solar battery maintenance

**drivesafe** · 6th November 2016, 10:50 AM

Hi Magnet, and sorry this is so long but here is a bit more info on using unregulated solar panels to charge and maintain batteries with safety and how it works.

There is a lot more to it but I will try to keep this in an understandable form.

All batteries have an internal resistance and this resistance is the reason for the amount of current a battery draws, while charging.

With all forms of resistance, as you increase the voltage difference across the resistance, there is a corresponding increase in the amount of current travelling through that resistance.

This current travelling through a battery is commonly termed as the current load, and from this point on, to keep this as basic as possible, I will not use resistance and only refer to the current or current load.

To charge a battery you need both a voltage higher than the battery's State of Charge voltage ( SoC ) and some amount of current.

If you are charging from an alternator and you have a very low auxiliary battery ( in most D4s, this will be an Optima 55Ah AGM ), then there is going to be quite a high current being drawn by the auxiliary battery, and this current draw can initially be as much as 100+ amps.

In the above case, your auxiliary battery can only draw such a high current because the D4 alternator is capable of producing those sorts of currents.

With your 20w solar panel setup, the VERY maximum current available current, and this will only be for a few hours a day, is around 1.25A.

Over an 8 hour period of good sunlight, you would be very lucky to average as much as 1A. With no load applied to a "12v" solar panel ( called open circuit ), the panel can generate up to 24v

The reason for the difference between the rated wattage and the actual wattage you end up with is because of two related factors.

The first is that to try to get as high an output from a solar panel all day long, you need to have the solar panel track the sun. This means, the solar panel must continually be repositions so that it is facing directly at the sun, as the sun moves across the sky, from horizon to horizon.

Your setup will, like the vast majority of solar panel setups, will have the panel facing directly at the sun for no more than a few hours.

And as this wasn't enough to reduce the average amount of power coming from the panel, at the point where the panel is facing directly at the sun, the sun's energy will heat the panel and as a solar panel's temperature rises, it's total wattage output reduces.

So at the very time you think you are getting the maximum POTENTIAL ( 20w ) output from your solar panel, it will actually be down by as much as 30% ( down to around 14w ) .

OK so that covers the very basics of battery charging and solar energy production. Now to put the two together.

As above, a 20w solar panel has a maximum current output of just 1.25A and this will be for a small portion of a sunny day.

So at best, you have a charge current of 1.25A, and if you battery is low, it will be trying to draw all the current it can get.

Now if you apply a high current draw to a solar panel, the panel will produce all the current ( watts ) it can but the much higher current draw of the battery will pull the solar panel voltage down to pretty close to the battery's internal voltage.

So if the battery voltage is at say 11.0v, the solar panel voltage will be around 11.2v at the battery's terminals.

With this sort of very low current charge, it will take quite a few days before the battery gets anywhere near a fully charged state. But once fully charged, the battery will now draw very little current, so the solar panel voltage will rise at a greater rate than when it was being used to charge the low battery.

With a fully charged battery ( around 12.7v ), even a small current applied will raise the battery's internal voltage considerably.

So at the peak of the day, when your solar panel is putting out it's maximum current ( wattage ) the voltage at the battery will now rise substantially.

But as the solar voltage at the battery rises above the battery's internal voltage, the higher voltage difference will cause the battery starts to try to draw more current, and this higher current draw only has 1.25A available ( the maximum output the 20w solar panel can ever produce ) so the solar panel voltage can only rise until it reaches the solar panel's maximum current ( wattage ) output.

The batteries literally become their own voltage regulator.

Magnet, with your setup, the maximum likely voltage, at the high point of the day, is not likely be much over 14.0v

NOTE, if you use a larger unregulated solar panel, while the batteries will still act as their own voltage regulator, the high current ( wattage ) available will mean the voltage at the batteries will rise.

For instance, I use an 80w unregulated solar panel to recover badly sulphated batteries and a 100Ah battery will rise to 17.5v using an unregulated 80w solar panel. PS, do not carry out this type of charging on a good battery.

