Gary
-- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | | s[first posted a few years ago] I've been doing some research on lead-acid batteries with an eye towards using them to provide power for our ham radio repeater site.
Our site is difficult to get to, and the commercial AC mains power goes away at times. Everything in the site runs off a nominal 12 volts DC. During idle periods, the equipment may only draw a few amperes, but most of the transmitters can draw up to 10 to 15 amps each. A maximum drain of 100 amps isn't out of the question, although it would probably be only for a few minutes at a time. Some systems (such as the digital communications equipment) key on and off quite regularly, with perhaps as much as a 50% duty cycle, whilst others may not key for hours and then stay on for as long as an hour or two (voice repeaters during drivetime). We do not want there to be any interruption of power when the mains fail. We don't believe that most of the outages are of a duration that a generator will be necessary - a few hours is sufficient.
It is clear that a good solution to our problem is a bank of lead-acid batteries capable of supplying the peak current, floating across a supply that can recharge them and supply the standby and perhaps one or two transmitter's demand.
Ok, that's the problem. Here's what I've found.
Lead-Acid batteries commonly available today can be roughly grouped into three categories by construction and intended use: 1. Automotive starting 2. Traction 3. Stationary
Automotive starting batteries are formulated with thin pasted plates and are designed to supply high peak currents for brief periods of time whilst cranking an engine. They are not expected to be discharged to more than perhaps 75% of capacity, and are expected to be recharged immediately after discharge. If used in deep-discharge or float service they will not last long. (I.e., the capacity of the battery will diminish fairly quickly. While it will still act as a battery, it will not be able to supply its rated capacity soon after being placed in the wrong kind of service.)
Traction batteries are made with thick pasted plates and have very rugged separators between the plates to make the battery more immune to physical shock and vibration, and to reduce the chance of failure due to dendritic growth during recharging. These batteries are sold for use in electric forklifts, golf carts, marine trolling motors, and RV power. They are designed to be discharged nearly fully each day, and recharged each night. Because there is some tradeoff in battery life by using the pasted plate construction to keep the size and weight of the battery down, they are not used in applications where extremely long life is required. The commonly-available Deep Cycle Marine batteries are of this general type.
Stationary batteries are made with thick solid plates. They are designed to be used as standby power, supplying minimal power and kept in a state of nearly full charge until needed. They can take deep discharge. Because of the solid plate structure, they are bigger and heavier, but their lifetime is much longer. One source suggests that 10 years is not unusual. Some photovoltaic storage batteries (for solar-powered homes and such) are of this type.
The best battery for our application is the Stationary battery, but they are not commonly available. Much more readily obtained are the Marine/RV batteries, at about $50 apiece.
Charging and discharging these batteries is a big question. I posted a query to the net and received about a dozen replies, most of which contradicted each other in one or more points. However, there is some consistency in the information available in our library, and I'll try to summarize it below.
Note that all the voltages given below are for batteries at working temperature - typically 80F (27C).
DISCHARGE:
Batteries are rated at an Amp-Hour capacity at a specific rate. For traction type batteries, this is typically a five hour rate, so a fully-charged 100 Ah traction battery in good condition can supply 20 amps for 5 hours before it is exhausted. Stationary batteries are usually rated at a 10 hour rate, and automotive (if rated in Ah at all) are given for a 20 hour rate. The discharge curve is NOT linear; if you double the current drain, you will get less than half the time. Similarly, if you halve the drain, you will get more than twice the time.
Each type of battery has a specified voltage at which it is considered completely discharged. If discharge continues below this voltage, the battery life may be considerably shortened, and repeated abuse of this kind can result in a battery which cannot practically be recharged. Each battery manufacturer specifies this voltage; in general, the final voltage for the three general types of batteries are
automotive 1.75 v per cell traction 1.70 stationary 1.85
Thus a typical 12 volt marine battery with 6 cells should not be discharged below about 10.2 volts.
Another way of looking at it is that no cell should be discharged more than about .3 v below its full-charge rest voltage.
A typical cell will show the following voltages:
fully charged, open circuit, at rest with no charge/discharge for at least 12 hours 2.12 v/cell
As soon as load is applied (internal v-drop) 2.00
fully discharged, under load 1.70
fully discharged, open circuit 1.99
beginning of charging 2.10
70% to 80% charge (gassing begins) 2.35
full charge 2.65
CHARGING:
Liquid-electrolyte lead-acid batteries can be recharged at any rate that exceeds internal and surface discharge rates, and which does not cause excessive gassing (liberation of oxygen, hydrogen, and steam).
In non-float service, there are several simple chargers.
A single-rate (constant-current) charger limits its charge rate to about 7% of the Ah capacity of the battery; for a 100 Ah battery, it would charge at a rate of 7 amperes. Since the battery will start at about 2.1 v per cell, and finish at about 2.7 v per cell, the charger must be able to vary its voltage over this range. For a "12 volt" battery with 6 cells, the charger will need to supply between 12.6 and 16 volts over the duration of the charge. Charging is complete when the battery reaches 2.65 to 2.7 volts per cell.
