Deficit reductionDec 16, 2013
More capable batteries with faster recharge rates can lower a battery bank’s charge deficit
A 144-volt bank of EnerSys SBS batteries in Nigel Calder’s boat Nada as part of his HYMAR hybrid power experiments.
(page 1 of 2)
For as long as I have been experimenting with boat electrical systems, the limiting factor in DC systems design (and, indirectly, much AC systems design) has been the low charge acceptance rate of traditional lead-acid batteries (whether wet cell, gel cell or AGM). This is a particularly acute problem on cruising sailboats which, much of the time, have little need of an engine for propulsion purposes and therefore have extremely limited engine run hours available to power an alternator for battery charging. It is also a problem for any powerboat that spends a significant amount of time at anchor. Both classes of boat tend to have a more-or-less permanent energy deficit.
Here’s the problem. As a conventional battery comes to charge, its charge acceptance rate steadily declines until it is accepting little charging current, regardless of the power that is available. The only way to pump relatively large amounts of energy into a battery bank in a relatively short period of time is to operate the batteries in a well discharged state — say between 50 percent state of charge and 20 percent state of charge. However, if batteries are operated on a regular basis at these kinds of states of charge, they fail prematurely.
Calder found that high output alternators also produce substantial heat that can reduce output. Ducting away the hot air from the alternator helps it maintain a higher output.
In order to avoid premature failure, the general rule of thumb is to prevent discharges below 50 percent state of charge, and to regularly charge to 80 percent state of charge, with periodic charges to 100 percent state of charge. Unfortunately, the charge acceptance rate at 50 percent state of charge is, at best, 40 percent of a wet cell battery’s rated capacity, declining to 15 percent at 80 percent state of charge, and just a dribble of amps above 90 percent state of charge. Gel cells have a moderately higher charge acceptance rate, and AGMs an even higher rate, but nevertheless all suffer from low charge acceptance rates at higher states of charge.
The dilemma is clear. You either run a fossil-fueled engine long hours lightly loaded to periodically fully recharge batteries or you kill the batteries in short order. In practice, cruising sailboat owners at anchor often end up running their main engine, or an auxiliary generator powering a battery charger, for two to three hours a day, while powerboat owners run a generator lightly loaded for even longer. In both cases, the average charge rate into battery banks is typically no more than 1 kW (approx. 80 amps at a nominal 12 volts) and generally less than this. With wet-cell batteries, up to 40 percent of this charging current is then lost during the battery recharge and discharge cycles, and with gel cells and AGMs up to 30 percent, so the effective net charge rate is only 600-700 watts. If you run through the numbers, including the amortization cost on the engine or generator, plus fuel and maintenance costs, it is not uncommon for boat owners to be paying $5 to $6 per kilowatt hour (kWh) for power generated on the boat (as opposed to 10 cents to 20 cents at home).
For the past couple of years we have had breakthrough battery technologies that more-or-less completely eliminate this problem. In the lead-acid world, we have thin plate pure lead (TPPL), most widely distributed by Northstar under the Energy1 brand label, and EnerSys under the Odyssey brand label, but also available in the telecoms world in the guise of SBS batteries. In the future, we may have lead-carbon batteries. Beyond this we have an ever-expanding range of lithium offerings.
What these technologies have in common is truly astonishing charge acceptance rates, the lithium even more so than the TPPL. Lithium, for example, will accept a charge rate that is equal to 100 percent of the rated capacity of the batteries (e.g., 100 amps into a 100-amp-hour battery) more-or-less up to 100 percent state of charge. In practical terms, with either technology the limiting factor in a DC system now becomes the output of whatever charging devices are available.
I conducted experiments on my brother Chris’ 40-foot Malo sailboat to better understand the implications of these technologies. These experiments were conducted in a typical off-the-grid cruising environment, gunkholing in the Bahamas. We installed a set of four Odyssey PC2250 TPPL batteries and wired them in parallel to produce a nominal capacity of 500 amp-hours at 12 volts. We replaced the 80-amp alternator on the Yanmar engine with a 120-amp Balmar alternator controlled by an external Balmar 614 voltage regulator. (Note that we chose TPPL over lithium because of the high cost of lithium as compared to TPPL, which, itself, is about a 30 percent more expensive than quality AGMs.)
The lower the current input rate, the higher the cost of battery charging per kilowatt hour while at anchor.
The first operational task was to establish an appropriate charging regime for the TPPL batteries. This technology is too new for existing regulators to have a built-in program. The Balmar 614 was an excellent platform for our testing in as much as it has numerous user-settable parameters (via the ‘advanced’ programming mode) which enabled me to develop customized charging algorithms.
The European Union funded HYMAR (HYbrid MARine) project, of which I was the technical coordinator, conducted extensive destructive testing of TPPL batteries at the EnerSys factory in Wales (UK). Based on this testing, a preliminary conclusion is that in cycling applications the Odyssey and SBS batteries are more likely to suffer a loss of cycle life, and thus life expectancy, through undercharging than through overcharging. Another preliminary conclusion is that in order to optimize cycle life, the batteries need to be periodically pushed to a full state of charge (a ‘conditioning’ cycle) which, even with the TPPL technology, requires up to four hours at low charge acceptance rates, declining to as low as 100 milliamps per 100 Ah of battery capacity at the end of the cycle.
This latter target — 100 mA per 100 Ah of battery capacity — creates a problem not just in terms of the very low charge rates, but also because the traditional amperage-measuring shunts used in most popular battery monitoring systems cannot accurately measure at these levels. For example, a ‘typical’ shunt that creates a 200 mV drop at 200 amps current flow will see 1 mV (1/1,000th of a volt) drop per amp, and can measure reasonably accurately to maybe as low as an amp, but no lower. It would require a resolution of 0.1 mV (1/10,000th of a volt) to read 100 mA.
Given the goal of minimizing Chris’ engine run time for battery charging at anchor, I programmed the Balmar 614 regulator for aggressive charging based on modifying an existing flooded deep cycle program. And given the inadequate resolution of his amp measuring device, I built in a minimum four-hour absorption phase as a mechanism to maintain charging until the charge acceptance rate was down to the milliamp level. My programming relies on intermittent periods of extended motor sailing (for several hours) and infrequent dockside hook-ups (enabling the battery charger to be plugged in) to create the extended charging periods needed for the conditioning cycle.