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Posts published in “Renewable Energy”

Battery States of Charge

State of Charge: Here are no-load typical voltages vs state of charge

(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell – if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated.

These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher – the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.

State of Charge

12 Volt battery

Volts per Cell


































Battery Charging: Battery Charging takes place in three basic stages: Bulk, Absorption and Float

Bulk Charge – The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no “correct” voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it’s main purpose is to keep an already charged battery from discharging. PWM, or “pulse width modulation” accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called “pulse width” because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts.

Chargers: Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods. Among the regulated chargers, there are the voltage regulated ones, such as Iota Engineering and Todd, which keep a constant regulated voltage on the batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called “taper charge” – as if that is a selling point.

What taper charge really means is that as the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 20 amps may only be supplying 5 amps when the batteries are 80% charged. To get around this, Statpower (and maybe others?) have come out with “smart”, or multi-stage chargers. These use a variable voltage to keep the charging amps much more constant for faster charging.

Charge controllers

A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types.

Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no “best” controller for all applications – some systems may need the bells and whistles of the more expensive controls, others may not.

Battery Charging Voltages and Currents:

Most flooded batteries should be charged at no more than the “C/8” rate for any sustained period. “C/8” is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde AGM batteries are a special case – the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, we don’t recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell.

Never add acid to a battery except to replace spilled liquid. Distilled or deionized water should be used to top off non-sealed batteries. Float and charging voltages for gelled batteries are usually about 2/10th volt less than for flooded to reduce water loss. Note that many shunt-type charge controllers sold for solar systems will NOT give you a full charge – check the specifications first. To get a full charge, you must continue to apply a current after the battery voltage reaches the cutoff point of most of these type of controllers. This is why we recommend the charge controls and battery chargers listed in the sections above. Not all shunt type controllers are 100% on or off, but most are.

Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes “stratified”. You can have very strong solution at the top, and very weak at the bottom of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most – check the manufacturers recommendations, especially on gelled.

Battery Aging

As batteries age, their maintenance requirements change. This means longer charging time and/or higher finish rate (higher amperage at the end of the charge). Usually older batteries need to be watered more often. And, their capacity decreases.

Mini Factoids

Nearly all batteries will not reach full capacity until cycled 10-30 times. A brand new battery will have a capacity of about 5-10% less than the rated capacity. Batteries should be watered after charging unless the plates are exposed, then add just enough water to cover the plates. After a full charge, the water level should be even in all cells and usually 1/4″ to 1/2″ below the bottom of the fill well in the cell (depends on battery size and type).

In situations where multiple batteries are connected in series, parallel or series/parallel, replacement batteries should be the same size, type and manufacturer (if possible). Age and usage level should be the same as the companion batteries. Do not put a new battery in a pack which is more than 3 months old or has more than 75 cycles. Either replace with all new or use a good used battery. For long life batteries, such as the Surrette and Crown, you can have up to a one year age difference.

The vent caps on flooded batteries should remain on the battery while charging. This prevents a lot of the water loss and splashing that may occur when they are bubbling.

When you first buy a new set of flooded (wet) batteries, you should fully charge and equalize them, and then take a hydrometer reading for future reference. Since not all batteries have exactly the same acid strength, this will give you a baseline for future readings.

When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel – the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast. Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this “memory” is totally false and will lead to early battery failure.

Inactivity can be extremely harmful to a battery. It is a VERY poor idea to buy new batteries and “save” them for later. Either buy them when you need them, or keep them on a continual trickle charge. The best thing – if you buy them, use them. Only clean water should be used for cleaning the outside of batteries. Solvents or spray cleaners should not be used.

Electricity Prices about to Skyrocket

With President Obama’s so called Cap and Trade system, it is being reported by his own cabinet that electric bills will increase $700-1200 a year per family, beginning in 2012. So much for the breaks promised for ‘middle class’ Americans huh? This is directly in line with the speech given by Obama back in January that was buried by the leftwing media.

How will this really work? Simply by hammering companies that produce green house gasses, those producing less will get a break and can sell their ‘credits’ to others that produce more. For example coal fired electricity plants… while the idea of producing less pollutants, in this example CO2 should be a good thing this is definitely not the way to do it. Will operations for coal plants really change, perhaps in time, however in the short term they will simply pass along the increased operating costs onto the consumers. Currently coal fired plants produce electricity for the grid at a cost of roughly $.06 per KiloWatt Hour (KWh), and sell it wholesale to utility companies who in turn sell it to consumers…. the trickle down effect is replaced with an avalanche effect for consumers, imagine paying up to $.25/KWh.

What does this really mean for the folks? Brace for impact! In less kinder words start stretching those hamstrings to grasp your ankles firmly, Obama is going to give it to you right up the ass!

What can we do about it? Directly, not a lot, Obama is going to do whatever he wants for a while, with a Democratic Congress to help push his agenda there are no check and balances left to hold him back.

Indirectly, find ways to start curbing your electricity use, actually all fuel usage (we know any fossil fuel usage will get hammered as well).

