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«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»

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2.3. The most common types of lead-acid battery 2.3.1. Lead-antimony and lead-calcium Lead is alloyed with antimony (with the addition of some other elements such as selenium or tin in small quantities) or with calcium to make the material harder, more durable and easier to process. For the user it is important to know that compared to lead-calcium batteries, batteries alloyed with antimony have a higher rate of internal self-discharge and require a higher charge voltage, but also will sustain a larger number of charge-discharge cycles.

2.3.2. Wet or flooded versus starved (gel or AGM) electrolyte The electrolyte in a battery is either liquid (wet or flooded batteries), or starved: formed into a gel (the gel battery) or absorbed in microporous material (the AGM battery).

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In batteries with starved electrolyte oxygen gas formed at the positive plates migrates to the negative plates where, after a complicated chemical reaction, it is “recombined” with hydrogen into water. No gas will escape from the battery. Hydrogen gas is formed only if the charge voltage is too high. In case of excessive charge voltage oxygen and hydrogen gas will escape through a safety valve. That is why these batteries are also called VRLA (Valve Regulated Lead Acid) batteries.

Then batteries may be distinguished on the basis of their mechanical construction and purpose:

2.3.3. The flat-plate automotive battery (flooded) This is the battery used in cars. Not suitable for frequent deep discharging as it has thin plates with a large surface area – designed purely for short-term high discharge currents (engine starting).

Nevertheless flat-plate heavy-duty truck starter batteries are often employed as house batteries in smaller boats.

2.3.4. The flat-plate semi-traction battery (flooded)

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The deep-cycle battery must be charged, at least from time to time, at a relatively high voltage. How high depends on chemical and constructive details and on the charging time available.

Note: The high charging voltage is needed to reconvert all sulphate into active material, and to help prevent stratification of the electrolyte. The sulphuric acid (H2SO4) produced as the battery is being charged has a higher density than water and does tend to settle downwards so that the acid concentration at the bottom of the battery becomes higher than at the top. Once the gassing voltage is reached, charging is continued with plenty of current (and therefore a high voltage). The resulting gas generation ‘stirs’ the electrolyte and ensures that it becomes well mixed again.

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The tubular-plate battery is extremely robust and accepts a very high number of charge-discharge cycles. It is an excellent low cost substitute for sealed gel- or AGM batteries.

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Here the electrolyte is immobilised as gel. Familiar as the Sonnenschein Dryfit A200, Sportline or Exide Prevailer battery.

2.3.7. The sealed (VRLA) AGM battery AGM stands for Absorbed Glass Mat. In these batteries the electrolyte is absorbed (“sucked up”) into a glass-fibre mat between the plates by capillary action. In an AGM battery the charge carriers, hydrogen ions (H2) and sulphate ions (SO4), move more easily between the plates than in a gel battery. This makes an AGM battery more suitable for short-time delivery of very high currents than a gel battery.

Examples of AGM batteries are the Concorde Lifeline and the Northstar battery.

2.3.8. The sealed (VRLA) spiral cell battery

Known as the Optima battery (Exide now has a similar product), this is a variant of the VRLA AGM battery. Each cell consists of 1 negative and 1 positive plate that are spiralled, thereby achieving higher mechanical rigidity and extremely low internal resistance. The spiral cell battery can deliver very high discharge currents, accepts very high recharge currents without overheating and is also, for a VRLA battery, very tolerant regarding charge voltage.

2.4. Function and use of the battery

In an autonomous energy system the battery acts as buffer between the current sources (DC generator, charger, solar panel, wind generator, alternator) and the consumers. In practice this means cyclic use, but in fact a quite special “irregular” variation of cyclic use. This contrasts with the forklift truck example where the duty cycle is very predictable.

As boats are often also left unused for long periods of time, so are their batteries.

For instance on a sailing yacht the following situations can arise:

- The yacht is under sail or at anchor in a pleasant bay. Those aboard would not want any noise, so all electricity comes from the battery. The main engine or a diesel generator is used once or twice a day for a few hours to charge the house battery sufficiently to ride through the next generator-free period. This is cyclic use, where, significantly, the charging time is too brief to fully charge the battery.

- The yacht is travelling under power for several hours. The alternators on the main engine then have the time to charge the battery properly.

- The yacht is moored at the quayside. The battery chargers are connected to shore power supply and the battery is under float charge 24 hours a day. If the DC concept is used (section 8.2) several shallow discharges may occur every day.

