A Discussion on Using Primary versus Secondary Batteries
LV2 Battery requirements:
To summarize the now out-of-date analysis of the avionic's power requirements, (see attached to BatteryPack) we should be planning on a 5 hour "avionics on" time with various power levels (full on, wait for recovery, emergency power, etc.).
The calculated need was for ~ 45WHr, or ~ 4AHr at 12Vnom.
Give our stringent weight requirements, only two battery technologies made sense: primary Lithium cells (think camera batteries) or rechargeable Lithium-Ion cells (think laptop batteries). The primary Li cells came in at ~ 200g for the battery pack and the Li-ion cells came in at ~ 500g.
Why we should use primary Li
1) Weight.
We save ~ 300g, which is 6% of the entire budgeted avionics weight. That's nothing to sneeze at, when grams matter.
2) Simplicity.
Wire up the pack, test the voltage, and away we go. We could even work out some sort of "fast-swap" mechanism so that we could easily swap out battery packs. The only battery circuitry necessary would be a voltmeter and an amp-hour circuit.
3) Availability.
We can get the Li batteries almost anywhere.
Why we should use rechargeable Li-ion:
1) Known charge status.
Using a smart battery, we should be able to tell the status of the batteries down to a few Watt-minutes (after a few cycles to calibrate the smart battery system).
The primary Li are a giant unknown - we can use voltage cutoffs, and try and measure the power out, but it's still going to be a larger unknown. This is an important point - the last launch was panicky simply because we had no idea how long the batteries would last. We could test many packs under flight profiles to get a good idea, but then we'd have to waste the packs in the process and if we had a bad battery pack we might not know it until it was too late.
2) Cost
Primary Li batteries are expensive. Although Matt may have some for cheap, in the very long run this could be a problem. Especially given the amount of testing we're going to want to do on the system, we could be burning up battery packs at an alarming rate. We'll use shore power for most tests, of course, but towards the end we should be testing off of batteries to get the full benefit of the test. Li-ion are expensive up front, of course, but then we have the pack for years.
Perhaps the biggest argument against the Li-ion is the up front cost of learning the system and getting it to work. But I claim - with no data but an opinion - that in the long run the payback is that we'll have more hassle-free, long term returns on the system as a group than constantly worrying about a primary pack. It would hopefully be a build-once-and-forget type of system.
3) Long-term and multiple-use Solution
After we figure out this system, I see no reason to switch it for LV3 or LV4 or whatever - until micro-fuel cells or micro-fusion comes on line ;) , I think Li-ion is going to be the best solution to this kind of problem. This means any mobile applications we have in the future - gliders, payloads, possibly repeater base stations, etc - should be able to directly use this solution.
4) Because it's cool.
Primary Li batteries are for wimps like Lockheed Martin who don't get their vehicle back when they launch it ;)It's a waste, at the very least environmentally speaking. Posting this solution to the net would be a huge win publicity wise as well - an open source smart battery manager for your system? That's pretty cool, and useful for a lot of people.