Building a 560AH LiFePO4 House Battery for Under $1k
In summary, the new HOUSE battery occupies the same location as the previous HOUSE batteries, weighs less (95 lbs vs 125 lbs), but has 5 TIMES the capacity. In other words, this LiFePO4 battery is equivalent in capacity to over TEN (10) 105AH Deep Cycle batteries but with much much better performance and expected longevity.
Based on reviews and recommendations the original plan was to purchase a JBD-SP04S020 4S 120A LiFePO4 BMS with Bluetooth from Overkill Solar. This BMS has two temperature probs for controlling the MIN/MAX operating values and Bluetooth operation with available iOS app. Temperature monitoring is important as the LiFePO4 cells should NOT be charged at temps below freezing as doing so would damage the cells. Unfortunately, Overkill Solar was out of stock and all attempts to contact Overkill Solar failed. Fortunately, on the diysolarforms.com forum, a link to the manufacture's website was provided and so the BMS was ordered directly from lithiumbatterypcb.com. While buying directly from the manufacture does not provide the same service and value of dealing with a local company in the USA especially in regards to warrantee related issues. Given the lack of response from Overkill Solar and the fact that buying direct was half the price, buying the BMS directly an easy choice that worked out fine. asked on the positive experience, a second identical BMS but with the additional RS-485 communication module (i.e., both the Bluetooth UART and RS485 communications).
Testing, Charging, and Top Balancing
Main Component List
- Battery cells purchased from Alibaba.com
- Lishen LFP 280AH cells
- $75 * 8 + $250shipping
- BMS purchased from LLT Power Electronics
- JBD-SP04S020 120A BMS 4S w/ low&high Temp w/ bluetooth
- $60
- Odds and Ends
- $40
Battery Cells
Purchase & Delivery: A total of eight (8) 280AH Prismatic LiFePO4 (Lishen Model LF280) were purchased from Shenzhen Jidian Technology Co., Ltd. on Alibaba.com. The shipment took nearly 60 days to arrive via the slow boat from China. While this delay is rather excessive, the order was placed just before the Chinese New Year Holiday. This, along with COVID-19 impacts, likely added to the delay. The battery cells arrived without any buss bars and mounting hardware. These items did eventually arrived in a separate shipment.
The battery cells arrived in two separate shipping boxes (see photo below). Each cell was wrapped well and arrived with no visible damage or imperfections. The voltages for all cells were within a few mV of 3.260V indicating that they were shipped at or near 50% charge capacity as recommended by the manufacturer's specifications.
BMS
First, cells were combined in pairs in parallel (positive to positive, negative to negative) then the 4 two-cell units were combined in series (positive to negative and negative to positive) to obtain a 2P4S configuration. The advantage of parallelizing first is that the 8 cells are configured to one battery requiring only one BMS. The alternative 4S2P would be making two individual 12V batteries requiring two BMS units and then connecting these two 12V batteries in parallel (like commonly done for 12V FLA batteries).
8 3.2V cells configured in 2P4S for a 12V system
Since the BMS controls the whole battery charging/discharging based on any one cell performance, it is important to balance the cell capacities to achieve optimal performance. Cell balancing can only be done with the cells nearly full (top balancing) or nearly empty (bottom balancing). This is because the cell voltage is nearly independent for most of the cell capacity except for the top or bottom capacities. See discharge curve below. Connecting a cell with 60% capacity at 3.30V with a cell at 95% capacity at 3.30V would not equalize and balance out since both cell voltages are nearly identical.
A simple method was used to top balance the cells by first connecting the BMS to the 2P4S cells and verifying BMS/battery operation and settings, then configuring the BMS maximum cell voltage to 3.650V and then charging the whole battery with 14.6V bench charger. With the BMS max cell voltage set, the cells will receive charge until anyone of the 4S cells exceeds the 3.650V setting. At that point, the BMS stops the charging for the whole battery system.
