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.
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.
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
ID Voltage sg1 sg2 sg3 sg4 sg5 sg6 1 12.86/7 1.250 1.250 1.250 1.240 1.240 1.240 2 12.88/9 1.255 1.260 1.255 1.250 1.250 1.250 3 12.88/9 1.250 1.250 1.240 1.255 1.250 1.250 4 12.82/3 1.245 1.250 1.245 1.242 1.237 1.235
Only one cell reached the 1.260 I was hoping for. Battery 4 was not charged as well as the others
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.
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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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.
If the level of the electrolyte is permitted to fall below the tops of the plates the exposed surfaces will harden and become sulphated.
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 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.
Thorough and careful attention to the following steps often will restore a sulphated battery to good operation condition.
- Clean battery;
- Bring electrolyte level to proper height by adding water;
- Put battery on charge at the prescribed finishing rate until full ampere-hour capacity has been put into the battery based on the 8 hour rate. If at any time during these procedures the temperature of the battery exceeds 50° C. reduce the charge rate to maintain temperature at or below this point. If any cell gives low readings (0.20 V less than the average cell voltage of the battery), pull and repair the cell before continuing with the procedure;
- After full ampere-hour capacity has been put into the battery, continue the charge at the finishing rate until the specific gravity shows no change for a 4 hour period with readings taken hourly. Record voltage and specific gravity readings. Correct specific gravity readings for temperature. These readings indicate the state of charge;
- Place battery on discharge at the 8 hour rate and during the test record individual cell voltages and overall battery voltage at the following time intervals: 15 minutes after test is started, then hourly until voltages on one cell reaches 1.80 and from then on at 15 minute intervals. From this point on the cell voltages should be under constant observation and the time recorded when each cell voltage goes below 1.75 volts. Terminate the test discharge when the majority of the cell voltages reach 1.75 volts, but stop the test before any single cell goes into reversal.
- If the battery gives rated capacity no further treatment is required other than normal recharge and equalization of gravity.
- If the battery does not deliver near-rated capacity, continue the discharge without adjusting the discharge rate until one or more cells reach 1.0 volts.
- Recharge the battery at the finishing rate as described above, again charging until there is no further rise in specific gravity over a 4 hour period, readings being taken hourly.
- Discharge again at the 8 hour rate and if the battery gives full capacity, recharge and put into service.
- If this procedure does not result in full capacity, repeat once more as noted above.
- If battery does not respond to this treatment, it is sulphated to the point where it is impractical to attempt further treatment and battery should be replaced.
Thorough and careful attention to the following steps often will restore a sulphated battery to good operation condition using a MMF charger and a Discharger/Analyzer.
Bring electrolyte level to proper height by adding water;
Initial Charge Current (I) : 4-5% of the battery capacity,
Constant Voltage (V): 2,4V/cell
Time of charge: from 12 to 24 hours
During the charge process keep under control the battery temperature that must not exceed 50°C.
If any cell gives low readings (0,20 V less than the average cell voltage of the battery), pull and repair the cell before continuing with the procedure.
4. The sulphated batteries have an high internal resistance; when a sulphated battery is put on charge the battery voltage reaches in few time the value of the Constant Voltage set and the charger current is very low.
Continuing to keep the battery on charge for long time with this very low current the lead sulphated is removed by the plates. The battery voltage would tend to decrease but the MMF charger increases the charger current to keep the voltage constant to the value set.
The charge process have to last a long time, from 12 to 24 hours.
If it's possible, keep under control the specific gravity of the electrolyte. If, during the charge the specific gravity doesn't increase for about 4 hours the charge process can be stopped.
After the end of the charge process, record the specific gravity of the electrolyte. The density of the electrolyte indicates the state of the charge.
Place battery on discharge at the 8 hours rate.
If a unit is used to discharge the battery it's advisable to set the discharger with the following parameters:
Discharge Current: 1/8 of the battery capacity
Time of discharge: 8 hours
Final Battery Voltage: 1,70V/cell
The discharger is fully automatics and stops the discharge process if the time of discharge or the minimum battery voltage set are reached. During the discharge, keep under control the individual cell voltages and overall battery voltage at the following time intervals: 15 minutes after test is started, then hourly until voltages on one cell reaches 1.80 and, from then, on at 15 minute intervals. Stop manually the discharge process if the voltage of one of the cell falls below 1,60V .
If the battery, with reference to the discharge parameters set, gives at least the 80% of the rated capacity no further treatment is required other than normal recharge and equalization of gravity.
If the battery does not deliver near-rated capacity, execute a new charge/discharge process.
Again charge the battery for long time between 12 or 24 hours and/or until there is no further rise in specific gravity over a 4 hour period, readings being taken hourly. Keep under control the battery temperature.
A sulphated battery is one which has been left standing in a discharged condition or undercharged to the point where abnormal lead sulphate has formed on the plates.
When this occurs the chemical reactions within the battery are impeded and loss of capacity result.
This document does not cover all the electricity theory and technology involved in the process of sulfation in battery operated system. For more information, please refer to specialized literature.
S.N. HQ141168 MRR
Emailed this to Wind and Sun
>Surprised to see no LED on the Victron when I returned today at 6pm. When I left yesterday at 1pm it was registering a float charge to two Rolls 120Ah batteries that I had just connected in parallel. See [b]comment 1 on https://calstock.org.uk/elf.php/2001/01/01/rolls-batteries[/b]
>I soon discovered the 20A fuse had blown. I can not understand why. The Victron may well control the voltage and current but it cannot take much more than 8A from the panels. It may voltage adjust internally the 30+ voltage to 15 or less and have the ability to pass 16A to the battery but why it has channelled over 20A through itself is a mystery. I am about to email the vendors.
>I have repaired the fuse and all seems fine. The power light has been flashing since indicating the Battery Life algorithm is being run.
>I note, according to the manual, the only situation where the fuse should blow is when the battery is connected with reverse polarity.
and this to Victron
>On 6th Nov I bought
> a Victron 75 | 15
> an REC 250w panel of 60 cells (nominal: volts 30.2; amps 8.3) (max open volts 37.4) (short circuit 8.86 amps)
> and two Rolls 120Ah batteries.
>On 12th Nov I connected the two batteries in parallel, so have 240Ah at 12v. The batteries are fully charged so when I connected them to the Victron and then the PV after 1 minute the yellow LED stayed on showing only a float charge. An hour or two later, around 1pm, I went out for the day and the weather was quite sunny
>I returned home this overcast and rainy day, 13th Nov at 6pm, to find to my surprise and disappointment, no LED light on the Victron. I discovered the fuse had blow. Once replaced all is fine.
>Could you tell me what may have happened?