Cut heating and cooling costs now! The largest bang for the buck comes from insulating your home, a tremendous amount of heating/cooling energy just escapes the household envelope lost to the environment! Insulate and grab the caulking gun and seal up those leaks, savings of over 40% have been reported. A short term investment can yield some very impressive long term results, aka cost savings directly visible on your monthly bills. Payback times can vary, but if you are looking to make an immediate change this is by far the largest bang for your buck that can be realized.

Replace those drafty old windows and doors, not the cheapest route, but the savings are there too, after a much longer payback window. If this is out of the budget now, seal up any leaks around them with caulk, and during the winter simply covering the windows with tight fitting shades or blinds can dramatically slow down the heat transfer in or out of your home.

Replacing old inefficient appliances, if your water heater or refrigerator is older than 10 years, consider replacing it, new ones are far more efficient and will a significant difference that you can see on your utility bills.

More energy saving articles will follow, as well as the Grey Fuzz’s venture into using the sun to replace the electric grid and heat/cool our home!

Battery Charging Process

Written by Steve Spence (website at bottom of article)

Batteries are complex mechanisms that can even fool the experts at times, so it comes as no surprise that non-technical people have a hard time understanding the charge process. Ask a typical crowd of battery users when their batteries are full charged and at least ten answers will surface.

In both Living on 12 Volts with Ample Power, and Wiring 12 Volts for Ample Power the authors explain that a battery is fully charged when the voltage is about 14.4 Volts and current through the battery has declined to less than 2% of the capacity of the battery in Amp-hours …2 Amps for a 100 Ah battery.

That information is substantially correct, however, a more intuitive feel for the charge process is necessary, not only to understand when the battery is full, but also to know when the battery is not behaving normally. It is the intent of this application note to provide enough information about the charge process so that the average user can judge how well the batteries are charging.

The Bulk Charge Step
When a charge source is first applied to a well discharged battery, charge current begins to flow, typically at the maximum rate of the charge source. If a true 40 Amp charger is connected to an 8D battery which is completely discharged, about 40 Amps of charge current would flow for some period of time. Because most of the charge is delivered at the maximum charger rate, the first step of the charge cycle is called the bulk charge step.

NOTE: During the bulk step, battery voltage will steadily rise.

The Start of the Absorption Step
At the instant battery voltage has risen to the maximum allowable voltage of the charge source, current through the battery begins to decline. This simultaneous event of reaching maximum voltage and the start of current decline marks the beginning of the absorption step.

For instance, if the 40 Amp charger is set to 14.4 Volts, then when battery voltage has risen to 14.4 Volts, the charger will now hold the voltage constant. Current through the battery will begin to decline.

NOTE: The charger, (or alternator), is not limiting the current at this point. The battery is `absorbing’ all it can at the voltage setpoint.

The End of the Absorption Step
The absorption step should continue until current through the battery declines to about 2% of battery capacity in Amp-hours as mentioned above. Without knowing what the current is through the battery, you can’t know when it’s full. Just because that fancy charger, (or inverter/charger), has kicked out to float is no sign that the battery is full …there is no charger on the market that measures battery current!

It’s a given, then, that you need to measure battery current to know when the battery is full. With a battery current meter, you can discover some very interesting details about the charge process. For instance, you can discover that once the charger voltage limit is reached, battery current begins to decline. If the current decline is rapid, either the batteries are nearly full, or they are NO GOOD! If the current decline is slow, then either the charge source has more output than the batteries can reasonably absorb, or the batteries are NO GOOD! Here’s where Amp-hour instrumentation is particularly valuable.

Given enough time at the absorption voltage, charge current will decline to a steady-state value, that is, a low current that either stays constant, or declines very little. At the point where charge current has gone as low as it is going to, then the batteries are truly full. While 2% of Ah rating is close, good batteries will reach a steady state current at less than 1% of Ah rating.

The Float Step
Once a battery is full, a lower voltage should be applied that will maintain the full charge. Depending on the type of battery, (liquid, gel), and the age of the battery, 13.4 – 13.8 Volts is appropriate as a float voltage.

Temperature Compensation
The voltage given above are good only at F, (C). For high temperatures, voltage will be less. It is important to charge batteries with temperature compensation. To learn more about this aspect of charging, refer to page 70 in the revised edition of Wiring 12 Volts for Ample Power.

A Very Common Problem
Your batteries are only four months old. You discharge them until their voltage is less than 11 Volts and then crank up the engine. The alternator brings up the voltage to 14.4 Volts very quickly, but the current begins to decline immediately and in a few minutes is down to a few Amps. You:
*suspect your voltage regulator and immediately call the factory and ask for a replacement to be sent out; OR *realize that something has happened to the batteries because the alternator and regulator are operating as expected.

Conditioning Batteries
How do batteries that are only four months old die? Perhaps they weren’t broken in properly; maybe they sat deeply discharged for a few days or more; perhaps they were allowed to self-discharge over the last four months …there’s plenty of ways to murder batteries.
All batteries that refuse to accept a charge are not necessarily ready for the scrap heap. Often, a deep discharge followed by a slow charge will recover lost capacity and charge acceptance. For more information, refer to Wiring 12 Volts for Ample Power.