One of the most important design decisions you’ll have to make when building your battery is to decide how big to make your battery. This idea of “how big” your battery should be can mostly be broken down into two factors: voltage and capacity.
Choosing the voltage of your battery
The voltage of your battery will mostly be decided by the device that you’re trying to power. For example, if you want to power a 48V ebike, you’ll need to make a 48V battery (or possibly a 52V battery, which fits most 48V ebike controllers). If you are powering a 24V golf cart, you’ll want to make a 24V battery. If you have a 12V system, you’ll want to make a 12V battery.
The main exception is if you’re planning on using a voltage converter or inverter. For example, you might build a 48V or 52V battery for a DIY powerwall to power your home, and then use an AC inverter to turn that into 110V or 220V AC or a DC-DC converter to turn it into 12V DC.
To get the correct voltage for your battery using li-ion cells, simply multiply the nominal voltage of each cell by the number of cells in series. (Don’t know what series or parallel connections are? Click here to read about it). For example, most li-ion cells are between 3.6V to 3.7V nominal. That means that three cells in series will give us 3 x 3.7V = 11.1V nominal. Ten cells in series would give us 10 x 3.7V = 37V nominal, which is why most 36V li-ion battery packs use a 10s (10 cells in series) configuration. Here’s a list of voltages and their corresponding cell counts:
|Number of cells in series||Nominal voltage of battery pack|
The most common configurations are usually 3s, 4s, 7s, 10s, 13s and 14s, as these are the closest configurations to 12V, 24V, 36V and 48V. Remember though that the actual voltage of your battery will be both higher and lower than the nominal voltage during the discharge cycle. To get the full voltage of your battery when fully charged, multiply the fully charged voltage of each cell (4.2 V for li-ion) by the number of cells. So a 10s battery pack is actually 42.0 V when fully charged. The fully drained voltage would be approximately 3.0 V multiplied by the number of cells in series, which would be 30 V for a 10s pack. In actuality, most li-ion cells can be drained down to 2.5 V, but there is very little energy left between 3.0 V and 2.5 V and it is healthier for the cells to stop discharging at 3.0 V.
Choosing the capacity of your battery
Next you’ll need to decide on the appropriate capacity of your battery, which is usually measured in amp hours or Ah. The more amp hours in your battery, the longer it will last before running out of charge.
The capacity of your battery depends on how many cells you have in parallel (remember, you can read up on parallel versus series connections here). Adding more cells in parallel will result in higher capacity. For example, if you use 3.5 Ah cells and have three cells in parallel, you’ll have 10.5 Ah of capacity (3.5 Ah x 3 cells = 10.5 Ah). But if you used 5 cells instead, you’d have 17.5 Ah total.
In general, the best way to achieve better performance and upgrade your battery is to build a larger capacity battery with a higher Ah value. This is because the more cells you have in parallel, and thus the higher the capacity of the battery, the less load is placed on each individual cell. Think of it like you and your friends carrying a heavy refrigerator. Would rather pick up the refrigerator with three people or five people? It will be much easier to lift with five people, since each person will carry less of the load. It’s the same thing with battery cells! The more cells you have in parallel working together, the larger the capacity and the less load each cell has to support by itself.
With the VRUZEND kits, each end cap will add a small amount of resistance to your battery due to the spring connector and the bus bars in between them. This adds a little more load to each cell. To use our analogy from earlier, it’s like lifting that fridge while wearing a backpack with a couple books in it. Therefore, you have all the more reason to plan to build a larger capacity battery with a higher Ah value. That way each cell can carry less load and remain happier and healthier.
The caps in your VRUZEND kit with stainless steel spring contacts are rated for a peak current of 5 A each, and it is better to aim for 3.5 A of continuous current. That means that you’ll want to make sure that you plan to have enough cells in parallel to handle your total current load. A single cell would be able to support a peak of 5 A when using the VRUZEND terminal caps. If you had two cells in parallel, you could support a peak load of 10 A when using the VRUZEND terminal caps. Four cells in parallel could support a 20 A peak load, and so on. So if you know that your load, such as your ebike controller, draws 25 A, you’ll want to make sure you use at least five cells in parallel to give you 25 A of peak current that you can support. More cells in parallel is always better though, as it reduces the load per cell.
Remember that the current load that your battery can support depends entirely on the number of cells in parallel, and not series. So if you’re building a 36 V battery with 10 cells in series, that doesn’t mean you can support 50 A. You have to calculate your current carrying capacity based on the number of cells in parallel, not series. If that 36 V battery you are building has four cells in parallel and 10 cells in series (making it a 10s4p pack), it can support 20 A when using the VRUZEND terminal caps, since each cap can carry 5 A each.
In conclusion, the two main factors to consider are your voltage and capacity (measured in amp hours). Your voltage will probably already be decided for you, based on the device you are going to power with your battery. Your capacity is usually up to you, but it is most often best to go bigger than you think. Not only will it give you more run time (and range, if it is for an electric vehicle), but it will keep your cells healthier and lasting longer too as each cell will work less hard.
|About The Author|
|Micah Toll is a mechanical engineer, lithium battery builder and ebike educator. He’s written multiple books including DIY Lithium Batteries and The Ultimate DIY Ebike Guide. When he’s not tooting around Tel Aviv or Florida on his ebikes, you can probably find him reading, writing, running or vegging out on the couch.|