Battery Impedance and Charging Current/Voltage Monitoring.

Hello Ladies and Gentlemen,

I am new to this forum and it is great to see so many interesting topics and well educated and experienced professionals and hobbyists. I would just like some confirmation of information I have researched regarding a charging circuit project I am embarking upon. I am entirely new to circuits and design however I like to arm myself with as much knowledge as possible before attempting any circuits involving batteries due to their limitations.

At the moment, my plan is to use a DC-DC step down linear voltage regulator I have recently purchased to charge x4 1.2v NI-MH 600mAh batteries in series I found in some christmas lights. The output of the regulator is adjustable between 15v - 3v and the input is a 12v 1A power wall adapter that is non regulated I believe. From research these batteries should be optimally charged at C/10, or 60mAh at somewhere around 4.8 - 5.2v for 16 hours. However, I am becoming confused when factoring in voltage drop and impedance within the circuit and researching about optimal charging currents. I could not find a clear definitive answer however battery university provided some good information and it contradicts my current thinking.

- According to Ohm's law, shouldn't the internal resistance of the battery with a fixed voltage input determine its charging current?
- Do NI-MH batteries specifically attempt to draw much more current due to their chemistry than optimal for battery life and temperature control?
- Are there any batteries where keeping voltage constant with a regulator is suitable for charging due to the above questions? e.g. lead acid or Li-ion?

I am aware over charging these batteries can be extremely dangerous and everything must be timed and monitored constantly in a ventilated area due to noise and limited feedback, however a simple confirmation of the above would be suffice for me to move on from this project. Thanks.

The Hermit said:

- According to Ohm's law, shouldn't the internal resistance of the battery with a fixed voltage input determine its charging current?

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Not as regards charging, and the internal resistance isn't a fixed value anyway.

- Do NI-MH batteries specifically attempt to draw much more current due to their chemistry than optimal for battery life and temperature control?

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Be a pretty useless battery if it didn't draw, and provide, far more current than it's recommended charging current.

- Are there any batteries where keeping voltage constant with a regulator is suitable for charging due to the above questions? e.g. lead acid or Li-ion?

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No, you ALWAYS need some kind of charging control, or the batteries (or/and charger) will be damaged. Although this could be quite crude, such as a suitable series resistor to limit the current. However, with such a crude scheme you need to manually limit the amount of time spent charging, and NiCd/NiMh are notoriously to monitor their charging status.

Li-Ion is a LOT easier, just requiring a constant current charger that switches to constant voltage once the threshold voltage is reached (4.2V per cell), the current then reduces, and when it's down to about 10% of the constant current it's considered fully charged. You can easily charge them with a bench power supply, simply set it to the threshold voltage (so 8.4V for two in series), and the current limit to your required charging current (I use 520mA for 2.6AH cells). When the current falls to 52mA they are fully charged - this is a 'nice' charge, so charges them fully, you can charge faster, but they don't take as much charge then.

Thank you very much for your response.

Nigel Goodwin said:

Be a pretty useless battery if it didn't draw, and provide, far more current than it's recommended charging current.

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This made it so much easier to understand in my mind, I feel silly for not realizing this with its discharge capacity as well.