Moar Battery Calculations: Star (or “Radar”) Charts

Screen shot 2015-01-04 at 10.04.56 AM

Since batteries have so many things to consider when comparing them, this type of a chart is really helpful.  You can do this with Excel, and simply choose “other” when you go to make a chart, and pick “Radar”.

This is using parameters that I consider important, via this spreadsheet: battery star chart master.  Use the first sheet tab to calculate the values you want to see, in this case, power density (both weight and volume), energy density (both weight and volume) and cost per watt.   Then you copy and paste the values (Paste Special > Values) into the second sheet.

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I’ve plugged in the data from the three contenders I’m looking at – the Turnigy 40c lipo, the A123 20ah pouch, and the Farasis 25ah pouch.  Here are some notes, though.  The Turngy I’ve rated at 20c, simply because I don’t believe them.  The A123 prices I got from eBay for some sort of ballpark.  The Farasis prices I just put at the same number as the A123, since I have no way of finding them online.  I’ve also massaged the numbers with a multiplication factor to get them into some sort of similar amount, so they’d graph well.  I’m pretty sure this is kosher, since we’re not looking at absolute specs, just relative specs.

Still with me?  Now at a glance I can see the relative merits of each cell.  The Farasis is expensive, for the performance you’re getting, and they fall short in the power density department.  The Turnigy cranks the amps/weight (and for that matter, the price), and the A123 are kind of a mid-point in all this.

These are all pretty similar high-performance batteries, but let’s, for yuks, plug in some numbers for an old standard, the CALB 40ah packs.  There you get a result I wouldn’t have predicted – they’re hugely expensive for the wattage you’re getting out of them.  I knew they sucked, but I didn’t know they were so costly per suckage.  Here’s that chart:

Screen shot 2015-01-04 at 10.04.30 AM

Just an aside – from what I’ve been able to glean, the only difference between the CALB and the newer GBS are in the C-rate, and it’s not significantly better.  So those guys who want me to believe that the GBS are all kinds of awesome?  Sorry, not buying it, unless someone can show me where I’m wrong.

 

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5 responses to “Moar Battery Calculations: Star (or “Radar”) Charts

  1. Just checking – are you only looking at power (kW) as opposed to energy (kWh) in these axes? I would imagine specific energy (Wh/kg) and volumetric energy density (Wh/l) would be as important a metric as specific power (kW/kg) and energy cost ($/kWh).

  2. Up to you, but I suspect there’s a typo in your legend then. Specific energy’ is in kWh/kg, not kW/kg (lower right).

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