Solar Electrics

[b]In response to a request for help I had these views:[/b]

I mentioned a number of options to alleviate your dependence on the generator. Given a spend of £600 you could buy 250W panel, 120Ah battery and a MTTP controller and peripherals. The ones I mentioned would be very good quality. You could save 100 or 2 maybe but quality would drop.

The cheapest option is just to have more batteries so that when you do run the generator you will be storing more for the next day. However after you gave me the details of the battery charger, at 10A max. it wouldn't charge much more with another battery or two and you would still have to run it for some hours. You may be able to get similar batteries to that you have for £60 (70Ah)

My latest view is that you would be better off with a single solar panel of 100W. This would give a 8A charge to your battery every hour of good sunlight, compared to 10Amax from the generator.

I have contacted Wind and Sun in Hereford and they have a new 100W panel from India, the cost is £126 including VAT and Carriage. It has a general junction box so standard cable can be used to connect it.

A more substantial option is to buy two such panels, two more batteries and a controller and you would be unlikely to have to use the generator, but that would cost another £350. This would give you 200W input, per good sunshine hour, 210Ah battery.

The outcome as of 20th June 2015 is to buy a single 100W panel as in the penultimate paragraph above. I am encouraging the user to connect it themselves and monitor the battery charge with a pre existing voltmeter. The panel can be manually adjusted to receive maximum radiance or alternatively reduced to ensure the battery doesn't over charge. (*Once a voltage of 14.5 has been sustained for two hours the maximum should be reduced to 14.* - This only applies to a continuous charge for example from a mains charger 24/7)

**26 Jan**
Today I noticed a 0.1v drop between the Victron and the batteries through 1 metre of 10mm^² cable. At and 10A that's a resistance of 1/100 ohm

Although there are limits to what voltage can be placed across a battery to charge it is largely governed by the current that flows through the plates. Too much and they will buckle, melt or even explode.

The first consideration in a Lead Acid battery is it's AmpHour rating commonly referencing what a battery can supply over a 20Hour period. Hence the rating is the 20h rating. For example, a 120AH @ the 20h battery can provide 6amps for 20 hours. So a bulk charge of 12 amp one tenth of the 120Ah would be fine, more may be acceptable, as much as 20 to 30 for a short period if the battery is not flat and has high resistance to a charge.

However the current is usually a by-product of the voltage which in a 12v battery is unlikely to be higher than 16v

The charge at this initial, bulk, level will be set to a voltage and any current that can be put in will be accepted. The low output from a 100w solar panel producing 19v going into a 120Ah battery that is only 50% full will have it's voltage dropped by the battery as a load and will take about 8 amps if full sun is available etc.

A maximum voltage will be set and when the battery reaches that it is assumed to be largely charged, 80% is an often quoted figure but this can be misleading in the case of batteries with a high resistance due to sulfation. Nevertheless at this point the battery charge controller will now reduce the power to the battery, that V x I and keep the voltage at (14.5) say.

The battery is now in what is called an absorption mode and will stay this way until it is 'charged' but see the post on absorption as it is not quite that simple. This was mentioned to end the concept of Bulk charging.

[b]NOTE:[/b]
When using a charge controller such as a Victron MPPT 75|15 there is an yellow LED that flashes twice every second to indicate Bulk charging mode. However this does not mean the battery is charging it means the voltage from the solar panels are recognised. The following two scenarios occur:
1. The sun has risen or it's really cloudy and the output from 2 x 250 watt panels may be 0 or 1 watt. This is not enough to maintain a battery but the indicator can be misread as though there is bulk charging. So unless you monitor the charge you could be sufating the battery.
2. The power in is little more that, equal to or less then the load. Again the Victron indicates bulk charging when as above sulfating could actually be occurring.
If the above happens over a day or two in winter or more then damage may be caused that is not recognised. This has been a problem for me until I bought the Victron VE direct cable to not only monitor the charge rate but adjust the absorption voltage to compensate for low charge rates on many occasions

Drop in voltage with constant current

The usual process is for the voltage to rise with a constant current hence the idea that when a battery is highly or fully charged the voltage is regulated.

**Feb 7th**

But I have found that although the voltage is regulated at 15.63 the current feeding 3 x 120Ah batteries has gone from 11.2A to 13.4 over 80 minutes. This could be problem with the Victron Direct readings but I have had the same problem with a single 120Ah battery on a traditional mains charger. In this case the current ranges from 8 to 2 for periods of many hours.

