Using Bench Power supply to charge NiCd pack

Question

I have an 18vdc battery powered drill hat works fine but the charger has died and a replacement is $54. Almost double what I paid for the drill plus battery plus charger two years back. Battery was still holding a good charge.

The label says, 18vdc 1.2Ah Battery pack

I did some searching and am using a bench power supply charging it in Constant Current mode at 0.750A but the voltage to get that current rate is 24vdc and rising slowly. I have it right alongside me and cannot feel any appreciable heat build up after 30-minutes.

The rising voltage bothers me, is it OK to let it charge like this or should I clamp it at a set voltage. I can't do both this power supply as one setting limits the other.

Answer 1

Don't be fooled by early results when checking temperature. They heat rise past 70% charge accelerates quickly from internal gas pressures and charge transfer inefficiency dissipates more heat.

from batt U.

Figure 1 shows the relationship of cell voltage, pressure and temperature of a charging NiCd. Everything goes well up to about 70 percent charge, when charge efficiency drops. The cells begin to generate gases, the pressure rises and the temperature increases rapidly. To reduce battery stress, some chargers lower the charge rate past the 70 percent mark.

Therefore do not exceed 1.46V per cell * 13 cells = 19V

The charger from Dewalt is only rated at 18V with a current rating of 2A. Dewalt DW9116 7.2V - 18V NiCd Battery Charger New for DW9057 DW9094 DC9071 $34.90 Ebay.

The fact that you need to drive 24V just to get 0.7A indicates your batteries have aged significantly and are on their last legs.

I know from past experience manufacturing and testing burp chargers this can rejuvenate NiCd batteries. Can you make a pulse switch with low <10% duty cycle at 24V ? Then track your Amp-seconds of charge and useage and see if that improves it.

http://batteryuniversity.com/learn/article/how_to_restore_nickel_based_batteries

Which reminds me I have to fix my Hitachi LiPo cordless drill after left out in the rain.

Answer 2

Your NiCad batteries will likely have a terminal voltage (fully charged) of 20.8 V. This is 13 cells in series.

You should NOT set your power supply to 24 V as this will severely overcharge the batteries.

Set your power supply to 20.5 V (there are other complexities such as temperature and voltage drop on charge, but I'll ignore them for the moment).
Set your current sense OC/CC to around 120 mA (as WhatRoughBeast suggested).
When you connect a discharged pack the OC/CC will trip and reduce the voltage.
When the terminal voltage of the pack approaches 20.5 V the current will reduce to a trickle.
Under these conditions the pack will NEVER fully charge, and to explain that here is the typical charge graph for a single NiCad cell:

Notice that to get to 100% charge you have to manage a terminal voltage DROP as the battery approaches fully charged.

Using your simple power supply charging, we need to stay on the left of the peak.
I've said above to use 20.5 V, this represents about 1.6 V per cell in your pack. At this terminal voltage you can expect to be about 75% charged. You could select any voltage between about 20 V and 20.5 V to get reasonably consistent results with little chance of damaging your pack.

You could also of course build yourself a simple charge controller to connect to your power supply and get closer to 100% charge ...however that's possibly not in your DIY capability.

For example, this might be all you need:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

I would not use a bench power supply to rapid charge any chemistry of batteries.

NiCd however usually respond well to a trickle charge at C/20. They absorb about one half of the supplied charge, and can dissipate any over charge and hold a stable temperature and remain undamaged for several weeks.

For a 1.2A.h (1200mA.h) NiCd battery, a constant current charge of 1200/20 = 60mA for 40 hours should fully charge the battery.