Tech Wiki

C34 Battery Selection

Jim Moe Windseeker

There is no "best" type of storage battery for all purposes. It depends on what your requirements are. Essentially you have three types to select from and several subsets of these types. Your "house" battery must be a deep-discharge design and starting battery should be either a marine combination starting/deep discharge or an automotive starting battery.

For another point of view, see this from Sterling, an English charging system designer: http://www.sterling-power.com/support-faq-2.htm [Stu 1/23/2011]

The currently available marine battery types are:

Flooded lead-acid

These have removable caps on the cell tops so you can check the electrolyte level and will vent gas freely. Most "marine" batteries are flooded (you can always tell because they gurgle when you rock them). Flooded deep-discharge batteries are relatively low in cost, compact for the Amp-hour rating, fairly robust, have a 150-250 charge-discharge cycle life and are quite capable of withstanding overcharging. A subset of these is the golf cart battery which is designed for 250-400 charge-discharge cycles and is more robust in deep-discharge applications.

Flooded batteries are generally excellent for engine starting as they can deliver the high current required but golf cart batteries are somewhat less effective for this since they trade off high starting current for deep discharge capability. However, with four connected in a series-parallel arrangement, the current draw per battery for starting is so low this isn’t a problem, particularly so for our small auxiliary engines. Disadvantages for all flooded batteries are a fairly high self-discharge rate (10-15% a month at 70 degrees F) and, of course, they spill if tipped or inverted. For best battery life, marine deep cycle batteries should be regularly discharged to no more than 40-50%. Golf cart batteries are designed to be discharged to 25-30% without affecting their life.

Flooded starting batteries will readily deliver the high current required for engine starting but will withstand only 10-20 charge-discharge cycles to 50% or less without affecting their capacity. Consequently, they should never be used as house batteries except in an emergency.

“Maintenance Free” flooded lead-acid

Another subset of flooded batteries which has become popular in recent years is the so called "maintenance free" style with fixed caps, or even without caps, on the cells. So far as battery function goes, these are essentially identical to flooded lead-acid batteries. Many of these are merely standard flooded batteries without caps but with some extra electrolyte. The temperature compensated regulators in today’s vehicle alternators control charging voltage so well that outgassing rarely occurs. Thus the reserve electrolyte isn’t depleted, or is depleted very slowly. However, if you do overcharge them, gas will escape through the built-in pressure relief vents and if this continues until the reserve fluid is lost, the plates will dry out and the battery will fail.

Some maintenance free batteries use a “recombinant” technology to recover the gasses released during minor overcharging, as do gelled electrolyte and AGM batteries (see more on this below). They will tolerate a fair amount of overcharging but if it becomes serious, gas production will outpace recombination and venting will occur. The electrolyte level can't be checked or water replenished if it is lost due to overcharging. Also, if the battery is tipped, electrolyte is often discharged through the overpressure vents since the recombinant process doesn't work unless the battery is upright. Consequently, maintenance free flooded batteries offer no advantage over lower priced conventional flooded batteries since either will leak if tipped or inverted and conventional batteries can be refilled if overcharging, outgassing and venting occur.

Gelled electrolyte lead-acid

These are typically called “Gel Cells.” They are sealed and can be used in any position. They’re a lot higher in cost and a lot larger for an equivalent Amp-hour capacity. Self-discharge rate is much lower (2-3% a month at 70 degrees F) though. They typically will provide 250-400 charge-discharge cycles and this varies considerably between batteries from different manufacturers. They can be discharged to 25% of capacity without significantly affecting battery life.

They use the same recombinant technology as described above and have pressure relief vents but very little extra liquid electrolyte. If overcharged to any degree they will outgas and vent. When the gelled electrolyte dries out, there is no way to replace the lost water and the battery is finished. Periodic small overcharging events progressively shorten battery life and one severe overcharging can kill a gelled electrolyte battery in a matter of hours or even minutes.

Needless to say, very well regulated automatic chargers (ac line, alternator, solar or wind) are absolutely essential for these batteries. However, with proper care they are robust and capable.

