Necessary voltage to charge NiMH using Solar Panel

Question

I'm trying to understand which voltage I need to charge my NiMH batteries so I can purchase the components to do this.

Actually I have 2x AA Ni-MH 1.2 V 1900 mAh and 4x Solar Panel 2 V 220 mA (0.44 W).

I read Solar Cell - Preventing overcharge of an NiMH battery and Voltage input for charging NiMH Batteries but I'm still confused as I'm not an expert in this field.

I gathered this information:

• Batteries should not drop under 1 V or they will be damage also not go over 1.78 V;
• Constant voltage doesn't work to recharge them (not sure).

I checked out some components to stabilize the current and I found a LM317 in order to stabilize the current to (C/10) 200 mAh and use a MAX471 to check out these values.

But the problem will be the voltage that during the day will drop or go up due to weather so the voltage will never be stable, it is a positive thing?

In the case I need a stable voltage I also found these components (M2578A, TL497A).

I hope to find somebody that can help me. I'm free to other solutions if this one doesn't fit my purpose.

Answer 1

You must have at minimum a battery charge controller because you need a constant current mode. You also need something to make sure you don't overcharge the battery and start a fire. To adequately use a circuit like this you will also need something to stabilize the voltage from the solar panel called an MPPT tracker to make sure the voltage is constant.

Source: https://www.mpoweruk.com/chargers.htm

Answer 2

Quickly charging Ni-MH from a solar panel is very hard to do. Unlike lithium ion or lead acid, you can't reliably depend on the cell Voltage to determine when to stop charging, and depending on the brand, Ni-MH can be damaged by over charging much more so than Ni-MH.

The easiest way is to just set the charge current at the safe value of 0.03C for all types and just let it trickle charge. With NiCd you can go up to .1C and it'll handle that for a couple of years just like a cordless phone battery does with continuous charging.

Since the solar panel doesn't produce electricity continuously, full charge detection methods like negative Delta V or temperature is difficult or impossible. Do you know of a battery charger that can reliably charge Ni-HM cells if it is unplugged every 10 minutes?

One method that can work is to use a type of counter logic to track the total discharge and charge going in and out of the cells. Take in to account that Ni-MH charges with 2/3 efficiency and you have a good idea of the state of charge. Trickle charging can ensure full charge is reached eventually.

MPPT is just a method of adjusting current being taken from solar panels in order to keep the solar panel Voltage at its optimal level for given temperature and sun conditions. MPPT is a good investment if the cost of the solar panels exceeds the value of adding MPPT by a couple of times. It has nothing to do with battery charging, even though MPPT devices are often associated with battery chargers.

Answer 3

A low trickle charge should be safe. Low is about 0.03 - 0.05C. If C = 1200 then C/40 = approx 30mA. If this is supplied from a solar cell, low level sun exposure is likely to 8-10 hours per day max.

This means direct charging off a solar cell is only semi-continuous. A solar cell at low current draw (relative to its capacity) is essentially a voltage source (See Solar cell IV graphs) The voltage is a function of solar irradition. High irradiation has a higher voltage.

Choose a resistor to provide the safe trickle current at maximum irradiation. Say you have a 6v cell to provide 30mA to charge 3 cells in series.
R= (6v -3.6v )/0.030A =2.4/0.03 = 80ohms Cells in parallel should have (6-1.2)/0.03 = 160 ohm each.

Any current provided at less than that at maximum irradiation current will be less than the safe level. Also the current will also be intermittente s (because the sun is not constant). This means you could probably safely reduce the resistors to so that they provided up to about C/10 for short periods of time.