T1 Terry wrote: ↑Sat Nov 14, 2020 12:54 pm
native pepper wrote: ↑Fri Nov 13, 2020 3:03 pm
T1 Terry wrote: ↑Fri Nov 13, 2020 1:46 pm
Note: These batteries can not be used in parallel, T1 Terry
Terry could you explain why these batteries can't be used in parallel, I have an idea why, but an explanation would be great help to many looking to go down the path of lifepo4 I feel so they can avoid the scams. Unless it is so technical you need a degree to understand it
My feelings with regard to the charge parameters manufacturers are providing, is to create redundancy after a few years, I note most give less than 10 year warranty, when they should be giving 20 years with proper charge parameters.
Here goes
As a starting point, I'll explain why a battery becomes unserviceable (can't be fixed and won't do the job required)
All batteries die at the single cell level, it is only one bad cell in an AGM or any lead acid battery that makes it no longer operational. All big batteries are built with the single cell terminals available so the voltage can be measured to see if each cell is within its safe operating zone. If the single poor performing cell can be brought back into a serviceable state or replaced if it can not be recovered, the whole battery can be returned to service rather than junked.
A single cell dies because:
1) it has either a low level or run out of electrolyte and this damages the plate surface
2) It has been excessively discharged to zero volts and the higher voltage from the other cells forces current through the 0 voltage cell actually reversing the current flow .....this destroys any chemical cell, but is an absolute death sentence to a lithium chemistry cell.
3) The cell is charged beyond full capacity causing the voltage to rise excessively and closely followed by a serious over heat, boiling the electrolyte away and buckling the plates or (in the case of lithium cells) causing them to build up metal transferred from one plate to the other eventually bridging the gap between the plates creating a short circuit within that cell.
To avoid cell damage, the cell voltage should be monitored and maintained within the same operating voltages, look after the cell voltages and the battery voltage will look after itself (yeah, I stole that from the pennies and pounds line)
The drop in batteries have very small current shunts for balancing each cell and small cabling and contactor or just single contact relays or mosfet transistors to control the battery isolation if the battery voltage goes too high or too low. They rely on all the cells remaining perfectly in balance and just measure the total battery voltage, not the individual cell voltages so any cell can go over voltage or under voltage (even within the same battery pack) yet the total voltage of all the cells remains within the 10vdc to 14.6vdc range that so many of the drop in lithium batteries use for their BMS shut down. A few minutes with a pen and paper and you will see how many combinations of 4 numbers will still add up to either 10vdc or 14.6vdc. Any number outside the 2.8vdc and 3.6vdc is a cell being damaged, any over 4vdc is a cell being murdered, the same for under 1.5vdc. If any cell goes to 4.5vdc or 0vdc, that cell is irreparably damaged or dead.
Time for a coffee and possibly a re-read to get all this sorted, or time to go and look some where interesting
OK, for those still with us, why can't the 12v batteries be paralleled. All lithium batteries have very low internal resistance, this means the battery with the shortest cable length to the load or the charging point, will do all the work until that battery can't do it any more without the voltage going high or low (at the terminals, not just at the cells) then the next battery with a slightly long cable length takes over the job and so on down the line.
Now think about the small cable size and current carrying ability of the battery isolation used that I mentioned before. To power a 2400w appliance (standard 10 amp power point plug and lead is anywhere up to 2400w, maybe a bit more sometimes) the inverter needs an extra 10% to do the conversion from DC to ripple free AC @ 230vdc allowing for the losses to heat as well (cause nothing ever works perfectly, there are always losses) the battery needs to supply 2640w.
Watts = amps x volts, divide 2640w by 12v at the battery = 205 amps. To carry 200 amps without the resistance causing so much heat the wire burns and the voltage drops so low the inverter shuts down, you need heavy sized battery cable even for a short run of a mtr or so, even bigger if the cable length is longer. We are talking cable the thickness of the average male middle finger ..... can you imagine that size cable being used inside a drop in battery? Not likely is it. Next, to disconnect the battery at 200 amps @ 12v, contact faces about the diameter of the widest part of the average males thumb are required and one at each end of the contactor bar to ensure the circuit is broken and there isn't just a big arc forming and burning the faces of the contact. This size contactor in the Gigavac range is around the $300 to $350 range and around the size of an open hand, heal to finger tip ..... can you imagine them spending that much money and having room for that in a drop in battery ..... unless you are paying around $1500 each for them.
There is no method of power sharing that sort of current in any drop in battery, the size required would be substantial, the heat generated would be substantial so couldn't be installed inside a sealed case, the sophisticated electrics required would be more than the price of the battery ....
All this just to product enough power to boil the kettle or power the microwave, what about running the air conditioner and other appliances at the same time, so a 3,000w or bigger inverter .....
Those in the "know" on those other forums told us it couldn't be done, but we have been doing it for more than 10yrs and the batteries are still performing fine so .......
T1 Terry
Hopefully all those that suffered serious effects from reading the loooong post recover in time for Christmas ...
