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18650 Li-ion batteries - tech question/s

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When I bought my Kayak, I found, it, just a little low in the water for my 112Kgs. So in an attempt to raise the waterline I'm stripping weight from my setup.

I need to know about using Li-ion batteries from power tools etc, to power fish finders.

So I found a lot of these recycled 18650 Li-ion batteries on the interweb, that are the recycled good ones, from dead, power tool battery packs, tested and working and - wait for it - cheap!

I've previously repacked two laptop battery packs, with these and they work just fine.

So I got some more, to solder together, to make a pack, for my fish finder. Using a weighing machine I found that I'd save about 2.5Kg if I used these, and not my current (geddit?) lead acid battery

I'd completed the soldering and then I put the battery charger on the pack and the needle shot off to the right, > 7 Amps, a dramatic orange glow started to come from the inside of the charger. I quickly disconnected everything to prevent fire, danger, or a fission event.

I am hesitant to plug in my expensive fish finder to see if it works.

Then I got to thinking.  

When I repacked the laptop battery packs, there was a little circuit board in them, that I assumed, did a bunch of things including, perhaps - provide some resistance in the circuit.

My 12 V battery charger is one of the old ones - transformer, plug, clips, needle/meter, switch, nothing in the way of electronics. Do I need to provide some resistance in the circuit with these 18650 cells ?

It may be that my fish finder will have the necessaries to do this, if I connect it straight to the battery pack. 

But then how to recharge? I am hoping that someone here will be able to give me advice on this.

Some specs ::

18650 Cell Features and Technical Specifications 
Nominal Voltage: 3.6V. 
Nominal Capacity: 2,850 mAh. 
Minimum Discharge Voltage: 3V. 
Maximum Discharge current: 1C.

Garmin Striker plus 7sv specifications 
Transmission power: 500W (RMS) 
Voltage range: 10 V to 28 V input voltage. 

Help please !
See diag. below for the arrangement I made.


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Belay that, I'm an idiot.

I put on two pairs of close-up reading glasses and re-examined my soldering work.

Underneath one of the connecting wires, on the +ive terminal of one of the cells, a stray piece of solder had melted through the cell's plastic covering and allowed the +ive terminal to be connected to the -ive metal casing of the cell, i.e. a short circuit.

After I remedied that, everything works as desired. Shit hot !

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Well, I ran the fishyfinder on Simulator all day, connected to the Li-ion pack.

It started out at 16.8 V and after 10 hours it got down to 14.4 V

This has exceeded my expectations. It maybe a different story when the transducer is connected and it's pumping out watts, but I reckon it should go for a while, and I'm not likely to be running it for more than about 6 hours on the water.


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The thing transmits at 500 watts? 😳

How does this thing work?

If it runs on 12 volts, and puts out 500 watts and is 100% efficient (it's not!) that's about 41A !!! eek !!!

I'm guessing this is some sort of pulse device so fairly minimal average power.





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Yes,  Sonar pulses.

Sonar Features & Specifications


Traditional: 50/77/200 kHz

CHIRP (mid and high)

ClearVü and SideVü 260/455/800 kHz



If you're really interested, heres the Garmin support/Sales video




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