Solar charge controllers specifications

On a sunny day (Mon, 26 May 2014 03:00:49 -0700 (PDT)) it happened "A. B." wrote in :

been working on solar for some time now. I started by buying cheap solar cells, to make my own panels, and recently bought a huge solar panel, should go on a boat. Its not much power, I measured about 45 W from a max 100 W panel. Anyways the battery, and the type of charger, is an interesting subject. There are many types of batteries, and as many types of charge curves. I did a search on ebay for solar charge controllers, and most looked like crap. So I ordered some of these:

Those are voltage and current adjustable bucks that I will use to charge a 12 V battery, now lead gel, but possibly later LiFePO4, but those are very expensive indeed. My panel gives about 19 V or more open voltage, so for a 12V battery I need to use a buck and set the maximum voltage per cell for the battery, and the maximum charge current can be set in the same unit. In short, whatever battery you use, look up its graphs, and then it is simple. All batteries I know about will require a max voltage per cell monitoring when charging, and a limit on the charge current (my panel puts out about 5.5 amps, I'd rather use a lower charge current). this brings me to an other aspect you need to investigate, will you deep-discharge your batteries (what do you use it for), if so notice the short lifetime of lead acid or lead gel. LiFePO4 wil give you up to 2000 charge - deep discharge cycles, versus a few hundred at most for special lead based deep charge ones.

Then there is the 'other side' do you convert back to normal 50 or 60 Hz mains? I decided not to do that and got some boost converters to power my laptop, Samsung HD monitor, other stuff, the Samsung monitor uses an external 14V adaptor, added some xlr connectors as power connectors, and the laptop is 19V and the other something like that. The losses in these simple boost converters should be lower than going via mains voltage. I got these:

already tested, excellent, very little RFI (probably because of the ring cores), about 100 kHz.

As you can see all these things are Fery Sjeep, and great fun to play with. This is my experience so far, I ordered some smaller LiFePO4 to play with and test charge - discharge, those have a very stable 3.2 V per cell, even until completely discharged.

So the best of luck, there are so many parameters, but this is what I know so far. When the bombs fall at least I can still have light and radio :-)

Never bet one one horse, fix the race if you can.

This makes perfect sense since a large solar PV array is usually configured with the panels all in series to be a relatively high DC voltage at modest current to avoid resistive lisses in the downleads. This solar voltage can pose a threat to firemen in the event of a fire.

The controller must be able to handle the maximum current that the solar PV can deliver on the sunniest day with an unobstructed direct sun and thin white cirrus everywhere else (and a bit of safety margin since things run right at their rated limits tend to die early). It must also be able to survive the open circuit voltage of the array which can be considerably higher than the under load nominal output.

Obviously the charge controller must output a voltage and current compatible with the destination battery ratings and charging regime and/or dump any excess when the battery is fully charged.

You can get roughly twice the output for a given solar PV by the addition of two plane mirrors either side forming a hexagon. eg.

\_/

Wind loading is a bit of a nuisance and some cheaper PV plastics do not handle the extra heating very well.

--
Regards, 
Martin Brown

often stated that "solar charge controllers are rated and sized according

to the solar array system voltage and current"! Normally charge controllers

are used to charge batteries then why they are not rated and sized according t

o battery's charging voltage and current witch is more logic in my opinion?

Because you can vary the charge current to a lead battery without losing too much 'nergy (you'll lose some from charging anyway). You will, however want to run your solar panels at maximum possible efficiency, and as I understand it this is somewhat dependent on the operating point(voltage and current). For mechanical generators, the control logic will adjust according to a simple search algorithm to leech the most power out of it. I imagine it is not much different here..

Well, in addition to the comments other people have made...

It's not all one or the other, in terms of "what do you design for"? Certainly, the charge controller must be aware of the charging-voltage requirements of the battery - e.g. what's the proper "float" voltage once the battery is fully charged.

As far as the current question - in most cases, a solar system's lead/acid battery bank would be able to accept a higher charging current than the panels will deliver - the charge rate is going to be limited by the panels. So, designing your controller to handle a

200-amp current flow into the battery, and then using it with a panel which can't deliver more than 50 amps in that application, is sorta a waste of money (unless you're planning to add more panels later).

