NiCad Battery/Charger

[Brown]

So I have a 12V NiCad battery for a drill with no charger and a 19.2V NiCad drill battery and charger. My question is will the 19.2V charger work with the 12V batter? The charger's output voltage is 12-24V 1.5A. Thanks!

[Paul Price]

Most drill mfg's customize the socket/battery shape so that only their correct drill battery can be inserted into the charger.

Ni-Cd batteries can be overcharged and overheated by excessive voltage applied to the battery. You can make a charger circuit that can limit the current and voltage fed to the 12V battery  you want to charge using (a LM317 regulator on a small heatsink) and connect the 12V battery using alligator-clip jumper wires to the 19.2V battery charger.
 
You set the LM317 max output voltage with a resistor to be somewhere about 12 to 12.6V and you are in business.

[Brown]

Now I have two functioning drills, THANKS!! 

[Brown]

I have another question, can I make a 12V NiCad charger from a old computer PSU? I imagine I'd need some protection circuitry? Thanks

[Paul Price]

Yes you can use an old PC P/S.  The 12V output (usually yellow wires, in rare cases white wire) is often rated at >8Amps, so plenty of current is available. The only caveat is that a 12V Ni-Cd battery may require a charge voltage >13.5 volts to pump enough electrons into the battery for full charge and the PC power supply is fixed at +12V.

Discarded laptop power supplies mostly yield a 17-21V output range with up to 4.5 amps with high efficiency. Add a LM317 to keep things regulated both current and voltage-wise.

I seem to always to come across discarded wallwart power supplies discarded from powering various PC equipment and 12V unregulated output voltages are very common. Add a LM317 Regulator to limit the max current (to not overload the wallwart as well, some can only deliver some large fraction of 1-amp) and then you only need to set max charging voltage and you are in business.

[Brown]

I have a old laptop power supply and what voltage should it be adjusted to? 12.5V?

[Paul Price]

Try setting the LM317 max output voltage with a resistor to be somewhere between 12 to 14.4V.  If you set the LM317 output voltage too low, the battery will not be fully charged. If you set the output too high, but in this range, you might overheat the battery after fully charging it.

The way to find what the correct voltage to set the voltage regulator to is first try 12V until the battery is charged to that voltage. Then increase the output voltage to 12.5V and check the battery temperature again after 15 minutes. If the battery still does not get warm, try 13.0V. The correct regulator output voltage is one that just gets the battery to become warm at full charge. Once you get it right, you won't need to try this procedure again.

A NI-Cd battery actually becomes cooler with charging until it is fully charged, after which the extra energy going through the battery becomes heat. Ni-Cd batteries can withstand quite a bit of overcharge if they are not allowed to get very hot.

[benjamin545]

LM317 should have overcurrent protection at 1.5 amps. you need a regulator that can handle more than that i would think. the lm350 is a 3 amp regulator, and the lm338 is a 5 amp with the ability to handle <1sec high current draws. those 2 would be better options for a regulator i would think. all 3 regulators can be bought on ebay for super cheap. like $5-$8 for 10 or so.

[Paul Price]

Charging at a moderate 1.5 amps (LM317 max) would ensure that the battery pack would less likely overheat at full charge or damage any single battery that has been fully discharged. Slower charging could be safer and better.
The individual batteries might be able to be fast charged and have a high mAH capacity, you would have to know what the mAH of the individual cells of the battery is to before opting for higher charge currents.

[benjamin545]

yes. you would have to check what max current you should charge the batteries at. keep in mind that the lm317/350/338 will not regulate the current for you when you are using them in voltage regulation mode. they will just shutdown when they are driven at their over current protection limits. what you would have to do then is use another regulator in current regulation mode right after the voltage regulator and that way you can keep the current from going too high as well.

[grumpydoc]

Some confusing advice in this thread.

NiCd cells are usually charged from a constant current source rather than a constant voltage source.

