“Short” Discharge Time: Battery Comparisons

Fig-3-1356973810

Davide Andrea (…author of the book “Battery Management Systems for Large Lithium-Ion Battery Packs”, designer of the Lithiumate and Lithiumotive BMS for Elithion, designer of the Prius and Ford Escape Hybrid PHEV conversions for Hybrids Plus) posted this white paper on using “Short Discharge Time” as a way of comparing batteries.  That is, you short-circuit the cell and see how long it lasts before it’s dead.  Find that here: Short Discharge Time as a Characteristic of Battery Technology

I’m not going to pretend that I can offer an opinion on whether this is a viable way to measure batteries, (there seem to be some differing opinions about the tests out there on the interwebs, but hell, there are differing opinions on whether the sky is blue, too) but there are a couple of things I like about it.

First, it shows the importance of battery construction, especially internal resistance, in how a battery can physically deliver current regardless of ratings like ah and so-called C-rates.  It’s a simple metric.  Just short the frikkin cell out and see how long before it dumps.  That’s my kind of test.  The only thing that would make it better is if you blow stuff up in the process.  Wait.  Yes.  I’m pretty sure that happens too.  Which leads me to: Second, the visual image I have in my head of this guy sitting down with some bench rig and shorting out cells with some makeshift plasma-inducing rig, over and over again?  Priceless.

Here, as a Public Service, is a little video of what you can do with a few lead-acid batteries in a Short Discharge test:

(Edit: It has come to my attention that the “Short Discharge Test” does not actually involve shorting out the cells, plasma, or anything else fun at all.  Hell. Rereading the story, he’s talking about the “theoretical” time to full discharge, which, of course, explains why he can do a full discharge and then charge to 50% since, as far as I know, once you do that you’ve toasted the cell. So yeah.  More stories from “Engineers on Math”, I guess…  but still interesting.)

Advertisements

3 responses to ““Short” Discharge Time: Battery Comparisons

  1. I don’t think the test really tells anything but max discharge rate, which is hardly interesting unless you are using way too small cells to begin with. Or drag racing.

    • Well, having very recently experienced the vast difference between the discharge rates of RC lipo compared to CALB LiFePO4, I’d have to disagree. Strongly.

Comments?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s