A very heavy discharge current such as a large inverter or short circuit can destroy gelled electrolyte batteries it in minutes as well as they are not able to provide extremely high current. However, they can be used for starting small auxiliary diesels but should have a 50-100% higher Amp-hour rating than a flooded or AGM battery if used regularly in this role.

AGM (Absorbed Glass Mat)

These are somewhat larger in size for a given Amp-hour capacity than flooded batteries, but not as large as gelled electrolyte batteries. Price originally was about equivalent to gelled electrolyte batteries for an equivalent Amp-hour rating but has come down as AGM technology has improved and they have become more popular. They are sealed and may be used in any position. Life is typically 200-300 charge-discharge cycles. Self-discharge rate is low (3-4% a month at 70 degrees F) and they can be discharged to 25-35% without significantly affecting battery life.

High discharge current capability is excellent and they can be used as starting batteries or house batteries. Tolerance to overcharging is a lot better than for gelled electrolyte batteries since they have more reserve electrolyte, but not as good as for flooded batteries. Still, well regulated ac or alternator charging is a requirement. Another unique advantage of AGM batteries is that they can take very high charging current for fast recovery without overheating or damage through outgassing..

The Choice

So what to use? If you are going offshore and can possibly suffer a knockdown, then any type of flooded battery is definitely out. Today most ocean cruisers are using AGM or gelled electrolyte batteries for this reason. Their ability to handle daily charge-discharge cycles for months on end, longer life, deeper discharge capability and freedom from spilling make these batteries worth the considerable additional cost and battery bulk for the serious cruiser. However, if you install gelled or AGM batteries, really well regulated chargers (ac, alternator, solar or wind) are a must.

Today, the better choice for offshore use would be AGM as they’re more robust, provide a higher Amp-hour capacity in the restricted space available in a C34 and can provide more current for starting. If you do use gelled electrolyte batteries for the house bank, consider using AGM for the starting battery as it is much better suited to high starting current and will turn the engine over at a much lower state of charge. However, for this you will need an Echo Charge unit to charge the starting battery since the two types have different optimum charging and float voltages.

On the other hand, for near-costal or sheltered waters, the flooded battery gives you a lot more Amp-hours in the C34's restricted battery box, and a lot more Amp-hours for the buck as well. Generally, the reduced charge discharge cycle life isn't so much an issue here, as these boats tend to get used a lot less frequently. The problem is more typically ageing and sulfation due to periodically remaining discharged for long periods between use. If you have a alternator with an internal or manual regulator and/or a ferroresonant or "economy" shore power battery charger, the flooded battery's excellent tolerance to overcharging is essential. Having this type of charging system essentially rules out any “maintenance free”, gelled electrolyte or AGM battery type. As to the higher rate of self-discharge, if you charge the battery periodically (every month or so) this won't be a problem and will minimize sulfation. If you are in a cold climate, it's not a problem either, as no battery self-discharges to any extent below about 20 degrees F.

As the above indicates, choosing the "right" battery type for any application always involves a lot of trade-offs and compromises and you have to know what the end use is going to be to make an educated choice. The bottom line is: unless you are going offshore or a knockdown is a potential in your cruising waters, your best bet by far is flooded batteries for both house bank and starting.

Battery Recombinant Technology

The thing that makes all sealed batteries, and some “maintenance free” batteries, at all tolerant to overcharging is the recombinant process. To some extent in charging, and particularly in overcharging, water molecules are broken down into their component elements, hydrogen and oxygen, which is called outgassing. You can see these bubbles when charging a flooded battery with vent caps. These two gasses will recombine into water if the circumstances are right.

When the battery is in a discharged state, very little outgassing occurs, as most all the electrical charging energy is converted into chemical reactions between the electrolyte and the plates. However, as the battery becomes more fully charged, outgassing increases since there is less energy going into the chemical charging reaction and more into dissociation of water molecules. If charging current continues after the battery is fully charged, all the electrical energy from the charger is converted into heating the battery and outgassing.

