I don't mean to be a wet blanket, but I'd like to offer a few thoughts before you play with these very cool new batteries.
I do safety stuff for a living, but that doesn't mean that I don't take risks or don't enjoy cool things that blow up and stuff. I am working with some researchers trying to keep these things from taking down commercial airliners. There is broad consensus (everyone except the battery manufacturer's lobby) that these things have taken down 4 commercial airliners so far.
So here are a few facts, observations and suggestions.
- There are more of these things in the world than anybody could imagine. They are everywhere and they do fail if abused or damaged.
- The charging and protection circuits are vital It would be best to use an integrated and proven package "re-purposed" as a back-up with minimal modifications.
- Cheap Li-ion batteries fail a lot more than expensive brand-name units.
- A short in a Li-ion battery will trigger thermal runaway. The same thing happens if you get a short in an unprotected circuit hooked to a Li-ion. Once that occurs it is very difficult to stop it without cooling it with massive amounts of water. If a cell goes into runaway it will catch on fire and explode burning metal. If you get a fire, it cannot be extinguished by conventional means.
- If one cell goes into runaway, the tests show that it will ignite all other cells in the proximity.
Risk is a combination of probability and consequence. If you understand that, you can do seemingly crazy things safely (and that is called fun).
So lets look at the PROBABILITY of a short causing a thermal runaway in this application.
1. Cheap batteries from ebay ++
2. Hundreds of people doing DIY with various skills ++++
3. Equipment placed next to saltwater sump exposed to splashes, leaks, overflows and salt creep. +++++
It is pretty clear that the probability of somebody having a short is extremely high. If you mitigate those factors and things get safer.
No lets consider CONSEQUENCES;
- Minimum outcome if an unprotected short occurs.... Fire that destroys stand and tank but is extinguished when the tank or sump fails (serious).
- Worse outcome (but still probable).... Fire that destroys dwelling with possible injury or loss of life (catastrophic)
So when you have a very high probability combined with serious or catastrophic consequences, it doesn't mean you don't do it. It just means you have a serious think about it.
That is all I am saying.
. Now just gotta do it without the wife seeing ha ha.
