Low-cost Portable Power Station From Old Laptop Batteries

• Mobile power stations are probably not new. This is a very useful device for outdoor activities or use when there is a power outage (220v).
• I take advantage of the waste, easy to find and replaceable parts in the future to complete a mobile power station at a low cost.

1. Daly BMS 7S Li-ion 24V 40A
2. 24V 80A PWM Solar Controller
3. 24V To AC 220V Pure Sine Wave Power Inverter
4. 4 Slots 18650 Battery Holder
5. Fuse

• The battery is the most expensive thing in the system if you buy new batteries. 1 genuine battery costs about 3$. With a system like this, you will spend 168$ on battery, a pretty big number.
• Therefore, we will choose the old laptop battery. Old laptop batteries you can ask for from friends. We all know batteries are bad for the environment, so no one should throw them in the trash. 1-2 batteries are difficult to send for recycling, but no one will buy them when they sell them. Therefore, people will always leave it in a corner of the house.
• If you don't have enough batteries, you can buy old laptop batteries at a computer store for about 1$. You will receive 4-6 good batteries. Pretty good price for a genuine battery.
• To process all 56 batteries also takes a certain amount of time, you won't have time to go to the bar and save a decent amount of time.
• Due to using old batteries, we will need to do some work as follows:

1- Check battery capacity. After having the battery capacity, we will group each 8 batteries into 1 block so that the battery capacity between the blocks is as close to each other as possible. Each of my battery blocks has a capacity of about 17AH, you can see more in the clip.

• I use the Vapcell S4 Plus charger priced at 700K above Banggood to charge and measure the battery capacity. This charger can charge 4 independent slots with 3A charging current per slot, saving quite a bit of time.
• When measuring battery capacity, I use 1A discharge current for all batteries. And write them on the battery for easy grouping of battery blocks
• You can see that I write 1A -2613, which means that I tested the battery capacity at 1A discharge and the measured battery capacity is 2613mAh - equivalent to 72.2% (the new battery has a capacity of 3500mAh). The batteries with a capacity less than 70% I will remove.
• Below the battery capacity test section, you see that I write 4.17 and 1. That means I measure the voltage of the battery when fully charged is 4.17v. After 2 weeks, I went back to measure the voltage of the battery, it was 4.16, meaning the voltage only decreased 0.01. This is a good, reusable battery. If the battery voltage drops a lot, I will take it to recycle.

2- Another important factor is the internal resistance of the battery.

• The Vapcell S4 Plus charger can measure the internal resistance of the battery but it is not very accurate. So we can build a simple internal resistance measurement system with a 5W 10ohm resistor
• We will measure the battery voltage when there is no load and the voltage when the load is resistive. For example, when testing with a load we have a voltage of 4.12V. Current at 10ohm load will be 4.12/10=0.412A
• We will calculate the internal resistance according to the following formula: IR = (No-load voltage - Load voltage) / load current.
• For example no-load current is 4.18V then IR = (4.18 - 4.12 ) / 0.412 = 0.145ohm = 145 milliohm
• This is the reference value because we also have the resistance of the conductor. But it's even between the batteries. We will pair battery blocks with capacity and internal resistance as close together as possible. At this time, the BMS will work most effectively on our system to be optimized.
• When pairing battery blocks, you should also pay attention to the voltage and internal resistance between the battery cells as little difference as possible.

• I use Daly's BMS 7S which is a pretty good brand with a reasonable cost.
• BMS has the effect of protecting the battery from overcharging, over-discharging, over-current protection, short-circuit protection, and voltage balance between battery blocks.
• I choose a BMS with a discharge current of 40A. With a discharge current of 40A we have a maximum power of 1000W (battery voltage 25.2V x 40A = 1000W). BMS with 40A discharge current is suitable for continuous use at 500W.
• Moreover, this BMS has a very good charging current of 20A with 56 batteries. If the charging current is too small, the charging time will be longer.
• Let's try it out. With a charging current of 20A divided by 7 battery blocks, each block will have a charging current of 2.85A. Divide 2.85A by 8, each battery will have a charging current of 0.35A, it's fast and safe enough for old laptop batteries.
• When choosing Daly's BMS, you should also note: they have 2 types for li-ion and LiFepo4 batteries. Please select the type for li-ion battery.
• In addition, Daly's BMS has public and private gateway types. I like the common port because we will charge and discharge the same port for it to be compact.
• Wiring for BMS is also quite simple, you can refer to the attached diagram.
• There is 1 very important note you need to keep in mind when connecting the BMS to the battery:

1. Remove the balance jack from the BMS
2. Solder the equal jack to the battery according to the diagram
3. Measure the voltage across the balanced jack. From the black wire, the voltages are 4v, 8v, 12v… to 24v, respectively. When you are sure to solder the correct wire to the battery, plug the balanced jack into the BMS.

• I have 14 18650 battery holders with 4 batteries and 1 base. We can attach 56 cell or 224 cell when soldering 4 cell together like this (1 base will attach 16 cell)
• The battery holder you can solder as the attached photo
• We will solder 2 bases together so that 8 batteries can be attached in parallel. Form a block of batteries with 8 batteries.
• Next we will solder the battery blocks together to save space and be neater.
• We will have a total of 7 blocks in series and each block has 8 cell (up to 16).

• I bought this standard 24V to 220V pure sine wave inverter on Banggood.
• I choose 24V inverter because its efficiency is higher than 12V. Save money on electricity…
• For example: With a capacity of 1000W, a 12V system has a current of 1000:12=83.3A, a 24V system has a current of 1000:24=41.6A. As you can see the 83A current is very large we will need large wiring. It's expensive, and big wires are much harder to tidy up than small ones. High-current devices are also more expensive than low-current devices.

• I use a solar charge controller because it's pretty cheap. You can use a dedicated 29.4 charger for 7S batteries at a slightly lower cost. But in the long run the solar charge controller has more advantages such as:

1. We can use with solar panels in the future.
2. Wider input voltage range. You can take advantage of a 36v charger supplied through the solar port to charge the battery instead of having to buy the correct 29.4 . charger
3. The solar charger has many protection modes such as overcurrent protection, short circuit protection, over temperature protection… combined with the protection feature of BMS our system will be safer.
4. The solar charger provides more intuitive information allowing for more parameter settings, has a convenient DC output…

• The way to connect the solar charge controller is also quite simple, see the attached photo

• Devices such as inverters, charge controllers all have their own fuses, but I still use an extra fuse to protect the device and this fuse also acts as a switch to close/disconnect the battery system.
• You should use a fuse or a DC aptomat for the DC system. In particular, AC aptomat should not be used for DC systems because their structures and working methods are different.

• The connection system is quite simple with only a few wires. You can press cos like me or you can attach it
• Because I only use 300W, I choose 14AWG wire. If you use 700W, you should choose 10AWG wire, 1200W use 6AWG wire. Big strings are always better but too big costs money
• Wires in the system if you are not clear, you can see more in the clip. I did it in great detail
• We connect the positive terminal of the battery through the fuse. Then connect to the positive pole of the solar charger and the positive pole of the inverter.
• The P- port of our BMS connects to the negative pole of the solar charger and the inverter. “P” stands for power and “B” stands for battery.
• You may find yourself attaching 2 handles on the battery and inverter system for easy handling and they can be hung up to save space.

• I use the old suitcase. You can close the box for the system. The power cord and screwdriver I put in a plastic box to carry and ready to use when needed. Enough space for us to carry 56 spare batteries.
• 2 boards to fit in 1 suitcase for easy storage and carrying