Bob613
New Member
The battery is for off-grid storage, replacing a 790AH 24V Crown industrial battery that is actually working quite well but the maintenance is tiresome.
The maximum charge rate is 90 amps from a Midnite Classic controller fed by 3600 watts of panels in two fixed arrays. The maximum discharge rate will be 135 amps (continuous), 250 amps peak during pump starting, through a Magnum MS4024 inverter.
Ordered 24 REPT 280 A cells from Shenzen Continent Technology Co. Leadup to the buy went well, with all my questions answered, shipment arrived nicely packed with no damage 60 days after order. There was a 10 day or so delay because of a backup at the Vancouver port.
As delivered, the cell voltages were uniform, within 20 mv, slightly below the values they sent me before shipment. The internal resistances are all within 2 milliohms of each other. I won’t share the actual values because I measured them with a YR1035+, which according to the manual isn’t accurate at low resistance values, but I trust it as far as cell to cell consistency.
I opted for welded on female terminals. They are 6mm, like the drilled terminals, but are nice and consistent. I went this way instead of the stud type because I want to control the metallurgy, using aluminum bolts and nuts to make sure there is no galvanic action. I want this to last a long time.
The cells will be connected using 3/16 aluminum bus bars. I wanted 6101 but it is really hard to find so used 6061 and went from 1/8” to 3/16” to account for the higher resistivity of 6061. The bus bars are 7” wide to cover the terminals on three cells; the total bus calculated voltage drop is 10 millivolts at 125 amps.
The terminal is 12.5mm in diameter, with a 7mm hole, so the contact area looks small but is 75 mm^2, between 2/0 and 3/0 cable, more than enough for the current that any individual cell will see. I found a couple of papers discussing the resistance of busbar connections, and they suggested while the area needs to be large enough to carry the current, the joint resistance is greatly affected by the contact pressure.. The target contact pressure is in the 15 to 30 N/mm^2 (2000 to 4000 psi) range. For my terminals, this means a force of 1125 to 2250 N (250 to 500 lbs).
To secure the terminals I am using a series stack of 3 belleville spring washers with a spring rate of 4000 N/mm (900 lb/mm). By compressing the spring stack by 0.50 mm (3 flats on the nut) the joint is loaded to 2000N. I am screwing a nut onto the bolt, hand tightening the bolt into the terminal, then holding the bolt as the nut is tightened 3 flats after it is snug. Experimentation and calculation shows that this load is within the yield strength of the 7075T0 bolts.
I annealed the 6061T6 bus bars to soften the contact area, to give the bus bar and the terminal the best opportunity to become intimate. BTW, an oven self cleaning cycle does a pretty good job annealing aluminum – not perfect but pretty close.
The batteries are installed without compression. I considered the various information about compression and decided, after some communication with REPT, and some other research, that given my low C rates, there would be minimal value. On the other hand, by leaving space all around each cell prevents any stress on the battery terminals, which I judged to be a more likely source of unreliability.
A number of sources suggest that a cycle is defined as a cumulative use equal to the batteries' capacity; that is, using 25% one day, 50% the next, and 25% the next equals one cycle. Using that information, if a typical 280 AH battery is good for 3000 cycles, then its life can be calculated as an accumulated 280 * 3000 = 840,000 AH. In my 3P8S battery, that equates to 70,560 kilowatt-hours. Since we use about 6 kw-hr/day, the expected life is 32 years. Affecting that one way or the other by compression is, for me, unnecessary.
Time will tell.
The control system is an Electrodacus SBMSO BMS.
An output turns the Midnite controller on/off through its AUX 2 port (I am abandoning the WB Jr and the ability to absorb to end amps) when the battery reaches 28.4 volts or any cell reaches 3.55. The Midnite will be set to 29 volts, which it will never reach. Instead, it is always in either bulk mode or resting, controlled by the BMS.
Another output turns off the inverter if the voltage is low; this output resets and the inverter will restart if the voltage recovers. The inverter interface is a magnum ME-RSA adapter.
A third output provides an alarm signal if the SOC gets low (likely will set it at about 35%) to alert to turn on the standby generator.
A fourth is a fault output in case the battery reaches dangerously high or low voltages. This output will trip all the breakers (inverter, solar in, charge controller out, generator). I don’t expect this to ever be used but the system is unattended for part of the year, and I decided this extra safety is worthwhile. It took a while to find shunt trip breakers I was happy with, and in the case of the inverter breaker it involved a bit of invention – I designed a simple add-on to trip the 250 amp breaker.
The generator is not really a standby generator but rather a 24 volt 60 amp propane fueled battery charger. Setting the run timer to 5 hours will take the battery from about 30% to 70 % or so. We don’t expect to run the generator more than a few days a year, if at all, and so decided that this approach would be OK. If it turns out we need to run the generator more, I can easily connect it onto the same charging circuit that controls the Midnite solar controller.
I haven’t done any battery capacity testing; the vendor sent me some data and in any case the battery is large enough that is the cells are a bit off in capacity it doesn’t matter.
The battery cabinet is nearly complete, so I expect that within a week or so I should be ready to swap in the new battery pack. The cell voltages are all close so I am not going to separately top balance. Instead, I’ll keep an eye on the cell voltages on the BMS and if anything looks out of whack I’ll deal with it then. I suspect that in a fairly short while the BMS will take care of any imbalances.
