Charging Two 6 Volt Batteries from a 12 Volt Solar Panel

No it wont work. Follow your lines from the positive to the negative of the first battery and from the negative to the positive of the second battery.

You have two dead shorts in that circuit!

TCMTECH;

Perhaps this why a Radio Shack Electronics book and a pencil should never be given to a monkey. Daaaaa. Thanks - I might have burned my play house down.

If I put one blocking diode(d2) between the positive terminal of battery two and the positive rail this should stop the battery one short. If I put another blocking diode between the negative terminal of battery one (d1) and the negative rail this should stop the short in the second battery. Will my batteries stay balanced? Will my load rail voltage now be 5.4 volts instead of 6 volts?

Gavilan

You are supplying 12 volts on the 6 volt rail.....that might not be quite what the OP wanted, I nearly over-saw it myself!!!

The 0.6 volt loss over the diode will only drop a charged 6 volt battery from around 6.6 volts to around 6 volts.....not a problem.

People forget that the batteries are only "nominally" 6 volts, as car batteries are only "nominally" 12 volts, but in actual fact much higher......

99% of the time a healthy battery is much higher in voltage than its "nominal" number may suggest......

Man, do I feel stupid.

You must switch from charging mode to discharging mode for this to possibly work. This will also mean that you must have some circuitry to either decide or manually switch which mode you are in. I also wonder if your 12V solar cell is sufficient to charge two 6V batteries in series. First, most photovoltaic panels are rated at the peak performance time of "full noon sunshine". Much of your daylight hours will probably not be producing a full 12V with 1 and 2/3 amperes of current. Second, a nominal 12V car battery typically requires 14.5 volts from the alternator to charge current into the battery. This overvoltage for charging will depend on the chemistry of your battery, but all require some.

If this were my project I'd keep the batteries wired in parallel and use a micro-controller controlled switching circuitry in a buck configuration to step down the solar panel voltage to charge the batteries. After I had gotten this to work I'd consider putting and ultracapacitor in parallel with the solar panel and then modify the switching circuitry to a boost/buck configuration. This way one could maximize all of the light energy captured by the solar panel on cloudy days.

In addition to the problems listed above, I might also add that leaving the lights on dawn to dusk will likely deplete your batteries during cloudy days resulting in damage to the batteries from prolonged "0" charge condition without auto off battery protection circuit...You might also consider an earlier 2am cutoff...

Lot of good info on this site, do some reading...

http://backwoodshome.com/blogs/EnergyQuestions/2009/02/01/charging-a-6-volt-battery-with-a-12-volt-solar-panel/

First I want to thank you for taking the time to comment. It is much appreciated.

Quoteing; "You must switch from charging mode to discharging mode for this to possibly work."

Note The load circuit is in parralel with the solar panel and batteries.

The load on off switch is in the load circuit. I have been running a set of lights on a solar/battery system for several months now but I want to drop the voltage to 6 volts. It is shown in the link. It works. And it works well; but I either have to waste wire by putting two 6 volt assemblies in series or waste power by using a resistor to drop the current.

The system is being designed for use in tropical areas. Lots of solar insolation. But it works quite well here in East Texas. No fancy circuits please. Keep it nice and simple.

I used the term 12 volt solar panel but it is a panel designed to charge 12 volt car batteries so yes the voltage is greater than 12 volts while charging. The charge controller protects the panel from battery discharge at night.

Each assembly pulls about .25 watts of power. A ten assembly system will light about 100 feet of pathway. Thats 2.5 watts. I expect a reading station assembly will draw about 1 to 2 watts of power. I have not yet tested a reading assembly.

Using a 16 hour on cycle would require 40 watt hrs.(way more than required) Each of the 6 volt batteries are rated at 14 amp hours so that appears to be far in excess of what I would need but I want to keep the discharge cycles reasonably shallow.

1. The load circuit is powered off by the P Channel based MOSFET circuit as shown in the link - it works well. I have had one solar based and 3 plug in models working for several months. No problems so far.

