A few beginners questions

[commander768]

Hi,

Lately i have been working on a project, trying to reasemble a cordless drill battery from 2 old dead battery packs. I already have a working drill with good battery so i am doing this project for fun trying to learn as much as i can in the process. I have a few questions.

- I tried to charge 4 NiCd 1.2V batteries. I used an adapter that i got with a cordless drill that uses 4 batteries. Its 6V 350mA. I connected it directy to batteries creating a circle and noticed that the current is 600mA. Since it was over 350mA i disconected it but i am wondering why this happened. Was this because batteries were bad and had less voltage, therefor the difference between voltage of them and the voltage of adapter was bigger, resulting in bigger current ?
Or maybe i shouldnt connect the batteries directly to adapter anyway ? Looking at the inside of a drill ( its a cheap 4.8V ) i am pretty sure the adapter connects directly to batteries without any circuit controlling it .. and if it is rated for 350mAh and drawing more, it is not ok ..

- if i wanted to figure out the approximate mAh of the battery, could i just hook it up to a resistor ( 1 Ohm for instance ) and then check voltage of the battery at different periods, for instance at minute 0 and minute 10. Lets say voltage at minute 0 was 1V and at minute 10 0.9V, then it suddenly got empty. For the sake of argument lets say that voltage drop was linear. So we get average voltage of 0.95V, 1Ohm resistance and time 10 minutes. So calculating ( 0.95V / 1Ohm ) / 6 = 158mAh. Would that estimate be good ?

 

[commander768]

edit: about the first question it would appear that i accidently connected plus of adapter to negative of battery instead of plus to plus. It is ok now, drawing 0.26A

 

[KrisBlueNZ]

Hi there and welcome to the Electronics Point forums

I tried to charge 4 NiCd 1.2V batteries. I used an adapter that i got with a cordless drill that uses 4 batteries. Its 6V 350mA. I connected it directy to batteries creating a circle and noticed that the current is 600mA. Since it was over 350mA i disconected it but i am wondering why this happened. Was this because batteries were bad and had less voltage, therefor the difference between voltage of them and the voltage of adapter was bigger, resulting in bigger current?

Yes, that's pretty much right. There's quite a bit of variation between adapters - a specification of 6V, 350 mA doesn't really tell us a lot about its characteristics. But connecting it to a series group of four 1.2V NiCd cells, especially ones in unknown condition and state of charge, is quite likely to have that effect.

Or maybe i shouldnt connect the batteries directly to adapter anyway ? Looking at the inside of a drill ( its a cheap 4.8V ) i am pretty sure the adapter connects directly to batteries without any circuit controlling it .. and if it is rated for 350mAh and drawing more, it is not ok ..

Right, that's probably not a good idea. Recharging batteries in cheap simple products like cordless drills is not an exact science. Here's a quick summary.

The adapter will be one of two types: a mains-frequency transformer type, or a switching supply.

The mains-frequency transformer ones feed the AC mains into a transformer, and feed the transformer's output through a rectifier (usually 2 or 4 diodes) and into a smoothing capacitor that gives a "bumpy" DC output. These adapters are relatively heavy, and often fairly large. They seldom have regulated output voltages, which means that the actual voltage you measure at the output will vary depending on the AC mains voltage, and especially, depending on how much current you draw from the adapter. A specification like 6V, 350 mA means that the output voltage is nominally about 6V if you draw 350 mA from it; if you draw more, the voltage will drop, and if you draw no current (i.e. disconnect the load), the output voltage could be as much as twice the specified voltage!

This kind of adapter is sometimes connected directly, or through a resistor, to the battery. This gives a poorly controlled charge, and the user manual may recommend that you disconnect the charger after a certain amount of time, to prevent overcharge. Or the charging system may be designed (if that's not too strong a word) so that as the battery voltage increases, and the difference between the adapter output voltage and the battery voltage decreases, the current will decrease to the point where it's effectively a trickle charge and will not damage the battery if left connected indefinitely. This is called float charging and is acceptable for lead-acid batteries but not recommended for NiCd and NiMH batteries.

These adapters will not give a properly controlled charge under any circumstances, unless the appliance includes a proper charge control circuit. This is not likely with a cheap cordless drill.

