Nigel Calder explores the pros and cons of current battery technology
For the past few years I have been part of a project experimenting with massively powerful alternator-type devices; the one I have now will generate up to 9 kilowatts (kW). To put this in perspective, that’s 750 amps at 12 volts! There’s not a lot of point in having this kind of generating capability if there is nowhere to dump and store the energy, so the corollary to the generating testing has been battery testing.
The Holy Grail for which we have been searching is a system ca- pable of generating and storing, during normal propulsion engine run times, sufficient energy to run the house systems for 24 or more hours. In our case, normal engine run times would be the time it takes me to set or pull up the anchor, or to get on and off a dock. For those with air conditioning, the objective is to be able to run the air conditioning overnight without having to run a generator, and then next day to be able to replenish the energy used during normal boat operations (i.e.
without requiring a stand-alone generator).
We have the generating and control piece of this solved (more on this in a future article), but the batteries are a work in progress. What’s needed is a battery that can store substantial amounts of energy in a relatively compact and lightweight format (a high energy density), and which is capable of absorbing extremely high charge rates to high states of charge. It must also be able to tolerate deep discharges so that the full capacity can be used at each cycle. This is the battery a typical cruising sailor needs, albeit on steroids in our application. There are various lithium-ion offerings that meet these requirements, but in general these are shockingly expensive, so initially we have been concentrating on lead-acid.
LEAD-ACID LIMITATIONS All forms of lead-acid batteries have several major drawbacks in a cruising environment. First, they don’t like deep discharges. In order to minimize the depth of discharge at each cycle, you need at least double the capacity you will use, which translates into a lot of volume, weight and cost. Then when it comes time to recharge, once you get up to around a 50-60 percent state of charge, the batteries’ ability to absorb charging current steadily tapers off and can’t be forced without doing damage. Not only that, if you get fed up with the extended engine or generator run time resulting from the low charge acceptance rates and repeatedly terminate the charge before the batteries are fully charged (this is known as operating in a partial state of charge), the batteries suffer damage from sulfation.
Lead-acid batteries are also relatively inefficient at converting electrical energy into chemical energy and vice versa. This inefficiency is manifested as heat. If you force the pace on the charging side, a lot of heat is generated. If the battery gets too hot, it goes into a condition known as thermal runaway in which the internal electrolyte boils, producing hydrogen and oxygen. In a worst-case scenario, the battery blows up, or the vented hydrogen accumulates inside the boat, is ignited by a spark and the boat blows up.
AGM RULES Of all the lead-acid batteries on the market, the absorbed glass mat (AGM) family has the best overall properties for the operating environment on most cruising boats. In particular, these batteries have the highest charge acceptance rate of any lead-acid battery to the highest states of charge, and the highest efficiency (around 85 percent, as opposed to around 60 percent for wet-cells), and as such generate the least heat during fast charges and discharges.
Within the AGM family, we have a sub-genre described as thin plate pure lead (TPPL), the best known of which are the Odyssey and SBS batteries from EnerSys and the NSB Energy1 batteries from Northstar. These batteries have the highest charge acceptance rate of any AGM battery with the best efficiencies. They will tolerate repeated deep discharges (down to around 20 percent remaining capacity). Over the past eight years, I have participated in aggressive testing of dozens of these batteries. In general, they have performed well, except that unfortunately they do not like being operated in a partial state of charge, suffering from sulfation. This means you regularly (ideally, once a week) have to have an extended charge cycle at low rates of charge.
If you have to run an engine (the main engine or a generator) solely for the full charge cycle, it is extraordinarily inefficient. An antidote to this is to have sufficient solar on board, configured so that it is providing the necessary extended charge cycle, as opposed to being consumed by
house loads. An even better antidote would be to eliminate the sulfation issue and thus the need for the extended full charge cycle.
THE MAGIC OF CARBON Some years ago it was discovered that if you sprinkle carbon dust into the active material in the negative plates of an AGM battery the carbon inhibits sulfation. There is now a family of these batteries available from Northstar, known as NSB Blue batteries. I have yet to test any, but this looks to be a significant step forward for partial state of charge operation.
The ultimate battery in the carbon doping world comes from Firefly. In a “normal” lead acid battery, there are plates with a grid composed of lead (this conducts the energy in and out of the battery), into which is pasted the active material (which absorbs energy on charging and gives it up on discharging). A Firefly battery dispenses with the lead grid in the negative battery plate and replaces it with a foam formed from carbon. The active material is pasted into the pores in the foam. The carbon acts as the conductor of energy in and out of the battery plate.
This foam ushers in some excellent properties. The normal consequence of discharging a battery is to turn the active material into lead sulfate. If the battery is left in a discharged state, the lead sulfate slowly morphs into large crystals that cannot be recovered by normal charging processes (the sulfation mentioned above). However, the pore structure in the Firefly foam is too small to allow the lead sulfate to crystalize to this extent, and as a result these batteries are more-orless immune to sulfation. Does this sound too good to be true? I have operated these batteries in a partial state of charge for months at a time, and I have discharged them to 35 percent state of charge (SOC) and left them for eight months without recharging them. I have then restored them to 100 percent SOC.
The carbon-foam plate grid is a remarkable step forward in the lead-acid battery world. The technology is patented and as such these batteries are only available from one company. Unfortunately, I have found the quality control to be somewhat uneven.
ULTRACAPACITORS Finally, for a decade various experiments have been made with capacitors embedded in lead-acid batteries. A capacitor is a device that can absorb very high energy spikes, but with a miniscule storage capability. In theory, a capacitor/lead-acid combination can absorb high rate charges in the capacitor and then over time dump the charge into the battery for storage, maintaining a high average state of charge and minimizing sulfation. The first of these batteries is just now becoming available in the United States from East Penn Manufacturing (the Ultra battery). The jury is still out on whether or not this represents a significant step forward in marine cycling applications.
FROM LEAD-ACID TO LITHIUM-ION After more than a century of development, you’d think we’d know everything there is to be known about lead-acid batteries, but exciting discoveries are still being regularly made; lead-acid technology should not be written off. However, there is nothing on the horizon that suggests lead-acid batteries will absorb the kind of charging currents I would like to throw at them up to high states of charge and at efficiency levels that will limit the internal heat generation in the battery. For this, we have to search elsewhere, with the only game in town currently being lithiumion. We’ll take a look at these batteries in the October issue. s
“Sealed” AGM batteries will still vent if pushed too hard
The aftermath of a hydrogen explosion from venting lead-acid batteries
The carbon foam technology used in Firefly batteries is promising
The author puts a TPPL battery bank through its paces
An Odyssey TPPL battery used for engine starting, paired with an AGM house bank