Knowing your batteries - Part 4, Some battery charging guidelines

by Larry Janke on 13 Dec 2009 This is Part 4 of a multipart series by http://www.semarine.com/store/home.php!Larry_Janke from Southeast Marine Services. This week he offers some battery charging guidelines

SOME BATTERY CHARGING GUIDELINES:

The underlying chemistry set out in Part 1 illustrates what happens during a discharge/charge cycle. PbSO4, lead sulfate is formed as a normal part of the reaction. It is widely if erroneously, believed that lead sulfate is somehow a negative occurrence.

It is not, any more than exhaling carbon dioxide is a negative aspect of breathing. It is part of the normal chemistry. The problem arises when the sulfate is allowed to remain on the surface of the battery plate and become a more complex crystal.

Just as you can easily write your name in wet concrete with your finger, but need a chisel to do it after a few days, so too does the sulfate become more dense and difficult to break down. The longer it sits the more difficult it becomes and as a practical matter if allowed to go on long enough will be virtually impossible by normal charging methods. .

That’s why we need to convert it back to lead, lead dioxide and sulfuric acid while it is still soft. 30 days is as long as it should sit. Contrary to the claims of some manufacturers of battery charging devices, particularly 'pulse chargers', hardened lead sulfate does not 'go back into solution'.

Lead sulfate is insoluble in sulfuric acid and nearly insoluble in water. The hardened sulfate crystals are broken into smaller less complex crystals and are shed from the plates exposing new active material which had previously been unavailable for reaction.

Specific Gravity versus charge at 78F. Open circuit (no load) voltage versus charge **

100% 1.265 - 1.275 100% 12.7 VOLTS
75% 1.225 - 1.235 75% 12.5 VOLTS
50% 1.190 - 1.200 50% 12.2 VOLTS
25% 1.155 - 1.165 25% 11.9 VOLTS
0% 11.6 VOLTS


* Specific gravity cannot be checked on gel ,AGM or sealed batteries

ADD 0.004 FOR EACH 10 DEGREES ABOVE 78 F.
SUBTRACT 0.004 FOR EACH 10 DEGREES BELOW 78 F.
EXAMPLE: 88F. SG=1.275+.004 ACTUAL SG =1.279


State of Charge/ Specific Gravity
Temperature compensation conversion

To further complicate the matter, how you use your boat changes charging requirements too.

If you only use your boat on weekends and it sits in a slip hooked to shore-power most of the time one set of rules apply.

If you are cruising and deeply cycling your batteries every day then another one does.

If you live aboard a third set of circumstances dictates and while there is crossover between these circumstances the purpose of this article is to help define and choose which charging system or systems is best suited to your application.

AC battery chargers vary widely in design and cost. The least effective and naturally the least expensive is the simple single stage charger you can purchase at your local discount or automotive outlet. These chargers are not appropriate for charging deep-cycle battery banks as they charge slowly and cannot fully recharge to rated battery capacity in any reasonable period of time. There are no automatic current or voltage adjustments during the recharge cycle and with out constant monitoring, the single stage charger will excessively gas the battery causing electrolyte loss and may overheat and warp the plates causing premature failure.

The graph in Fig.1 illustrates a charge cycle from a single stage charger. As you can see charge current drops quickly necessitating very long charge time and if you are not there to monitor the process your batteries will pay the price. Additionally these chargers usually have steel cases not well suited to the marine environment, are not ignition protected and utilize inexpensive minimal specification transformers to drop the voltage from 120 to 12 volts rather than isolation transformers utilized in quality chargers designed for marine applications .A further problem in that the DC negative and AC neutral are connected, with the subsequent potential hazard of electric shock, severe injury or death. Neither are they ignition protected, especially important in gasoline engine powered boats. A final and important drawback is that these types of chargers will allow substantial amounts of alternating current known as AC ripple to pass through into the batteries, a condition which is very destructive to battery plates. AC ripple should not be more than .5% of float voltage. These chargers do not belong on boats.

So, if we can’t use the cheap stuff how should we choose an AC powered charger? The answer again depends a lot upon how the boat is used. For a cruising sailboat which depends upon periodic charges and recognizing that most cruising boats also rely on solar, wind and engine driven alternators for much of their recharge capability an onboard AC powered charger may seem unnecessary or at best a redundant luxury. There are, however some very real reasons to consider one. If space and budget allow, the ability to produce AC from an auxiliary generator has some distinct advantages. In addition to a redundant source of battery charging which is never a bad plan, many appliances operate more efficiently on generator produced higher voltage AC than on 12 volt DC or AC produced by an inverter. A sailboat particularly larger ones and a powerboat which spends a lot of time at anchor away from a shore-side source have a lot in common in terms of electrical requirements. Both need an adequate bank of deep-cycle capable batteries and a means to completely recharge them in a reasonable period of time. Since many of these vessels have refrigeration, freezers, water-makers and water heaters, all consumers of substantial amounts of power it is usually more efficient to replace their requirements with AC driven systems.

