Aha! Found some useful charging info from Black & Decker (haven't been able to find a manual online)...
Quote:
Our cordless lawn mowers use a two stage charging process. The first stage of charging takes about 3 hours (depending on depth of discharge) and will recharge the battery to about 75% of its full capacity. The second stage of the charging process will charge the battery to its full capacity. Approximate times to reach full charge from a fully depleted battery are listed below:
12 Volt models: 18 hours
24 Volt models: 24 hours
In order to maintain optimum battery life and run-time, please fully charge your lawn mower’s battery after each use. (Source: Charging time for a cordless lawn mower)
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That fits with what I saw today.
The charger itself is "dumb" - it just puts out a constant 1.7 amps. But the circuit board in the mower itself shunts charging current into a finned heat sink, presumably once battery voltage rises above a set point ("75% of full capacity"), so as not to cook the battery during the final 25%. Not a very efficient use of charging power.
So around lunch time tomorrow, it should be fully charged. Well, sooner than that since this "replacement" pack is smaller than OEM, plus the actual capacity is lower because the batteries are roughly 2/3 worn out.