Solar power setup suitable for charging batteries

[Harold Davis]

OK, I've taken on board the responses to my other question and I'm persuaded that solar would be more suitable for my needs than windpower, even here on Lewis where this afternoon (when it wasn't especially windy for this area) the wind at height 2m was averaging 23mph and gusting up to 35mph.

What pieces of kit would I need for a small quick and dirty solar setup suitable for charging AA, AAA, C and D-sized batteries?

I was thinking as follows:

• small solar panel, producing maybe 100W

• load going to 12V leisure battery and, when battery is fully charged, to somewhere else, such as a bulb - with load directed by charge controller

• from the battery, run either a 12V battery charger if I can find one or an inverter with a 230V battery charger plugged into it

So this comes down to needing a solar panel, a battery, a charge controller, some cable, and a circuit with a resistor (bulb) in it for dumping unwanted charge.

Ideally I could fix the panel to a wooden frame containing a cupboard for the battery (security isn't an issue here) and bulb and then run a cable to inside the house where the batteries would be charged.

Is there anything else I would need?

Thanks!

Harry

[Theo]

Look at a Maximum Power-Point Tracker (MPPT) device. They exist for 12V lead acid and for lithium batteries:

You could probably use an MPPT in CC-CV mode, like this one: It depends whether you need to charge batteries when the sun isn't shining, at which point you'd need the bigger battery to buffer the charge. If so, an MPPT plus a 12V lead acid and a 12V NiMH charger would do it.

Don't run an inverter, it'll waste your energy.

Theo

[Theo]

By which I mean, you set the constant voltage (CV) to be the fully-charged voltage of your battery (about 1.55V for a single NiMH), and the constant current (CC) to your desired charge current. The battery charges in constant-current mode until it reaches the set voltage, when the current tails off:

The modules have potentiometers to set these values - you'd have to calibrate them with resistors as a dummy load.

Theo

[tabbypurr]

You're surely joking. What's needed is a small solar panel, nothing like 100w, and a diode. The diode prevents the panel flattening the cells at night. If the panel is sized large enough to exceed a wise charge current, add a shunt voltage regulator.

NT

[dennis]

With something as unreliable as a solar panel you need a boost convertor to get a sensible voltage out to charge things whenever there is some light, just like an inverter for solar panels.

[tabbypurr]

At the risk of stating the obvious, you only need a boost convertor if your panel is low enough voltage that you need a boost convertor. If you're charging 6v of rechargeables from a 12v panel a boost convertor is not needed. By the time its output drops to 6v it's not worth transforming anyway.

NT

[Brian Gaff]

Why would you want to dump unwanted charge? Brian

[Brian Gaff]

I'm not sure if they all do this, but one of mine has a kind of pulse mode where it checks the battery every so often and keeps it topped up. I only know as I had this weird interference on the radio which sounded like a Sick ferret every few seconds! :-)

Brian

[Martin Brown]

Lets take a step back.

How many hundred of these batteries per day are you intending to charge?

D sized batteries are about 4.5Ah so to charge 4 of them at C/5 rate you would need about 1A each and 1.5v which is a total of 6W for 5 hours.

AA batteries are around 2Ah which is about half that.

You can get cheap AA solar chargers that are intended for camping and pretty much double their effectiveness with a couple of handbag mirrors either side at 60 degrees \_/ pointed at the sun.

Or perhaps something like this:

It really isn't clear what you are aiming to acheive!

You would be better off buying a 10-20W solar panel intended for maintaining a car battery and a commercial battery charger intended for plugging into a car cigarette lighter. Different chemistry batteries need different charging regimes if they are to live a long and healthy life. Certain Lithium types are inclined to catch fire if abused.

A small solar panel will not object to an open circuit load you don't need to dump the power it generates anywhere.

A windmill you do or the rotor will spin too fast and out of control.

[rmlaws54]

Take care what type of diode - you need one that does not drop too much voltage in the forward dircetion. In general the leakage (in darkness) of a PV panel is very small. I lived on a boat for 10 years using solar panels for electric power and never needed a diode. I just had the PV panel connected directly (with a fuse) to the lead-acid accumulators.

I do agree that you need some kind of regulator (in my case it was me disconnecting the battery when fully charged). it is a bit easy to fully charge and then 'boil' (i.e. electrolyse) off the water if the battery is left on charge for long periods withjout being used.

I used several 100 W panels wired in parallel.

Robert

[tabbypurr]

For a serious solar install you'd use a schottky diode. For a small 12v panel charging some AAs it's pretty immaterial.

Panels with a diode built in won't drain your batteries. Panels without do, and need that external diode.

NT

[Rob Morley]

That happened to one of mu Minis when the regulator died. I couldn't figure out what the disgusting smell was, I suppose it was some sort of sulphur compound.