Originally Posted by electric1
Those 12V batteries are meant to replace car starting batteries, where brief high current
surges are accompanied by long charging
sessions via alternator
. That is why individual cells are not exposed just like in Lead Acid batteries. If you use these for propulsion
or house power, you will quickly kill them since you have no way of monitoring individual cells. Most Lead Acid batteries die prematurely because you can't cater to individual cells, but no one cares since they are cheap
to replace. In LFP we care for each cell because the batteries cost $$$$.
electric 1, I (respectfully) disagree. This is not quite true.
Many of the Chinese LiFePO4
manufacturers are starting to copy our original engine
starting/cranking batteries which we have been producing for close to 3 years already.
12V cranking batteries are simply made up of 4 LiFePO4
cells in series to provide the nominal 12V. There is no such thing as a 12V LiFePO4 cell!
A 12V LiFePO4 battery
is equally well suited to engine
starting as it is for house power. This is why these batteries are so wonderful since these are a truly "multi-application" battery
The critical factor is the C-rate. The cell must be capable of providing the peak discharge current
as per the cell manufacturers specifications.
This is why the low C rate cells ie less than 5C such as all the brands commonly made for the DIY market are best suited for house power. Only if the battery bank is of sufficient size should it be used for engine cranking. For engine starting applications is is far better to use a high power cell or specific engine starting LiFePO4 battery made especially for this purpose.
In most of these packs a BMS is not used. Generally only LVC protection is used to prevent damage due to accidental over discharge. There is no need for cell balancing since very high quality "matched cells" are used (also sometimes known as "racing cells"). Since these cells are virtually identical to each other in terms of internal impedance and actual capacity they have very little tendency to become out of balance.
I will have more to say about this issue shortly.
Our primary manufacturing plant is in Taiwan
and we have been supplying high power engine cranking batteries for the Taiwanese fishing
fleet for 2 years now and a much smaller and lighter engine cranking battery can be used to provide the same power as a lead acid engine start battery. The Taiwanese fishing
boats are powered by a 380HP marine diesel
engine and the original lead acid battery they used was rated at 12V 240Ah.
We supply them with a LiFePO4 battery one third the capacity (12V 80Ah) and less than half the weight (less than 34lbs). This battery does an equally/better job compared to the original lead battery.
In another case my own Toyota Camry has had a 12V 20Ah LiFePO4 engine starting battery which weighs 9lbs fitted for nearly two years (two years in June 2010). It has worked faultlessly day in, day out and has saved me a considerable amount of fuel
in that time.
Neither the Taiwanese boat batteries nor my car battery contain a BMS!
It all comes down to the likely hood
of cells becoming unbalanced. There are a few factors for this which I will explain in much greater detail shortly.
Nor do our LiFePO4 booster/jump starting lithium packs (pictured below) contain any cell balancing and we provide 2 years full warranty on these packs. We wouldn't be providing this warranty if we thought that cells would be getting out of balance and failing!
As you can see below this is what two BMI 12V 10Ah LiFePO4 batteries look like-almost identical. The battery on the left with the battery data interface connectors fitted has a BMS fitted internally while the engine cranking battery on the right with the clamp type auto terminals has no BMS (only LVC).
Both battery types are a direct drop in replacement and have no external BMS components. All the battery management electronics
(where required) are built into the battery case.
Both batteries USE THE SAME CELLS.
So lithium batteries are different to lead acid batteries in respect that lead acid battery cells for engine cranking and house power are distinctly different from one another while with LiFePO4 batteries they are the same.
I have just ordered a small but very high power (35C peak) LiFePO4 engine cranking battery for one of my major electric
boat projects. This battery which weighs in at 13.4lbs will be the cranking battery to start the 20kVA Yanmar diesel
engine/generator for charging
the main LiFePO4 propulsion
battery bank when there is insufficient solar
power on cloudy days and for extended run times with all motors at full power. The same battery will also be used to power the anchor winch
as well as be a source of emergency
12V power in the unlikely event of a DC/DC converter failure for house power.
If there is anything which is not understood please let me know so I can explain in greater depth