Operating fish finder using lithium battery

[gmwahl]

It appears to have Bluetooth monitoring?
A buddy bought a battery thinking it had Bluetooth monitoring from  wording on ad online. He couldn’t get it to work. Called the factory, turned out he purchased a non smart battery!! No Bluetooth monitoring!

It says right on the battery that it has bluetooth but I'll try calling them to make sure, maybe they can give me an idea why it isn't working. After charging both batteries individually, I now have it hooked up in parallel. After charging them separately, I rechecked the first one again and the voltage had dropped from 13.46 to 12.36. I'm not sure if charging them for 24 hrs was enough but it worries me that it drop at all.

 12.36 volts after charging, assuming that it is not under ANY load, would indicate a possible faulty cell.  It is normal for LiFePo4 batteries to go from 13.6ish down to 13.2 or SLIGHTLY lower, but 12.3 is not normal.  Is the other battery holding between 13.0 and 13.2 volts after you balanced them in parallel?

LiFePo4 batteries will have a very flat discharge curve and won't tip you off to being discharged (by being weak) until you are at the point of damage usually, and that is usually around 2-2.5 volts per cell.  A lead acid battery will discharge in a constant downward slope and you will know when you are getting close to discharge because the battery will get gradually weaker.  If you let a LiFePo4 battery go until it just stops, you may have damaged cells.  

Because those are smart technology,  I suspect that the battery BMS won't allow it to be either overcharged or drawn down too low and will have a cutoff, but it is still possible.  That may be why your battery is acting the way that it is.

Mike

Thanks Mike. When I took it in to be tested they said the one battery was toast, the other one was good but can I even buy one battery to replace the bad one or do I need to buy two at a time for my 24 v system? From everything I'm reading on this thread, I'm guessing I should buy two but that will cost over $600, so I'm looking for a deal and so far, I'm finding nothing less than that.

Your "good" battery will be ok with a new one as long as you are at more than 90+% or better of your rated capacity after a full recharge or cycle.  Below that and the old battery will draw on your new one as it attempts to equalize and balance and would accelerate the wear and degradation of the new battery cells.

Example being, if your old battery is rated for 100 amp hours and after draining it down and doing a full, deep recharge, or a series of cycle charges, you only have 90 or less amp hours capacity,  then you wouldn't want to parallel that with your new battery. I don't know if you have a charger that let's you know capacity or not....not too likely with on-board chargers, but a battery company can do a cycle test on it and let you know it's final capacity.

Mike