Advice please on balancing LiPos

[Edgeflyer]

I have 2 Lipo chargers and a variety of LiPos, some pretty dated but seem to charge OK and are not puffed. One charger has a balance socket, the other no ne. The charger with balance has separate programs for charging or balance charging. What is the difference? I have some idea that the balance plug gives the direct voltage at each cell. Presumably the charger is supposed to top up each cell to the same level I. E. 4.2 volts. However the charge current, maybe 2 amps or more, is supplied through the main power connector which connects to the set of cells wired in series. How can the charger separate the current at each cell? The balance plug must allow current to leak out of the highest voltage cell until the lower one's catch up! A bit too clever, but at least if one cell has become too low in voltage, say around 3.5 volts, the charger can then generate a fault code and stop. This implies my charger with no balance socket is not safe, unless I use a separate checker to ensure each cell is above 3.7 volts before starting. Can any experts throw more light on these mysteries?

[Edgeflyer]

I know folks will be horrified, but when I had a lipo that had fallen below 3.5 volts per cell, at which the charger refused, I used the NiCd program, with the lipo outside and watched carefully, to charge the battery to above 3.7 v per cell, then Switched back to the safe lipo program. This lipo has performed properly ever since. I Am NOT ADVISING this procedure, but it shows that the below volts state is not always fatal. I know others who charge puffed lipos, in a fire proof bag, and seem to get away with this. I ditch any puffed cells as this seems to indicate they have been drained at too high a current. I wonder what the gas released is?? Could it be poisonous and or flammable? Hydrogen cyanide springs to mind, the nasty stuff released from burning polyurethane insulation and other polymers.

[Frank Skilbeck]

Re balancing, when you are in balancing mode the charger diverts a small amount of the current around the higher voltage cells via the balance cable, so they charge up slower than the lower voltage cells, typically this is around 100 ma, which is why it can take a long time.

[Peter Christy]

A normal charge will simply monitor the cell voltages and cut off if one or more cells are outside the tolerance range set in the charger (assuming a fairly modern charger).

A balance charge will allow the charger to partially by-pass high voltage cells by siphoning off some of the current via the balance plug. So, although the charger may be passing (say) 2A through the pack, on individual cells, some of this current (typically up to 0.5 amp, but it depends on the charger) may be drawn away from the cell via the balance lead.

Think of it as putting a lowish value resistor in parallel with the high voltage cell. This way the cells either side still get the correct current, but the high voltage cell gets less.

You can't draw *all* the current away, because the balance lead isn't man enough for the job, which is why balance current is typically limited to around 0.5A.

(Frank beat me to it!  )

--

Pete

 

Edited By Peter Christy on 30/01/2019 09:06:57

[Dave Hess]

The common iMax type chargers balance the cells by charging the pack to a fixed voltage then the charge is switched off and bleed resistors are opened to drain down the higher voltage ones, then the charging is switched back on to bring the pack voltage back up again. That process is repeated until all cell voltages are close enough to each other. There is a bleed resistor on each cell channel. The voltage difference on each channel is read by a microprocessor. The microprocessor can open each bleed resistor separately via transistors on each channel.

There are other chargers that manage and charge each channel separately. In effect, you have X number of single cell chargers in one device. They charge through the balance leads and dont use or need the main power leads.They have the advantage of quicker and more reliable balancing, but have the downside that you can't see if you have a weak or damaged cell unless you buy one of the expensive ones that show how much charge has gone into each cell.

[Josip Vrandecic -Mes]

Hi Philip ,maybe this also helps : **LINK**

All the best

Jo

Edited By Josip Vrandecic -Mes on 30/01/2019 09:34:02

[Nigel R]

"below volts state is not always fatal"

"Below volts", as in properly 0% charge, is 3.0V per cell for normal lithium chemistries, as far as I know. It's not good at all for lifespan to let cells go that low, and below about 3.7V the voltage would fall off extremely quickly.

The 3.7V level represents an easily detectable "safe minimum".

As to balance charge...

Most "charge" cycles consider only the pack total voltage. Not that great a method by itself, as a weak cell will not properly be brought up in line with the stronger cells in a pack.

A "balance" charge will (generally) tack a second phase on after the previous "charge" cycle, on where the cell voltages are slowly equalised by adding charge to the weak cells and draining charge from the strong cells.

Balance charging will improve both pack performance and lifespan.

As for charging using the nicad program, lipos can be charged from almost any DC power source you like, providing the cell voltages are monitored closely and none are allowed to drift over the fully charged figure of 4.2V. The nicad program will not self-terminate properly on lipo, but I guess you knew that already.

That said, I wouldn't recommend using anything but the proper lipo program - unless you are very happy you know exactly what you are doing.

"I ditch any puffed cells as this seems to indicate they have been drained at too high a current."

Any source evidence for that?

As far as I know, a puffed cell only indicates that the cell is puffed.

Ditch them when they lose capacity and current capability. i.e. when they give weak performance on the motor.

[Nigel R]

"then the charge is switched off and bleed resistors are opened to drain down the higher voltage ones, then the charging is switched back on to bring the pack voltage back up again."

Just minor correction Dave, process is, 'standard charge' performed as you say, then

pause a bit, 5 seconds (ish)

then read cell volts

then switch in bleed resistors over "high cells", whilst:

(a) if high cell is lower than target, applying balance current

or (b) if high cell is above target, do not apply balance current

wait 15 or 20 seconds

repeat

and like you say, once the cells are all close enough, charging ends

I don't believe any imax clones attempt to balance during the standard charge phase at the start of the process.

[Edgeflyer]

Many thanks to all who have contributed excellent advice.

[Edgeflyer]

On the subject of lithium cells gassing, no I don't have direct evidence of this being due to high drain rates, but some suggest overheating, which could be due to overly high current drain, produces toxic gases. Here is a link in the context of lithium ion cells. (how do you put a link in a post?)
https://eandt.theiet.org/content/articles/2016/10/lithium-ion-batteries-found-to-produce-toxic-gases/

[Nigel R]

nice one, thank you

links, edit window, the second row of icons, it is the sixth from left, highlight some text, click the icon, and you can add the web address in the popup window that appears.

[PatMc]

Posted by Philip Baker 3 on 30/01/2019 17:21:45:
(how do you put a link in a post?)
https://eandt.theiet.org/content/articles/2016/10/lithium-ion-batteries-found-to-produce-toxic-gases/

Try this :

Copy URL ...

Left click link icon ...

Paste URL in dialogue box then press OK...

 

Voila ... **LINK**

 

 

Edited By PatMc on 30/01/2019 18:08:15