I have this idea that the batteries are sulfated and as such provide a high resistance to charge and hence a high voltage is needed for it to absorb wattage. But after prolonged periods it is if a layer of sulfate is broken down and the resistance drops leading to a higher current. This seems to be happing multiple times over the last few weeks where I have been trying to recover capacity in what in all accounts are heavily sulfated batteries. By heavily I mean a first test that showed 10% capacity and a second that showed 25%. Since then I have had single batteries charged either for 1, 2 or thee days continuously by a mains charger that can produce 12A, or any number up to three in parallel fed by 15A at up to 15.94V for whatever sun is available. The max so far has been 1.25Kw in one day. This has been going on for some two weeks and the anomaly is still here.

**13:30** Just spoken to Phil at Bardens Uk who has confirmed that the dropping voltage is the sulfate being broken down.

Thursday 6th Nov 2014
On receipt of the batteries, after checking the containers were intact, I measured the voltage of each and found one at 12.83 the others at 12.88. I then numbered each one and measured the specific gravity of the acid to obtain a better measure of the state of charge.

The hope was that each was fully charged as they are new models and shouldn't have been hanging around the retailers for long and who should have fully charged them before sale. I was clearly already a bit concerned that one was at 12.83.

The expected sg of acid in a fully charged battery would be at least 1.26. On an older battery an sg of over 1.27 may indicate water loss and concentration of acid.

Results of the tests: Battery temp estimated at 11°
sg1 to sg6 are results for specific gravity from negative to positive terminal

Only one cell reached the 1.260 I was hoping for. Battery 4 was not charged as well as the others

Sat 8th
Connected Batt 4 via Victron MPPT controller. At the end of the day the LED indicated float charging i.e. battery at or over 80%. I used this as load battery.

Sun 9th
Once Batt 4 reached the float charge today I disconnected it and connected Bat3 which is now the load battery.

When Batt 3 reaches float charge early one afternoon I will disconnect it and reconnect Bat4. Once I have done this to each a few times I will check the specific gravity and if they are compatible will connect them in parallel.

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CAUSES

Most cases of sulfation are caused by:

UNDER CHARGING OR NEGLECT OF EQUALISING CHARGE

When a cycled battery is charged repeatedly at low rates but not fully charged, the acid is not effectively driven out of the plates, particularly the lower parts, and sulfation results.

Repeated partial charges which do not effect thorough mixing of the electrolyte also result in sulfation. It is difficult in normal battery operation to determine just when sulfation begins, and only by giving periodic equalizing charges and comparing individual cell specific gravity and voltage reading can it be detected in its early stages and corrected or prevented.

Troublesome sulfation does not occur in less than 30 days.

STANDING IN A PARTIALLY OR COMPLETELY DISCHARGED CONDITION

Permitting a battery to stand in a partially discharged condition for long periods allows the sulphate deposited on the plates to harden and the pores to close.

Batteries should be charged as soon as practicable after discharge and not allowed to stand in a completely discharged condition for more than one month. During freezing weather the battery should be recharged immediately following discharge to prevent freezing.

LOW ELECTROLYTE

If the level of the electrolyte is permitted to fall below the tops of the plates the exposed surfaces will harden and become sulphated.

ADDING ACID

If acid is added to a cell in which sulfation exists the condition will be aggravated.

HIGH SPECIFIC GRAVITY

In general, the higher the fully charged specific gravity of a cell the more likely is sulfation to occur and the more difficult to reduce. If in any battery there exists cells having specific gravity more than 0.015 above the average, the possibility of sulfation in these cells will be enhanced.

HIGH TEMPERATURE

High temperatures accelerate sulfation, particularly of and idle, partially discharged battery.

All cells of a sulphated battery will give low specific gravity and voltage readings. They will not become fully charged after normal charging. An internal inspection will disclose negative plates having a slate like feeling, sulphated negative-plate material being hard and gritty and having a sandy feeling when rubbed between thumb and forefinger.

The internal inspection should be made after a normal charge, since a discharged plate is always somewhat sulphated. A good fully charged negative plate is spongy and springy to the touch and gives a metallic sheen when stroked with the fingernail or knife. A sulphated positive plate is a lighter brown colour than the normal plate.