2. During daylight hours all of the panel power except for the parisitic power going to the control circuit, which is less than .0003 watt, feeds the battery.

I will be taking this circuit with me when I start traveling thoughout SE ASIA and South Pacific.

I know; absolutely that the on/off control circuit works; with several hundred hours of operation.

I do not know for sure - yet - the batteries are enroute - whether I will be able to adapt the two 6 volt 14 amp hour batteries to my solar charger designed for 12 volt batteries.

These low voltage, high power lighting systems have considerable potential for folks who pay 38 to 45 cents a KWH for power -or in some cases - can't afford a generator and fuel. It will make a difference in their lives and gives me something constructive to do in my travels. If I can work the bugs out I will teach some young people at each of my stops to assemble the systems for local use.

It needs to be simple in assembly and operation.

The solar insolation will be adaquate even with the small 20 watt panel.

The same control circuit works for plug in and solar powered systems.

The questions now remaining:

Will the second circuit as shown charge two 6 volt batteries in series from a 12 volt charging circuit that is supplied from a 20 watt array designed to charge a 12 volt battery? I believe that they will.

Is there any shorts in the circuits or will the blocking diodes solve that issue?

What will the voltage drop be across the leads of the load as shown if the supply voltages of the two batteries is nomilaly 6 volts each?

Any comments would be greatly appreciated.

My motto will be - lighting the path while still seeing the stars. Its an anti light pollution thing as much as it is a give back thing. In any case it should be fun; but I need to get it right before I leave.

Please no curve balls.

Thank You

Sincerely;

Gavilan

A man without a tool is just another monkey.

Hi Gavilan

What about using a solid state double throw relay to connect your load to the battery with the highest charge. (with a time relay to prevent pulsing)

Eliminate the short circuits, otherwise the wiring will suffer and the charge controller will get damaged. Also, the batteries will die.

Simply put it is impossible to run the two batteries in a combination of series 12 volt and parallel 6 volt at the same time. You can have one or the other but not both at the same time.

Solar panels as I understand them are not fussy about voltage pull down from loading. being the can run at any voltage between short circuit zero volts and what ever there maximum open circuit voltage is without harm.

Just wire your batteries as a 6 volt parallel system and let your solar panel dump into that. At most you would need to possibly add a 6 volt charge controller into the system where the original 12 volt one is now.

"Just wire your batteries as a 6 volt parallel system and let your solar panel dump into that. At most you would need to possibly add a 6 volt charge controller into the system where the original 12 volt one is now." - tcmtech.

Thanks!!!!

This looks like the best solution so far. I simply buy 6 volt charge controlers instead of 12 volt.

Do I need to do any rewiring on the solar panel?

Gav

Thank You everyone.

The control circuit of the lights has a photo-cell controlled P Channel MOSFET that breaks the circuit continuity between the 6 volt rails when there is ambient light.

This occurs as the voltage begins to build in the solar panel.

This means that the 6 volt circuit "switch" is effectively open while the batteries are charging. (I hope - but I am not ready to bet $40 dollars in LEDs on it so I am going to test it with a single assembly first for a few days.

Here is the load circuit - showing the photo cell controlled P Channel MOSFETs switch.

The leads are the 6 volt rails from the batteries in a solar powered system or the transformer from a plug in system. (Fuse protected - not shown)

It works wonderfully in the plug in models. The solar model uses the panel voltage to bias the gate circuit without the use of a photo-cell.

In the plug in model the diode is probably not needed since the source and gate are at equal potential. It may not even be needed in the solar model where the gate voltage is the panel voltage; but I wanted to make sure the battery could not discharge into the panel through the gate. All I know is it works; but the lights come on too early when I use the panel voltage without tweaking the resistance of the gate lead. The photo-cell was a not too expensive monkey solution.

In the solar model I have been dumping current through a resister and wasting power; thats why I want to go to a six volt supply voltage. I could just wire it up for 12 volts (putting two - two LED assemblies in series) but the assemblies are about 10 feet apart.