Why not recharge your batteries at the same time? While a high capacity engine driven alternator is one way to do this you are using a sledge hammer to drive a tack. Why run your main engine to make a kilowatt or two of power when an efficient AC purpose built generator will do the job more quietly, and efficiently.

This is not to say that there is not a requirement for a properly regulated, high-output alternator, there definitely is, but if you are going to do al the other things mentioned above, higher voltage AC is more efficient and you may as well recharge at the same time.

It is very common and perfectly acceptable from an engineering point of view to discharge/recharge your deep-cycle batteries in the 50-85% range for periods of time up to 30 days. But then, it is important to bring them all the way up to 100 %. Failure to do this will result in hardening of the sulfate formed as a normal part of the discharge process to the point where it can not be broken down by ordinary chargers and battery capacity is reduced. Continued repetition of this partial recharge cycle will result in premature battery failure. To avoid this requires a charging system with the ability to hold charge rate at the level recommended by the battery manufacturer for enough time to completely charge the battery bank. In the popular three stage chargers it is necessary that the absorption stage of the cycle is long enough so that not only is voltage driven up to the recommended level but that it is held there
for sufficient time to also raise the specific gravity which lags behind voltage particularly in the thick plate batteries best suited for extended cycling applications. (See fig 2)

Many popular 3 stage chargers including the chargers in most inverters limit absorption time to 3-4 hrs, while acceptable for smaller battery banks, larger installations of 400-amp hrs and above require longer absorption times to completely charge the batteries. A few chargers can be adjusted for absorption times appropriate to battery bank capacity. While they have a higher initial cost, these industrial quality chargers are usually a once in a lifetime purchase and over the years will actually be less expensive than replacing several cheaper chargers. Additionally they have many important features which translate into faster more complete charging and greatly increased battery life, another substantial saving. And while it is true that this complete recharge can be accomplished with a properly regulated engine driven alternator it is going to take long engine run times to accomplish, whereas the AC charger can be used whenever the AC generator is operating or shore-power is available.
Chargers which operate primarily from generators need special consideration especially since thee majority of the chargers available today are of high-speed switching design. Most mass market switchers are not terribly compatible with the generators commonly found on boats because since they use a portion of the line cycle to achieve voltage conversion and the regulator in the generator is being subjected to load and no load conditions at a much higher frequency than it can interpret. Consequently most switching chargers including inverter chargers will operate at decreased output and charging times go up proportionally. Again some industrial quality chargers are designed to eliminate or at least minimize these problems.

For a boat which spends much of it’s time in a slip or as a live aboard some other considerations come into play when choosing an AC charger. While three stage chargers, assuming adequate absorption time, will give the fastest and most

Fig.3 Courtesy Analytic Systems Fig.4 Courtesy Analytic Systems

complete charge in the least time, they are best suited to charging batteries with out constant or transient loads for the reason that a three stage charger can not tell the difference between a discharged battery and a light bulb. If your boat has been sitting in its slip for a while and the batteries are fully charged and sitting at a float voltage, a load such as a light, pump or refrigerator can fool the charger circuits into pushing voltage above the level required for float charging, in other words you start charging a completely charged battery which results in heat and gassing with subsequent loss of electrolyte, plate warping and premature failure. These problems are especially
exacerbated in lead-calcium grid batteries (AGM and Gel) and when you add the AC ripple component to the DC voltage the problem gets even worse.

For these situations, chargers such as the Analytic Systems BCA in two stage mode series and VMI are ideal. These chargers will carry the house loads up to their output capacity in amps with out raising voltage above the recommended float level with the damaging results discussed above.

Many chargers advertise multi-bank charging capability. If the requirements of appropriate battery charging are observed; multi-bank charging is not a particularly desirable procedure. This is not because the charger is poorly regulated, in fact, the contrary is true. The better regulated the charger the worse the multi-bank charging result. If the charger is connected to batteries where loads are similar, such as two starting banks, multiple bank charging is acceptable and in fact quite successful. The rub comes when the charger is connected to a starting bank and a house bank which is subject to periodic loads.

If for example, the charger has voltage regulation of +/= 0.5% as quality industrial chargers will, and is connected to a start bank and a dynamically loaded house bank, one which has periodic or constant loads such as lights, refrigerators, pumps etc.. When a load is placed on, the house bank, the bank voltage will drop. The regulator in the charger is trying to hold voltage to the charger specification. Although the majority of the current will flow to the now lower voltage house bank, however some will flow to the non-loaded start bank, causing it’s voltage to rise To satisfy the regulator the house battery voltage must now drop at the same percentage the start battery rises. In a sufficient amount of time, the start batteries, because of their mow superior voltage will be supporting house loads and as the star battery voltage rises the batteries will begin to gas, and dry out and the house battery voltage will decline as they slowly discharge.

