|From Ocean Navigator #92 |
Like wet-cell lead-acid batteries, nicad batteries have plates awash in electrolyte. Nicads, like the SPH unit from Saft America above, are reportedly highly resistant to abuse and can last 25 years in aviation and trucking applications.
The literature for boat electrical systems either ignores nicad cells, or, while mentioning them as the ideal battery system, does not give any details beyond saying that they are very expensive. Our land-based cousins are, however, as cost-conscious as we are, and they have resolved the cost dilemma by using reconditioned cells. As for performance, the claims made for nicad cells are very impressive: longevity, high voltage under load, current delivery, ease of charging, invulnerability to overcharging or deep discharge, performance in low temperatures, low self-discharge rate, self-equalization, etc. A few years ago, I installed some reconditioned nicad cells in my boat, used them for 20,000 miles of voyaging, and can now report on the results.
Wet nicad cells use a dilute solution of potassium hydroxide (KOH) in water, with small amounts of lithium hydroxide (LiOH). This facilitates an alkaline reaction, rather than the acid reaction of a lead-acid cell. But, unlike the lead-acid cell, the nicad electrolyte acts only as an electron-transfer medium, and does not change chemically as the cell is charged or discharged. Therefore, a hydrometer cannot be used to monitor the state of charge (SOC). A metering system is needed for tracking SOC (which is of course more convenient). An accurate voltmeter is the minimum requirement, but ideally there should also be metering of current flow and an amp-hour calculation.
The voltage of a fully charged nicad cell is 1.65 volts, direct current (VDC), falling to between 1.00 and 1.15 VDC when fully discharged. Ten cells are therefore linked in series to make up a 12-VDC system. Nicad cells of various capacities are available that have the same voltage characteristics but differ in their amp-hour capacities and physical size. Cells linked in series should be identical (however, different strings of cells can be paralleled as long as the cells within each string are the same). My installation uses the popular Edison ED-160 cells, which have a capacity of 16 amp-hours each. Each cell weighs 21 pounds and is 6.4 inches wide, 18.3 inches tall, and 3.4 inches deep. Two strings of 10 cells each (connected in series by the supplied steel links) are paralleled to make up my boat’s 12-VDC, 320-amp-hour house bank. The total footprint size is 13 by 34 inches, and the aggregate weight 420 lbs (1.3 lbs/amp-hour).
Voltage: Graph I shows the voltage provided by my nicad bank as it is discharged by a ten amp (C/32capacity divided by 32) load. One immediately notices that the nicads have a higher and more level voltage profile under discharge, than lead-acid batteries. Typical voltages on my vessel are between 12.7 VDC (10% discharged) and 12.2 VDC (50% discharged). What this means is that equipment on board the boat works betternavigation and interior lights are brighter, power tools spin faster with more torque, etc.
Graph II shows the voltage of the bank while being charged by my engine alternator, which, when charging the nicads, has an average output of 40 amps. (A smart regulator is used; the alternator is rated at 50 amps). It will be seen that the voltage stays relatively flat (hence the need for accurate metering) until it rises quickly at 70 to 75% SOC, reaching 16.0 to 16.5 VDC at full charge.
Charging: Nicads are designed to accept charging rates and currents higher than those of the typical boat (e.g., a full recharge in four to six hours). This means that you can charge them more quickly than lead acids (less engine run time), and fill them “fuller.” It is difficult and time consuming to get the last 20% of capacity into lead-acids. Charging currents from environmental energy like photovoltaic arrays (solar cells) and water and wind generators are easily handled by nicads, and in fact, by recharging the cells at lower than design rates, cell efficiency is improved. (When selecting solar panels to charge nicads, choose panels with 36+ series PV cells, as these panels have a higher output voltage: 16 to 21 VDC. Smaller panels will perform poorly due to their lower charging voltages.) However, when environmental energy is insufficient and you need to run the engine or generator, the cells can easily accept the charging rates of high-output alternators.
Charge regulation: Lead-acid batteries ideally need to be regulated to avoid overcharging. This is not necessary with the nicads, which accept high rates of charge and are very resistant to overcharging. For example, my 320-amp-hour bank of medium-rate cells is designed to be recharged at a rate of up to 80 amps. This means that I have not needed to regulate my photovoltaic array (maximum current, seven amps) or the wind/water generator (max. current, 12 amps). Only the engine alternators, which put out 50 amps each, are regulated.
However, one must consider the electric equipment on board. DC appliance manufacturers have finally become aware that many DC systems are running on nicads, so most 12-volt equipment accepts 11 to 17 VDC, while 24-volt appliances accept 22 to 35 VDC. Some equipment has automatic acceptance of anything between 11 VDC and 50 VDC. Before installing nicads, however, it’s worth checking the specifications of each piece of equipment, as some older appliances may be specified to a maximum of 15.5 VDC. I have some such equipment on my vessel but the occasional overvoltages do not seem to have caused them any harm.
