Comparing 2-Battery Chargers

Many years ago, Practical Electronics magazine published a charging circuit that could be used to re-charge "non-rechargable" Zn-C batteries. The output waveform was a lop-sided AC, i.e. the negative-going pulse was smaller than the positive-going pulse.

Having built the circuit, and found that it works, it occurs that maybe the Duracell charger is working in the same way. What happens when an oscilloscope is attached to it?

Does that help?

Hi PWSlack,

I could be that, it's hard to tell. I had to use the 10A range on my multimeter for the previous current measurements to keep the charger from shutting down. With a scope across the battery I see 2 positive voltages alternating about twice per second.

S

You need to deep discharge each cell about once a month to avoid so called memory- this applies to NiMH & NiCd- I use a single cell holder with a 2.5v torch globe- the discharge is complete when the (dim) glow goes out(about 20 mins)- then use whatever charger you have to charge 1 or more. The charger you are referring above seems to be a "burp" charger- these are great for recharging most cells incl "non- rechargeable cells"!. A new cell available now is alkaline rechargeable to 1000 times, at a reasonable price- can be charged by NiMH charger. This reminds me- glug- glug- Burp!.

Never discharge a single cell battery of the types mentioned below about 1 volt, it will shorten its life.

Never buy a battery charger that charges two cells at the same time, for one of the cells, that will represent an overcharge at best.

Also, return the other charger or charge only one cell at a time.

A good charger has a switch to select Nicad or NIMH. It charges each cell individually and correctly, some cells take longer than others.

NIMH in spite of being similar to NICADS, do not need to be discharged if you are using a good charger.

Ansmann are very good, mine has even a fan to keep cell temperature down when quick charging.

Hi Andy,

"A good charger has a switch to select Nicad or NIMH. It charges each cell individually and correctly"

Have you measured the current for the two types? What is recommended? Is the algorithm different for the two types? Mine are inexpensive and small. I thought about designing one, but with the low price, it may not be worth the effort. I also have a power supply with constant current mode that I have used for 6V lead acid batteries. It seems like 1/10 of the amp-hour rating would be a good charging current. Does that sound reasonable to you?

S

NiMH & NiMH suffer from similiar effects because they use Nickel. The safest way to charge NiMH & NiCd is C10- ie io% of ah rating- constant current- but the rising cell voltage MUST be monitored- it will reach a high level, then drop- this is delta- if charge is continued beyond this point, temp in cell will rise rapidly in 1 or 2 minutes, safety venting of cell contents can occur- the cell is damaged- will never be as good. Good chargers have temp measuring on cells,& will terminate charge when cell temp rises quickly. The old never discharge below 1v per cell refers to a general battery-ie a collection of cells, as reverse charging of the weakest cell/s can occur- this does not apply to individual cells, which is what I was referring to- NiMH cells have a life of 300 r/c & lower current than NiCd- which, if treated as I have said, can achieve 2,ooo r/c. I agree that modern chargers(15 min!) from Varta with fans etc are great- but need special matching Varta cells ( I inserted AAA cells incorrectly and BANG)!. An ESR meter as used to measure electrolytic caps is great to measure the internal resistance of cells- as with all r/c, i/r rises as cell ages- use my discharge regime& i/r falls.

Although I design my own Lead Acid battery chargers and have found out quite a lot, the latest one has run for 18 months with no trouble for my caravan battery. Due to the high cost of made ones (good ones) I can actually save money!!

I do not design for NIMH or similar batteries. But if I did, I would buy some of the many excellent chips that are on the market today and with a hand full of other components, I would make a charger....

But as you say, the finished ones are so cheap, its not worth really doing that!

With regard to Lead Acid, it depends on your approach, I charge quickly to 13.2 volts and stop, let the charge sink to 12.6 volts and charge up again. That way although the battery is never 100% full, it also never gases, which could be dangerous in some situations, though I vent all gases outside of the caravan....

The current to charge is dependant on the battery size.....max current 1/10C is about right, but could be less. Therefore a 100 AH Battery can be charged with up to 10 amps...

I do not personally agree with trickle charging, but many do.....

Regards all !

#3 & #6

<< You need to deep discharge each cell >>

I fully agree to your comments

I like to add:

NiMH & NiCd batteries are not charged like old Lead-Acid [open or Sealed] batteries.

In Lead-Acid [open or Sealed] batteries Charging Voltage is set to:

7.3V for nominal 6V & 14.6V for nominal 12V batteries.

[Each cell is of 2.2V; 3 cells in so called 6V & 6 cells in 12V battery]

Initially Charging current is Hi which decrease as the battery builds-up potential.

Sealed batteries need more care in charging as the vapours recombine within to maintain

water level. If over-charged safety vent function & water is wasted & battery damaged.

But NiMH & NiCd batteries are charged @ Constant-Current [C5 or C10] for 7 & 14 Hrs respectively.

After which the Charger should go to TRICLE-CHARGE [~C20 ]

The manufacturers recommend to Hi-Rate-Discharge NiMH & NiCd batteries for initial 5 to 7 discharges to aviod MEMORIZE to discharge @ low-rate-discharge & upto a hi-end-discharge-limit.

After that you can use at your will.

I have a serious-experience of losing a lot of NiCd batteries in 1970s & contacted the supplier who contacted the manufacturer for advice.

Further recommendations included:

1. Min Discharge for initial [5 to 7] = C5

2. Charge [5 to 7] = @ C5 for 7 Hrs

later:

for Quick charge @ C5 for 7 hrs

for normal charge @ C10 for 14 Hrs

for trickle charge [standby] C20

At that time he also supplied a Timer-controlled Charger having having all the above facillities.