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Help with simple 18650 Li-ion project

Tripp_22

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I'm trying to make make kids toy that normally takes 3 AAA batteries run off of a rechargeable 18650 Li-ion battery.

This is the toy:
RC Bumper Cars

1659776069171.png


I purchased this board from amazon:
MakerFocus 6pcs 2A 5V Charge Discharge

1659776081413.png


As well as these micro USB breakouts in case I need them to charge the battery:

The cars run on around 4.2V-4.8V. I would like them to be ran on a steady voltage of around 4.7V as that is when they have the best performance. I also need to make sure the battery doesn't get ran below its minimum safe voltage (probably around 3.6v or so).
I tried the black charge/discharge/boost board from the amazon link above but it seems to have some sort of protection circuit that triggers as soon as I connect it to the rc car. I have one of the same motors that the car uses laying around the house so I tried that as well and the board turns off as soon as I connect the motor. The board works fine if I hook it to a simple load like 8 x 3.3v LEDs.
Here is a link to what the motor is like:

I'm guessing that it is trying to pull to much current from the board but I don't really know for sure. The board says it can handle 2A so that seems strange The car pulls about 200-300ma when operating normally but it must have an amp spike or something when the motor first starts turning because the car just stutters when I hook up my power supply to it with 400ma limit. It starts running properly around a 800ma limit on the power supply (4.7V), but I only ever see 200-300ma on the readout.
So my question is, can someone offer me some better equipment examples to get this thing converted to run on an 18650 battery? Or tell me what I'm doing wrong with the stuff I already purchased.
My son and I will greatly appreciate your help as we really love playing with these things but they go through batteries too fast.
Thanks.
 

d123

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Hi,

Suggestions to maybe give a go:
1) If there isn't one already, put an 100nF to 1uF non-polarized capacitor (anything but tantalum capacitors or electrolytic capacitors - 'though there are non-polarized 1uF electrolytic capacaitors) across the motor terminals.
2) Place a diode like a 1 amp Schottky diode (ideally, 200mV to 300mV voltage drop, instead of 700mV of a silicon diode) from the output to the motor' s positive terminal, anode connected to output, cathode to motor positive.
If motor is bidirectional, must be, the diode option won't work as the motor will be driven by an h-bridge.
3) A small value resistor in series with a capacitor to ground to lessen turn-on spike. Resistor from output to positive motor terminal, capacitor from junction of resistor and motor positive terminal and other end of the capacitor connected to the ground terminal.
Utterly guessing blindly, 10 Ohms (7 Watts minimum, boohoo, but 300mA would be 0.9W for the resistor, so even 2W might be enough if datasheet shows it can handle brief turn-on spike of 800mA) resistor and 10 uF capacitor would more or less make voltage into motor take 0.0005 seconds to go from 0V to just short of 5V, and limit current into motor to 500mA. More-or-less maths, but should do for starters.

No guarantees any or a combination of 1 and 3 would work, only possible suggestions given you already have the boost board.
 

Tripp_22

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Thank you @d123 !
I was dead in the water and now I have a few things I can try that sound promising. I have another similar motor sitting around that has one of those non-polarized capacitor already attached that I can steal.
I will try the resistor and capacitor in series as well to, but I will need to buy some capacitors on amazon so it'll take some time.
I will give an update after trying a few things out.
Thanks again
 

d123

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Hi,

I hope you can get it all to work like you want it to, and then have lots more fun racing them with your son.

My suggestions are just top-of-the-head, rule-of-thumb style thoughts based on the little experience I have of little motors, I really couldn't know if they will solve the problem, please be aware of that. The 10R might need to be smaller to accommodate 0.8A start-up current, 4.8V/0.8A = 6R 1.2W, so 2W. But only a spike so, 4.8V/10R = 0.48A 2.3W, so 3W. 4.8V/0.3A = 16R 1.44W so 2W. As 0.8A is brief, a 2W is safe margin, and even a 1W resistor for all above currents should cope if motor on only in brief spurts, if on for e.g. >0.1s, use 2W or 3W, whatever the maths tells you, W = (A x A) x R.

Maybe before buying components that may not solve problem, if you dare, or know how to do it, taking a car apart and taking a photo of the internals and any pcb it has would/could be useful info. Bitter and/or highly stressful experience tells me taking something apart can be far easier than the sometimes nightmare of trying to put finickety devices back together. Even a computer mouse can be a small challenge, so a toy car may be harder to fit together again, or not - it might be easier, who knows.

Good luck!
 
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Audioguru

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The circuit board is made for use as a power bank. It charges the single Lithium cell at a high current (reducing the battery lifetime) then it boosts the output voltage to 5V and maybe regulates the voltage.

The circuit board specs say it has battery "drain protection" which might not be a disconnect when battery voltage is low. Then the battery is ruined if the voltage gets too low.
An electric motor has a very high stalled current when it starts running or when it is stalled. Since the "power bank circuit" boosts the voltage to 5V then the maximum motor current is much higher with 5V instead of with the lower voltage from 3 AAA cells.
 

d123

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Hi,

Thinking of the 200 - 300mA you mentioned in first post, fun maths:

New AAA = ~1.65V
1.65V x 3 = 4.95V
4.95V/0.3A = 16.5R

Used AAA = e.g. ~1.1V
1.1V x 3 = 3.3V
3.3V/0.2A = 16.5R

Must be motor coil resistance.
AAA batteries don't have a static resistance, they have an output impedance that changes, it increases ('a lot', as far as I remember, on datasheet graphs) as it gets drained.

Yes, agree with above, do check Li-ion will cut out safely before discharging too far. Maybe it is one with an incorporated protection circuit?
 

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There is a big difference in the maximum current from a cheap AAA carbon-zinc Super Heavy Duty battery from China and an alkaline Name Brand battery. The protection circuit for the lithium battery might limit the max current and not allow the motor to start running.

Measure the resistance of the motor and subtract the resistance of the meter leads. Then calculate the maximum turn-on current.
 

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