Battery is good for something else!

Joined
Jul 4, 2022
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15
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?
 
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?
 
However much 12V current you need, get a DC-DC converter that's rated for at least twice that many amps. Those things are notoriously overrated.

What's your plan to keep the battery and the converter dry? If it were me, I'd use something dependably waterproof like a Pelican case with a sealed bulkhead connector added for 12V out. I'd also add a waterproof master switch on the 52V side so the thing can be turned on and off without opening the case.
 
Chalo said:
What's your plan to keep the battery and the converter dry? If it were me, I'd use something dependably waterproof like a Pelican case with a sealed bulkhead connector added for 12V out.
I could be wrong, but unless the heatsink for the converter, and any fan airflow vents it may have, are outside the case, a converter for this much current could overheat. (in my experience, would certainly do so; at the least it would heat up the battery excessively if they are in the same case).
 
amberwolf said:
Chalo said:
What's your plan to keep the battery and the converter dry? If it were me, I'd use something dependably waterproof like a Pelican case with a sealed bulkhead connector added for 12V out.
I could be wrong, but unless the heatsink for the converter, and any fan airflow vents it may have, are outside the case, a converter for this much current could overheat. (in my experience, would certainly do so; at the least it would heat up the battery excessively if they are in the same case).

I looked up the model name the OP provided and found the manual.

It says it consumes 500 mA at 10-20 volts. But it also says its output is 500W RMS or 4000W peak to peak. So there's definitely some horse manure being peddled there. I guess most fish molesters are unlikely to do the math.

However, even 10W of waste heat (500 mA times 20V) could cause problematic heat buildup in some circumstances.

I have considered cutting a window into a waterproof case, cementing an aluminum plate to the inside with silicone, and then thermal taping the heat sensitive components onto the plate. But so far I've not had to exercise that option.

Screenshot_20220704-152709.png
 
Chalo said:
It says it consumes 500 mA at 10-20 volts. But it also says its output is 500W RMS or 4000W peak to peak. So there's definitely some horse manure being peddled there.
That is definitely some marketing mumbling in there. :/

If the OP can use a wattmeter on the system, they can determine the actual power usage, which will help them find an appropriate DC-DC that will do the job for the minimum cost.

If it is really only half an amp, less than 10W at 13.6v, then a much cheaper and smaller converter can be used, and the battery will last a much longer operational time for each charge.

To determine how long, the actual current used by the system would be needed.
 
Why even step it down? It's only a motor. You most likely can get away with a PWM controller. Much simpler, cheaper, and close to 100% efficient.
 
Comrade said:
Why even step it down? It's only a motor. You most likely can get away with a PWM controller. Much simpler, cheaper, and close to 100% efficient.

It's not a motor. It's a sonar unit.
 
Thanks so much.
I was in a kayak and ran it on 12V lithium battery, they went about 7 hours before I had to turn them off.

Yes, there is not way it was pulling 62A :shock: thank you for the math reality check. 500mA sounds right especially given the wiring I had on it.

If it makes anyone feel better I used it for mapping the bottom of rivers and lakes in the search for lost children's toys. The older I get the more guilty I feel about it.

So assuming the down conversion is so great what am I looking at when it comes to a DC/DC converter?
 
Chalo said:
It's not a motor. It's a sonar unit.

"I want to dual use between the ebike and my kayak" and mention of 62A @ 12V sure made it confusing :lol:
 
I am using this DC-DC converter, just to knock down 36V to 12V. I believe it will take up to 60V DC input. Claims 300W output, but I was using it to power a small AC inverter.

Claims to be waterproof. I killed a battery on it though. The BMS on the 48V pack was acting up, so I bypassed it and what do you know, it ran the pack down to 20V.

https://www.amazon.com/gp/product/B07ML5ZML2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
 
endlessWork said:
If it makes anyone feel better I used it for mapping the bottom of rivers and lakes in the search for lost children's toys. The older I get the more guilty I feel about it.
You might find a few old bicycles as well :D
 
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