Has anyone seen reviews that measure the efficiency of NiMH chargers? I haven’t.
I was surprised last night when I determined that my LaCrosse BC-700 wastes more than 50% of the power it consumes. When it’s pushing 700mA into each of four NiMH batteries it consumes 2.8A from the 3V supply So in just over 3 hours of charging to fully charge 4 discharged Eneloop NiMH batteries it will use about 25Whr to put about 10Whr of energy into the 4 AA batteries. So charging them is only 40% efficient. If I’ve got an unlimited supply of 120VAC power, it’s no big deal. If I’m off the grid using solar, a power bank, or SLA battery, a 40% efficient charger is certainly less than ideal.
Is there something inherent to NiMH charging that makes it inefficient or are the manufacturers just lazy?
Well, what you described does indeed seem like charger inefficiency; inefficient power conversion.
But charging NiMH chemistry also has some inherent inefficiencies. I’m not sure what the actual efficiency is, but the temperature rise while charging indicates wasted power. As the cell becomes full some of the input current is used to split water molecules. The water molecules then reform which causes the temperature rise.
Charging Li-ion, on the other hand, is very efficient; cell temperature doesn’t rise much while charging.
I use a larger solar set up to run my chargers so it got me thinking and I just compared some of the chargers I use. I think your findings are unfortunately fairly standard for most chargers.
I checked with a Watt’s UP meter on the input and when available I used the chargers internally displayed voltage and current for the output so there could be some error there. I did my tests with 1 AA Eneloop in the charger.
Maha MH-C9000 Wizard One 0.3W idle, 1.48W out, while charging the input current fluctuates between 1-5W Probably averaging around 3W.
Foxnovo 4S 0.2W idle, 1.3W out, 3 W in.
Nitecore D4 0.3W idle, 1W out, 2.7W in.
Old Rayovac PS4 1 hour charger 0.6W idle, 2.2W out, 4.8W in.
I would say the heat a charger generates when doing it’s job should be a good indication of it’s efficiency. I have yet to see a charger that does not get quite warm when doing it’s job.
Yes, I see that for some of the test of some of the chargers like the MC3000. It looks to use about 2.5W to charge a NiMH at 1A. Hard to say exactly since the current consumption is not a flat line. The baseline consumption is 1.1W, so it may be more efficient when charging 4. I will see tonight when I test.
So, it took me a lot longer to test this since I had to get a 5.5/2.5mm barrel jack, but the MC3000 uses about 9.8W with the backlight on and 9.5W with the backlight off when charging 4 depleted AA Eneloop batteries at 1A each at the start of charge from a 12V power supply. This is measured at the 12V input to the charger and factors out any power consumption of the current meter. I also checked to see if the current draw increased or decreased as the cells charged, but it wasn’t very much. At the 7/8th’s point in the charge power consumption with the backlight off was up to 9.85W.
So, it used ~19.5Whrs to put ~10Whrs into the 4AA batteries. Just a hair over 50% efficient.