The most important part of the charge cycle is the absorption charge. Since the bulk charge only recharges the battery bank to an 70-80% level, the absorption charge completes the charging cycle.
If the absorption charge is not completed fully a part of the battery plates will not cycle and are likely to become sulfated, which in turn leads to shorter period of bulk charging until the absorption level is reached. If this is done often then the battery will loose capacity and it can be quite dramatic in that after a few months it may be reduced to 50% capacity. One process to remove sulfation is to over-charge or use an equalisation voltage once the battery appears charged.

Some chargers have a timer that allows the user to adjust the duration for the required time to return the battery to full charge. In order to set the correct time, a simple calculation is required. With the help of the 20 AH capacity, you can figure out the remaining charge required for the battery bank using the following equation:

If the actual absorption time is significantly less than as calculated below then the likelihood is that the battery has reduced capacity.

If a charger measures the standing voltage, for example after 12 hours of no load, it may adjust the time of absorption noting the battery is nearly fully charged. This again can be misleading as a sulfated battery not only will have reduced capacity but a higher resistance and so the voltage will appear high when there is little charge in terms of capacity.

t = 0.42 x (C/I) and with an 80% charge
Where:

t = Absorption Charge Time (Hours)

C = 20 hr Rated Capacity (AH) [ex: 2 strings x S-550 models (428 AH) = 856 AH rated capacity]

I = Charging Current (Amps) [charger output min 5% up to max 10% of 20 hr rate]

**Example** For a single 120Ah battery charging at 12amps that equates to 0.42*(120/12) which is 4.2 hours and I doubt I reach half of that. Given that the recommended voltage is over 15v I have developed quite a problem maybe using 14.5v Oops!

Good info on https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery :: wikipedia Lead acid battery

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.

An obvious thought is that a larger battery requires a larger charge. First however it is important to note that apart from common usage applied to AA and AAA batteries, for example, the word means a collection of units working together, as in a battery of guns.

Battery Cells: Nominal Voltage
Electrically chargeable cells that are used to form batteries range from a nominal voltage of between 1.2 and 2 volts. AA and AAA are 1.5v, Lithium Ion and Lead Acid are 2v. The common Lead Acid battery, used to start a car, is 12v; comprising six 2 volt cells connected in series (end to end).

Power Available: Amp Hours x Volts
Batteries have an amp hour rating which is shown on each battery be it single or a collection of cells. The label may show a value such as [48Ah] in the case of a car battery or [1500mAh](1.5Ah) on a rechargeable Lithium Ion. This figure is a measure of how much charge the battery can release over a given period of time, the most common period is 20 hours called the [20 hour rate]. And unless a battery displays an alternative rate it is assumed it is the 20 hour. So a 48Ah 12v battery has ten times more power than a 48Ah 1.2v.

The power in a battery is roughly Ah x V and in a 48Ah 12v battery is 576 watts or 0.576Kw. In comparison to mains electricity that is enough power to run a one kilowatt single bar electric fire for just over half an hour or a 50w laptop for just under 12 hours. However power output is not linear to load applied, as the quicker a battery is discharged the less efficient it is, hence the 20 hour rate parameter.

The 20 hour rate means that it will produce 48Ah at 12v only if the current drawn is constant and by calculation, at a nominal voltage of 12 that would 4 (48/12). It would power a 48 watt (12v x 4a) load for 12 hours and whereas a lighter load of 24w would run for over 24 hours a larger load of 96 watts would not run for 6 hours. The differences in the power obtained from the 20 hour rate in the examples above are not likely to be huge but the rates of discharge can be much greater. For instance some times I may use only 1 watt when running a single LED for reading but 400 watts when using an electric drill.

The lower the rate of discharge the more efficient the battery is and the less damage that occurs in a complete cycle of charge and discharge thereby extending the number of times that a battery can be recharged, varying from a few hundred to a few thousand.

Charging > Page 2

Pages: 1· 2· 3

For lead Acid Batteries

There are two processes where the battery resistance will change.

a) The normal operation of charging a battery will increase it's resistance, whilst removing sulfation, and the resistance will drop as sulfation occurs. This follows the formula that V=I × R.
With a constant current (I) the battery charges the voltage (V) rises. Is simple terms as an equation must balance then if one side increases then so must the other. So if V rises then I &times R must also rise and of I is constant then R must rise. The reverse is the case when the battery discharges.

b) Although sulfation and de-sulfation are the normal reactions in a lead acid battery sulfation can become ingrained and not subject to being processed by normal charging. In this case the battery appears charged and has a high resistance. To overcome this, or at least to try to, a high voltage can be applied in an attempt to force the sulfate to return to the electrolyte.

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

The main purpose of equalisation is to balance cells that may have developed different capacities which limits the battery's capacity and encourages sulfation. However equalisation or more properly, charging at high levels, can breakdown sulfation.

See http://support.rollsbattery.com/support/solutions/articles/430-corrective-equalization-instructions

From: http://rollsbattery.com/wp-content/uploads/2018/01/Rolls_Battery_Manual.pdf
>**Float**
When the Absorption charge is completed, the batteries require a certain amount of voltage to maintain a full charge when no load is applied. This Float voltage maintains the battery bank at a constant full state of charge. To prolong battery life, the Float settings on the power supply should be adjusted to the voltage indicated in Table 2 (a) & 2 (b) Flooded Charging Parameters. Higher or lower voltage settings may result in unnecessary overcharge or sulfation.

>**End of Charge**
As batteries near full capacity the charge current drops. End Amps or Return Amps generally refers to the lowest amount of current (Amps) running from the charger when the batteries have reached full capacity and are no longer accepting a charge. Some charges will measure the actual current output. If the charge current drops to the End Amps or Return Amps set point, this will trigger the charger to shut off. This setting is typically 2%-3% of the 20 Hr AH rating (C20) of the battery bank. Rolls recommends setting this at 2% for new installations

For the ELF set up of some 500Ah this would be 10A ????

Chemical reaction is slow at lower temperatures and so a higher voltage will be required to charge. See first post under charging: http://unveiled.info/index.php/solect/charging/