Your batteries can tolerate 14.7v for a number of hours each day, with little effect on the batteries. For example, your D4 alternator can run at 14.7v for long periods of time, depending on the state of charge of your cranking battery, and this high voltage has no effect on other batteries connected to the alternator, even if they are already fully charged.

Solar charging in some ways is very like charging from an alternator. When charging from an alternator, you have to be driving and few people drive long enough for even 14.7v to effect battery longevity.

With solar, the similarity is that it only charges the batteries while the sun is shining and even then, it only charges at the solar panel's maximum possible current ( wattage ) for a short period of time.

As Dave posted, test your battery/solar voltage at the high point of the day, and you will then know for sure that you will never over charge your batteries, no matter how many weeks/months you leave them in this type of setup.

**Ooops** · 6th November 2016, 01:03 PM

Originally Posted by drivesafe

... sorry this is so long but here is a bit more info on using unregulated solar panels to charge and maintain batteries with safety and how it works.

Drivesafe that's a brilliant explanation, many thanks
It even helps me understand what the wattsup meter's telling me

Originally Posted by drivesafe

All batteries have an internal resistance and this resistance is the reason for the amount of current a battery draws, while charging.

I get that lower battery SoC causes higher charging current draw i.e. the battery's starving and gorges itself and that the process slows up as the battery fills. Its the concept of ?internal resistance? that I'm not seeing. I can't quite grasp the principle that's in play here, do you have an analogy for this?

We've been talking about the D3 dual battery system with AGMs which I assume the panel treats as one big battery with the isolator open and one smaller battery with the isolator closed. Imagine a situation where the panel wasn't connected and the D3 battery voltage had dropped sufficiently to cause the SC80 to separate the aux from the crank battery. If I were to connect the panel through my rear Anderson would the isolator reopen when the aux was sufficiently charged?

In general non D3 use, does the size & type of the battery matter? Can I use this system to maintain individual lead acid 12v batteries on the bench or should they be wired to form a bank? Keeping them wired together would certainly be more convenient.

Does 14.7v remain the max acceptable voltage at the high point of the day or does it drop for non AGMs?

**Blknight.aus** · 6th November 2016, 11:10 PM

Originally Posted by Magnet6x6

Dave many thanks for the input.
In order to understand you properly I have a few more questions, I'm not second guessing you I'm trying to make sure that I've understood you correctly .....

Great thanks. Hibernation occurs as soon as the hand brake light goes out which is about 3 minutes

My Australian based reading tells me people base their calculations on 5 to 6hrs quality sun per day.
I'm currently in the Middle East so the panel sees full sun every day. Should I adjust the 5 - 6hrs sun upwards to perhaps 8 hrs a day for the purpose of calculations?

So you've calculated
total parasitic draw per day as 0.28aH x 24hrs = 6.7aH
panel output per day as 1.70aH x 6hrs sun = 10.2h or nearly double
Have I understood correctly? Where did 20ah come from?

As a battle field measure to get the 10w performance can I simply cover up half the panel?
I realise a reg is the answer but just so that I know.

I'm having trouble interpreting the dta that the wattsup meter is giving me. I'm in danger of having to read the instructions

On a completely different tack what would be the duty cycle for a Waeco CF50 run at decent beer temps in an ambient ranging between 40 and 50?C

The 1.7A I came up with for yours was from one quick google search for 20w panel info and is the dead short value for some panel somewhere.

Its not what you will get out of the panel for charging purposes and calculating how much you can put into the battery for working out how long the fridge will work for. But youre not using the panel in that way. your using it for a maintenance charger so the worst possible case is the maximum current at the maximum voltage the panel can produce.

20aH is a typo I didnt pick up. it should have been 10Ah.

as for the waeco question.. Id expect it to be in the 70% duty range depending on airflow what bag its in, if its in direct sunlight the humidity......

