Adding an external battery to your electric scooter: Parallel connection and precautions
An external battery can extend the range of an electric scooter without modifying its original battery: it is connected in parallel, using a Y-shaped wiring configuration, and both batteries discharge and recharge together. The absolute rule is that the external battery must have exactly the same voltage as the internal battery, meaning the same number of cells in series. When installed correctly, this setup adds range, reduces the strain on the internal battery, and minimizes voltage drops at the end of the charge cycle. However, if installed incorrectly, it can cause strong currents to flow between the two battery packs.
The essentials
- At the same time, the energy sources combine: an internal battery with a capacity of 2,880 Wh and an external battery with a capacity of 1,080 Wh result in a total of 3,960 Wh.
- Same voltage requirement: the same number of cells in series, and the same battery chemistry.
- The two batteries must have the same charge level when they are connected.
- Each one retains its own Battery Management System (BMS); the Y-connector wiring must be sized appropriately for the current.
- The mounting is just as important as the electronics: vibrations, water, and impacts can all cause problems.
The principle of parallel wiring
Connecting two batteries in parallel involves connecting their positive terminals together and their negative terminals together. The voltage remains the same as that of a single battery, while the capacity is added together. The controller perceives a single, larger battery.
During operation, the current is distributed between the two battery packs according to their voltage and internal resistance. Therefore, the internal battery no longer supplies all the current on its own: it heats up less, its voltage drops less during acceleration, and it ages more slowly. This is one of the lesser-known benefits of the external battery, in addition to the increased range. The effect on voltage drop is explained in our guide why the voltage drops when you accelerate.
The rules to follow:
The same voltage, without exception
An external 72V battery connects to an internal 72V battery, or a 79.2V battery to a 79.2V battery. If the voltages differ, the battery with the higher voltage will discharge rapidly into the battery with the lower voltage, resulting in currents that neither the cables nor the Battery Management Systems (BMS) can handle. It's also crucial to use batteries with the same cell type (e.g., lithium-ion with lithium-ion), as their voltage ranges must match from fully charged to fully discharged.
The same charge level when connected
Even with the same nominal voltage, a fully charged battery connected to a completely discharged battery will cause a strong current to flow from one to the other at the moment of connection. Therefore, the two battery packs should be connected when they are at the same level, typically when both are fully charged. The spark that can occur when connecting a large battery is another topic, covered in our guide on pre-charging and anti-sparking.
Properly sized wiring
The Y-cable must be able to handle the current from the external battery, with appropriately sized power connectors that are properly crimped or soldered and insulated. An undersized connector can overheat and melt. Our guide on power cables and connectors explains how to choose the right ones.
The charging process
Once connected, the two batteries charge together via the scooter's charging port. The charging time increases proportionally to the amount of energy being added: with the original charger, expect a significantly longer charging time than before. All the usual charging rules apply: use a charger with the correct voltage, ensure the batteries are at room temperature, monitor the charging process, and disconnect them when the charging is complete.
Autonomy or Performance: Two Different Use Cases
We offer two external battery packs, both 72V and 15Ah (1080Wh):
- The Autonomie 72V 15Ah external battery prioritizes capacity while maintaining a reasonable weight: it's ideal if your controller is the original one and you want to achieve greater range
- The Performance (S) 72V 15Ah external battery uses high-discharge cells: it relieves the internal battery during high current demands, which is useful if you have increased the power or are riding aggressively.
For scooters operating at 79.2V, our 79.2V 10Ah external battery in a 5L bag adds 792 Wh, utilizing LG M50LT 21700 cells. The difference between these two approaches is explained in Performance vs. Range Battery.
The mounting
An external battery operates in a harsh environment: constant vibrations, impacts, rain, and splashes. Here are a few guidelines:
- A bag or a secure mounting system attached to the robust frame, which prevents the battery from moving and rubbing
- There's nothing else in the bag: no tools, no keys, nothing that could puncture or cause a short circuit
- Cables are protected at points of passage and friction, and connectors are shielded from splashes
- No placement that obstructs steering, folding, or braking.
| To do: | Things to absolutely avoid |
|---|---|
| Same voltage, same number of cells in series | Connecting two batteries with different voltages |
| Connect the two battery packs to the same charge level | Connect a fully charged external battery to an empty internal battery |
| Y-harness wiring and appropriately sized connectors | Small connector adapters |
| Secure attachment, dedicated bag | Loose battery in a bag with metallic objects |
Workshop tip
Connecting two lithium batteries in parallel leaves no room for error. We provide the Y-connector wiring with our external batteries and offer complete installation in our workshop as part of our assembly package.
Frequently Asked Questions
Does an external battery increase the power?
It doesn't increase the controller's set power, but it helps the battery deliver that power: it reduces voltage drops and minimizes cut-offs at the end of the battery's charge cycle. This is especially true with a high-discharge external battery pack.
Can I disconnect the external battery when I don't need it?
Yes, provided that you then reconnect it to the same charge level as the internal battery. The simplest method is to disconnect and reconnect both fully charged batteries.
My internal battery is 60V. Can I add an external 72V battery?
No, never. Both batteries must have exactly the same voltage. Switching to 72V is a separate project, covered in our guide on upgrading to a higher voltage.
Do you need a more powerful charger?
It's not mandatory: the original charger works at the same voltage, it just takes longer. A more powerful charger is only feasible if the batteries and their charging port can handle it.