Switching adapters are internally complicated. They convert the AC mains directly to high-voltage DC, and drive a high-frequency signal through a compact, light-weight transformer. They are always regulated - sometimes quite accurately, sometimes fairly loosely, but always better than the first kind of adapter.

These adapters generate a fairly constant voltage as long as you don't exceed the specified output current. Because the voltage is accurate, they can be used to float charge NiCd or NiMH batteries, by adding a resistor and/or a diode between the charger and the battery. Or the appliance may control the charging process properly; this is what cellphones, cameras, tablets and other portable electronic devices do. Again, a cordless drill is not likely to do this unless it uses a battery type where this is essential.

These explanations are based on my limited experience with rechargeable products. Other users here will probably be able to correct and/or add to my comments.

If i wanted to figure out the approximate mAh of the battery, could i just hook it up to a resistor ( 1 Ohm for instance ) and then check voltage of the battery at different periods, for instance at minute 0 and minute 10. Lets say voltage at minute 0 was 1V and at minute 10 0.9V, then it suddenly got empty. For the sake of argument lets say that voltage drop was linear. So we get average voltage of 0.95V, 1Ohm resistance and time 10 minutes. So calculating ( 0.95V / 1Ohm ) / 6 = 158mAh. Would that estimate be good ?

The capacity of a battery is normally specified at a discharge current of 1C. In other words, if the battery says it's 2.4 Ah, if you discharge it at 2.4A it should last an hour before its charge is depleted to the "fully discharged" point.

You might expect that if you discharged it at C/2, i.e. half that current, 1.2A, that it would last for 2 hours, but it will actually last longer than that. The faster you discharge it (the higher the discharge current), the lower the calculated capacity will be. Check the data sheet; some manufacturers claim higher capacity than their batteries deserve by specifying that the capacity is calculated at a lower discharge rate, e.g. C/2, C/4, C/5 etc, so their product looks better than it is.

You also need to ensure that the battery is fully charged before you start, and ideally you should discharge it at a constant current, instead of into a constant resistance. Using a constant resistance, the discharge current will fall as the terminal voltage falls.

Again this is based on limited experience. Other users may fill in the gaps. This info is general only.

 

[Arouse1973]

Look pretty good to me Mr super fast typer! That took you less than 30mins God I am slow!

Adam

 

[commander768]

Thank you for your replies.

Using only adapter is not really the best option but from what i can tell, but if batteries are bad, it cant do much more harm anyway.

I was wondering, did any of you ever try to zapp batteries back to life ? I did a few tests, with proper protective gear ofcourse and i got some interesting results:

battery NiCd 1.2V, resistance 1.25 Ohm

before zapping: starting voltage 0.58V, after 1 minute less then 0.20V
after zapping: starting voltage 0.80V, after 13 minutes 0.47V ( stopped the test here )

Battery is still very bad ofcourse but the difference is clear. I will try zapping it a few more times because this looks very interesting. I also managed to get a few 0.00V batteries back to life with it. Crystals ..

 

[Arouse1973]

I have tried this with varying success, but it was a long time a go when NiCDs where expensive.
Adam

 

[commander768]

I have been charging 4 AA Ni-Cd batteries to find out which one is bad. I found the one. But now i found out that the other 3 appear bad also. They are less then a year old, from some mini cordless machine i bought.

When you charge batteries in series, if 1 of them is bad, others wont charge properly. So to get around that ( to find out which batteries are good or bad ) i used my charger, i thought it is charging in paralel. But now i am not sure it is anymore, it looks like because of 1 bad battery other 3 arent charging properly. What do you think ? This is my charger:

 

[BobK]

Does the charge light come on if less than 4 batteries are inserted? If not, it is probably charging in series.

Bob

 

[commander768]

Each one has its own light and you can only charge 1 battery. I did notice though that when i insert another battery it affects how strong other batteries shine. The battery in far left is a bad battery. If i insert it, the light of the battery next to it will light a lot less strong, but the light for 3rd and 4th battery will shine a bit stronger. I might have to try and charge those batteries 1 by 1 to really see what the condition of the batteries is. Will take lots of time though since it is charging with about 0.18A

 

[BobK]

Okay, it is charging in parallel then. The bad battery is probably eating up most of the available current.