A simple taper charger is a constant-voltage source set to 2.8 volts per cell with a series ballast (typically a resistor, but a choke or the internal resistance of the supply can be used) that limits the output current to 7%C when the battery is started charging at 2.1 v/cell. Again, charging is complete when 2.7v/cell is reached.
Trickle-charging of a fully-charged battery can be done to keep it charged. This is done by supplying .5 to 1 mA per Ah capacity. Trickle charging should be discontinued when it has continued for at least 24 hours and the battery has reached 2.25 v/cell. Typically, trickle chargers are set to run perhaps once a week. Because of their thin plate construction, automotive-type batteries will deteriorate if trickle-charged for more than perhaps six months.
An interesting research result was that using pulsating rectified AC or superimposing a small AC current on pure DC charging current increased battery life by up to 30%. Apparently the mechanism is that is reduces gassing and leads to a more porous lower-resistance plate, and lessens the tendency to form dendrites during charging.
In float service, where the battery is in parallel with the mains supply, the supply voltage must be set to 2.15 to 2.20 v/cell. This will charge the battery, and avoids excessive gassing, but does not serve to "freshen" the cells - there is not enough gassing activity to move electrolyte around and clear the beginning of deposits from the surfaces of the plates. It is recommended that batteries in float service occasionally (perhaps once a month) be charged to 2.65 v/cell to freshen and equalize the charges. In large installations, this is done by switching parts of the battery banks out of service in rotation. In smaller systems that can tolerate the voltage excursion, it can be done by simply boosting the output of the mains supply.
Charging inevitably leads to some water loss due to gassing; 100Ah of a gassing charge (2.4v or more per cell) will yield about 1.2 oz of water loss. Hydrocap Corp [975 NW 95th St, Miami Fla, (305)696-2504] makes a replacement filler cap that contains a catalytic material that recondenses emitted steam, and recombines the hydrogen and oxygen gasses into pure water that then dibbles back into the cell, greatly reducing the required maintenance. With the available flame arrestor option, they sound ideal for unattended battery systems, and should greatly reduce the danger of fire and explosion from liberated hydrogen. They're about $5-$10 per cell.
To read further:
Smith, George. Storage Batteries, including operation, charging, maintenance, and repair. ISBN 273 43448 9, TK2941.S57 1968
Aguf, I.A. and M.A. Dasoyan. The Lead Accumulator (translated from the Russian by S Sathyanarayana). Calcutta, 1968
Longrigg, Paul. Rapid charging of lead-acid batteries for electric vehicle propulsion and solar energy storage. DOE/NTIS 1981.
Aren't libraries wonderful? - Brian
In article <3262fs$4ht@mozo.cc.purdue.edu> mconner@rain.atms.purdue.edu (Mark D. Conner) writes: >I am looking for some info on charging gel cells. At what point >should you recharge the cell (drop to what voltage), and how do you >tell when the cell is fully charged? I put my cell on the charger >when it dropped to 11.6V, and monitored the voltage while charging. >The cell came back up to 13V fairly quickly, and after 2-3 hours the >charge current was down to about 70 mA. Since I was going to bed, I >disconnected the charger rather than leaving it on all night. Is >there a voltage or current threshold for determining when the battery >is fully charged? The cell has a 7Ah rating.
Here's a long, but concise reply that will help you properly care for these fine batteries:
The generally accepted way to charge a lead acid battery (gel just means gelled electolyte, BTW) is bring it up to charge using 14.3 vdc until the charge current begins to drop off, then to "float" it at 13.6 VDC.
This float voltage will not overcharge the battery due to its internal resistance.
We usually put a battery discharged to 11.0 volts (full discharge) on 14.3 vdc for .5 hr for each AH of rated capacity, then float it. If the battery gets hot, drop to float voltage immediatly, you'll boil away the electrolyte. They do get warm while recharging initially.
You can recharge them at a "trickle" rate just by using 13.6 vdc as well. After a series of charge/recharge cycles, the cells will discharge unevenly and eventually you'll need to "equalize" the cells with 14.3vdc charging as above.
Lead acid batteries emit hydrogen gas during recharge. Adequate ventilation is required. Discharged batteries should be recharged before storage, and kept in a cool place, on a shelf. Recharge stored batteries every three months, they tend to discharge due to internal cell resistance.
If the terminal voltage reads 13.4 vdc when you remove the charger, its probably charged. Most batteries will drop to 13.0 vdc in the absence of charging voltage pretty quickly. The only accurate way to determine a battery's status is through discharge load testing.
We use two 1157 auto lamps on clip leads for a quick test of a battery's capacity and status. With all four filaments lit, the load is 4 amps, a good test for 4ah to 12ah cells. If you get bright illumination for a couple of minutes, that's a pretty good indication you have a working battery. Don't buy used gel cells unless you apply this test first.
Anyway, we buy and use small gel cells by the case here (F M Systems is an alarm company) and that's what works for us. Much of our info came from the fine people at Powersonic and Yuasa/Exide. They publish some excellent tech notes on these topics, BTW. The charger schematic in the Powersonic notes is useful for a basic charger design.
Note that Ni Cad batteries are an entirely different chemistry with very different charging and operating characteristics. Don't use this info with Ni Cad batteries, it doesn't work, and you'll damage the cells.
Regards,...