From: Rick Willoughby

To: harryproa@yahoogroups.com.au

Sent: Saturday, March 09, 2013 9:12 PM

Subject: Re: [harryproa] Re: Lithium batteries?

The method of preventing accidental short circuit needs to be strong enough for preventing something like a spanner making short circuit if dropped on the battery. Car batteries often have a plastic flip cover over the terminals.  Grease or wax probably lack the mechanical strength.

I have had two accidental shorts with lithium batteries and both cleared by vapourising the short.  In one case it was a small guage copper wire that produced a lot of smoke from the insulation burning but no fire or personal injury.  The other was a 1.6mm X 20mm piece of aluminium flatbar but only made contact on an edge that instantly vapouraised enough of the aluminium to break the circuit.  The metal spatter did not do much damage to the plastic case.  

When I have accidentally shorted a lead acid battery there is a small melt mark on the terminal.  The lead melts readily and helps break the circuit.  Also lead/acid batteries have localised sulphate depletion near the plates that limits the power output based on time and current - meaning they will recover power capacity somewhat if left rest.   The Li-poly batteries have reduced internal resistance due to temperature rise  from high current so their power capacity rises on shorting.  (I have had a fire in the engine bay of a car when the braided earth lead bridge to the other terminal.  The plastic case of the battery caught fire and the burning damaged the paint on the bonnet before I could get the car away from property and open the bonnet.)

I have both small lead acid batteries and lithium batteries for testing.  Two 12V 12Ah lead acid batteries that weigh combined 7kg can sustain 24A at 18V.  My 0.7kg 24V lithium battery can sustain 100A at 18V.  So power density in the limit is about 40 times higher for the little Li-poly battery.  The larger LiFePO4 cells I have are not quite as powerful relative to rating but still many times the power density of lead acid.

Handling lead acid batteries is hazardous mainly due to their weight - lifting or dropping.  If unsealed, hydrogen gas can be an issue for confined space.  With lithium the most hazardous aspect is the power density combined with ease of causing a short.  

Lithium batteries have some bad press regarding fires associated with cell damage or immersion in water.  There is not much discussion regarding accidental short, which I consider to be a more probable cause of a fire and injury.   

Rick

On 10/03/2013, at 1:41 AM, bjarthur123 wrote:

 

is there no way to waterproof the terminals? electrolyte-free grease? melted wax??

ben

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> This link shows the type tests done on the cells I am using:
> http://mybridge-evbattery.weebly.com/battery-performance.html
> There is no full immersion test. There would be chlorine and
> hydrogen production if immersed in saltwater due to electrolysis and
> these gases are likely to be more hazardous than any fire risk from
> water getting to the lithium inside the cell.

Rick Willoughby

rickwill@bigpond.net.au

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