A charger designed for a 19.2V pack will be OK for a 12V pack if we are talking about trickle charging the cell - i.e a charge rate of 1/10th the rated capacity (i.e 100mA or less for a 1AH battery). Of course you need to make sure that the trickle charge current is appropriate for the 12V pack.

If we are talking about rapid chargers then it is easy to damage a cell by overcharging. There are two ways of detecting that charging is complete - either by detecting the slight dip in voltage as the cell reaches full charge or by detecting the rise in temperature that occurs when one continues to charge a full cell. The 19.2V charger might have difficulty detecting when a 12V battery is full either because of a physical constraint such as the cell needing to fit where a thermistor is in close proximity or an electrical one such as the charger expecting a delta-V around 25V rather than 16V. In fact if the 19.2V charger is smart enough it might refuse to even start charging the 12V pack thinking it is too deeply discharged.

It is not generally a good idea to charge NiCd's from a constant voltage source - you will risk damaging them - and fiddling around manually increasing the voltage bit-by-bit can't be anybody's idea of fun.

NiCd's are easy to charge if you have a bench PSU though - just set the PSU to constant current mode, set the current limit to Capacity/10 and stick the pack on charge overnight for about 16 hours. Simples.

[Paul Price]

A lm317 has current limiting and this means that it can constant-current charge a battery just because it limits it own output current to a safe value that will charge the batteries at a current limit that is safe for a large Ni-Cd battery pack.

[madires]

I'd get an universal charger for RC models (around 50 bucks) which supports a sufficient number of NiCd/NiMH cells. Most of them are (much) better than the tool's original charger. Usually I take the cheap chargers, rip out the charging circuit and put in some 4mm binding posts.

[grumpydoc]

A lm317 has current limiting and this means that it can constant-current charge a battery just because it limits it own output current to a safe value that will charge the batteries at a current limit that is safe for a large Ni-Cd battery pack.

That doesn't strike me as the best way to approach a NiCd charger - especially as making a constant current supply from an LM317 is not especially hard.

For one thing you're stuck with a single charge current, which is probably not that well specified¹ and by definition running the LM317 at it's maximum rating.

Why not spend the extra few minutes to do it correctly?


[1] the TI data sheet for the LM317 says max current 1.5Amin 2.2Atyp which suggests the current limit could be anywhere between those two values.

[Paul Price]

Because it's just a battery for a drill!

The idea is to get the job done and get on with life. You need a drill to work, you are not making a life-support system.

[grumpydoc]

The idea is to get the job done and get on with life. You need a drill to work, you are not making a life-support system.

Yet you suggested a complex pick-the-right voltage set-up when googling "LM317 constant current" would have shown you how to build a constant current source with an LM317 and half a minute with the battery specs would have indicated what current to charge at with no trial-and-error.

And as I also said if you have a bench supply with a constant current mode you already have a NiCd charger - an even easier way to "just get the job done"

[Paul Price]

K.I.S.S.

[benjamin545]

an LM317 Constant current is different than over current protection. if you setup the regulator in constant current mode, you can regulate up to 1.5amps through the regulator, but this configuration ignores regulating voltage.

if you setup the lm317 as a voltage regulator, you cannot regulate the current going though it with just that one device. they do however have internal over current protection that will turn off the regulator for like a second or so if they see more than 1.5(or possibly up to 2.2 as grumpydoc stated) they will shutdown, similar to how they shutdown if they get too hot. so you cannot use this feature as a way to regulate current, only as a way to protect your circuit from over-current conditions, like a short. if you want to regulate both voltage and current, you would need to either use 2 regulators, one configured for voltage regulation, one configured for current regulation, or you would need to design your own external current regulation, similar to how Dave did in his power supply design video.

[grumpydoc]

K.I.S.S.

Simple is one thing, broken entirely different.

[Paul Price]

You don't need two LM317 regulators, just one with current limiting.

[G7PSK]

Bosch and many other tool manufacturers make their chargers multi tool. The one I have in front of me right now is for battery packs from 4.8 volts through to 14.4 volts. I have another that goes up to 24 volts. As far as I can see there is nothing to tell the charger what voltage the battery is.