Help wanted for 12v (3s) pack to 9.6 and 7.2v

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unclebob

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Jul 11, 2018
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Hi All,

I'm slowly gathering 18650's here in the uk. while testing them, the ones that are not up to scratch are to be used in other projects. For example my daughters remote control car.

At present it uses a 9.6V ni-cd 1000mAh pack which is dead in 4-6 minutes at present.

So i want to replace it with a 3s 18650 pack - i have some high c dyson cells which i will be using.

I have the 3s bms which outputs 10a, and with charged cells i have an output of 12.4v as expected.

Question i have is how do i drop from from 12v to 9.6v and 7.2v? I tried the following buck converters, but it seems to only output upto 4.65v. Do i need to go bigger in s to get higher voltage to be able to produce my 9.6 and 7.2v?

https://www.ebay.co.uk/itm/MP1584-A...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

Can anyone provide me with a link to something else that can drop the 12v from bms to 9.6v and 7.2v?

PS - no idea why the car uses both 9.6v and 7.2 - the battery pack it came with is wired with three wires - one is 0v, then one is 7.2v and the other is 9.6v as does the board on the receiving side in the car. maybe its used for the signal side/lights etc, while the 9.6v is used for motors?
 
So typical 18650 cells voltage operating range is 3.5 to 4.2 V = (3S) 10.5 - 12.6V
The right buck (step down) converter should be able to drop that to 9.6V & have another one doing 7.2V
Depending on how much current the motors draw, you may need a bit more current than 3A like this one:
https://www.ebay.co.uk/itm/XL4015-5A-DC-DC-Voltage-Step-Down-Buck-Converter-4-38v-input/263168983882

But you might also look at 1S/3P (one cell) & step up with one like this:
https://www.ebay.co.uk/itm/DC-DC-St...ter-3-5V-30v-to-5v-6v-9v-12v-24v/263325286995
 
The buck converter you linked is a 3A model, and that is probably a generous rating that won't be sustainable without additional cooling.

A motor's startup current draw can be quite high ... it might be drawing down the current so much that the circuit is unable to power itself.
 
khauser said:
The buck converter you linked is a 3A model, and that is probably a generous rating that won't be sustainable without additional cooling.

A motor's startup current draw can be quite high ... it might be drawing down the current so much that the circuit is unable to power itself.
Click to expand...

Hi Khauser,
at present, neither are even connected to anything as load, just the inputs, and i soldered leads onto the outputs. I was adjusting the voltage before i soldered to the board but thats where i noticed my issue.
 
Hmmm, not sure then. You should not need a different buck converter ... the one you list should be able to do it.
 
Likely you don't need to buck the 3S Li-ion pack to replace a 8S 9.6V NiCd pack because they can go up to 8x1.5V = 12V so probably can handle 12.6V. Just be sure that the cells are healthy enough to provide the needed (burst) current (and the BMS will allow it).
 
Some buck converters may need a small load eg 220 ohm resistor to behave normally.
 
Thanks for the responses, Im going to try and disconnect one, so leaving it as one, and see if it makes a difference, and also will try a resistor to see if it needs a load, i will also try with a 12v lamp from the car and see if that works.

gausee163, thanks for the response - i may get away with the 9.6, but will still need something for the 7.2v so same applies. In terms of the cells, i have a set of dyson cells which i need to take apart - have about 8 packs so hopefully should get atleast one set of three which can handle the burst load, but just testing the wiring side with standard sony G5's to set voltages.
 
unclebob said:
Thanks for the responses, Im going to try and disconnect one, so leaving it as one, and see if it makes a difference, and also will try a resistor to see if it needs a load, i will also try with a 12v lamp from the car and see if that works.

gausee163, thanks for the response - i may get away with the 9.6, but will still need something for the 7.2v so same applies. In terms of the cells, i have a set of dyson cells which i need to take apart - have about 8 packs so hopefully should get atleast one set of three which can handle the burst load, but just testing the wiring side with standard sony G5's to set voltages.
Click to expand...

