fill-channel tries to move funds from several of your channels into a single specified channel.
fill-channel finds your channels with a surfeit of funds and tries, one by one, to move funds from them into the channel you specify. Unlike auto-balance, this command doesn’t remove the destination channel from processing after the first successful move.
The nature of Lightning routing means it may not be able to find such a route, or the route may not actually be capable of routing the funds, or it may just fail with a TemporaryChannelFailure, or it may time out, or your Lightning node may be using outdated policy/pricing information from one of the nodes, or... Well, there are lots of reasons routing funds might fail.
Here’s a simple example of running the command:
lntoolkit fill-channel --to 1629436649948643328 --max-fees-msat 5000 --max-amount 1000000 --max-from-channels 25 --verbose
This tries to move up to 1000000sats (from --max-amount 1000000) from up to 25 of your channels (from --max-from-channels 25) to your channel with ID 1629436649948643328 (from --to 1629436649948643328) and it will not use any routes that cost more than 5000msats (from --max-fees-msat 5000). It will start with the channels it believes has the most surplus funds, and it will process each of them until it runs out of channels or it the --to channel becomes full.
Here’s the list of parameters it can take:
Usage of fill-channel: -certificate string path and filename of the TLS certificate to use when connecting to LND. (default "~/.lnd/tls.cert") -dry-run if specified, route(s) will be calculated but no funds will be transferred. -exclude value specific channel IDs to exclude from 'to' or 'from' channel lists. This parameter can be specified multiple times per command. -final-cltv-delta uint final CLTV delta to use in the routes from the 'from' channel to the 'to' channel. (default 144) -from value specific channel IDs to move funds from (overrides heuristics determining capable channels). This parameter can be specified multiple times per command. -from-fullness-threshold-percentage uint minimum percentage full the channel must be to be deemed capable of being a source of balance. (default 75) -macaroon string path and filename of the macaroon to use when connecting to LND. (default "~/.lnd/data/chain/bitcoin/testnet/admin.macaroon") -max-amount value maximum amount (in sats) to try to move from one channel to another. (default ㋛100,000) -max-fees-msat value maximum fees (in msat) to be spent on the balance move. (default m㋛10,000) -max-from-channels uint maximum number of channels to try to use as a source of balance. (default 5) -max-hops int maximum number of hops in routes used for the balance move. (default 20) -max-moves uint maximum number of balance moves to perform. (default 5) -max-routes int maximum number of routes to try (cheapest first) when trying to move the balance. (default 20) -max-search-time duration maximum length of time waiting while searching for the next route. (default 30s) -max-to-channels uint maximum number of channels to try to use as a destination for balance. (default 5) -quiet only show errors and critical messages. -server string hostname and port to use when connecting to LND. (default "localhost:10009") -target-empty-from-percentage uint percent-fullness of the 'from' channels that the command should aim to achieve. (default 50) -target-fill-to-percentage uint percent-fullness of the 'to' channel that the command should aim to achieve. (default 99) -timeout duration timeout to use when connecting to LND. (default 30s) -tip value tip amount (in satoshis) to the developer as a thank-you (Tip is only paid if an action is performed.) -to value target channel ID to try to move funds to. -verbose show additional logging information. -version report the version being used.
Like many LN Toolkit commands, fill-channel can take a --dry-run parameter to go through the motions of connecting and deriving routes, but without actually sending the funds. This will show you what the command will try to do, but fill-channel has no way of knowing which, if any, routes will succeed so it just keeps going until it reaches another termination condition.
Like all LN Toolkit commands, fill-channel can take a --verbose flag to greatly increase the amount of diagnostic output it creates (including JSON representations of routes, if you like that kind of thing). It can also take a --tip <amount> parameter if you want to tip me some satoshis.