Gardner

The lithium batteries are impressive compared with lead/acid.

Some cell types are not particularly safe around water because lithium reacts with water.  The LiFePO4 cells are not known to burn or explode if immersed.  This technology can also be overcharged without exploding but it stuffs the cells.

The cells have very low internal resistance and no means of self balancing when used in a battery bank.  Hence you need to monitor and control the voltage of each cell.  This is usually done automatically with a Battery Management System.  A BMS will have means to monitor and balance the cell voltage during charge.  This means that there is a sensing wire to each cell and the charge current has to pass through the BMS so it can cut the charge if any cell reaches its maximum - usually 3.75V for a LiFePO4 cell.  My BMS has a balancing current of 72ma and can control up to 20A charge current.

The BMS should also be able to prevent complete discharge of any cell if not protected by other circuit.  That means it should be able to cut off discharge current if any cell gets below the allowable minimum - usually 2.4V with LiFePO4 cells.

Some BMS have high current cut out as well so the maximum discharge current can be limited.  This works faster than a fuse and can recover automatically once the load is reduced.

For a house bank you could get a solar charger that stops charging once the maximum battery voltage is reached.  If the cells are already balanced and all have the same capacity then  the BMS does not need to control the charge current.  A solar charger will also usually have a load circuit with a lower voltage limit that cuts off the load so it does not completely drain the battery.  So, again, if the cells are in balance then the BMS does not need to control the discharge current.

Hence the functionality of the BMS can range from just cell balancing up to the four functions needed for complete battery protection -cell  balancing, charge control, discharge control and overcurrent protection.   

The BMS on my 48V battery has cell balancing and charge control.  The motor controller protects against under voltage and peak current.  I did get a full function BMS supplied with the cells that had overcurrent protection of 100A but it made a bad smell and produced smoke when I first connected it to the cells. I did not get any useful advice from the supplier on repairing it and the mailing cost made it uneconomic to return.  I just purchased the simpler BMS from another supplier that has proven reliable so far.

I have a plastic cover over my cells.  There are 32 exposed terminals in my battery.  They have stainless bolts for the terminal connectors and you can get impressive fireworks if you short the terminals - similar issue with any battery but the lithium cells just keep giving until the short disintegrates or vapourises. 

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 

On 09/03/2013, at 11:33 AM, Gardner Pomper wrote:

 

Does anyone have an opinion on using lithium batteries for the house bank on a Harryproa? I would like to have a 250Ah house bank (2x4D) and that weighs a ton (well, 250lbs or so). Since I should not discharge below 50%, I only really get 125Ah from it. With lithium-ion, I think I can discharge all the way, so I just need to buy 150Ah of lithium-ion, which weighs somewhere around 30lbs.

So, other than the cost, what are the downsides? Am I in danger of them catching fire? Anyone have any experience with these?

- Gardner

Rick Willoughby

rickwill@bigpond.net.au