Since the cells were shipped with about 50% capacity (as suggested in the specifications), this initial charge took over a day to complete (560AH * 50% / 10A = 28H). At this point the battery was mostly charged but without capacity balanced cells. To top balance the cells, the BMS disconnected and the cells disconnected and reconfigured putting all cells in parallel. The cells were then charged with bench charger set to 3.65V (see photos below). As the battery cells approach 3.650V, the supplied current drops to 0A resulting in a top balanced battery system.
Initial BMS charging of cells Top balancing all cells in parallel
Build
Since the buss bars that came with the cells were late in arrival, it was decided to construct new beefier buss bars from sold copper stock. For the nuts &bolts hardware, 1 inch long M6 bolts were used rather than M6 studs, along with lock washers, and M6 nuts for mounting to the battery terminals. The contact area of the buss bars and the cell battery terminals were first cleaned using 1,000 grit paper followed by an alcohol wipe, then a small amount of NOALUX (an anti-oxidant and anti-seizing compound for aluminum to coper conductor connections) was applied to the contact surfaces, and then secured with the M6 bolts via M6 nuts and lock washers. Lock washers are important as they maintain an optimal tightness securing a good electrical connection.
A battery box was constructed by first compressing the 2P4S configuration lengthwise. End plates, made from 1/2" plywood, were connected with 7 threaded 1/4-20 rods. Between the end plates, the rods were cover with clear vinyl hose to provide chafe protection for the individual cells. The rods were connected with tee-nuts on one end plate and acorn nuts with washers on the other end plate. Two rods were located just below the battery cells, two on each side lengthwise, and one rod centrally located just above the cells. A bottom plywood base was added and screwed into the plywood end plates. A 1"x2" board was added lengthwise to the top edge. Side panels were made from 1/8" thick FRP wall panel and mounted with screws. A separating sheet of FRP was placed inside and above the battery cells to which the BMS was mounted onto. A 90° copper buss bar brings the battery positive up to a side mounted terminal. An ANL fuse mount for the negative voltage terminal were located along the top inside edge. A FRP panel was screwed on the top providing a closed battery box. And a 1" x 2" oval hole in the corner of a side plate provides battery cable access to the internal battery cells. External battery cables are connected to the negative ANL fuse post and to the positive terminal post. See photo of completed battery box at the top of this post.
Battery cells under compression with buss bars. BMS mounted on a FRP panel.
BMS Bluetooth iOS App
The Xiaoxiang BMS app is available for both Apple iOS and Google Android devices. The iOS version will connect and read BMS parameters via Bluetooth. In order to change parameter values, requires the in-app purchase for the "Pro Version" of the app for $6.99. Get the Pro Version!
Operations
BMS Configurations
Below are are screen shots from the BMS app showing the current setting which I have been configured for the BMS. The first page contains general information. While there are 8 battery cells in a 2P4S configuration, there are only 4 "effective" cells in series and so the BMS monitors cells pairs. The capacity configuration parameters are used for estimating the state of charge (i.e. battery capacity). This settings basically tell the BMS the max battery capacity and some capacity voltage dependence. Over a few cycles, the BMS will learn and update how capacities are calculated. The second page allows for additional functional configurations. The BMS has a built in limited balancer. This balancer will not efficiently perform a top balance, but can help maintain a battery which is slightly out of balance. Do a good job at top balancing, and the BMS balancer is likely not to be needed. The 30mV delta is a recommended setting. In this battery, the cell voltage deltas have been found to be 4mV or less for capacities below 98%.
The most important BMS parameters are the Protections Settings. The third page (photo below) shows cell over & under voltage values. These are the two most important parameters of the BMS monitoring. The corresponding battery over/under values should just be 4 times the cell values (i.e. 4*3.650V = 14.6V). As required for LiFePO4 cell protection, the charge temperature must be above freezing (0°C). The NTC1 and NTC2 checkbox tells the BMS that there are two temperature probs.
No comments:
Post a Comment