How to implement ship mode in Li-ion battery designs

By: Texas Instruments Gautham

Do you remember that many battery-powered electronic toys had a small plastic pull tab on the battery (see Figure 1) that you pulled to make the toy start moving? This was a way to close the battery's connection to the product's active circuitry and was the original "ship mode." 

This article will explain what ship mode is and how to use this feature in your product to provide the best user experience. Although this article will focus on using Texas Instruments battery charge management integrated circuits as examples, you can apply these concepts to any low-power system you are developing.

Figure 1: Pulling the tab on a battery-powered product

What is Ship Mode and why do I need it?

Shipping mode is the state in which the product consumes the lowest battery current. Consumers expect to use battery-powered products immediately after purchasing them. This means that the battery must maintain a certain capacity during shipping and during its shelf life, which may take several months or even longer. 

Lithium-ion batteries have become a popular choice for designers because they are rechargeable, support high power requirements, and are extremely lightweight. However, unlike non-rechargeable batteries, you can’t put a plastic pull tab on a product that uses lithium-ion batteries. So for safety reasons, you want to avoid using these batteries. This means we need to find alternative solutions to implement shipping power saving features in products that are both on and off.

You might wonder why you should care about ship mode when "ship" only happens once, but that's not entirely true. Ship mode is a state where the product consumes the lowest quiescent current while waiting for the user to press a button to turn the product on. For example, the Texas Instruments BQ25120A actively monitors for an adapter or push button input to be inserted while consuming 2 nA typical current.

We often recommend that designers implement a shipping mode when a product is about to be boxed out of the factory, when the product is in use and the battery is low, and when the product is in use and the user wants to turn off the product.

The BQ25120A series charger’s ship mode is implemented using a push button interface, as shown in Figure 2. The push button input (/MR) is internally pulled up to the VBAT pin. When the device is in ship mode and the user presses the /MR button, the device exits ship mode.

You do not need to use a capacitor on this pin because the signal is internally fault-free, but it is often visible in some schematics. If the switch is exposed to the user, you may choose to protect it with a transient voltage suppression diode. A low voltage reading on the /MR pin translates to a "button pressed" action. Be careful when connecting to a microcontroller (MCU) to drive the /MR pin because the voltage on the /MR pin is pulled to the battery voltage itself. N-channel metal oxide semiconductors are commonly used to simulate the "press" action of the MCU in a buttonless system. 

Figure 2: Internal pull-up on the /MR pin

When the product is about to be boxed, the EN_SHIPMODE command is sent via I 2 C and the device waits to enter the ship mode until V IN (charger) is disconnected, as shown in Figure 3.

Figure 3: Illustration of power saving mode for transportation on a factory production line

I see a lot of challenges on the production line. What works well in the lab may not be the same when running a production line with hundreds of products per hour. The product is on a fixture and the assembler cannot properly pull the product out if the V IN adapter does not spring back. When the V IN is seemingly plugged back in, the charger will come out of ship mode. How do you ensure the product is in ship mode in this case?

The answer is simple: When V IN is present, the EN_SHIPMODE command is set. Once the product leaves the fixture that powers V IN , the /CD pin is brought high to execute the ship mode command and put the device into ship mode before packaging, as shown in Figure 4. In all of these sequences, /MR is pushed to show that the device is indeed in ship mode.

Figure 4: Illustration of Ship Mode in a factory with a flexible adapter

When a product runs low on battery, it is often best to power down to retain some reserve capacity and then go into ship mode. The device can read the battery charge when the user presses a button. If the battery is too low, the product may display a warning and go back into ship mode, as shown in Figure 5. For example, when using the BQ25120A , a single register write can be used to send the ship mode command, as shown in Figure 6.

Figure 5: Product wake-up procedure

Figure 6: Entering shipping mode via I2C command

Additionally, to create a power on/off button, you can still use the /MR pin, by configuring the MRREC register to enter shipping mode, and programming the duration of the button to determine how long it needs to be held low; you have just created a power-off button. To wake up again, press the button again, as shown in Figure 7.

Figure 7: Power on/off button

Hopefully this article has given you an idea of ​​how to use some of the features on your charger and you'll be ready to implement ship mode in your next product! If you have any questions, visit the TI E2E Support Community forum.