mikehoulder

After Red's comment added to my last post, I'd like to see if my little chinese NiMH charger gives any trickle charge at all.

To put my multimeter in series with one of the cells under charge would need a destructive modification of the charger. Is there any way to measure in parallel?

What I'm thinking is to put a known resistance in parallel with the cell under charge and measure the voltage across both the resistance and the cell separately and then solve two simulatanous equations in the two unknowns: cell internal resistance and current passing through the cell. Just an idea! Can something like this be done?

Best wishes
Mike

hebertjj

Why would you have to destroy your charger to put your meter in series with the battery and charger? Cut one of the lines going to the battery and put your meter (in amp reading mode) in series with the charger line.

exeter_acres

My Feedback: (2)

I use a Watts Up watt meter for my electrics.... and I made an adapter so I could put it between the charger and batteries....
wanted to see if my charger and the meter agreed.....

richrd

Ampere is by LAW… Culombs per second of time. Something must be in series or measure byproduct of current.
Buy a DC current probe and measure Flux density with your DMM…but...best way build an adaptor to connect in series like most of us mortals do.
Rich

Rodney

There are clamp on ammeters available. A DC clamp on is a little harder to locate than one for AC but I purchased one a Sears a couple of years back for about $50. It is accurate enough for my use, probably about 1%. Nice thing is, it will measure quite small currents if you loop the line through the coil a few times, multiplies the reading by the number of turns. This comes in handy when you do not want to break a lead for measurements.

Red Scholefield

My Feedback: (9)

Where you are using a charger that has pressure contacts for the cells you can slip a simple current probe between the cells. See picture. It consist of an insulation material between two tabs to which you can connect your multi-meter to read current.

SacramentoJoe

I used to use a clamp on ammeter to load test 12-volt car batteries back when I worked at a service station. My car timing light also uses one. I always wondered if they made a smaller version. Never thought about looping the cable for small currents, though. That's smart!

mikehoulder

Sorry, I should have said that I was using a small digital camera battery charger with four sets of pressure contacts and there are no external cables to intercept. I would have break open the charger and damage the pressure contacts. Why take the trouble? Many of my NiMH cells are damaged using a variety of the cheap low-tech chargers that are available here (millions of miles away from adequate resources). I want to know why so as to avoid risk to my aircraft in the future. Shortly, I will buy from the USA a decent charger and I want to be as certain as I can that I won't damage my cells.

Thanks to all of you for your replies and suggestions. Thank you, Rodney. I had to find out what an ammeter clamp was and that lead me to an explanation of the Hall effect which was very welcome.

Thank you, Red. You are telepathic. You guessed that I was using a pressure contact charger. Your suggestion is the right one for me here and now. If I use a small square of plastic sheet to separate a battery contact from its charger contact, how thick does it have to be?

Best wishes to you all
Mike

KW_Counter

Mike,
Yes, in theory your idea of a resistor in series will work.
What you need is very low resistance with high current capabilities, 1 ohm at 10 watts for example.
The more accurate your resistor the more accurate your answer.

Personally, I would make up a charge cable where I could put a meter in the line.

Good Luck,
KW_Counter

mikehoulder

Stone the crows!!! kw_counter. I didn't expect that. You've made my day.
Mike

Red Scholefield

My Feedback: (9)

1mm is OK, I use double back sticky tape to hold the whole thing together.

KW_Counter

Mike,
What does "Stone the crows" mean?
Did I do good?
Thanks,
KW_Counter

mikehoulder

KW_counter, you did very good for me, Thanks a lot. I have a big grin now.
"Stone the crows" is an expression of intense surprise. It is probably a sanitized and polite mutation of something irreligious such as "Gor Blimey" which means 'May God blind me'. I'm a bit out of date now; perhaps I'm from the wrong century.

KW_Counter

Mike,
Glad I could help.
I'm also from the wrong century.
KW_Counter

mikehoulder

I have some very limited and provisional results. Many thanks to all, especially Red. My charger is charging two NiMH cells at 800mA until the the peak detection trips. Thereafter, it tricle charges at 50 mA. Could it be pulse charging? My meter seems to show a drop in current every second.

I'm waiting for some new equipment. When it arrives, I have a long list of experiments to do and I'm sure I will be boring you all with a host of resulting questions.

One thing I must do before the new stuff arrives is read through all 49 pages of the threads in this forum. I've already found the exact same question there to one of mine and with a very good answer. Sorry, P-40 Driver.

Best wishes to all
Mike