The National Aeronautics and Space Administration released a report on February 18, 2019 concerning its Mars rover batteries. The NASA update on lithium technology was positive about battery life. The primary battery specifications for Spirit and Opportunity were 300 cycles over 90 days. Yet despite this, the rovers are still trundling along after 2 years.
How the Energy Conversion System Works (Simplified)
The solar energy system comprises deployable solar panels that open after landing. Initial conversion efficiency was around 1000 watt hours per sol, the latter being a Martian day of just over 24 hours. However this power gradually reduced due to Martian dust accumulation.
That said, frequent Martian winds have partly swept the panels clean. Thus, they are still good for over 500 watt hours. The NASA update on lithium batteries confirms they operate well at sub-zero temperatures, thanks to a special, low temperature electrolyte for night time operation. Moreover, they chose these over alkaline systems because of superior capacity.
Battery Temperature Control Revealed in NASA Update on Lithium
It gets extremely cold at night on Mars, because there is no sheltering atmosphere. Hence the lithium batteries are in “aerogel warm electronics boxes”. Here resistive heaters, radioisotope heating units, and thermal switch activated heat rejection combine to maintain a -20ºC to +25ºC range.
Each Mars lithium battery assembly comprises “two parallel Li ion batteries, each containing eight prismatic, 10 Ah, Li-ion cells.” However, each battery has its own “independent battery control board” to keep charge and discharge within 3.0 to 4.1 V. This is because it is essential to prevent overcharging, and the dreaded possibility of thermal runaway. This could lead to fire and explosion.
We are delighted that all continues to go well with the panels and batteries on Spirit and Opportunity. There is so much more to learn about Earth’s ‘sister planet’. A sensation could be over the next rise.
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