How do you know when to terminate charging when using Solar Panels?

[DisplacedTexan]

Great Forum!!

I want to build a portable solar powered AA/AAA NiMh charger.

I have been reading up on the standard ways of charging/terminating NiMh cells, e.g. fast charge at a constant current until you notice a voltage drop and then slow to a trickle charge...

But how would you do something similar with solar panels given that there current varies so much with the available light?

Anybody have any ideas?

Thanks!

 

[Mr Happy]

:welcome:

Unless you are planning on really big solar panels, they likely will not generate enough current to fast charge an NiMH battery. This is good, because if you slow charge it, you don't strictly need much charge control. As long as you keep the charging current below 0.1C (i.e. a current that would take more than ten hours to charge the battery) then the battery can "shrug off" the extra charge after it is full without coming to harm.

If you want to do better than this, you will probably need to have a switch that stops charging on maximum voltage. For many NiMH batteries a voltage in the range of 1.45 to 1.50 V per cell will indicate a more or less full charge. However this voltage can vary from one make and model of battery to another, which is why it is not typically used in normal battery chargers. For a solar charger, however, it might work out nicely.

 

[AlexLED]

Howdy, Texan !

You read almost correctly, except for that a trickle charge is not advisable for NiMH.
And beware that in order to detect the voltage drop, you need a charging current of at least 0.3, better 0.7 C. So you need quite powerful solar-cells....

You could use one of the standard chargers which have a 12V input and hook them up to a solar-panel capeable of providing enough current.

For example, use this equipment:
Charger:
http://www.batteryjunction.com/v1ec-bc2hu.html

Foldable Solar Panel:
http://www.21st-century-goods.com/page/21st/PROD/GSE12

I'm using the latter with my Orbit charger and it works well.

 

[wapkil]

Howdy, Texan !

You read almost correctly, except for that a trickle charge is not advisable for NiMH.

It depends on definition of what "trickle charge" exactly means but, if I remember correctly, in all of the meanings it can be used with NiMH cells.

In the first meaning (that I think the OP was referring to) the trickle charge is the current that is applied to the battery after the rapid charge (0.5C-1C) phase is finished. This current is much lower (0.005-0.1C) than quick charge current. As far as I remember some manufacturers advise to use it while others suggest terminating the charge immediately after the quick charge phase. The well regarded here Maha C-9000 uses this trickle charge (top-off, top-up) method. I think this phase is somehow similar to the constant voltage phase present for Li-Ion cells.

Another trickle charge meaning that I encountered is the maintenance current that can be applied to NiMH batteries to compensate for the self discharge. This method can also be used with NiMHs (but not with Li-Ions). Appropriately low current should be selected and I don't know what is the impact on the cell longevity.

I also saw recommendations to trickle charge the battery before starting the quick charge phase to ensure that it is in good health (similarly to what can be done for Li-Ions).

 

[Radiophile]

Out of curiousity - why?

If it were necessary for me to charge batteries with a solar panel, I'd use the panel to charge a 12V battery, and then use a charger capable of 12V operation to charge the NiMH batteries. Why? Because of voltage variations as you mentioned. A 12V battery wouldn't care about voltage and current variations. You probably wouldn't even need a charge controller. The NiMH batteries would then get a good charge and terminaton.

There are solar powered battery chargers for sale, but none I've seen are quick chargers, and charge termination happens when you take the batteries out of the charger.

 

[AlexLED]

Out of curiousity - why?

If it were necessary for me to charge batteries with a solar panel, I'd use the panel to charge a 12V battery, and then use a charger capable of 12V operation to charge the NiMH batteries. Why? Because of voltage variations as you mentioned. A 12V battery wouldn't care about voltage and current variations. You probably wouldn't even need a charge controller. The NiMH batteries would then get a good charge and terminaton.

There are solar powered battery chargers for sale, but none I've seen are quick chargers, and charge termination happens when you take the batteries out of the charger.

Yes, fully agree !
Drawback of my setup is that charging even terminates when somebody walks between the solar panel and the sun, so you need to position the gear wisely.

