You'll need a DC-DC converter that takes the full voltage range from full to empty of your battery on it's input, and outputs 13.6v (the typical actual voltage of "12v" systems), and can supply at least as many amps (preferably two or three times as much) as your fishfinder system requires.
If the 12v 750w is from it's label for it's power requirements, then you divide the watts by the volts to get the amps, meaning 750W / 12v = 62.5A. That is a very high current, and explains why it would deplete your 9Ah batteries so quickly--at that rate, a 9Ah SLA battery would last about one minute of operation. (might get two if it was lithium)
A converter for this much current is probably fairly expensive and large (and will have a large heatsink as it will get hot and need a fair bit of airflow); I poked around for a few minutes on google but didn't find anything useful. Most likely one that will take up to 60v input will work on your battery pack even down to it's empty state of around 48v, but you'd have to verify that when you find one that meets the other requirements, which are a minimum of 62.5A / 750W (preferably at least 100-120A / 1500W), at 13.6V. It's ok if it can output more watts or more amps, as the fishfinder will only draw what it needs to, but it does need to be 13.6v on the output (12v will "work" but the fishfinder will probably draw more current at that voltage than at the higher voltage; it may work at up to 14-15v, since those are common high-limit voltages for "automotive 12v" systems, but you'd need to check it's manual or manufacturer to be sure--13.6v is a good safe voltage, however).
A 17Ah 52v battery would have about 800Wh of capacity (17 x 52, rounded down). Since the converter won't be 100% efficient (probably 75%, perhaps better), then you will likely have about 600wh actually available to run the system (.75 x 800). If it takes 750w to run the system, you *might* get just under an hour of operation, (600wh / 750w = 0.8 x 60 minutes = 48 minutes), before your battery is emptied.
If the battery doesnt' have a built-in shutoff, you should add one so it does not overdischarge and damage the cells.
endlessWork said:
It is good for my fishfinder, or so I hope.
I have a big kayak and a 12V 750W - 598 Fishfinder SI that is a power hungry monster that would chew through my 12V 9aH batteries
Those batteries are gone but I now have this 52V ish 17Ah battery. I want to dual use between the ebike and my kayak (really more of a boat in kayak marketing.
I figure someone else here has done something similar down converting from 52V.
Can you tell me how it went and what I can expect?