How to balance Ni-MH batts in a batt holder?

[tino_ale]

Hi guys,

I am planning to use 6 or 9AA batt holder from Fivemega in custom mods. The cells are wired in serie in the packs.

I do have a RC charger (Duratrax ICE) that can easily charge the whole "pack" at once.

But I don't know how to make sure the batts are equally charged in the holder. I don't have a multi-slots Ni-MH charger to make sure they are all fully charged. If I charge them one by one with my RC charger, the time it will take to charge them all, the first cell will have started to auto-discharge...

I'm not talking of everyday use... But I want to balance the pack at least the first time, and after every 5-6 recharges.

Can I use a normal charge followed by a trickle charge at the end? What current should I use then?

Also, should the pack be discharged down to 0.9Vper cell before recharging?

Thanks for your help!

 

[mzzj]

Hi guys,

I am planning to use 6 or 9AA batt holder from Fivemega in custom mods. The cells are wired in serie in the packs.

Also, should the pack be discharged down to 0.9Vper cell before recharging?

Thanks for your help!

I would recommend 6 batteries in series, not 9. Less possibility for turning reverse-polarity one off the cells compared to 9x series connection.

4 cells in series is pretty much foolproof, anything above that and at some point there is increased risk that one of the cells is going reverse-polarized. Or at least selecting correct cut-off voltage for high-drain use is tricky compromise.

For example 9 cells in series with cut-off voltage of .9*9=8.1v

pack with weaker cell(unmatched cells or aged cells): 8x1.1v+ 1x0v =8.8v
-->great possibility for reverse-polarizing one cell.

 

[SilverFox]

Hello Tino ale,

If your cells have already been formed, then just charge at 0.5C using a sensitive termination value. Let the cells cool off and use a 0.1C for 4 - 6 hours. That will bring the cells into balance.

Tom

 

[LuxLuthor]

Hello Tino ale,

If your cells have already been formed, then just charge at 0.5C using a sensitive termination value. Let the cells cool off and use a 0.1C for 4 - 6 hours. That will bring the cells into balance.

Tom

I had that same exact question. Thanks Tom!

 

[eebowler]

Silverfox, so trickle charging a series pack wouldn't accomplish the same thing? What you're suggesting above seems to require a charger with different charging rates and that isn't available to the average joe so I'm wondering if there's another way...

 

[lasercrazy]

Just use a slow charger set to a few hundred ma.

 

[mrsinbad]

You can get a pretty full charge on each cell if you charged at .1C for 12-14 hours. If you use a delta charger, it could sense a false peak and shut off earlier than necessary.

 

[SilverFox]

Hello Eebowler,

You are correct. Charging at 0.1C will charge the pack and bring all cells into balance. It does take longer, but only one charge rate is required.

Tom

 

[tino_ale]

Thanks for all good advise. I'm getting real used to deal wit Ni-MH, what to do, what not to do... thanks to your help :twothumbs

 

[rizky_p]

Good info for me too, is this how i should charged the 15.6v pack i got from you Lux if i want a balanced pack?

 

[SilverFox]

Hello Risky p,

Yes. You should balance the cells in a battery pack every 10 - 20 charge/discharge cycles.

Tom

 

[adamlau]

Charging at 0.1C will charge the pack and bring all cells into balance. It does take longer, but only one charge rate is required.

Does this also apply for Li-ion packs?

 

[SilverFox]

Hello Adamlau,

No.

Lithium chemistry can not handle an extended trickle charge. The voltage keeps on climbing and eventually the cell is destroyed. NiMh chemistry can handle a slow rate of overcharge. During the 0.1C charge, the "fuller" cells reach full charge first, then they are overcharged for a while allowing the "emptier" cells to become fully charged. In the end, all of the cells reach the same state of charge.

To achieve the same thing with Li-Ion chemistry, you either need to charge each cell individually (or with balancer circuits) or in parallel.

Tom

 

[lightforce2]

Thanks Tom for explaining this, I'm using 6 eneloops in series and was wondering the same thing.

cheers,

 

[rizky_p]

@SilverFox: Thanks. what is battery forming?

 

[SilverFox]

Hello Rizky p,

Forming is an old term that came to be during early lead acid battery manufacturing. In its current use it refers to getting an even distribution of electrolyte within the cell, and allowing the separator to soak up all the electrolyte it is capable of. Both of these processes are necessary to get full capacity from a cell.

I believe the leading cause of battery failure is separator damage. The separator can be damaged by high heat, and by localized hot spots that result from areas of starved electrolyte. These localized hot spots occur both during charging and discharging.

A 0.1C charge for 16 hours is the best way to prepare the cell for use (i.e. the forming charge), and it also is effective in balancing the cells within a battery pack, and should also be done after the cell or pack has been in storage.

If you go through a charge/discharge cycle every day, and charge your cells in a independent channel charger, the only time you have to consider using the standard charge is when your cells are new, or when you are checking your cells for capacity.

Tom