Intro: Lithium Field Charger Battery Pack
In this instructable, I'll show you how to make a battery pack from inexpensive Panasonic NCR18650B cells. While those aren't good for usage in an RC model for their low C rating, they are good for recharging your model battery packs in the field.
The NCR18650B cell has 2,5-4,2V usable voltage, 3400mAh capacity, can be discharged at 2C (6,8A) and charged at 0,5C (1,7A) in warm and 0,25C (0,85A) in cold surroundings <10°C. Yes, the voltage span is correct. Although this is a LiIon battery, it still goes up to 4,2V and does not take damage from discharge to 2,5V.
The final battery pack will be a 6S 22,2V 3400mAh battery pack with 0,5C charge and 2C discharge rating. And the big advantage is the weight. This battery pack has only about 300g and holds 75Wh. The same amount of power in a lead acid battery, which is what many people use for in-field charging, weighs about 2kg.
Step 1: Bill of Materials
So to build one of these, you need:
- A battery connector, I chose XT60
- Cables, I chose AWG14 silicone wires
- Shrink wrap to cover the solder joints
- Shrink tube to encase the battery pack in, they are readily available in the correct size for this battery pack
- A balancer wire for 6S because we want to create a 6S battery pack
- The NCR18650B batteries
Step 2: Connecting the Battery Cells
In order to make a 6S battery pack, the 6 battery cells must be aligned in series. Meaning the "+" of one cell connects to the "-" of the next, all in one row. The ends of that row are the connections where you charge and discharge the pack.
I used short strips of the silicone wire, stripped of its silicone cover, to make those connections. For soldering the cells, I suggest you first sand off the coating of the "+" and "-" ends of the batteries. Soldering gets a lot easier then. Just a quick scrub with sanding paper to get lots of scratches in the ends of the battery, that's enough.
Take care not to heat the batteries too much. Not more than 2-3 seconds with the soldering iron. If you're not comfortable with soldering explosive, flammable lithium batteries, let someone help you with it or use readily available battery holders instead.
Step 3: Adding Wires
Next up: the open ends of the batteries, meaning the "+" of the first and the "-" of the last one, are connected to the red and black wires. Both on the discharge lead and on the balance lead. They share the same connection.
The remaining leads of the balance wire are connected between each two batteries. The one next to the red one is soldered to the joint of battery 1 and 2 and so on. The idea of the balance connector is that each end of each battery has a wire connected to the balance plug, in the same order the batteries are soldered in the pack.
Step 4: Shrink Wrap
Finally, pull the shrink wrap tube over the battery, shrink it with a hot air gun and you're done. Balance charge the pack and you're done.
Step 5: Usage
So I made four of these, connected them in parallel (both the discharge lead and the balance lead) which makes a 6S4P battery pack with 300Wh in 1,2kg. I don't charge it to the max (only 4,1V per cell, not the full 4,2V) and don't completely drain it, this increases the cycle count a lot. And by using several in parallel, I reduce the discharge current which the batteries also benefit of.