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Help wanted for 12v (3s) pack to 9.6 and 7.2v
#11
(07-12-2020, 04:49 AM)Wayne\s World Wrote: [...] 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.

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

Unless one has sufficient expertise and experience to reliably avoid such, then one should always incorporate adequate protection in Li-ion packs (and even experts have oversights so they should too should use a BMS, esp. if the device puts others at risk).
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#12
Apologies - i missed the updates on this.



(07-12-2020, 04:49 AM)Wayne\s World Wrote:
(07-06-2020, 08:53 AM)unclebob Wrote: 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-Adjust...2749.l2649
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?

(07-06-2020, 08:53 AM)unclebob Wrote: Can anyone provide me with a link to something else that can drop the 12v from bms to 9.6v and 7.2v
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 Tongue

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.


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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