Hopefully I won't burn my playhouse down.

Gav

I assume that the six triangles shown on the right represent LEDs. I don't see any current-limiting resistors. Unless the MOSFET itself acts as a limiter, you will likely blow the LEDs. You need at least one current-limiting resistance.

The supply voltage is low enough I don't need a limiting resistor. The two LED assemblies are in parallel. The two LEDs in each assembly are in series.

Thats a three volt drop across each LED. There is no excess current to limit. Resistors are power wasters.

Sorry, I've been using LEDs so long I keep thinking of the older colored ones that had less than 2V drop each...

It takes no more wire, and sometimes less, to wire two of your 6V modules in series for 12V than it does to wire them in parallel for 6V. It does mean you must use an even number of modules, but it also eliminates the switching from charge mode to use mode.

KISS! (I assume you are familiar with that expression)

Why do you want to reticulate at 6V?

Why not stick with 12V reticulation/loads?

Just one more item...

Some method of charge regulation other than just connecting the batteries in series might make life easier (and longer) for them. Here are some articles on the subject that are available online:

Life Extension Through Charge Equalization of Lead-Acid Batteries
http://energy.ece.illinois.edu/balog/images/intelec02.pdf
Battery Cell Balancing: What to balance and how
http://focus.ti.com/download/trng/docs/seminar/Topic%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.pdf
Battery Equalizers-Installation Instructions

http://www.allbatterysalesandservice.com/graphics/52109-52204-52206-52208-52210.pdf

...and in case you are considering Lithium Ion batteries

Multi-Module Equilizer Circuit for Series-Connected Li-Ion Batteries

http://pearlx.snu.ac.kr/Publication/[DC91]Multi-module_Equalizer_Circuit_for_Series-Connected_Li-ion_Batteries.pdf

Nobody is born knowing how to do all this stuff. Good luck!

At these low output and charging densities will cell balancing be a problem?

The batteries themselves should balance on discharge?

In your posts, you seem to always know better than the Engineers that have replied to you, or is that just my take on it. My apologies if that was not your intention.

The Engineers here are right. You can charge or use, but not both at the same time.

Or switch to a 12 volt lighting, usually a change out of all 6 volt bulbs to 12 volt will do just that and then run a 12 volt line, or do the same, but run two 6 volt bulbs in series with each other if you have a lot of them to use up......

May I suggest that you at least add a fuse in each Battery + line, to safeguard you and your equipment, that seems to have been forgotten.

Do buy a good supply of fuses, I somehow think you may need them......

Best of luck and remember - Wear safety glasses when working.

Use series connected pairs of 6 volt led assemblies and just have a 12 volt system, or use 12volt led assemblies. If you are making your own led assemblies just make them to operate on 12 volts.

Well what I would consider is if you leave the battery in series for 12 volt charging & with the 6 volt lights just divide the load up in 2 even circuits & operate each off a seperate battery no need for extra switching for series/ parallel etc that way if 1 battery starts to fail it is not in parallel & will not drag the other down with it.

Just a thought but there are possibly down sides to this arangement that I am sure some 1 will inform me about

Tapping from sub batteries results in differential discharge and can lead to early system degradation of the complete bank due to the unbalanced charging that is consequentially required.

Technically possible but not a recommended best practice.

In addition to this performance issue, undesirable fault conditions can arise. A short circuit in 1 6V loading branch will result in 12 V being presented to the other branch for the duration of the short circuit. An open circuit battery fault condition will result in the charging current to flow through the loads of the effected branch acting as a series resistance with the other battery. If not correctly fused a fire may ensue.

GA

in common you don't want to load batteries in series
coz they differ ... physically ... develope differences in time

so in your suggested case you'd need regularly chek for matching pair

step up/dn converters would loose some 5-th of your charge (but you do have excess) ... buing the wire is cheaper

just fusing the load . . . doesnot that cause batteries overload (what that charge controller can do) [i constanly kill my components i have no problem with that - just next time be more careful :D]