In a vented battery, these gasses escape through the caps into the surrounding air. In a sailboat the lighter than air hydrogen dissipates through gaps in the overhead into the atmosphere. In a sealed battery utilizing a recombinant process, pressure is allowed to build up and the battery is built so the hydrogen and oxygen will recombine into water. Thus ideally, nothing is lost. Problem is that this process won't handle a whole lot of outgassing and certainly not as much as is caused by significant overcharging.

To limit the gas pressure there's a relief valve which vents off the gasses if the rate of outgassing exceeds the rate of recombining for long. When that happens, you’ve lost the water permanently and if it happens often, and for an extended time, the battery drys out and fails. Sealed, flooded "maintenance free" batteries have an extra quantity of electrolyte to allow this to occur occasionally without harming the battery.

Both gelled electrolyte and AGM batteries use essentially the same recombinant process to recover these precious hydrogen and oxygen atoms. They both have pressure relief valves as well, so what's lost in overcharging is lost for good. While this is bad for a flooded battery where you have lots of reserve liquid, it's disastrous for a gelled electrolyte battery and bad for an AGM battery since they have much less extra liquid to spare.

AGM batteries are better in this respect, since the liquid electrolyte is absorbed in a fiberglass mat which separates the active electrodes. With this type of construction, it’s possible to have quite a bit more excess liquid on hand, while still preventing spillage, than is possible in a gelled electrolyte battery.

Both shore power and alternator three-stage "smart" charging is designed to fit this sealed battery characteristic. They charge heavily at first when virtually all the electrical energy is going into the chemical reaction that recharges the battery. When the terminal voltage rises to a predetermined point, they cut back on the charging rate to minimize outgassing and finally cut way back to prevent overcharging so little energy goes into outgassing. While this is essential for all sealed batteries, it is very desirable for "maintenance free" flooded batteries and extends their life. While it is less of a requirement for flooded batteries with vent caps, these will benefit as well since they will require less water and it prolongs their life too.

Since we've come this far, I'll finish up with some more of the details on what gives each type of battery its’ unique characteristics. In the flooded battery cell, either vented or recombinant, the negatively charged electrolyte molecules can move freely between the plates since the electrolyte is a liquid. In fact, when very high currents flow, such in starting or heavy charging, electrolyte circulates between the plates sloughing off some active material. This keeps internal resistance of the cell very low but does gradually erode the plates and is the main limitation for battery life. Because of this freedom of movement of charged molecules, the self-discharge is relatively high.

In the gelled electrolyte battery cell, the same liquid electrolyte is used but is constrained by lots of inert material (diatomaceous earth) which considerably slows the migration of these charged molecules between the plates when the battery is being charged and discharged. Thus the battery cannot deliver as much starting current and internal resistance is higher. However, a benefit of this more sluggish movement of charged molecules is that the self-discharge rate is lower and less active material is sloughed from the plates during charging and discharging. This allows for a deeper discharge and extends the life of the battery. However, the extra gelling material in the electrolyte is inert and doesn’t become involved in the operation of the cell. Consequently gelled electrolyte battery cells are larger for the same Amp-hour capacity than for a flooded battery.

For AGM battery cell, a thick fiberglass mat retains the liquid electrolyte but, unlike the gelling material, doesn't significantly restrict the movement of charged electrolyte molecules. Consequently, ability to deliver high current for engine starting is excellent and internal resistance is very low, similar to that of the flooded battery cell. Since the fiberglass mat does restrain the flow of the liquid electrolyte during charging and discharging, it reduces sloughing of the active material from the plates, thus allowing deeper discharge and extending the life of the cell. There is less inert bulk in the fiberglass mat than there is in the gelled electrolyte cell, so AGM batteries are not as large as gelled electrolyte batteries for the same Amp-hour capacity.

As you can see, there is no "perfect" battery. Each has its own advantages and disadvantages. Needless to say, manufacturing quality and care are very important too, so stick to batteries made and sold by companies you know and trust. Typically, the specialty battery companies (such as Trojan) are better at building the specialty batteries that we require than are the volume producers (such as Exide, Delco or Interstate). Another thing that’s always true is that a properly designed charging system will improve the life and performance of any battery. It can even make a flooded, vented battery “maintenance free.”