IT could go as high as 100%

and thats assuming you like your beer at 4 deg C.

for the towel on the panel trick...

that depends on how the panel is set up some panels will provide partial power in partial shade some wont.

usually the panel will have a bunch of smaller cells wired up in a series/parallel configuration, if you block off a complete row of cells that are in series with each other the panel should put out a lower total amps but the same voltage.

if you block off a set of cells that are in parallel the voltage will drop but the amps should stay the same.

**Ooops** · 7th November 2016, 04:21 AM

Dave, many thanks for the explanations and the 'towel on the panel? trick

Originally Posted by Blknight.aus

thats assuming you like your beer at 4 deg C.

absoballylutely, anything else just isn't civilised

The fridge is kept in a Waeco travel bag. The bag has some insulation but not enough to be of real use. The fridge is never in direct sunlight but I do need to find a way to upgrade airflow around it

**Ooops** · 7th November 2016, 05:32 PM

I've been experimenting with a single 55Ah Optima
A day or two back, at the start of the experiment, the voltage was 11 something v. Today the wattsup meter was showing just over 15v on the load side.
When I disconnected the panel I got 12.3v on the multimeter

Dave & Drivesafe, based on your instructions the battery's seeing too much voltage but when disconnected from the panel its below full charge at 70%

Should I keep charging until the disconnected reading is 12.7v?
If so what sort of reading should I expect to see on the load side of the wattsup while its all connected?

**drivesafe** · 7th November 2016, 07:44 PM

Hi Magnet and I use similar devices for doing basic tests.

But no mater what type of meter you use, because of the effects of voltage drop, the meter needs to be as close as practical to the battery, and you need to use decent cabling between the solar panel and the battery.

Where is you meter connected and what size cable are you using?

**Ooops** · 7th November 2016, 10:14 PM

Originally Posted by drivesafe

the meter needs to be as close as practical to the battery ..... Where is you meter connected

Its 12" of wiring away from the battery

Originally Posted by drivesafe

what size cable are you using?

I'm using a 10m piece from a 220v ext cable, the thinnest wiring is 2mm across excluding the insulation

**drivesafe** · 8th November 2016, 03:26 AM

If you have a separate multi meter, try measuring the voltage at either end of the wire, while the solar panel is connected and the sun is at it's brightest.

Also, your Wattsup shows watts and current going passing though it. Note both of those at the maximum sunlight.

As for your batteries, 20w is way to high for charging a single battery ( without using a solar reg ) . But an unregulated 20w panel is fine when charging your two batteries at the same time, plus you will be powering your isolator as well.

**Ooops** · 8th November 2016, 04:02 AM

Originally Posted by drivesafe

try measuring the voltage at either end of the wire, while the solar panel is connected and the sun is at it's brightest.

20v at both ends with no load
I haven't tried it at both ends while under load

Originally Posted by drivesafe

Wattsup shows watts and current going passing though it. Note both of those at the maximum sunlight.

I'll report back

Originally Posted by drivesafe

20w is way to high for charging a single battery ( without using a solar reg )

Noted, thanks. I have six old batteries in various states of decline. Am I right in thinking that I can bank these up and use the 20w?

Last question, there's some great deals to be had on cheap & basic controllers out your way but getting suppliers to post out of country is proving to be an issue. Do you know off hand of any firms that post overseas?

Thanks again

**Blknight.aus** · 8th November 2016, 11:21 PM

for unmonitered charging 20w is too high, for monitored charging go as big a panel as you like BUT you must manually monitor the voltages and current flow. (another thing drive safe and I have a differing opinion on is panel regualtion for some charging applications)

jump on the jaycar site and get the model off their cheapy regulator then go buy one from ebay if they wont still ship over seas.

No you cant just stick the 6 batteries on as a dummy load as they will drain the voltage over night.

the easiest form of regulation is a light bulb as you current limiter and a blocking diode.

BUT..

that wastes a lot of your available power.

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