Also, solar panels have a distinctly non-linear "current delivered vs. voltage at that current" curve, under any given set of lighting conditions. Peak cell voltage occurs when no current is being drawn (hence, no power delivered into the load). Peak current occurs when the panels are short-circuited... once again, no power delivered into the load, with all of the power being dissipated in the wiring or in the panels' series resistance (which isn't zero).

Assuming that you have panels hooked up in series (for a relatively high voltage, delivered at low current hence minimizing I^2*R losses in the wiring), you'll be using a charge controller which acts as a voltage step-down ("buck") switching regulator... delivering a higher current at lower voltage into the battery bank. A good controller will increase the amount of current it draws from the panels, and monitor the panel voltage (which will drop as the current flow increases) until it hits the "sweet spot" for that particular panel configuration... the point at which it's delivering the most power into the battery bank.

Doing that - finding the panels' maximum power point - may be easier if the controller is at least roughly matched to the panels.

ten stated that "solar charge controllers are rated and sized according to the solar array system voltage and current"! Normally charge controllers ar e used to charge batteries then why they are not rated and sized according to battery's charging voltage and current witch is more logic in my opinion ?

Hello Sir, We have built solar charge controllers for medium sized solar installations for some time. The cheapest/simplest/most rugged design is the shunt type. The most rugged battery is the lead acid or sealed lead acid. The basic rule of thumb is that if a solar panel is rated to supply 'A' Amps(maximum) at 'V' Volts maximum the maximum charging current is A/10 Amps. Lead acid/sealed lead acid batteries are immune to overcharging. If however, a delicate battery as the Li-Ion is used, the charge controller has to be much more sophisticated. Note that a full charging cycle for a Li-Ion battery consists of a constant current and a constant voltage phase, and these need to be controlled via a feedback loop. In such a case, please look up details at the Battery University. Hope that helps.

A rough guide is that a 3kW solar PV array will need a controller that can comfortably handle charging the battery array 14.4v at 200+A or

28.8v @ 100A etc. This is a typical UK size (despite the fact that the UK isn't very sunny and in winter they are a waste of time here).

Staying below 4kW is optimum in the UK for extracting maximum from the crazy feed in tariffs but the controllers are typically converting the output into mains electricity rather than storing it in batteries.

Most controllers track the optimum power for the lighting conditions. Partial shading of an array is very damaging to output.

So long as the controller can sense the I-V curve it isn't too hard with modern kit to track the optimum I*V as conditions change.

--
Regards, 
Martin Brown

usually the batteries are stronger than the panels (can handle more power) And the controller need only be as strong as the weaker of the two.

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umop apisdn 


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[snip>

I doubt it, with that EMP coming with them...

joe

1.25-127 V/cell for 'maintenance free' or VRLA (Valve Regulated Lead- Acid) batteries, and 1.35-137 V/cell fro 'wet' batteries.

Some temperature compensation applies, the coefficient of which I forgot.

joe

[snip>

Don't ever try to 'overcharge' a sealed lead-acid (VRLA) battery at 14.4 V during a prolonged time, because the valves will open and your liquid will come out as the gas, leading to a dry, and useless (if not replenished, which is difficult for VRLA batteries) battery.

Under normal conditions (not in Alaska) the maximum float voltage of such batteries is 1.27 V/cell.

joe

power)

No must be as strong as the stronger of the two. It is clear that if it is not the equal of the panels it will fry.

?-)

On a sunny day (27 May 2014 23:13:18 GMT) it happened joe hey wrote in :

Well, the proof of the pudding is in the eating...

so you're saying chargind a 12V SLAB (which has 6 cells) to more than

7.42V is going to damage it?

2.27V / cell would make a lot more sense.

--
umop apisdn 


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Ay, typo, sorry. 2.25-2.27 & 2.35-2.37 V/cell. Better?

Joe, rests in shame

Yeah, make that 2.27 :/

Joe

Yeah yeah, I know, should 2.27 V :)

Joe