Bob

 

[commander768]

I measured the amps going through batteries. Its 10mA going through each battery. If i disconect 1 battery, the amperage on other batteries goes to 13mA, incase i disconect a bad battery amperage goes up to 12mA.

Also i made kind of a zapping machine. I just connected 6 1.2V batteries together from my old cordless screw. Since those batteries are not in good condition ( under 1 Ohm load voltage drops to 0.5-0.7V ) i use 6 of them to zap a single 1.2V battery. I did manage to bring a few batteries from 0.0V back to life and improve voltage drop on few ( lesser drop ), but i didnt make enough testing to say what the overall improvement is.

I have been wondering what the normal drop is on average 1.2V high amp batteries when they are new. For instance if i connect my old 1.2V batteries to 1.25 Ohn resistor, they drop to about 0.5 - 0.8V, not a single one of them went over 0.8V. What would be a drop on a new good conditioned average battery ?

 

[Arouse1973]

How many times do you think you have you charged them in the time you have had them, once a week, once a month, everyday?. Are they fully discharged before each time you re-charge them?
Edit: Have they been stored for long periods of time in a discharged state.
Adam

 

[commander768]

Right now i am working on 4 batteries from some 4.8V cordless drill i bought 2 years ago. I didnt use it much, i charged it maybe 20 times, i used it when i needed it around the house. Sadly when i bought it i didnt know its not good to charge it when battery is still at like 50% ( battery memory ) and that its not good to leave the machine with completely empty battery ( crystal forming ). I really loved this cheap drill, the price was almost a joke and it was a great help to me at computer assembly/dissasembly. About 4 months ago when i charged the drill it only worked a few seconds and died. I opened it and checked voltage of individual batteries under load. I found out that 1 of them completely fails at load, barely puts out any charge.

 

[Arouse1973]

I think it might be the fact you left them in a discharged state for too long. I don't think memory effect is an issue in todays batteries as technology has improved some what. You might need to do a few charge discharge cycles to bring them back to life. Charging them at a higher rate could also be an option because the charger you show is probably a slow charger type.
Adam

 

[commander768]

Batteries seem to be worse every recharge cycle. Now even other 3 batteries have same problems. It is strange to me because now i have some batteries that are more then 10 years old and have been completely discharged for more then 6 years, but still hold the charge a lot better ( and more importantly, zapping improves them ).
I connected 3 of the bad batteries to the drills charger. Its designed for 4 batteries but for 1 cycle it should be ok. Its charging with 0.28A. Batteries are only 700mAh and i read that max charge rate should be at C/3 but like i said, for 1 charge i hope it will be ok. But those batteries really seem to be a bad quality. Such a shame. This little tool is great

 

[Gryd3]

Batteries seem to be worse every recharge cycle. Now even other 3 batteries have same problems. It is strange to me because now i have some batteries that are more then 10 years old and have been completely discharged for more then 6 years, but still hold the charge a lot better ( and more importantly, zapping improves them ).
I connected 3 of the bad batteries to the drills charger. Its designed for 4 batteries but for 1 cycle it should be ok. Its charging with 0.28A. Batteries are only 700mAh and i read that max charge rate should be at C/3 but like i said, for 1 charge i hope it will be ok. But those batteries really seem to be a bad quality. Such a shame. This little tool is great

If you have cracked it apart already, why not just swap the cells out?

 

[commander768]

The thing is that this small 4 battery drill uses small AA batteries and i dont have any spare ones. An option would be to buy new ones, but right now, since it looks like those 4 AAs are gone forever, i might modify this small drill a bit to acomodate 4 sub-c batteries.

 

[KrisBlueNZ]

I think it might be the fact you left them in a discharged state for too long.

Yes. That, and their age, and the probability that they're crap batteries - if the cordless screwdriver was very cheap, there's normally a reason for that.

 

[Arouse1973]

Yes right Kris. Cheap Chinese batteries. I have seen this before. I was told it was due to the large population in China needing work. And the policy is if a job can be done by hand then they won't automate it and apparently that applies to battery manufacture also.
Adam

 

[commander768]

So i might aswell throw those 4 batteries away ? It would appear not only 1, but all 4 are pretty much dead.