Current: Due to their low internal resistance, nicads can deliver current faster than lead-acids and with less voltage loss. In a test during which a 700-amp-hour lead-acid bank was replaced by a 160-amphour nicad bank, the same applied load of 5.8 amps resulted in 13.2 VDC, compared to 12.5 VDC on the lead-acids, even though the load on the nicads was four times greater in relation to their capacity. What this means in practice is that when the autopilot draws a large current the voltage drop is less, and so rudder response, and therefore steering, is improved. Again, high-surge devices like inverters are handled more easily by nicad power. The ED-160 can be discharged at rates of more than 500 amps (although this will reduce the amp/hour capacity).
My Ed-160s are “medium rate” cells, which means they are designed to have their total capacity of 320-amp/hours withdrawn in a seven to 48 hour period, or at a rate of up to 45 amps. A typical microwave requires a surge current (via an inverter) of 500 amps for 0.1 seconds at start-up, but even a small nicad pack of 160 amp/hours can supply this amount of power in the time required.
Self-discharge & equalization: Nicads have a similar rate of self-discharge to new lead-acids (about 10% per month). In the case of the nicads, this rate is maintained for the life of the cell, 25 to 50 years. A six- to eight-year-old deep-cycle lead-acid battery will lose about 30% of its charge monthly through internal self-discharge.
Equalization (the controlled overcharge of a full battery) is essential for lead-acid batteries to keep individual cell voltages similar. A typical equalization program would periodically require six hours of engine or generator running for a controlled-current overcharge. The cost and wasted energy of equalization is unnecessary with nicads. The chemistry of the cells is such that, in cells connected to work as a battery, the cell voltages converge. With no equalization over the past three years, my cells still display voltages within 0.01 VDC of each other.
Temperature: At 32° F a lead-acid battery will lose 25% of its available energy, while a nicad cell will lose only 8%. At 14° F the lead-acid will have lost about 50%, the nicad only 15%. Lead-acid batteries are destroyed if they are allowed to freeze. Nicads are not damaged by freezing, and although they will not function while frozen they will work as soon as they thaw. By adding more potassium hydroxide (KOH) crystals to the electrolyte to increase its density from the normal 1.19 grams/ml to 1.30 grams/ml, nicads will operate at temperatures as low as ?58° F.
Longevity and bank sizing: Lead-acid batteries are damaged by total discharge and only have a usable capacity of between 30% and 60% of their rated power, depending upon how long you want them to last. Nicads are not damaged by total discharge (even if left in this condition), and will last for 25 to 50 years, providing many thousands of cycles if properly maintained. A nicad bank can be sized 30% to 40% smaller than the lead-acid bank it replaces, without appreciable loss of performance. It is easy to add capacity at a later date if required.
Maintenance: This is minimal, but important. Electrolyte levels should be checked monthly and topped up with distilled water if necessary. Electrolyte loss is directly related to overcharging, which can also result in losses to the oil layer that must be on top of the electrolyte. The oil keeps the electrolyte from carbonate contamination through contact with the atmosphere, and it should be topped up if required. Only the correct oil should be used.
Electrolyte replacement is necessary at intervals. If oil levels are maintained, this should be once every 10 to 20 years. It can be done on board the vessel, by the user, if care is taken.
The cost of nicads makes it difficult to justify using them for engine starting, and so one has to deal with two different operating voltages on board.
Standard alternators will not provide enough voltage to fully recharge nicads. An external regulator will solve this problem, but it should charge up to between 15.5 and 16.5 VDC. The output voltages of other charging sources should also be checked. Remember that nicads will perform better, and last longer, if they are occasionally charged up to 16.5 VDC. Finally, check to see whether regulation is required for any on-board appliances: can they all handle the maximum system voltage?
New nicads are expensive. However, when you consider that the bank can be reduced in size and that it can last twice as long, nicads start to justify their cost. Reconditioned cells are much cheaperabout 40% more than sealed gel batteries.
Nicads certainly look good on paper: but how did they fare in practice?
Although my installation performed extremely well for the first year or so, it gradually lost capacity, and this became a serious problem. Eventually it became evident that the electrolyte was contaminated and that the bank needed reconditioning, but as I was not carrying the necessary chemicals (which are not readily available, even in North America, nor easy to ship due to their toxicity), this had to wait until I sailed to the U.S.
Meanwhile, investigation revealed that the company that reconditioned the cells had not been doing this properly, which probably contributed to my problem. Second, I came to the conclusion that, while the specified amount of oil on top of the electrolyte (1/8 to 3/16 of an inch) is sufficient for a land-based installation, it is insufficient on a small vessel at sea, and had allowed contamination to occur. I was therefore recommended to have 1/2 to 3/4 of an inch of oil covering the electrolyte and this is what my cells now have.
The reconditioning procedure (which I did in the cockpit, at anchor) took one day. Seventeen of the cells recovered their full capacity; three (representing 15%) did not and therefore needed to be replaced. My assumption is that these cells had a limited life when I received them.
Nicad cells are not sealed and will leak alkaline in the event of a knockdown or rollover. They require special placement and restraints. The potential benefits of nicads on a boat make them very attractive; against this is their cost when new. If you intend to sail for many years, new cells may be the way to go. Only consider used cells if you are prepared to recondition them yourself you’ll need to procure the dry chemicals and oil before departing the industrialized world. Add extra oil to all cells. The assistance of Richard Peres, author of a series of nicad articles in Home Power magazine, and the manufacturers of Windhunter Yacht Power and Steering systems is gratefully acknowledged.