Charging NiMh batteries

[omkar]

Hi,
I have a lot of brand new 1.2V 10000mAh NiMh batteries.
http://www.ebay.com.au/itm/6-x-size-D-Rechargeable-Battery-10000mAh-1-2V-Ni-Mh-/180739439290?pt=AU_Electronics_Batteries_Chargers&hash=item2a14e8faba

I am planning to make a 12V pack by connecting them together so the final pack will be 12V (or 14V) with a 10000mAh pack. I have read a lot of forums which say that connecting them together is possible but you need to be careful while charging them.

Any ideas on how to charge this pack? Any links to such chargers? Is it even possible to get a pack that gives me 12V and 10000mAH out of the batteries I have?

Cheers
Omkar

PS: Planning to use this battery pack on my bot which has two power hungry drill motors and a bunch of other electronics and one small motor for other controls and a servo.

[Soeren]

Hi,

I am planning to make a 12V pack by connecting them together so the final pack will be 12V (or 14V) with a 10000mAh pack. I have read a lot of forums which say that connecting them together is possible but you need to be careful while charging them.

Any ideas on how to charge this pack? Any links to such chargers? Is it even possible to get a pack that gives me 12V and 10000mAH out of the batteries I have?

First off, I'd test those cells, to find their real capacity, as Hong Kong cells are regularly quite optimistically rated (yes, they do write that the cells are Japanese, but anyway).

To make a 10 cell battery is easy, especially if you use D-cell holders. Soldering the cells is not a good idea, unless you have a very good soldering iron and experience in doing it - one thing is the danger of cell rupture, but even if they keep together, the heating will decrease their capacity some.

If you don't want to use holders, cycling the cells a number of times with a realistic current draw will help you find the weakest cells and put them aside for other use.

If you decide to go with soldering anyway, a "balancing" wire from each node to a connector will be a very good idea, as it's almost impossible that they should keep identically charged over time.

For charging a 10Ah cells (or a battery made of same), you need at least 1A for 15 hours. I cannot see what kind of charging they're rated for (perhaps you know?)
I wouldn't go over 3A to 5A of charge current, without any info about how they take on a charge.

There's plenty of chargers you can use, like this, this or this (or make one yourself), but do measure each cell at least every third charge or so, as an imbalance is so much easier to cure if taken care of at an early stage.
(The cell balancers found in the linked chargers cannot be used for nickel based cells).

The problem that some people may have mentioned is that, if a single (or a few) cells of a battery weakns enough, it will reach 0V while the other cells are still very active and thus it will be charged in reverse - and while this can be remedied, the cells in question will be even weaker than before and cannot be used in the battery.

Keeping a small notebook with the charger, where you write down each cells voltage (mark each cell with a number or whatever) to 3 decimals (plus the date) after charging, will make it much easier to spot when a weak cell is about to develop.
Measurements some way into the discharge (on the 'bot) is not a bad idea either.

[omkar]

thanks  i was definitely planning to use D cell holders as totally not a very good solderer....

"Cycling the cells to find weakest one": I assume this means that number the cells, and keep a track of which combination of cells gives you highest current/voltage?...

Also those chargers look great. Though the descriptions dont tell us that the NiMh cells cannot be balanced.

[Soeren]

Hi,

"Cycling the cells to find weakest one": I assume this means that number the cells, and keep a track of which combination of cells gives you highest current/voltage?...

Not quite.
Number each cell - check

But, then charge, discharge, charge, discharge, etc. for whatever number of cycles it takes, to get them to max capacity, keeping track of their fully charged voltage - right out of the charger and with a stable and repeatable load of 0.56 Ohm to 1.0 Ohm (5W) would be suitable.

First measure the unloaded voltage (and write it down), then, while still measuring the cell voltage, load with the resistor while counting eg. 5 seconds and write down what voltage it have at that time.

Then discharge the cell to say 1.1V with the same resistor, recharge and repeat until two sequential charges show the same (within a few percent).

You may see this as a bit too much work and admittedly, it is a lot of work if you don't have/make some kind of ATE for it, but the cells are gonna love you back

Finally, when you've been through all the cells, select the best (those with the least difference between unloaded and loaded voltage) for your battery and put a few of the best (of the rest) aside for later replacement of single cells.

Also those chargers look great. Though the descriptions dont tell us that the NiMh cells cannot be balanced.

Perhaps not, but they're made for Lithiums, so even if their balancer modules could handle the lower voltage of Nickels (which I'd think they cannot), there won't be enough lines for your battery (you'd need 11 lines).

However, as you're going with holders, you don't need balancers. Just check each cell with a voltmeter and if you have some that are either higher or lower in voltage, charge them in sets, so they all have a chance to go to the max.

When they're all at the same voltage (within say 10..15mV), you can equalize them by applying a current of 35mA to 50mA overnight (or up to say a couple of days). This can be supplied from a 15V wall wart with a series resistor of 22..27 Ohm, or a slightly higher voltage with a simple LED+transistor constant current generator.

If you have a spare PC supply and know how to up the 12V line to around 18V (or have any 18V/5A supply), you can make a simple taper charger that, while not being particular fast, will (per definition) end each charge with a low equalizing current.

[omkar]

hi ... one more question about these chargers you have mentioned above. Do I need to charge each cell in my battery seperately or can it charge the whole set of cells (12 cells making up my 10Ah / 12V battery) in one go. Will the wattage of this charger need to be higher if I want to charge the whole "battery"?

Essentially I will be using this charger as a base station charger where my bot will come in and sit itself on the charging station. So the bot will be carrying all the cells in the battery and all the base station will do is disconnect the bot circuit and connect the two wires to the battery. After the battery is charged, the base station will push the bot out and mechanically connect the battery back in to the bot circuit.

[omkar]

Thanks Soeren for valuable input.

To anyone reading this post. This setup worked:

iMax B6 charger set to charge NiMh batteries at 1Amp setting charging 8  1.2V/11000mAh Nimh batteries in series held together in D Cell holders.

I used D Cell holders to hold the cells. As there is no temperature sensor, i kept the whole setup in the middle of my garage away from everything else. But the batteries did not break a sweat. They got slightly luke warm. But when i did this its winter time and my garage is not heated.

Previouslly I tried with 0.3Amp setting and after 14 hrs the batteries were not charged at all. Same with 0.5Amp setting. I think 1/10C (which is 1Amp for a 11000mAh battery) is the minimum setting you need.

Hope this helps whoever tries this.

Cheers

PS: Before putting all 8 cells in series, I checked each one to see if its within +/-10% of each other for voltage output. There were a few which were reading 0.8V, I charged these seperately to bring them up to scratch with the rest of them.