Currently designing power system for Lunar rover. Solar power and RTG are the planned power sources during sunlit periods however secondary Li-ion batteries will be used to operate in PSR's, these will of course rely on solar power to recharge as the RTG produces only 30We. By calculation the batteries should take around 3-5hrs to recharge however I'm concerned about the power required to do this? Simply, P=IV and so if the 60Ah 28V battery has a 7 amp charge current it would require 196W (7*28). This seems high but I can't seem to justify another method of calculating this??
I also ask for advice on how to distribute the batteries throughout the rover? Fewer large packs with high Ah ratings or smaller packs but more of them?
Hi Shannon,
challenging task - Li-Ion-Batteries on the moon...
But your problem may be worse than you think.
How about an approach considering the charge?
To prevent aging, I suppose it only needs to refill the typical 80%.
So 48Ah of the 60Ah installed need to be refilled.
From 10% SoC to 90% SoC you can assume constant current charging at an average voltage similar to the rated voltage.
Voltage change of course depends on chemistry in use.
To transfer 48Ah in 5 hours demands about 10A and about 280W in average.
Try the Energy as a fundament...
48Ah at 28V resembles an energy of 1344 Wh. To transfer this in 5 hours, it demands..tataa - . 270W.
Besides the power demand - how do you intend to cope with temperatures from -200°C to 130°C with batteries that allow -45°C to +65°C core temperature at best?
Thermal control could consume more power/energy than you have available.
Cheers - Martin
Hi Shannon,
challenging task - Li-Ion-Batteries on the moon...
But your problem may be worse than you think.
How about an approach considering the charge?
To prevent aging, I suppose it only needs to refill the typical 80%.
So 48Ah of the 60Ah installed need to be refilled.
From 10% SoC to 90% SoC you can assume constant current charging at an average voltage similar to the rated voltage.
Voltage change of course depends on chemistry in use.
To transfer 48Ah in 5 hours demands about 10A and about 280W in average.
Try the Energy as a fundament...
48Ah at 28V resembles an energy of 1344 Wh. To transfer this in 5 hours, it demands..tataa - . 270W.
Besides the power demand - how do you intend to cope with temperatures from -200°C to 130°C with batteries that allow -45°C to +65°C core temperature at best?
Thermal control could consume more power/energy than you have available.
Cheers - Martin
Martin Schulz , Thank you for your thorough response. So given the time constraints I'm working under as this is MSc university moon mission proposal project the logical decision seemed to sway towards using secondary Li-ion batteries because of the flexibility in being able to reusable them there efficiency is far superior to the others on offer and given the flight heritage for other mission on the mars rovers. Aside from this, yes indeed to keep the batteries healthy i have assumed specific DOD , currently a maximum of 60% if requiring large run times and so the available capacity stems from this, thank you for clarification on this. The benefits are also that the rover is likely be able to operate during PSR's on the moon in search for water ice and can recharge once returned to sunlit region.
To answer your thermal management question, we are implementing an RTG (european only). This is still in development ofc (2028 likely) , with space and mass limitations the unit 3 has been selected and this will provide in excess of 500W waste heat to maintain thermal stability inside the rover, it will also help out with the electrical loads providing a constant 30We.
Artur Braun , thank you for your response. I appreciate the severity of these issues and losses such as temperature, radiation and dust have been taken into account in regards to the solar panels. We are also looking into possible cleaning methods or otherwise foldable hinges during hibernation periods.
As mentioned in a previous response we are looking to implement a european RTG that will provide excessive waste heat to thermally stabilise the rover
Dear Shannon,
I agree with colleagues above to calculate the energy demand for charging a battery with 28 volt, and 60 Ah. It is required to set the depth of charge such that to preserve its lifetime. May be 80 percent for lithium ion batteries are acceptable but you have to review it returning to the data sheets of the battery.
So need to charge the battery for .8x 60= 48 Ah.
The charging time depends on the charging current.
One must be aware there is specific nominal charging current for any battery.
Physically the such battery size is composed fro an assembly of battery cells.
For space it is always advisable to divide them into units which can better handled
and also that may allow a total more nominal charging current.
Such that you charge it in smaller time than the sun shine period.
So the period of charging is dictated by time of sunshine, then it remains to select the battery of the group of batteries that will satisfy the nominal charging current.
The Ah=48= Nominal charging current x sun shine hours. For charging form the PV generator you need to put a controller between the Generator and the battery
to grantee the safe operation of the battery.
For the best charging method of the battery please see the paper: Article Enhancing the design of battery charging controllers for pho...
The needed energy will be more than the stored energy because the charging process efficiency is less than one. So one has to divide the stored energy by the efficiency to get the total required energy from the charging electric generator.
As for the thermal variations, as the battery will be in a container with the electronics the whole container will be thermally regulated to reduce the temperature fluctuations to that can be accommodated by the components.
Best wishes
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