azvmax

Here's some info I found.
http://www.reed-electronics.com/ednm...98/01di_05.htm

http://www.angelfire.com/electronic/hayles/charge1.html

Red Scholefield

My Feedback: (9)

Overheating batteries can come about in a number of ways. First lets understand where the heat is coming from. An insignificant ammount comes from the internal resistance (I2R heating). Consider, charging a AA pack of Ni-Cds at 1 amp. Internal resistance of the pack is on the order of milliohms. Lets say for the purpose of argument it is 100 mohms. This means that a 1 amp charge would give you 100 mw of heating power from the internal resistance. Thats 1/10 of a watt - insignificant! Where does the heat come from then? When a Ni-Cd goes into overcharge the oxygen now being generated is recombined on the negative plate. Since energy can neither be created or destroyed (non an AMA rule) it must be accounted for. In this case we account for it by what is called the heat of recombination. At 1 amp into a pack that is reading close to 6 volts when fully charged this comes out to be 6 watts total or 1.5 watts per cell. Now wrap this in foam, pack it in a balsa wood box (like they use to insulate LP gas tankers) and you have your answer to where the heat is coming from. In this example my handy-dandy little xcell spread sheet program tells us we could expect a temperature to go to 200F. (93.2C)

Red Scholefield

My Feedback: (9)

Overheating batteries can come about in a number of ways. First lets understand where the heat is coming from. An insignificant amount comes from the internal resistance (I2R heating). Consider, charging a AA pack of Ni-Cds at 1 amp. Internal resistance of the pack is on the order of milliohms. Lets say for the purpose of argument it is 100 mohms. This means that a 1 amp charge would give you 100 mw of heating power from the internal resistance. Thats 1/10 of a watt - insignificant! Where does the heat come from then? When a Ni-Cd goes into overcharge the oxygen now being generated is recombined on the negative plate. Since energy can neither be created or destroyed (non an AMA rule) it must be accounted for. In this case we account for it by what is called the heat of recombination. At 1 amp into a pack that is reading close to 6 volts when fully charged this comes out to be 6 watts total or 1.5 watts per cell. Now wrap this in foam, pack it in a balsa wood box (like they use to insulate LP gas tankers) and you have your answer to where the heat is coming from. In this example my handy-dandy little xcell spread sheet program tells us we could expect a temperature to go to 200F. (93.2C)

Wayne Miller

Thanks Red S.

I understand your explaination of how the heat is generated from charging.

What I don't understand is why the same pack will charge OK many times, but will occaisionally overheat with this charger. It always seems the problem surfaces when trying to "top up" a pack with the MKII.

There have been other threads on this and such things as power source, cell problems, charge rate, etc.

I personally would like to find an explaination that explains what happens to cause the packs to overheat only once in a while. Once I know, I can take the necessary precautions.

Currently, I always drain a battery just a little before charging, about 15 sec, and I have not experienced the heating problem since. This leads me to believe the charger samples voltage and not internal resistance, and is trying to charge to "peak detect", which it can not reach and therefore over charges, but this is just a guess.

I am keeping an open mind while this thread educates me in hopes to find the true answer.

Thanks to everyone who is investigating the problem and hopefully an answer will be found.

Fly4Fun,

Wayne Miller

Red Scholefield

My Feedback: (9)

When a pack is already nearly fully charged the voltage profile (peak or negative portion) is considerably different than a pack that has not been fully charged or has been recently discharged a bit. In these cases the charger in question does not have the sensitivity to pick up the subtle change and keeps going until there is a significant drop in the voltage as a result of the heating the pack gets in overcharge. What you are seeing is quite common. Missed or delayed charge termination. In the worse case scenario you end up with a toasted pack. In more sophisticated chargers there is a back up termination mode if the peak is not detected, this can employ a thermal cut off or a cut off triggered when the voltage drops a given amount from the maximum voltage reached in the charge. Now if you consider a pack where the cells have a modest difference in capacity, one cell will be peaking while the one with less capacity will already be going into overcharge and its voltage is decreasing. The net effect can be that there is no detectable voltage peak at the terminals of the battery as seen by the charger.

Geistware

My Feedback: (16)

Red, I never thought about that argument. If the cells dicharge at different rates, the combined effect may overcharge on or more cells but not all.!

Red Scholefield

My Feedback: (9)

Yes, that is why a topping charge is recommended after fast charge, to bring all the cells to the same state of charge. Note this does not mean they will all have identical capacity, but this won't make that much difference on discharge as you would not normally in our use take the pack all the way down while flying. I think you can see what happens if you don't occasionally top off the packs with a slow charge. The cell with the lower capacity will be given a deeper discharge, the deeper you discharge a cell the poorer the charge acceptance on the next charge and the cells get further apart in the state of charge reached. If this is repeated over and over without a topping charge you get into a real unbalance problem. This is why people saw such miserable performance from their Cam corder packs, there was no provision in those products for a topping charge, just fast charge only.

RICK9303

My Feedback: (172)

The standard fast charge rate is 3 times the over night rate. So if you have a 1500ma pack that you would
normally charge at 150ma over night, you would charge it at 450ma or about .5 amps. The scale on the charger is amps., and I believe 1.5 amps would be way to much juice to charge a nicad or nimh of this size.
Just a thought.

Rick

JNorton

My Feedback: (2)

Red,
Thank you for your concise information on how nicads voltage profile differs from partially discharged to fully charged. Does this hold true for NiMh too?

This information truly explains why the Hobbico MKII sometimes overcharges on fully charged batteries. I have never had a problem with this charger and feel it is a great value and very versatile for the price, but I could never reconcile the horror stories till now.

Thank you once again for sharing your expertise.
John

Geistware

My Feedback: (16)

If I am not mistaken, Max charge is 1C. Fast charge is 1/5C and slow charge is 1/10C

Wayne Miller

Hi Red S.

Thanks for taking the time to help us understand the battery characteristics and charge problems. It has certainly helped me understand the problem.

Fly4Fun,

Wayne Miller

azvmax

Thanx for the info Red

My charger randomly fails or intermittently topping off, not topping off.
Im am now using a friends exact same charger with the same packs with absolutely no problems for several months now.
I am sending mine in again to Hobby Services.
I believe there are some units that are out of tolerance or bad components within.

Red Scholefield

My Feedback: (9)

Not out of the ordinary. Hobbico has not built the best reputation. When they work they are fine, but then there is the too frequent occurance of not working quite as well as they should.