# Flooded lead acid battery basics



## mdimarco (Oct 22, 2013)

Sure you can find this info by googling or probably digging through past posts where this info is buried. But are we going to be a resource or non-resource? We need to keep this type of info at our fingertips.

To start off I believe that lead acid are the best design of batteries on the market. They may not be the most fancy pants battery on the block, but they are the most reliable, safest (if properly maintained), understandable, scalable, reproducable, proven, buildable, and affordable batteries on the market. I feel that the most world changing uses for large scale battery banks in the future will be born from this basic technology of flooded vented cells. You can't escape flooded cells. It is just basic chemistry that the lower the surface tension of the electrolyte solution, the more contact it will make with the plates. You can't escape flooded, it is here to stay. The most ripe platform for innovation is the flooded lead acid design. You can tweak the plates, you can tweak the electrolyte, you can tweak the venting, you can tweak the form factor, you can automate the maintainance, You can inprove the electrolyte mixing, you can improve the charging ect. It is an excellent foundation for batteries of the future. It does not cut any corners (at least the 6v flooded). You can make the plates out of lithium and tweak the electrolyte and dimensions and you have a lithium ion battery. This platform is ubiquitous. People will hate on the technology. I say let them hate and leave em behind.

Lead acid batteries are the oldest modern day battery technologies. They were invented in 1859. They consist of a flooded design with vents, with lead plates in a liquid acid bath. Cells are roughly 2.1 volts each, so a 6 volt battery has 3 cells in series, and a 12 v battery has 6 cells in series. This is why 6v batteries tend to be better, because they can have fewer plates per battery and therefore the plate to acid ratio and dimensions can be better optimized.

Here are some values to keep in mind while charging:

These are general voltage ranges per cell:

Open-circuit (quiescent) at full charge: 2.10V
Open-circuit at full discharge: 1.95V
Loaded at full discharge: 1.75V
Continuous-preservation (float) charging: 2.23V for gelled electrolyte; 2.25V for AGM (absorbed glass mat) and 2.32V for flooded cells
All voltages are at 20 °C (68 °F), and must (for a 6 cell battery) be adjusted −0.0235V/°C for temperature changes.
Float voltage recommendations vary, according to the manufacturer's recommendation.
Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) is almost as detrimental as excessive voltage (causing corrosion and electrolyte loss)
Typical (daily) charging: 2.37V to 2.4V (depending on temperature and manufacturer's recommendation)
Equalization charging (for flooded lead acids): 2.5V for no more than 2.205 hours. Battery temperature must be absolutely monitored.

-wikipedia

Flooded lead acid like to be kept charged, but not over charged. As you can see from above, if you want to keep a charged battery charged, you do not want to charge it at a voltage higher than the reccomended float voltage at a given temperature. "Floating a cell" replaces charge at the rate of likely natural discharge. Since this battery tech likes to be kept just full, this is a great way to maintain a pack. Once used (even if not fully discharged) charge it back up to full as soon as possible. This will prevent sulfate crystals from forming on the plates and therefore a reduction in available power. Overcharging will cause corrosion and a loss of available power so be careful and only charge according to the above specifications or that provided by the manufacturer.

This sulfation can be removed by pulsing the battery, but that is beyond the scope of this article. Suffice it to say that to make a battery charger that did this as a normal part of a charging or as an option would be very valuable and could possibly increase battery life. Also building electrodes (plates) that resist sulfation would be another giant leap forward in the tech.

Methods of charging:

During the constant-current charge, the battery charges to 70 percent in 5–8 hours; the remaining 30 percent is filled with the slower topping charge that lasts another 7–10 hours. The topping charge is essential for the well-being of the battery and can be compared to a little rest after a good meal. If deprived, the battery will eventually lose the ability to accept a full charge and the performance will decrease due to sulfation. The float charge in the third stage maintains the battery at full charge.
The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate, and full charge is reached when the current decreases to the three percent level of the rated current. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to initialize the charge termination.
The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell. Setting the voltage threshold is a compromise, and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.
To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level. Chargers exposed to temperature fluctuations should include temperature sensors to adjust the charge voltage for optimum charge efficiency. If this is not possible, it is better to choose a lower voltage for safety reasons. Table 4-5 compares the advantages and limitations of various peak voltage settings.

low and slow:
2.30V to 2.35V/cell
high and fast:
2.40V to 2.45V/cell

Advantages

low and slow:
Maximum service life; battery stays cool; charge temperature can exceed 30°C (86°F).
High and fast:
Faster charge times; higher and more consistent capacity readings; less sulfation.

Disadvantages

low and slow:
Slow charge time; capacity readings may be inconsistent and declining with each cycle. Sulfation may occur without equalizing charge.

high and fast:
Subject to corrosion and gassing. Needs constant water. Not suitable for charging at high room temperatures, causing severe overcharge.

-Batteryuniversity.com

Like that said, it is probably best to start with a higher voltage at first to get the battery to 70% max in roughly 5-8 hours and then a lower voltage (and amperage I would assume) to get to the max charge in another 7-10 hours. This will give the best of both worlds; get the battery close to full asap to prevent sulfation, and finish the charge slow to increase battery lifetime. Floating after the charge is the cherry on the top and will keep your battery healthy during downtime.


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## mdimarco (Oct 22, 2013)

Hmm for some reason I can't edit.

Maintainance:

Aside from charge maintainance that has been outlined above, the flooded vented design also requires electrolyte maintainance. To do this first make sure you have distilled water on hand. Tap water contains levels of minerals which you don't want to introduce into a battery. Make sure that liquid covers the plates at all times. Any part of the plate that is not submerged will rapidly oxidize and reduce the total capacity of the battery. So keep the batteries upright. Open the vents every few weeks (depending on the humidity and tenperature) and make sure none of the plates are visible. Also make sure the liquid is not above the bottom of a circular ring below each vent. This allows the electrolyte to expand when charging and so it does not overflow and cause acid spilling out and corroding the posts or wires.

They have additives for batteries that prevent as fast of evaporation from the battery so you can go months without maintainance. If you ise this you will have to remove some of the acid to make room for the proper amount of additive. Also there are grease type products to prevent corrosion on the posts or wire ends.

Charge can also be checked via the property of specific gravity of the electrolyte in addition to measuring the voltage. This is done with a device called a hydrometer which can be purchased at any automotive shop.


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## piotrsko (Dec 9, 2007)

addendum: voltage at rest and hydrometer values are indicators but are NOT reliably accurate methods of determining actual battery SOC.


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## frodus (Apr 12, 2008)

We already have a resource started on our wiki. Please move this post there so people don't have to dig into it on the forums:
http://www.diyelectriccar.com/forums/showthread.php?t=5393&redir_from=15034#post47191

Keep it factual, not biased opinion.


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## frodus (Apr 12, 2008)

mdimarco said:


> You can make the plates out of lithium and tweak the electrolyte and dimensions and you have a lithium ion battery.


False.

Lead acid stores energy in the electrolyte using a chemical reaction.

Lithium Ion batteries store energy ionically in the lithium lattice structure. There is no chemical reaction taking place.


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