I use a UBEC on these toys. They're usually not for running the entire toy but since they're not as high drain as some of the hobbyist ones they work fine. Gets hot but by the time they're done my lipos are already drained. Never used 18650s since I have lipos.

https://fpvmodel.com/products/henge-5v-6v-7-4v-8a-ubec-7v-25-5v-input-2-6-lipo_g1083-html
 
unclebob said:
Question i have is how do i drop from from 12v to 9.6v and 7.2v? I tried the following buck converters, but it seems to only output upto 4.65v. Do i need to go bigger in s to get higher voltage to be able to produce my 9.6 and 7.2v?

https://www.ebay.co.uk/itm/MP1584-A...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
Click to expand...
These buck converters have an adjustment to vary the voltage - you haven't mentioned it, but I'm assuming you know this and have adjusted it?

unclebob said:
Can anyone provide me with a link to something else that can drop the 12v from bms to 9.6v and 7.2v
Click to expand...
Another of the same buck converter from the battery (not the 9.6v o/p from the first BC) will offer the same adjustment to get you 7.2v, or even a suitable voltage regulator.

I agree that the motor will likely need more than the 3A that the BC can offer, at least for startup. See if you can find some info on the motor, online, or do some tests of your own with the existing pack.

Also, I reckon you could get away with a 2S pack without a BC which gives you 8.6v, even the raw 3S pack at 12.6v. These motors will probably handle the higher voltage and even give it an added bonus :p

For these small projects, a BMS is really not a necessity - just add a balance lead and rely on the charger to do the balancing.

Happy to be proven wrong here - just my thoughts on this kind of application.
 
Wayne\ said:
[...] For these small projects, a BMS is really not a necessity - just add a balance lead and rely on the charger to do the balancing. Happy to be proven wrong here - just my thoughts on this kind of application.
Click to expand...

Skipping the BMS could provequite dangerous, esp. as here when bucking a 3S to 7V, since it's possible that as the cells age their capacity gets so mismatchedthat thedevice runs the cells down to the point where one cell ends up being reverse charged. And,asstudies show, even a small reverse charge reliably leads to internal short circuits - which can be very dangerous, likelyleading to venting and fires/explosions.

Unless one has sufficient expertise and experience to reliably avoid such,then one should always incorporate adequateprotection in Li-ion packs(andeven experts have oversightsso they should too should use a BMS, esp. if the device puts others at risk).
 
Apologies - i missed the updates on this.



Wayne\ said:
unclebob said:
Question i have is how do i drop from from 12v to 9.6v and 7.2v? I tried the following buck converters, but it seems to only output upto 4.65v. Do i need to go bigger in s to get higher voltage to be able to produce my 9.6 and 7.2v?

https://www.ebay.co.uk/itm/MP1584-A...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
Click to expand...
These buck converters have an adjustment to vary the voltage - you haven't mentioned it, but I'm assuming you know this and have adjusted it?

unclebob said:
Can anyone provide me with a link to something else that can drop the 12v from bms to 9.6v and 7.2v
Click to expand...
Another of the same buck converter from the battery (not the 9.6v o/p from the first BC) will offer the same adjustment to get you 7.2v, or even a suitable voltage regulator.

I agree that the motor will likely need more than the 3A that the BC can offer, at least for startup. See if you can find some info on the motor, online, or do some tests of your own with the existing pack.

Also, I reckon you could get away with a 2S pack without a BC which gives you 8.6v, even the raw 3S pack at 12.6v. These motors will probably handle the higher voltage and even give it an added bonus :p

For these small projects, a BMS is really not a necessity - just add a balance lead and rely on the charger to do the balancing.

Happy to be proven wrong here - just my thoughts on this kind of application.
Click to expand...


Hi Wayne,
I've adjusted the buck converter - the max is 4 v that i can adjust to. I'll disconnect one and try again to see if there is an issue with the way i have paralleled them or if they are conflicting.

I'll open up the car again to see if i can get any info on the board and motors (there are two!)
The cell's arn't fixed so are charged on my MegaCell/Opus or TP boards... so the BMS is more for cutoff before the cells get too low etc.
 
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