conditioning ni-cd battery pack

[Avatar28]

My oldest son and I are involved with his school's robotics team. One of the squads he is on is in charge of the control systems, including batteries, motors, etc. Anyways, we have two 7.2v 1500mAh Ni-Cd battery packs to run the robot from. We have a Hobbico 905 (I think) charger with a charge/discharge anmeter. What is the best way to preserve and condition these batteries to ensure they maintain their maximum life without frying them. Likewise, what steps should we take when charging/discharging them?

 

[Turak]

Hmmmm...

7.2V NiCd pack you say. That means it probably has has six 1.2V cells in it.

A few things come to mind....

Make sure you completely discharge the pack (about 5.5V-6.0V) every 20 cycles or so.

Since it is a multi-cell pack, there is a chance of the cells becoming unbalanced. Slow charge it every 10-20 cycles to balance the pack back out. You said it is a 1500mAh pack, so I'd hit it with about 150mA for about 12-16 hours.

DO NOT leave the packs 'trickle' charging for extended periods of time (i.e. over 24 hours).

NiCd's are very robust, so they will tolerate a lot of abuse (i.e. high current charges, high current discharges, deep discharges, cold temps, etc.). Just don't heat them up too hot and don't trickle charge them to death, it just shortens their life in the long run.

 

[qwertyydude]

Yeah ideally you don't want to discharge below .9 volts/cell to prevent reverse charging a cell in the pack. But old-school ni-cds are tough batteries. I've had the same pack going on on for 10 years and it still has 85% of it's original capacity, must have over 500 cycles through it. All I do is every few months I set it to several full charge/discharge cycles, again no less than .9 volts/cell, and on the last cycle I trickle charge it overnight. But my real favorite is my ultra high capacity 5000 mah nimh pack. It's just a tad wider than your standard 6 cell sub-c but fits in most cars, and the power output is crazy high, current surges go as high as 100 amps in my brushless setup so that even 12 guage wires get hot.

 

[Avatar28]

Thanks. It turns out we may have a bad charger. As soon as you set it to discharge the meter deflects to 3 amps even though it is not supposed to start until you set the meter. Later when I finally got the meter to go properly back to 0, it was showing a discharge current in the 1 amp range (which according to the charger means to stop discharging or risk killing the pack). When I set it back to charge after that, it showed a 5 amp charge which seemed excessive (that is in the red and it had previously shown a 3 amp charge which, if I am not mistaken, should be two 15 minute cycles, correct?)

 

[SilverFox]

Hello Avatar,

NiCd chemistry is different from NiMh chemistry. During charging, it doesn't heat up until the cell is fully charged.

Because of this, fast charging of NiCd cells and packs is often done at 2C, or sometimes higher, in performance applications. Your pack has 1500 mAh cells, so a good charging rate for normal charging would be 3 amps.

Battery packs can get out of balance, so you need to balance the pack with a 0.1C charge for 16 hours. This means charging at 150 mA. It is best to have a balanced pack, but fast charging gives you a bit of an edge on performance. This means that you have to alternate between the two charge rates. Usually you charge at the fast rate until you notice the run time falling off, or notice an individual cell heating up during the charge. Then you use the slow charge to balance the pack.

When storing the pack, discharge at 1C to 0.9 volts per cell and store the pack empty. If the pack is stored for an extended period of time (months), you may need to do a few charge/discharge cycles to bring it back up to peak performance. Every 30 days you should do a charge/discharge cycle, at 1C, on the pack.

This should keep your pack in tip top shape.

Tom

 

[Avatar28]

Thanks, Tom. I'm not sure that a slow charge is really an option with this charger. Ours is the 905 here.
http://www.hobbico.com/chargers/hcap0125.html

That will get us in the right direction though.

 

[Turak]

Took a look at the charger and the manual.

It is basically a simple 3.5A timed (15 min max.) charger that has an end of cycle trickle charge. Unfortunately, it does not mention the 'trickle charge' rate. You would have to check that with an meter. Cheap digital one would do for checking that. The unit is also equipped with an analog ammeter. Might be able to get an indication of the trickle charge rate with the built in ammeter, but it may be too low to get an accurate reading with an analog ammeter.

Would have been nice to have a 'loaded' voltmeter (1A load would have been fine) too. Better yet a selectable load (i.e. .5A, 1A, 1.5A) voltmeter. Would have been a little better/easier to tell when the battery was in a fully discharged state. Not to mention easier to tell if one of the cells in the pack shorts out (i.e. instant drop in voltage, both nonloaded and loaded).

As far as charging and equilizing the batteries....well, you can....sort of.

Look at the last bullet point from the manual's charging section;

Model 900, 905
• Set the Timer (6) to the desired amount of charge time.
• The Timer will automatically switch to Trickle Charge at the end of the quick
charge cycle The Trickle Charge Light will glow at this time The Model 900
is equipped with a Trickle Charge Light to indicate when the charger is in the
trickle charge mode
• If the Battery becomes warm to the touch, this is a good indication that it is
fully charged
• You may periodically wish to let the Battery trickle charge for a few hours after quick
charging. This will equalize the cells which increases capacity and run times.​

Keep in mind too, in the last bullet point above, when they say 'a few hours', they DO NOT mean 24+.... On a newer pack, when they get warm, it generally means they are full or nearly fully charged. On an older pack, it can also mean that the pack may be developing a higher internal resistance with aging.

Most of the NiCd's I have seen go bad usually either develop internal shorts so that they can not take a charge, or an internal resistance so high they will not charge.

With packs of NiCd's the usual problem will be that one cell in the pack shorts out, thereby reducing the overall capacity and the beginning loaded voltage (detectable with a simple digital meter) of the pack.

Because of the limits of the charger, one thing that I would suggest that you do....

Buy a cheap digital voltmeter, if you don't already have one. Even a $5 special from Harbor Freight or something like that would do fine. After you fully charge the pack, plug it into robot and measure the voltage of the pack while it is under load and fully charged. It will be a good indicator if one of the Ni-Cd's short out, or if they start developing a high internal resistance. The voltage should be measured while the pack is under load....just measuring the resting voltage is not a good indicator.