Gardner
The comparison will depend on the load. The rating of the lead/acid is based on 10 hour discharge so a draw of 20A. If you exceed that rate then you will not get less capacity. The Li battery is rated at 1C or 100A. If you draw at lower levels then the voltage during discharge will be about 2V higher than you will get from the lead acid battery so this translates to higher capacity for most loads. Incandescent globes would burn brighter rather than but no one would use
The Li battery rating will be given for an end voltage of 2.5V per cell. Note the red text at the bottom that warns against leaving the cell at this level. If the cells are balanced then you can safely reduce to 10V (2.5V per cell) and get 100% capacity but note that the cycle life is given for 80% discharge NOT 100%.
The battery seems to have over and under voltage protection but it is not clear on cell balancing. The link states Without_Balance so it is something you should check. The low voltage cut out states 10V not specifically 2.5V per cell. The high voltage 15.2V corresponding to 3.8V per cell if in balance. So I would ask if there is cell balancing. Note that you have stated below minimum voltage at 2V - THE MINIUM VOLTAGE IS 2.5V NOT 2V.
The undervoltage and over voltage protection can be provided by your solar charger. So these aspects are not particularly required in the battery. It is more important that the cells stay in balance otherwise you have to check periodically and have some means to discharge individual cells.
Ponts for comparison
1. The life of the Li battery will be extended by slow discharge and slow charge. The cycle life is based on 0.2C or 20A for a 100Ah battery. I have had my little Li-poly battery since 2008 and it is actually better now than when I got it but it has only been cycled maybe 30 times and then not complete discharge. It initially had one cell out of balance but the charger balanced it and it now stays in balance. I have not used my 16 x 50Ah cells in anger yet but I charged them when I got them about 9 months ago and they sit in the garage without much loss. I have a two-wire digital voltmeter permanently connected that is powered by the battery and takes months to drop 0.1V. I expect it would sit there for years before it needs recharging. My experience with lead acid is sad by comparison. The 100Ah cells we have on the proa have worked well although we have not installed the BMS supplied. We have been balancing manually.
2. I would not consider a 125lb single battery in a proa. The thought of getting that into a cramped spot would have me looking for alternatives.
3. If I could save 100lb in boat weight for $240 I would think that good value. However I would shop around for the battery. The going rate for LiFePO4 cell was USD1.1/Ah six months ago. There is transport and transaction fees that could be added but starting price is USD440 for your 12V (really 14V) 100Ah battery. You can make price enquiry here:
Cindy corresponds in perfect english. I would not get the BMS through her.
This link shows a full function 60A 4S LiFePo4 BMS:
A solar charger will also typically have over/under voltage protection for the battery but you cannot do without the balancing function of the BMS. I have tried the WELLSEE MPPT solar charger and am pleased with them so far. Here is a 12V 50A unit:
You need to make certain that it suits the solar panels you select and has sufficient load rating for your maximum connected load.
Rick
On 10/03/2013, at 1:39 PM, Gardner Pomper wrote:
Very informative reply. Thanks!I have a few more questions... My reading on Li batteries speak of 1C and 5C charging and also of constant current charging. Is there a requirement to charge these batteries as a constant current? I am planning on using solar (perhaps 400w or so) panels as my primary charge, with shore power as a backup and possibly a small gasoline generator for emergencies.Another question is about the minimum 2V requirement on the cells. I had thought that Li batteries could be drawn all the way down without harm, as opposed to lead/acid where you should not discharge them more than 50% or so. Is 2v 100% discharged?I went internet shopping, just to see pricing and availability. I found a 100Ah LiFePO4 battery (28lbs, $620) listed as a replacement for 12v LA batteries. Here is the link http://www.batteryspace.com/LiFePO4-Prismatic-Battery-12.8V-100Ah-1.28-KWh-10C-Rate-Without-Balance.aspx I was thinking of it as a replacement for something like this http://www.wholesalebatteriesdirect.com/marine-batteries/52236-12-v-200-ah-4d-deep-cycle-agm-marine-battery-ub-4d.html (200Ah 4D deep cycle AGM battery 125lbs $385). As a 12v battery replacement, it claims to have internal protection for the cells, so does that mean I would not need a BMS?Finally, on a more general note, if you were building a cruising boat, without electric motors, would you consider LiFePO4 batteries a safe, reliable solution for a house battery bank?Thanks,- GardnerOn Fri, Mar 8, 2013 at 8:56 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
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: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.RickOn 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?- GardnerRick Willoughby
Rick Willoughby
__._,_.___
.