Electrical – Charging 6 NiMH batteries in series via solar

battery-chargingembeddednimhsolar cell

I'm currently working on a project in which a Atmega328 is powered by 6AA NiMH batteries in series (7.5v, 2000MaH). It is in sleep mode most of the time, except to turn a servo a couple times a day, for a couple of seconds (Hence the need for the high voltage).

I have a solar panel at 12V and 1.8W, so maximum of 150mA. After doing a lot of research, and looking at this post: Charging NiMH batteries with 1W solar panel, charge controller needed? .
The maximum output for my solar panel is within the c/10 -> c/40 range that is safe for the NiMH batteries to be charged at.

My question is, none of the resources I could find explicitly say if what I am attempting is OK for multiple cells like I have.. Due to the remote nature of my device, charging each cell individually, or using a LiPo, is not an option.

Thank you very much.

EDIT: Below is a PCB design for this circuit. According to @BruceAbbott The voltage of the panel would be fine, but since my servo can only handle a max of 8v, would the batteries still be able to lower to voltage to not damage my servo?

I believe the reason it is safe is due to how the current and voltage is controlled from a solar panel, for example, this IV curve (NOT the panel i'm using, just an example.)

This should ensure that the voltage never exceeds the dangerous limit?

Best Answer

The panel will automatically limit the current to a safe 'trickle' charge, so all you have to do is connect it directly to the battery.

If the panel has high dark current then a small Schottky diode in series will block the reverse current. Your panel probably has pretty low dark current so the diode may not be necessary, though it could be a good safety feature if the panel is wired remotely (a short in the panel leads won't short out the battery).

Related Solutions

Battery recharging via solar panel

Vpanel >> Vbattery so panel acts as ~~ constant current source.

2 batteries in series or 4 batteries in series will still be << 9V so charge in the same time.

Imp < Icharge < Isc or
0.33A < Icharge < 0.43A

Say ~~= 350 mA.

T charge ~= Battery_mAh / Ichg x 100 / Battery_efficiency % NimH CURRENT acceptance efficiency probably 80%-90%

= 700 Mah / 350 mA x 100/80 ~~~= 2.5 hours

That's in full mid-day sun with panel square on to sun. Charge for longer than this and you will quickly "cook" the batteries.
Vbattery_max at that charge rate should probably not be more than about 1.5V/cell when charging.

Google: your_city_name gaisma
eg Bangalore gaisma gives

    http://www.gaisma.com/en/location/bangalore.html

to get SSH = sunshine hours per day (= kWh/m^2/day)
Your panel will produce ABOUT 350 mA x SSH mAh per day

Bangalore example -

Half a day on an average day will be more than enough in any month - see below.

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Chart from Gaisma entry for Bangalore as above.

Electronic – How does battery charging work

NiMH batteries should be charged by constant current until they are full. For your battery pack, since it is 2000 mAh, C = 2000 mA. For fast charging, the current should be around C/3 or higher, up to, say, C. You know they are full when the voltage levels off or dips (dV/dt), or the temperature starts to climb more rapidly (dT/dt). The voltage of the cell at this point varies depending on many factors. It could be as high as 1.6 V, but it will drop off rapidly when charge current is removed.

If the batteries are charged at a lower rate, for example C/10, it may not be possible to detect the dV/dt or dT/dt signal at end of charge. Usually, when charging at this low rate, the charge is terminated by a timer instead.

If you use a NiMH cell until it is "dead" or "flat," you will probably see that the open circuit voltage is around 1 V/cell.

So, when charging NiMH cells, current is what is important, not voltage, per se. You can expect the voltage to vary from low (could even be zero, in theory) to 1.6V. For a battery pack with 6 cells, the nominal voltage would be 7.2V. When discharged, the pack voltage might be around 6V (open circuit). And while being charged, the highest voltage might be around 9.6V (while charging).

I hope this satisfies your curiosity about NiMH. Most of this information is available on the internet. You can look at guidelines published by battery vendors, and also the people who make chargers and charger IC's.

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