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I am in the process of building a power bank with 18650 batteries for a trolling motor. The nominal voltage for this battery is 14.8 V. The motor specs are 12 VDC - 45 A max current draw.

Now the thing is that my battery pack will be 16.8V when fully charged.

  • Does that mean I will need a converter to lower the voltage output but my question is what happens with the load's current draw?
  • Does it affect the design of the converter?

So far I only found 10 - 15 A max for affordable buck converters.

  • Is it possible to build such thing?
  • Is it even worth it?
  • any other suggestions?
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  • \$\begingroup\$ As far as cost, there is a bit of a huge drawback to running high currents, so one thing to consider is whether it would be worthwhile to go to a 24V or other voltage motor. Given that some sort of controller is likely necessary regardless, bear in mind that your battery bank doesn't necessarily have to have the same voltage as your motor. Homemade series-parallel liithium ion packs can be VERY dangerous, so be advised that if you don't want to build an elaborate battery monitoring circuit, you may wish to go to larger factory made lithium ion battery packs. \$\endgroup\$ – K H Aug 13 '18 at 0:15
  • \$\begingroup\$ Your current motor takes (12V*45A=540W) and it's hopefully better than 80% efficient, so it should have about 432W output at least. 746W is one HP, so your motor is probably between 1/2 and 2hp. If it's built into the actual "Boat motor" I would investigate whether it's a standard type and whether switching it out would be cheaper than the parts list for giving the existing one 45A@12V. Definitely consider overspeccing your battery by a significant amount. Some 18650 cells are rated for 15A, but they will get extremely hot. \$\endgroup\$ – K H Aug 13 '18 at 0:25
  • \$\begingroup\$ Yikes, between 1/2 and 2/3 hp that is. Nowhere close to 2! \$\endgroup\$ – K H Aug 14 '18 at 4:10
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    \$\begingroup\$ I've been investigating this some more. It bears mentioning that most types of motor are not particularly sensitive to peak voltage, so it's likely your motor can take a peak voltage of 16.8V but at no more than (12V/16.8V) 71% duty cycle. This is a simple motor speed controller that can handle 50A. \$\endgroup\$ – K H Aug 14 '18 at 4:52
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I have built plenty of high power buck converters, and a single-seat airplane powered by a large bank of 18650 cells. If I were you, I would not bother with a buck converter for this application. Here's some justification:

  • It's best for cell cycle life to avoid charging all the way to 4.2 V/cell. Stopping at 4.05 V / cell only costs you about 10% usable capacity, approximately doubles the cycle life, and reduces the time it takes to charge. It also brings your open-circuit voltage down to 16.2 V.

  • Most 18650 cells have relatively high internal resistance compared to, e.g. LiPo packs for hobby radio control models. Even with 20 in parallel, there will be considerable voltage sag under load. If you happen to use the same cells as I use in my airplane, the effective internal resistance of your pack would be about 5 milliohms, and you'll probably have another 10 milliohms in wiring and connector resistance which bring the voltage of a "full" pack under a 45 A load down to 15.5 V.

  • Most "12V" rated motors are intended to be used with automotive alternator + lead-acid electrical systems which run at 14.4 V. So you're only exceeding it's design operating point by 1.1 V or 8%, and even that will only be for a brief period until the cells get down to the "plateau" region of the discharge curve. These things tend to be built pretty rugged; it'll be fine.

You didn't ask about this, but - how are you intending to switch the power to the motor? If by a relay/contactor or a heavy-duty switch, you may need to take some precautions to prevent the contacts welding.

If you're using an electronic speed controller, provided that is rated for the max battery open circuit voltage, you can run it at slightly reduced torque or RPM command and as far as the motor's concerned it will be as if it were running from a lower voltage battery.

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Does that mean I will need a converter to lower the voltage output but my question is what happens with the load's current draw?

Yes, unless you want your motor speed / dynamics defined by how full and warm our batteries are, you will need a controller of sorts.

Does it affect the design of the converter?

Well, yes.

Is it possible to build such thing?

Well, yes. I mean, people manufacture cars that draw a lot more power from their battery packs.

Is it even worth it?

45 A is a hefty current if you need to control it with semiconductors.

But, then again, that's about 500 W; not incredible little, but I could imagine it's somewhere in the region that Pedelec bicycles do.

I don't know what "worth" means to you.

any other suggestions?

All batteries are designed for different things. Most 18650 cells will seriously overheat and likely catch fire and explode if you drew 45 A from them! So you'll need to put multiple cells in series (to reach your 14.8 V), and then put multiple of those in parallel (to reach 45 A). That requires cell balancing, making your overall system even more complicated.

Also, remember, you're not shopping for the highest energy density per unit of money, but for the highest power density, very likely (depends on your use case, but yeah, at 500 W power, you won't be using a second market laptop battery).

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  • \$\begingroup\$ Thanks for your inputs Marcus,! The battery pack will be 4s20p. 80 brand new batteries 2000mA / 592 Wh. i still have to figure out the BMS im gonna use but its already considered as part of the device. I want to build the controller myself if that is possible and worth it. meaning that it is affordable and i can do it without drawning into compliacated designing techniques and math formulas, ect. \$\endgroup\$ – lehll Aug 16 '18 at 14:40
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You don't need to change the voltage as far as the motor is concerned. A DC motor doesn't care very much what voltage is on it - it will just go faster by 16.8/13.2 i.e. no significant difference.

Since trolling motors have several speeds, do you have any reason to do this electronically at all?

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  • \$\begingroup\$ That was part of the question. "Will this affect the motor itself?" Will it be damaged in the end? I assumed that i have to lower the voltage in order to not blow it up. \$\endgroup\$ – lehll Aug 16 '18 at 14:09

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