Balance Charges

[mgjackson]

Does anyone know how these really work?

The reason that I ask is I have a pack where the balance charger leads have broken. I can dismantle the pack and re-solder them, but before I do, can anyone tell me if they are really needed.

I can see how the charger can monitor the voltage of each cell, and if there's a problem with a cell not charging it can detect that, but I don't understand how the lead actually 'balances' the battery. Afterall, the cells are hardwired together in series and the charger can only put a current through them; It can't not charge the one in the middle if that one is already fully charged.....in that circumstance I would expect the fully charged cell to discharge into the other two.

Thanks,
Matthew

[Holst]

I think the way it works is that it adds a resistor over the cell that is at a higher voltage to reduce the charge to that particular cell. So the undercharged cells will still charge and equalise.

The ballance leads connect to the ends of each cell, you need to get them in the right order or you might damage your charger.

Its quite important to make sure the cells are ballanced when using Lipos as you can easily damage them by overcharging.

[SRH990]

The majority of balance chargers charge through the main lead. The balance lead connects to each individual cell in turn and balances by discharging the high er voltage cells through a resistance, dumping some of the charge current, in this was it keeps the cells balanced but if cells are badly out of charge most chargers reduce the charge rate to a very low level to give the low cells time to catch up.

The Cellpro chargers actually charge most packs through the balancing lead, actually charging each cell individually maintaining perfect balance, this is generally considered to be the preferred option but most chargers which monitor individual cell voltages during charging should do a pretty good job.

[sacko]

I hope that pack isnt the one you bought down the indoor club

As it was well past it.

I have Re-soldered them before but the packs have had a fair bit of use. Zippy packs are £17 delivered, so fail to see the point of reapiring.

[EtheAv8r]

Yes - balance charging of your LiPo batteries is very important, and balance chargers do "effectively" put more or less charge into individual cells to keep them in balance .

They do it in different ways, some by charging though the balance leads themselves (there are current limitations here because balance leads are thin) so acting like a series of mini chargers one for each cell, and some by bleeding off individual cells with a discharge load to keep them in balance. Better quality chargers hove closer tollerences and achieve better accuracy.

[snapshot]

The optimum voltage per cell of a lipo is 4.2V, a balance charger ensures that no individual cell goes beyond that voltage. LiPo's won't stop charging when they reach 4.2V and leach off power to the other cells the way nimh or nicad would.

If you don't balance and expect an end voltage of say 8.4V for a 2 cell pack, 1 cell could be at 4V and the other at 4.4V! This would cause pack damage and possibly pack swelling and the risk of explosion!

The other side is if you stop charging when 1 cell reaches 4.2V other cells could be less, this is obviously unbalanced too. As lipos generally discharge at the same rate, you could end up with cells going below their minimum voltage and then become irrecoverable during a charge cycle.

[mgjackson]

I hope that pack isnt the one you bought down the indoor club

As it was well past it.

I have Re-soldered them before but the packs have had a fair bit of use. Zippy packs are £17 delivered, so fail to see the point of reapiring.

Actually, I'm not so sure. I'm running the battery for 8 minutes and its needs about 1600mha to re-charge. I flown the heli about 10 times this weekend and its been fine everytime.

Something was playing up, but it may have just been the pitch/throttle settings or perhaps the main gear was binding

[mgjackson]

The majority of balance chargers charge through the main lead. The balance lead connects to each individual cell in turn and balances by discharging the high er voltage cells through a resistance, dumping some of the charge current, in this was it keeps the cells balanced but if cells are badly out of charge most chargers reduce the charge rate to a very low level to give the low cells time to catch up.

Thats what I would have thought, but if you put a voltmeter on the negative and the first pin of the balance conector, you get 3.5v or whatever that cell is. If you do the same to the second you get 7v (ie cell1 + cell2). So if you were to connect a resister to the second cell then you're be discharging cells 1 & 2.

(Must point out I'm interested in how this works, not that I'm particually bothered about a battery pack with a broken lead :-))

[sacko]

Ah, that doesnt sound too bad. I take it that it is a 2200mah pack?

Did you have a look through the Finless Bob videos, your pitch still needs some tweakng but unfortuantely we ran out of time.

[DIJ]

Must point out I'm interested in how this works, not that I'm particually bothered about a battery pack with a broken lead

Take a look at this thread, it gives a reasonable explanation

Voltages for LiPo cells / balancing / connections - WattFlyer RC Electric Flight Forums - Discuss radio control eflight

[snapshot]

Thats what I would have thought, but if you put a voltmeter on the negative and the first pin of the balance conector, you get 3.5v or whatever that cell is. If you do the same to the second you get 7v (ie cell1 + cell2). So if you were to connect a resister to the second cell then you're be discharging cells 1 & 2.

(Must point out I'm interested in how this works, not that I'm particually bothered about a battery pack with a broken lead :-))

You do it in pairs, so pins 1 & 2, 2 & 3, 3 & 4 i.e. pin 1 negative and pin 2 positive, so for the next cell pin 2 is negative and pin 3 is positive.

So reducing a cells charge would be between pins 2&3 for cell 2 etc.

EDIT Just added this image, it may help to explain.

[mgjackson]

Ah, that doesnt sound too bad. I take it that it is a 2200mah pack?

Did you have a look through the Finless Bob videos, your pitch still needs some tweakng but unfortuantely we ran out of time.

Yes, so 1800mah is the max. With my current gentle flying I'm burning 200 mah a minute, so I could squeeze another minute out of it, but I think I'll keep the congtingency for when I throw it into the sky.

Sussed the pitch problem - it was in the manual and I'd missed it...if the servo horns are perfectly lined up with the some little but hitherhto unnoticed oval holes in the fusalage then the specified linkage measurements all work and I can get the -11 to +11 movment. But thanks with your help setting it up anyway :-)

[sacko]

Top job.

With 1800mah packs, you shouldnt be taking anymore than 1440mah out of the pack.

[mgjackson]

Its a 2200, so 80% of that is 1800

[mgjackson]

>Take a look at this thread, it gives a reasonable explanation

Yes, thank you, thats useful, although the comment about batterys going out of balance every 100 charges is interesting.