RedOwl wrote on Jun 25^th, 2014 at 7:42pm:

My take on it is that there is some sort of low voltage protection or cutout to prevent damage to the batteries, which can only be reset when placed back into the charger unit. RedOwl

You got it.
There is a protection circuit that prevents over charging, over discharging and over current. On the protection circuit I purchased to reset the under voltage, voltage needs to be applied to the output of the protection circuit.  I was able to reset this protection circuit by putting a 100 K resistor across the protection circuit.  This allows the battery to continue to supply power even when the protection circuit is tripped but in such low current it would take years to drain the battery. To reset the circuit I needed to disconnect the fish detector and leave it disconnected for a few seconds to allow the output voltage to rise to the battery voltage then connect it back up. 
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I don’t think on the Ryobi battery you have access to the unprotected battery connection so you would need another voltage source (like a charger).  It would not need to supply a lot of current but just have enough voltage.  Problem is Ryobi batteries are kind of expensive and if the protection circuit is damaged the pack is useless. What I would try to reset the protection circuit is two 9V batteries connected in series with a 10K to 100K resistor in series. This would provide an 18 volte source that was only capable of supplying 1.8 mA or less.
How many electrical connections are there on the Ryobi battery?