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Battery Notification
When a Provider includes multiple components, it can be useful to notify the
Seeker of the battery level for each component. One example of this would be
when a earbuds case is opened and the Seeker needs to know the battery for
each individual bud and the case itself.
To accomplish this, a Provider can include additional information in the
advertisement, built on top of the Fast Pair Account Data described in
Advertising: When not discoverable.
In addition to the Account Data, the Provider should include an additional field
specifying the battery values. The packet shall contain the following:
Octet |
Data type |
Description |
Value |
Mandatory? |
0 |
uint8 |
Flags |
0x00 (all bits reserved for future use) |
Mandatory |
1 - s |
|
Account Key Data |
Mandatory |
|
s + 1 |
uint8 |
Battery level length and type 0bLLLLTTTT- L = Number of battery values
- T = type
|
0bLLLLTTTT
- length = 0b0011 = 3 battery values
- type = 0b0011 (show UI indication) or 0b0100 (hide UI indication)
|
Optional |
s + 2, s + 3, s + 4 |
uint8 |
Battery values 0bSVVVVVVV- S = status (charging or not charging)
- V = value
- The battery values should be ordered as left bud (s + 2), right bud (s + 3) and case (s + 4).
|
0bSVVVVVVV
- status = 0b1 (charging) or 0b0 (not charging)
- value = battery level ranging from 0 to 100 percent, 0bS1111111 for unknown.
|
Optional |
To prevent tampering, the Account Key Data above shall be slightly
modified to include battery information when the battery values are included
in the advertisement. Normally, when building the account key
filter, a value V is produced by combining the account
key with a salt. Instead, when battery information is also being advertised, the
value V should be constructed as follows:
- Produce a value V, where:
- The first 16 bytes are K.
- The next bytes are the Salt.
- The remaining bytes are the battery information (from s + 1 to s + 4
including the length and type byte from the table above).
As noted in the battery length and type field above, the type can be either
0b0011
or 0b0100
.
- 0b0011 - Use when the Provider wants the Seeker to show an indication in the
UI of the battery values;
- 0b0100 - Use when the Provider wants the Seeker to hide the indication if it
is already showing.
One common use case for this is to use 0b0011
when the case has opened and
0b0100
when buds have been removed from the case or it has been closed again.
//The sample code demonstrates that the headset only reports the battery level.
#define FASTPAIR_ACCOUNT_KEY_SIZE 16
// In the sample code, the size of salt is 2 bytes.
#define SALT_SIZE 2
// 1st byte - Battery level length and type
// 2nd~4th bytes - Battery values
#define BAT_LEVEL_SIZE 3
uint8_t V[FASTPAIR_ACCOUNT_KEY_SIZE + SALT_SIZE + BAT_LEVEL_SIZE + 1] = {0};
int v_index = 0;
// The first 16 bytes are K.
uint8_t K[FASTPAIR_ACCOUNT_KEY_SIZE] = {0};
fastpair_get_account_key_by_index(keyIndex, K);
memcpy(V, K, FASTPAIR_ACCOUNT_KEY_SIZE);
v_index = v_index + FASTPAIR_ACCOUNT_KEY_SIZE;
// The next byte is the Salt.
uint8_t randomSalt = (uint8_t)rand();
V[v_index] = randomSalt;
v_index = v_index + SALT_SIZE;
// The remaining bytes are the battery information (from s + 1 to s + 4 including the length and type bytes).
uint8_t battery_level_len = 0;
uint8_t battery_level[BAT_LEVEL_SIZE] = {0};
fastpair_get_bat_level(&battery_level_len, battery_level);
// type = 0b0011 (show UI indication) or 0b0100 (hide UI indication)
V[v_index] = (battery_level_len << 4 | (is_show_ui ? 0x3 : 0x4));
v_index = v_index + 1;
for (int idx = 0; idx < battery_level_len; idx++) {
V[v_index++] = battery_level[idx];
}
To prevent tracking, the Provider should not include raw battery data in the
advertisement all the time. It can instead be sent via RFCOMM when connected to
a Seeker, see Message Stream: Device Information.
Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
Last updated 2025-08-12 UTC.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-12 UTC."],[[["\u003cp\u003eProviders can include battery information for multiple components (e.g., earbuds and case) in the advertisement data using a specific format.\u003c/p\u003e\n"],["\u003cp\u003eThe advertisement data includes battery levels for each component and flags to control UI indication on the Seeker device.\u003c/p\u003e\n"],["\u003cp\u003eTo prevent tampering, the Account Key Data is modified to include battery information, requiring a specific construction process.\u003c/p\u003e\n"],["\u003cp\u003eProviders can choose to show or hide battery information in the Seeker's UI based on user interaction (e.g., case opening/closing).\u003c/p\u003e\n"],["\u003cp\u003eTo prevent tracking, raw battery data should not be constantly advertised but can be sent securely via RFCOMM after connection.\u003c/p\u003e\n"]]],[],null,["### Battery Notification\n\n| **Note:** Google recommends implementing the [Cryptographic Test Cases](/nearby/fast-pair/specifications/appendix/cryptotestcases \"Link to the Cryptographic Test Cases.\") to ease verification of these requirements.\n\nWhen a Provider includes multiple components, it can be useful to notify the\nSeeker of the battery level for each component. One example of this would be\nwhen a earbuds case is opened and the Seeker needs to know the battery for\neach individual bud and the case itself.\n\nTo accomplish this, a Provider can include additional information in the\nadvertisement, built on top of the Fast Pair Account Data described in\n[Advertising: When not discoverable](/nearby/fast-pair/specifications/service/provider#AdvertisingWhenNotDiscoverable \"Advertizing when not Discoverable\").\n\nIn addition to the Account Data, the Provider should include an additional field\nspecifying the battery values. The packet shall contain the following:\n\n| Octet | Data type | Description | Value | Mandatory? |\n|-----------------------------|-----------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------|------------|\n| 0 | `uint8` | Flags | 0x00 (all bits reserved for future use) | Mandatory |\n| 1 - *s* | | Account Key Data | Mandatory | |\n| *s + 1* | `uint8` | Battery level length and type 0bLLLLTTTT - L = Number of battery values - T = type | 0bLLLLTTTT - length = 0b0011 = 3 battery values - type = 0b0011 (show UI indication) or 0b0100 (hide UI indication) | Optional |\n| *s + 2* , *s + 3* , *s + 4* | `uint8` | Battery values 0bSVVVVVVV - S = status (charging or not charging) - V = value - The battery values should be ordered as left bud (*s + 2* ), right bud (*s + 3* ) and case (*s + 4*). | 0bSVVVVVVV - status = 0b1 (charging) or 0b0 (not charging) - value = battery level ranging from 0 to 100 percent, 0bS1111111 for unknown. | Optional |\n\nTo prevent tampering, the Account Key Data above shall be slightly\nmodified to include battery information when the battery values are included\nin the advertisement. Normally, when building the [account key\nfilter](/nearby/fast-pair/specifications/service/provider#AccountKeyFilter \"Account Key Filter\"), a value *V* is produced by combining the account\nkey with a salt. Instead, when battery information is also being advertised, the\nvalue *V* should be constructed as follows:\n\n1. Produce a value *V* , where:\n 1. The first 16 bytes are *K*.\n 2. The next bytes are the [Salt](/nearby/fast-pair/specifications/service/provider#SaltField \"Salt Field\").\n 3. The remaining bytes are the battery information (from *s + 1* to *s + 4* including the length and type byte from the table above).\n\nAs noted in the battery length and type field above, the type can be either\n`0b0011` or `0b0100`.\n\n- 0b0011 - Use when the Provider wants the Seeker to show an indication in the UI of the battery values;\n- 0b0100 - Use when the Provider wants the Seeker to hide the indication if it is already showing.\n\nOne common use case for this is to use `0b0011` when the case has opened and\n`0b0100` when buds have been removed from the case or it has been closed again. \n\n //The sample code demonstrates that the headset only reports the battery level.\n\n #define FASTPAIR_ACCOUNT_KEY_SIZE 16\n\n // In the sample code, the size of salt is 2 bytes.\n #define SALT_SIZE 2\n\n // 1st byte - Battery level length and type\n // 2nd~4th bytes - Battery values\n #define BAT_LEVEL_SIZE 3\n\n uint8_t V[FASTPAIR_ACCOUNT_KEY_SIZE + SALT_SIZE + BAT_LEVEL_SIZE + 1] = {0};\n int v_index = 0;\n\n // The first 16 bytes are K.\n uint8_t K[FASTPAIR_ACCOUNT_KEY_SIZE] = {0};\n fastpair_get_account_key_by_index(keyIndex, K);\n memcpy(V, K, FASTPAIR_ACCOUNT_KEY_SIZE);\n v_index = v_index + FASTPAIR_ACCOUNT_KEY_SIZE;\n\n // The next byte is the Salt.\n uint8_t randomSalt = (uint8_t)rand();\n V[v_index] = randomSalt;\n v_index = v_index + SALT_SIZE;\n\n // The remaining bytes are the battery information (from s + 1 to s + 4 including the length and type bytes).\n\n uint8_t battery_level_len = 0;\n uint8_t battery_level[BAT_LEVEL_SIZE] = {0};\n\n fastpair_get_bat_level(&battery_level_len, battery_level);\n\n // type = 0b0011 (show UI indication) or 0b0100 (hide UI indication)\n V[v_index] = (battery_level_len \u003c\u003c 4 | (is_show_ui ? 0x3 : 0x4));\n v_index = v_index + 1;\n\n for (int idx = 0; idx \u003c battery_level_len; idx++) {\n V[v_index++] = battery_level[idx];\n }\n\nTo prevent tracking, the Provider should not include raw battery data in the\nadvertisement all the time. It can instead be sent via RFCOMM when connected to\na Seeker, see [Message Stream: Device Information](/nearby/fast-pair/specifications/extensions/deviceinformation \"Message Stream: Device Information\")."]]