I forgot to add in my first post, I haven't seen decent solar-chargers out-of -the-box either. Using a easy-to-charge 12V SLA as buffer is a major advantage, apart from when you really are tight on space or weight to take with you.

 

[Illum]

Out of curiousity - why?

If it were necessary for me to charge batteries with a solar panel, I'd use the panel to charge a 12V battery, and then use a charger capable of 12V operation to charge the NiMH batteries.

+2
Solar panels work in some ways like a wind turbine [but without the variable frequency part]...the output is in DC but it varies greatly in current output with solar conditions; and it also depends on the type of solar cells you are using whether it be Silicon [Monocrystalline/Polycrystalline/Amorphous] or non-silicon.

When choosing a solar panel for residential purposes, sizing is important to ensure you can sustain your usage/charging when the solar cell is only working at partial capacity. Same goes here. Lead acid can take the abuse of an occasional surge of power, assuming that your not using a solar charge controller or your solar cells are lower than 7W combined.

The resulting power contained in the lead acid can be easily tapped and regulated...and its way cleaner than direct drive or through a switching power supply. Some lithium chargers, like the triton actually prefers to be ran off of a 12V Lead acid.

 

[SemiMan]

I am going to take a wild wild guess here and say that if solar charging IS important to you, then whether you get 500 or 200 charges out of your batteries is not going to matter much (since you said size and weight were important).

0.1C is not a trickle charge rate. You will kill your batteries and quickly if you maintain a charge at this level without some cutoff. For NiMh, you can maybe do 0.01C without long term battery damage. Your cell is likely capable of more.

I would look at a two step termination, one, maximum voltage (the just in case case), and two, voltage plateauing (if the charge current is above a level). NiMh voltage will plateau of go down when at the end of charge. If you are putting in reasonable current and the voltage is not going up, then you are likely done. Watch temperature effects.

Semiman

 

[mdocod]

I second, or third the recommendation to have a 12V SLA buffer on the power supply side. Or any other chemistry that can be voltage terminated with a simpler charge controller. Li-Ion in LiMn, LiFeP04, or even LiCo chemistry could all work pretty good for this. I would personally probably suggest 3xLiMn cells in series (26670 size), with a charge termination at 12V even, this would keep them charged to about 80+% which is a reasonable balance between storage condition and having plenty of supply on tap and should give years of trouble free service with hundreds or possibly thousands of cycles depending on temperature.

 

[DisplacedTexan]

Great responses!

Thanks for the input. Since it was my first post, it had to be approved by a mod, and I wasn't aware it became "live".

I might just end up using a 12-volt lead acid battery instead and use one of those purpose built cheap solar trickle chargers for boats/rv's that aren't used often.

As to the why part... I brew my own beer and am thinking about using a micro controller like the arduino to measure the temperature in the shed way out in the yard over the winter and send the info back wirelessly and recording it. It probably has more to do with me learning about how micro controllers work .

You guys no more about batteries than most any other group I know!


Thanks!

 

[baterija]

You could also look at using NiCd instead of a NiMh. It's got lower capacity but is very resistant to damage from overcharging. It's what you see in most of the solar lights.

 

[VidPro]

carefull selection of solar and battery voltages it IS possible to have the whole thing "self-regulated" wherin when the voltage is lower on the battery the charge is faster, and visa versa.
this is under the assumption that the power is going to be being used regular.
simple, use a voltage cutoff
increase the storage so the quantity is not an issue, but the capacity still exists.

 

[georgesymond121]

Well, I have a question to ask , I just bought a 500watt solar panel from a company and want to install it at my house roof but I don't know how to put all things together but company mechanics helped me in fixing my problem. I want to know how much power it can generate for one day because my work is on computer and I have around 30 computers which absorb a lot of power. Therefore I bought solar panel to reduce some expenses.

 

[shadowjk]

Your average desktop computer consumes about 100Watt. If you have 30 computers, that's 3000Watt, or 3kW. Running them 24/7 means 72kWh per day.

More if you have screens for them all.

As for energy produced, one solar calculator estimates 2.41kWh per day average over a year, assuming fixed mount panel that doesn't track the sun, but also assumes you have no cloudy days at all. This would amount to 3 percent of your energy use. Less if you don't actually live in the Sahara.