voltage divider on lipo?

jimmyhackers

10 kW
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May 11, 2015
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600
just wondering if a simple two resistor voltage divider would work on a lipo?

or would it just burn the resistors up?

ive put a 4s lipo in my icycbe robot dog and its fine, except two cells run down before the other two are close to empty.

this is because it takes a 4s....but uses only 2s for logic and the full 4s for the motors. its hardwired to each battery terminal for each voltage so three power terminals out the batt and into my dog (0v, 8v, 16v).

it all works fine untill the logic and motors go way out of voltage ranges and he starts spazzzing before shutting down.

i was hoping to counteract the cell imbalancing with a simple voltage divider...and am just double checking if its plausible.
 
markz said:
You would need some very big wattage resistors depending on the current divide.

I have some 3000w wire wound red hot 500mOh ones.

They are like 6 x 2 inches. Lol.

You would waste quite a bit of power, I would presume.
 
That probably won't work as the resistors would need to waste a ton of power and get really hot.

A small dc-dc converter that drops the full pack voltage to half, or whatever is needed, might be a better solution.
 
or would it just burn the resistors up?

IF:

They look like this, probably, yes they will burn.

1wres.JPG

10wres.JPG


IF they look like this, they will, probably, NOT burn, but you will be heating the room up all cozy like.

100wres.JPG
 
i kind of figured id need a proper dc-dc convertor. isnt much space in the dog batt compartment hence me asking bout a simple resitor divider.

even the original nicad batts were wired in such a way that one half of the pack would run down before the other.

i guess i can run the main 16v off the battery as normal....and use a dc-dc convertor for the logic parts 8v.

i guess it would be good to have/add a lvc circuit as the dogs originally set up for a 12v battery (10x1.2v)

hopefully it doesnt mess the dog up.
 
Resistors will not work, and I haven't seen a good DC-DC converter that would fit this application.

But since 4S is trivial to balance charge, the best solution is to collect data on relative power consumption among the first two cells (2S) and full (4S) pack, and then rebuild the pack with higher capacity cells on the 2S section.
So if the first two cells are empty but the last two have 50% capacity left, then rebuild the pack with the first two cells of 50% greater capacity, or 2p the first two cells in series.

Balance charge with a node charger.
 
Use two packs.

Typical RC Aircraft setup ( if you have no BEC) is a main power pack and an RX pack. My med heli 550e had a RX pack before I got a better speed control with a BEC.... I ran 1 (6s) 22.2v 5000mAh for the power system, and one (2s) 7.4v RX pack of 2250mAh for the radio and servo/ flight controller power. ....

This is so your plane or heli does not fall from the sky when the power system fails, or you are 500 feet up and the energy runs out... . You still have the 7.4v batt ( rx batt) powering the electronics.

You can glide the heli or airplane in. If you have control surface electronics ( servos) working. You dont need power to glide. I can land un-powered, controlled, but cannot land un-powered without control. Absolute crash.

Or just use a BEC for this, and it will pull power evenly from the main pack voltage.

BEC is designed for this. They come in 2-12s input ( 7.4-44v) and output 5A around 5v, 6v, or 7.4v.. the good ones youset the output voltage for your servos and control electronics. Runs off main pack.

A BEC is designed for this voltage range.

So use two packs. or a BEC ( Battery Eliminator Circuit).

A good BEC is 30$.
Here is a 6s max input Castle Creations..... that kicks out 10A in the voltages you need. 22$.


https://www.ebay.com/itm/310737894645?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=310737894645&targetid=1264870804704&device=c&mktype=pla&googleloc=9007417&poi=&campaignid=11612432550&mkgroupid=122767864272&rlsatarget=aud-762207186714:pla-1264870804704&abcId=9300456&merchantid=8123504&gclid=Cj0KCQjw8vqGBhC_ARIsADMSd1AQ21g8Gv9PD6M3muCuRDDvfMvfYbFVKKj1_QCKYSb65dwvEpy46nMaAp9lEALw_wcB
 
fatty said:
So if the first two cells are empty but the last two have 50% capacity left, then rebuild the pack with the first two cells of 50% greater capacity, or 2p the first two cells in series.

I would not intentionally build a pack with mismatched capacities.
 
Any idea how much current is used on the 8v line? They make some really small dc-dc converters. A linear voltage regulator would also work if the current is low enough.
 
DogDipstick said:
ot intentionally build a pack with mismatched capacities.

Why not? Mismatched power consumption + mismatched capacity = matched pack
 
ive had minimal/no success.....

i originally tried my linear voltage regulator on the 4s to make the 8v output....but on powerup for some reason the LVR output would switch to main battery voltage (16v) momentarily.

during this startup.... the dog would stop and wouldnt run properly, this eventually made him play a crying noise ive never heard before (sounds more like a chimpanzee)

so....next step is try a 2s for 8volts-ish for the logic part ....with the LVR outputting the 16v for the main power.

the LVR cutouts out at (7v in) so hopefully this will kill two birds with one stone as its a reasonable LVC for a lipo.
 
used one of those xl6009 modules...some succes

worked a treat, dog booted up, moved, worked for all of ten seconds.....then the xl6009 module pooped itself when the dog did a big moove.

max output from it now is 1.7V :D

i have two of the modues left so i could try doubling them up.....
other than that i cant find a small/good enough LVR for him :(
 
SUCCESS!!!

I ended u finding this on ebay, https://www.ebay.co.uk/itm/273944949695?var=573861889072

works a treat...does get rather toasty though, no lvc to speak of either.....

dogs happy, im happy. all good.
 
is rather confined for space to add anythign larger like a bigger heatsink or a fan.

the battery compartment is enclosed, with no ventilation, which doesnt help....

i might drill a a few holes.
 
Just adding more surface area of aluminum connected to the existing one will help. You can place it along the inside of the enclosure.
You can also try to thermally couple the existing heat sink to the housing. This way the heat can dissipate to the outside and still be water resistant. I've used silicone glue before with good results.
 
its an i-cybie....so plastic, not a lot of thermal conductivity.

the battery compartment is enclosed.....so a bigger heatsink ...... in the same enclosure will be negligable. and tbh if it works it works.

im starting to think i should just of added 1s bms protection boards to each cell of a 4s. battery would still come off unbalanced.....but not overdischarged.
the heating would be a non-isssue. plus id technically double the runtime of the dog.
 
jimmyhackers said:
technically double the runtime of the dog.

Just use a BEC for Christs sake. It is built for this.
 
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