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Lithium

by:HGB     2020-09-28

For a battery with a average load the discharge curve seems as follows. A little experimenting reveals that 13.6 Volt (three.4V per cell) is the reduce-off point; beneath that very little occurs, while above that the battery will get no less than 95% full given sufficient time. There is a potential concern when a number of lithium-ion batteries are related in collection. For example, two 12 Volt one hundred Ah batteries, each with their very own construct-in BMS, linked in series to make 24 Volt 100 Ah.


Besides the cells inside the battery, there's additionally the BMS, which is made out of digital parts. Lithium-ion batteries with a construct-in BMS are nonetheless too new, and we should see, but finally the Battery Management System has to outlive for so long as the lithium-ion cells do as well. Unlike lead-acid batteries, the Voltage of a lithium-ion battery stays very fixed throughout discharge. That makes it troublesome to divine the State-Of-Charge from Voltage alone.


Round-trip power efficiency for LFP is kind of a bit better than lead-acid as properly, meaning that much less energy is needed to replenish the tank after a sure degree of discharge. That results in sooner restoration back to a hundred%, while we already had a smaller battery financial institution, reinforcing this impact even more.


Now assume a kind of two batteries is close to-empty, the opposite pretty full, and you put a load on the batteries, to discharge them. For longevity lead-acid battery banks shouldn't be sized the place they frequently see discharging under 50% SOC.


On cycle 772 the financial institution delivered 432.1Ah’s and did so with an increased reduce off voltage of two.9VPC vs. 2.8VPC at cycle 550. Charge present was allowed to taper a bit lower at thirteen.8V, right down to 7.5A, but other than that the back to again one hundred% to zero% cycles appear to have allowed a bit more capacity to be realized from the bank. Between cycle 550 and 772 the query of LFP reminiscence effect got here up in some LFP discussion groups. LFP memory impact is famous in white papers and scientific data where a voltage-hump can happen in the cost curve after many PSoC cycles.


To depend Ah’s faraway from the financial institution I used a Victron BMV Ah counter. This is one of the few exams I actually have seen on an LFP battery where supposedly they took it to 100% DOD every cycle. The cells have been made by GBS and the cycle tests had been to 100% depth of discharge or zero% SOC. The checks have been carried out at room temperature and the cell was discharged at a .5C load or 20A on a 40Ah cell. If you've drop-in batteries, or cells you don’t know match extremely properly, you will want to cost into the “balancing range” with each cycle and hold this for the manufacturers specified length.


For the fist 11 capacity exams on this bank, which I conducted every 50 cycles I used the arrange pictured here. For hundreds I used an inverter and ceramic disc heater plus an incandescent bulb to get the current to approx 100A.


To experiment with this financial institution I ran numerous back to back capacity exams to see if this bank suffered from any “reminiscence effect” in an attempt to erase them, if they had been there. With costly batteries corresponding to LiFePO4 I believe it is a clever concept to create a baseline capability determine that you could repeat a minimum of as soon as per year.

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