1% and 10.8% of the total energy consumption, respectively [10; 11]. [PERSON] et al. did similar research and used the similar test principle. However, they also considered the energy consumption of the tilting cylinder. The test results showed that the lifting, walking, steering and tilting accounted for 41%, 44%, 12% and 3% of the total energy consumption, respectively [12]. It can be seen that the lifting system consumes most of the energy.  
[PERSON] et al. did a lot of researches on the PERS for the electric forklift. In the year 2009, the feasibility of load potential energy regenerated was predicted, and the influence of different loads and different descending velocity on potential energy regeneration was analyzed [13]. In the year 2011, the efficiency of the electric motor (EM), hydraulic pump and valves were analyzed [14]. In the year 2013, the potential energy regeneration scheme of direct servo motor control technology was proposed. The energy saving efficiency of the system was measured, and the advantages of the electric pump drive system were verified [15]. The same year, the PERS of the electric forklift with a lithium battery was built, and the regeneration efficiency of the system at a different velocity was analyzed [16]. [PERSON] et al. proposed a forklift with the power system of a battery-EM-hydraulic pump. The test results showed that the energy regeneration efficiency could be up to 40% [17; 18]. The efficiency of PERS was increased by 18% in the low speed mode and 30% in the high speed mode. [PERSON] et al. controlled the valve group to drive the hydraulic motor to make EM generate electric energy when the boom is descending, and stored the recovered energy into the battery of the forklift. The regeneration efficiency was up to 20% [19]. These research verified that the potential energy could be regenerated and the regeneration efficiency was high.  
Although the PERS is successfully applied to small type forklifts, there are a few PERSs developed for electric heavy forklifts (EHFs). A study on PERS of EHFs is relatively less. For an EHF whose rated load is more than 12 tons, there are some differences to the small type forklift. In this article, a double hydraulic motor-generators (HMGs) with PERS to regenerate the potential energy when the boom descending is proposed. The structure schematic and the working mode are introduced. The control strategy is analyzed according to the working mode. Then, the proposed schematic is tested through a 25-tons prototype EHF.