If these conditions continue for enough time as is common with a boat sitting in a slip, permanent battery damage will occur. The solution is to isolate the batteries from each other, but not with the common 'battery isolator' which is really just a set of diodes which causes voltage drop and does not allow the battery bank to achieve appropriate charging voltage. The Balmar Company of Arlington Washington, well known for their alternators and regulators, manufactures a 'smart' isolator the 'Duocharge' which when placed between the house and start banks allows simultaneous charging of both banks from a common charger either alternator or AC powered with out the problems outlined above. The Duocharge can be used with banks of different size and capacities as well as between flooded and gel or AGM construction.

For smaller battery banks requiring shorter absorption times there are many other chargers, mostly three stage available to the boater some good some not so. Professional Mariner LLC of Rye New Hampshire manufactures their reasonably priced Protech line in sizes from 10 -40 amps and in various output voltages. These chargers have the advantage of being very flexible as to input voltage and frequency which makes them useable almost anywhere in the world including at the end of the last dock in a large marina with ever present low voltage, and are completely ignition protected. Be sure that what ever charger you choose that these features are present and look closely at the warranty. In summary the required features of any AC driven charger are;

1.Voltage Control, What is the range of voltage adjustment, and can voltage be adjusted to within 0.1 volt and does it remain stable when a load is applied at float voltage? What mechanism is used for Current (amperage) Limitation?

2.Absorption Time, Does the charger remain at the recommended absorption voltage for sufficient time to achieve complete conversion of lead sulfate and 100% charge? At least 1 hour per 100 amp hours.

3.Amount of AC ripple not more than 0.5% of DC float voltage. (approximately 65 mili-volts ( 0.065 V) in a nominal 12 volt system)

4.Generator compatibility, what is the continuous output of the charger when operating from AC power supplied by a generator?

Items 3 and 4 do not generally apply to DC charging systems, however in the case of leaky diodes a DC alternator can cause AC ripple damage. I have seen alternators put out 35 volts of AC

One additional feature is available on some chargers. This is called the ‘equalize’ cycle. An equalize cycle is manually initiated by pressing a switch on the charger.

Equalization should not be initiated until batteries are fully charged. and really is necessary only when cell specific gravity varies by .015. or the batteries seem 'sluggish', do not appear to retain as much charge as previously or do not seem to fully charge. Once equalize begins, the charger applies a limited current of usually not more than 10% of maximum (i.e. 4 amps for a 40 amp charger) for a predetermined period of time (some chargers have adjustable equalization periods) or until the battery voltage reaches a preset level above nominal (usually about15.6 volts for a 12V battery).

The purpose of this cycle is to deliberately overcharge the good cells of a battery while allowing a weak cell to be fully charged. As this deliberate overcharging of the battery causes some water loss, it should only be performed once per month as discussed above, when battery capacity appears to be diminished or specific gravity readings vary by 0.015. Preferred equalization is at lower voltage, for example, at 14.7 volts for longer periods of time. However if sulfate has been allowed to harden, higher voltage may be necessary.

In addition, as the battery temperature is elevated by this cycle, a temperature sensor is supplied by some manufacturers to monitor battery temperature and modify charging voltage of all cycles according to battery temperature, as well as shutting the charger off if the battery temperature exceeds 120 degrees F or 49 degrees C. The drawback to temperature sensors is that they do not measure individual cell temperature and integrate them, but they are a step in the right direction and are especially helpful in an over temperature failsafe situation.

What about pulse charging? It has been known for years that pulsing between certain frequencies would fracture the bonds between lead sulfate crystals. The issue is however, what happens to the sulfate? Contrary to the advertising material from some manufacturers of pulse chargers, the lead sulfate does not go back into solution and 'rehabilitate' the battery. Lead sulfate is insoluble in sulfuric acid and water. It drops to the bottom of the case and fills the mud-space, or clogs the fiberglass mat on encapsulated or AGM batteries and impedes electrolyte access to the plate material. Further, in an AGM oxygen migration may be inhibited, a suspected cause of thermal runaway.

A final decision is selecting the right charger size. As a general rule a charger with an output capacity in amps between 16 and 25 percent of bank capacity is appropriate. For example 100 amps divided by 6 or 4 equals 16 and 25 amp charger out-put capacity, 20 % is a good all around number. Flooded lead acid batteries should not be charged at more than 25% of capacity.

It is claimed that gel and AGM types can withstand higher current charging but I am skeptical, having autopsied some of them which were subjected to these charging parameters. If you have a large bank say 1000 amp hrs then you could safely use a charger of 250 amp out put, but since that charger would be prohibitively large and expensive, the longer absorption times discussed above become even more important.

However smaller higher quality industrial chargers can be 'stacked' or paralleled to achieve desired capacities, however this is not advisable with most less expensive recreational quality chargers.

To read the previous Parts of this article click below:

http://www.sail-world.com/Cruising/Knowing-your-battery,-Part-1---10-common-battery-myths/63451!Part_1
http://www.sail-world.com/Cruising/Knowing-your-Batteries---Part-2/63700!Part_2
http://www.sail-world.com/Cruising/Knowing-your-batteries---Part-3,-Why-fewer-bigger-is-better/64165!Part_3

To make inquiries to South East Marine Services or learn more about them, click http://www.semarine.com/store/home.php!here.