Choosing A 12 Volt DC-DC Converter For Your Electric Bike

Choosing A DC-DC Converter

Skip to the bottom of this section if you don't care about the fine details and want a one-size-fits-all DC-DC converter.

A DC-DC Converter converts an input DC voltage (in this case, our battery's voltage) into a higher or lower DC voltage. In our case, we require a DC-DC converter that can convert the higher voltages of our ebike battery to 12 volts. Selecting 12 volts allows us to use commonly available lights and acessories available for cars and motorcycles.

DC-DC Converters may also be referred to as voltage regulators, buck regulators (convert to lower voltage) or boost regulators (convert to higher voltage).

Here are some of the criteria to be considered when choosing a DC-DC Converter

Input Voltage

Our DC-DC Converter has to cope with a wide range of voltages because the voltage of lithium-ion batteries varies with charge level.  An example would be a 36 volt battery which has a nominal voltage of 36 volts. However, at full charge it sits at 42 volts, and when empty it can be as low as 25 volts. Selecting the wrong DC-DC Converter would result in auto-shutdown due to under-voltage condition, or damage due to over-voltage.

Below are possible voltage ranges for common lithium-ion ebike battery voltages:

As the above shows, battery voltage can swing as much as 20 volts over the charge range. So we need to choose a DC-DC converter that can operate in a wide voltage range compatible with our battery.

Output Amperage

Amperage, or amps, is the amount of current that can be provided by the DC-DC Converter. In general, this ranges from as low as 1A (1 Amp) to as high as 30A for commonly available DC-DC Converters. Using Watts = Voltage x Amps, we can see that results in a wattage range of 12 watts to 360 watts.

We want a DC-DC Converter that can provide the minimum wattage or higher required for our application. However, that wattage can vary based on the items being powered. Below is a rough guide to estimate your wattage requirements:

Additionally, you may wish to use your battery to power household appliances in emergencies with an DC to AC Inverter. Inverters can range from 100 watts to more than 1000 watts. Also, know that an inverter's wattage rating is the maximum wattage the inverter can handle. The actual watts used is determined by whatever you plug into the inverter. I recommend a second dedicated DC-DC converter that you can plug in when the need arises. The DC-DC converters with the wattage required for an inverter tend to be bulky and are less efficient when used with low wattage loads.

Note that the higher your wattage requirements, the faster your battery will drain and the less energy available to propel you to your destination. As an estimation, every 25 watts used for an hour will reduce your range by 1 mile (1.6 kilometers).

Input Amperage

Lastly, we should note the input amperage. In general, unless you are running a high wattage inverter, the input amperage requirements should be no more than 1-2 amps. Take into account the maximum amperage rating of your battery, the maximum amperage draw of your ebike motor and the maximum amperage draw of your DC-DC converter. Do not exceed the maximum amperage draw of your battery!

(Motor Amps + DC-DC Converter Amps) < Battery Maximum Amps

To get DC-DC input amps:

Nominal Battery Voltage / DC-DC Converter Max Watts = Input Amps

Recommended DC-DC Converters

Taking all of the above into consideration, here are my recommendations for DC-DC Converters:

DC-DC converters with higher wattage ratings are available. They are commonly available in ratings up to 30 amps (360 watts). Keep note of the DC-DC Converter's input voltage range and make sure to select the right size fuse for your application.