While transporting the batteries, I couldn't help but notice that 24KW fit easily into the frunk; with a little finesse 50KW would fit easily:
But I have been warned that is not a priority project
The maximum charge rate is 90 amps from a Midnite Classic controller fed by 3600 watts of panels in two fixed arrays. The maximum discharge rate will be 135 amps (continuous), 250 amps peak during pump starting, through a Magnum MS4024 inverter.
Ordered 24 REPT 280 A cells from Shenzen Continent Technology Co. Leadup to the buy went well, with all my questions answered, shipment arrived nicely packed with no damage 60 days after order. There was a 10 day or so delay because of a backup at the Vancouver port.
As delivered, the cell voltages were uniform, within 20 mv, slightly below the values they sent me before shipment. The internal resistances are all within 2 milliohms of each other. I won’t share the actual values because I measured them with a YR1035+, which according to the manual isn’t accurate at low resistance values, but I trust it as far as cell to cell consistency.
I opted for welded on female terminals. They are 6mm, like the drilled terminals, but are nice and consistent. I went this way instead of the stud type because I want to control the metallurgy, using aluminum bolts and nuts to make sure there is no galvanic action. I want this to last a long time.
The cells will be connected using 3/16 aluminum bus bars. I wanted 6101 but it is really hard to find so used 6061 and went from 1/8” to 3/16” to account for the higher resistivity of 6061. The bus bars are 7” wide to cover the terminals on three cells; the total bus calculated voltage drop is 10 millivolts at 125 amps.
The terminal is 12.5mm in diameter, with a 7mm hole, so the contact area looks small but is 75 mm^2, between 2/0 and 3/0 cable, more than enough for the current that any individual cell will see. I found a couple of papers discussing the resistance of busbar connections, and they suggested while the area needs to be large enough to carry the current, the joint resistance is greatly affected by the contact pressure.. The target contact pressure is in the 15 to 30 N/mm^2 (2000 to 4000 psi) range. For my terminals, this means a force of 1125 to 2250 N (250 to 500 lbs).
To secure the terminals I am using a series stack of 3 belleville spring washers with a spring rate of 4000 N/mm (900 lb/mm). By compressing the spring stack by 0.50 mm (3 flats on the nut) the joint is loaded to 2000N. I am screwing a nut onto the bolt, hand tightening the bolt into the terminal, then holding the bolt as the nut is tightened 3 flats after it is snug. Experimentation and calculation shows that this load is within the yield strength of the 7075T0 bolts.
I annealed the 6061T6 bus bars to soften the contact area, to give the bus bar and the terminal the best opportunity to become intimate. BTW, an oven self cleaning cycle does a pretty good job annealing aluminum – not perfect but pretty close.
The batteries are installed without compression. I considered the various information about compression and decided, after some communication with REPT, and some other research, that given my low C rates, there would be minimal value. On the other hand, by leaving space all around each cell prevents any stress on the battery terminals, which I judged to be a more likely source of unreliability.
A number of sources suggest that a cycle is defined as a cumulative use equal to the batteries' capacity; that is, using 25% one day, 50% the next, and 25% the next equals one cycle. Using that information, if a typical 280 AH battery is good for 3000 cycles, then its life can be calculated as an accumulated 280 * 3000 = 840,000 AH. In my 3P8S battery, that equates to 70,560 kilowatt-hours. Since we use about 6 kw-hr/day, the expected life is 32 years. Affecting that one way or the other by compression is, for me, unnecessary.
Time will tell.
The control system is an Electrodacus SBMSO BMS.
An output turns the Midnite controller on/off through its AUX 2 port (I am abandoning the WB Jr and the ability to absorb to end amps) when the battery reaches 28.4 volts or any cell reaches 3.55. The Midnite will be set to 29 volts, which it will never reach. Instead, it is always in either bulk mode or resting, controlled by the BMS.
Another output turns off the inverter if the voltage is low; this output resets and the inverter will restart if the voltage recovers. The inverter interface is a magnum ME-RSA adapter.
A third output provides an alarm signal if the SOC gets low (likely will set it at about 35%) to alert to turn on the standby generator.
A fourth is a fault output in case the battery reaches dangerously high or low voltages. This output will trip all the breakers (inverter, solar in, charge controller out, generator). I don’t expect this to ever be used but the system is unattended for part of the year, and I decided this extra safety is worthwhile. It took a while to find shunt trip breakers I was happy with, and in the case of the inverter breaker it involved a bit of invention – I designed a simple add-on to trip the 250 amp breaker.
The generator is not really a standby generator but rather a 24 volt 60 amp propane fueled battery charger. Setting the run timer to 5 hours will take the battery from about 30% to 70 % or so. We don’t expect to run the generator more than a few days a year, if at all, and so decided that this approach would be OK. If it turns out we need to run the generator more, I can easily connect it onto the same charging circuit that controls the Midnite solar controller.
I haven’t done any battery capacity testing; the vendor sent me some data and in any case the battery is large enough that is the cells are a bit off in capacity it doesn’t matter.
The battery cabinet is nearly complete, so I expect that within a week or so I should be ready to swap in the new battery pack. The cell voltages are all close so I am not going to separately top balance. Instead, I’ll keep an eye on the cell voltages on the BMS and if anything looks out of whack I’ll deal with it then. I suspect that in a fairly short while the BMS will take care of any imbalances.
While transporting the batteries, I couldn't help but notice that 24KW fit easily into the frunk; with a little finesse 50KW would fit easily:
But I have been warned that is not a priority project