I have no idea why that would happen, but since   I was planning a similar set-up with my Ryobi battery, I hope you/someone finds out what happened?
I wonder if the issue could be that the battery when not in the "proper" receptacle on a powertool, somehow goes into some kind of "mode"?

My Ryobi battery for my new string trimmer has held a charge thru a couple of long triiming sessions and still has plenty of juice left. Can't beleive a fish finder would deplete a battery faster than the effort it takes to cut weeds.

I haven't bought a fish finder yet, after reading this, I might do a mock-up here at home and see if I get the same result while I have time to make a AA holder.

That protection feature is what I suspected as well, I haven't had a chance to connect the batteries to the tools just yet but I'll try that first before I charge them.
The rigid batteries do have the tabs labeled "+ & -" so know I've got them connected correctly.
Back to the battery LED feature which indicates how much battery life you have left, I'll have to test and see how much shows up with a volt meter, I will let you know.

RedOwl

Most locators display the voltage of the power supply, a handy feature when running off of small batteries.
I pay fairly close attention to this during a trip so that I can see if I have an unusual power drain occurring and may be able to correct it. Things like corroded  or loose contacts or leaving a backlight turned on for example.

I re-read my Ryobi manual and it says that if your power tool of choice stops working while your using it, you release the trigger and that will re-set the battery IF it still has a charge.

This makes me think that it might not be something that only the charger can re-set, unless the tool battery holder/trigger is set up like the charger?

Since you have your fish finder set-up made, did you try running it jerry rigged at home and when it dies, just turning the fish finder on/off to see if that would re-set the battery?

I hope our electrical guru MP chimes in, if anyone would know, it would be him.

Quentin didn't have a problem, so maybe it's exclusive to the Rigid battery, or else his Ryobi battery doesn't have that feature.

Now I'm really curious.

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


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?