Introduction: Adding a Battery Monitor to Bamboo Lithium Battery
Bamboo Lithium is a 12V 10Ah battery intended to replace SLA battery with better weight to power ratio and longer cycle life.
The battery is made with 26650 3.3Ah LiFePO4 cells. The battery is configured in 3P4S configuration, which gives 13.2V nominal and 10Ah total capacity
The battery is well made, feels very solid, but the battery lack balancing connector and a battery monitor.
The cells in the bamboo battery turn out to be not very balanced new, my first charge after adding the balancing connector shows one of the cell (3P) is about 10% more charge than all other cells in the pack. Thus, even without adding the battery monitor, adding the balancing connector has a good value by itself.
In this instructable, I'll be adding a battery monitor and a balancing connector to the battery pack. Thanks to the Bamboo container, it is easy to cut open and to glue them back together
Step 1: Cutting the Top Open
The top of the Bamboo battery is mostly filled with glue, so it can be cut opened fairly safely without hitting any of the actual cells. Be very careful when cutting with a saw as any damage to the actual cell can cause a short circuit, which is very dangerous with high current LiFePO4 cells. This battery can easily driver over 80A of current into a short circuit, so be very careful not to cut beyond the empty space from the top. I start my cut with about a 5mm deep cut along the joint at the top of the battery. Please refer to the picture to see how much clearance is available from the top.
The inside of the battery is filled with polyurethane glue. I'll be using the same kind of glue to put it back. After cutting through the case material, it takes a bit of prying to get the top cover off. Use a non-conductive material and make sure not to stick the prying material too deep and hit the battery.
Step 2: More Prying
Slowing removing the side of the battery using the same prying technique. There is a shorting plat connecting all three parallel strings of cells together at each of the connection point, this is very good as I only have to tap into 3 location to connect the balancing connector, saving me quite a bit of effort and the design also ensure all the parallel cells are balanced.
Step 3: Open Up the Other Side to Fully Expose All the Contact Points
It is easy to clean the polyurethane glue from the shorting plate. The glue does not stick to the metal strongly, so a bit of scraping with a plastic knife does the trick. Solder a wire from each of the shorting place for the next step, which is to add the battery monitor
Step 4: Battery Monitor
I use a battery monitor bought off Ebay. The battery monitor is specially designed for 12V LiFePO4 battery. The battery monitor protect the cell from being over charged, under charged and over current. I select the particular battery monitor because it has very low idle current draw. The down side of this particular battery monitor is the low current limit, which is 8A max. This is much lower than the continuous current capacity of the cell, which is rated at 80A. Though it put a limit on max power capability of the battery, it does what I want the battery to do, which is to run a small inverter and USB charging.
The low idle current draw is critical for any embedded battery monitor because nothing is worse than the battery monitor drawing too much current after the battery has already been drained, which cause the battery go dip below the safe operating voltage. This can permanently damage the cell.
The particular battery monitor also does not do battery balancing. Balancing is important as it ensure all the cells in a string is running at the same voltage. This maximize the available energy from the cell, also prevent over charging/under charging one cell in a string.
I attached an external battery balancing connector so I can balance the battery initially. LiFePO4 battery stays fairly balanced during use, so constant re-balancing will not be required.
Step 5: Wire Up the Battery Monitor
The battery monitor is fairly will labeled at the back. Wire the battery to the battery monitor starting with the positive side (B+ label on the PCM), next is the first connection between the cell, which is the (B1) connection. Follow by B2 and B3. Eventually reaching the last connection for the negative side of the battery string (B-)
I wire up the balancing connector at the same time as I put the battery monitor in. The wire to output terminal of the battery is cut, and a new wire is solder from the PCM to the output terminal (P+ and P- connection)
Taping the battery monitor to the top of the cells to ensure it does move around at the gluing process, and to ensure the polyurethane glue doesn't get into the electronic. This will allow removal of the battery monitor if needed in the future.
Step 6: Glue Everything Back
Since the battery was build using polyurethane glue, I decided to use the same kind of glue to put it back. There are many different make of the glue on the market, I decided to use the Gorilla glue, which is easy to obtain from any hardware store.
Lot of clamping later, the battery is back in one piece. The polyurethane glue expand as it cures, so it does the job of filling up all the empty space nicely.
The balancing connector is glue in place using silicone to allow the cable a bit a flex, this will prevent the cable from breaking due to flexing in the future