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A Cooling Fan for the Maha C9000 Charger

June 2011

It was time to buy a new charger for the stack of NiMH cells I use for portable gear. After reading reviews and recommendations on various battery forums I bought a Maha C9000 charger because of the following features.

A status display: Capacity, current, charge time, and voltage for charge and discharge status. The electronics geek in me would love this because I wouldn't have to calculate capacity myself after the fact, and seeing voltage real-time is very handy.
Multiple modes: Discharge, Charge, and a break-in mode that charges at C/10 for 16 hours and then discharges at C/5 to form the cell and determine the industry-standard capacity.
Selectable charge and discharge currents.
Four independent channels. One per cell.

It proved to be a great charger with enough status info to keep me happy. It's not perfect though: My biggest knock against the status display is that one has to wait for it to cycle through the four values it gives for each cell. If all that one wants to know is the voltage then one must be patient while waiting for it to come around. The other misfeature is that one can't set parameters for one channel and then assign them to all channels at once; however, if one only wants to charge all cells at 1000 mA then the default is fine.

A heat problem

There is an issue with heat. When I began testing the unit I became concerned because all four cells would get hot enough that I was uncomfortable letting the unit run unattended, wondering if the charger was missing the end-of-charge indications from the cells. I do not like my cells getting hot in a charger; I measured cell temperatures of 42° for the center two, which were always the hottest. I began to wonder why those two were warmer. The obvious explanation was that the outer two were cooled by airflow or by radiating heat but a gut feeling made me reject that. I thought it might be the individual cells but further cycles with the same cells in different slots showed that the center two cells were always the warmest.

I noticed that the top of the unit just above the postive end of the cells would always heat up. Was it conducted heat from the cells or were there electronics under the cover that were getting hot and heating the cells by conduction? I found some photos online that showed there was nothing inside the unit at that location that could be responsible for the heat but I became suspicious when I saw photos of the bottom side of the circuit board. That's where the power MOSFETs are located. Perhaps they were getting hot and the heat was being conducted to the cells via the positive terminals soldered to the circuit board.

I removed the bottom cover and quickly found the source of the heat problem. It wasn't the MOSFETs; in operation they only became warm, instead it was the 78D05AL voltage regulator. It's used to bring the 12V supply voltage down to 5V for some of the electronics; it became too hot to touch. It's a linear regulator dropping seven volts, which leads to an appreciable power dissipation if there's more than trivial current being drawn. I don't know what the current draw is but that regulator became too hot to keep a finger on. A temperature probe showed 48° but I expect that it was even hotter with the bottom of the case on.

That heat was being sinked by the circuit board, which heated up the positive terminals, which then heated up the cells. The regulator is in the center of the top section of the charger, placing it closest to the center two cells, thus their higher temperature. I had identified the problem, but how to fix it?

A cooling fix

The slots in the case are so narrow that there can't be much airflow so I doubt that adding a heat sink would help much. There's no room in the case for anything but the tiniest of fans but there is enough air space between the circuit board and the bottom of the unit for air to circulate if a fan were mounted externally underneath the unit. So that's what I did: I hacked a hole into the case to mount an external 40mm fan on the bottom and widened the slots on the bottom edges of the case to let the air escape. The fold-out stand must always be used now to give the fan clearance. The fan's power leads were routed into the charger and soldered to the 12V connector inside; simple and effective, although the fan does run continuously while the charger is plugged in.

The designers of the charger may have compensated for the regulator heating up the charger and cells so I have a nagging worry that the charger's cell temperature profiling could be affected by the unit no longer heating up. However, the fan is only blowing on the bottom of the circuit board. The board has a tight enough fit that there should be little airflow on the top side where the thermistors are located. The fan blows no air across the cells, so they'll still begin to warm up as expected when they become full. The thermistors should detect this. However, all of my cells reach the C9000's 1.47V charging cutoff limit so I haven't been able to test a temperature rise cutoff.

Results

With the fan in operation the regulator only gets warm to the touch and the cells now only get slightly warm during charging instead of becoming hot. This suggests that most of the heat in the cells was being conducted in from the charger rather than coming from the cell chemistry dealing with the charge cycle!



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