- The yacht is out of service during wintertime. The batteries are either left disconnected for several months, left under float charge from a battery charger, or are kept charged by a solar panel or wind generator.

The number of cycles per year, the ambient temperature and many other factors influencing a battery’s service life will vary user by user. The following briefly discusses all of these factors.

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The table shows that cost varies greatly dependant on the choice of battery, and particularly that wet batteries are less expensive than VRLA batteries.

VRLA batteries do offer great ease of use, they:

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On the other hand sealed batteries are very sensitive to overcharging (the exception is the spiral-cell battery).

Overcharging results in gassing (through the safety valve) which means water loss that can never be replenished, resulting in capacity loss and premature aging.

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This table very clearly shows how heavy and cumbersome batteries are.

Coming back to the comparison in section 2.1:

Compared to the energy released by combustion of diesel fuel, for example, batteries are simply no rivals.

Burning 10 litres (weight 8.4 kg) of fuel generates approx. 100 kWh of thermal energy. So when consuming 10 litres of diesel fuel a diesel generator with an average efficiency of 20% will be able to generate 20 kWh of electric energy. This is the energy needed to charge a 24 V 700 Ah battery. Such a battery has a volume of 300 dm (= 300 litres) and weighs 670 kg!

Another telling comparison is heating water. Bringing 1 litre (= 1 kg) of water to the boil in an electric kettle requires 0.1 kWh. To supply the required 0.1 kWh, approx. 4 kg of battery is needed!

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The tables show how capacity falls off steeply with increasing discharge current, and that AGM batteries (especially the spiral-cell battery) perform better than gel batteries under high discharge currents.

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Although most batteries will recover from a full discharge, it is nevertheless very detrimental to their service life. Batteries should never be fully discharged, and certainly not left in discharged state.

It should also be noted here that the voltage of a battery that is in use is not a good measure for its level of discharge. Battery voltage is affected too much by other factors such as discharge current and temperature.

Only once the battery is almost fully discharged (DoD 80% to 90%) will voltage drop rapidly. Recharging should have been started before this happens. Therefore a battery monitor (chapter 3) is highly recommended to manage large, expensive battery banks effectively.

2.5.6. Premature aging 2. Charging too rapidly and not fully charging.

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2) Cell unbalance.

Cells of a battery never are identical. Some cells do have a slightly lower capacity than others. Some cells will also have lower charge efficiency (see sect. 3.4.) than others. When a battery is cycled but not fully charged, these weaker cells will tend to lag further and further behind the better cells. To fully charge all cells, the battery has to be equalized (which means that the better cells will have to be overcharged, see sect. 4.3.).

Unbalance will increase faster in case of very deep discharges or a very high charge rate. In order to prevent excessive cell unbalance, a battery should be fully recharged at least every 30 to 60 cycles.

2.5.7. Premature aging 3. Undercharging.

As discussed in section 2.2.4, sulphation will occur when a battery is left in fully discharged condition.

Sulphating will also take place, although at a slower rate, when a battery is left partially discharged. It is therefore recommended to never leave a battery more than 50 % discharged and to recharge to the full 100 % regularly, for example every 30 days.

Batteries, especially modern low antimony flooded batteries, often are undercharged because the charge voltage is insufficient (see chapter 4).

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Finally, temperature plays a big part in charging batteries. The gassing voltage and consequently the optimum absorption and float voltages are inversely proportional to temperature.

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2.5.10. Self-discharge A battery at rest loses capacity as a consequence of self-discharge. The rate of self-discharge depends on the type of battery and temperature.

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During discharging the sulphuric acid from the electrolyte reacts with the active material in the positive and negative plates forming lead sulphate and water. This reduces the sulphuric acid concentration and consequently the SG of the electrolyte.

During discharging, the depth of discharge (DoD) of the battery can be tracked quite well by using a

hydrometer to monitor the SG of the electrolyte. The SG will decrease as shown in the following table:

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During charging the reverse process takes place and sulphuric acid forms once again. Because sulphuric acid is heavier than water, in batteries with liquid electrolyte (this does not apply for gel and AGM batteries) it settles downwards, so that the acid concentration increases at the bottom of the battery. However, above the plates the acid concentration in the liquid does not increase until the gassing level is reached!

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Battery voltage too can be used as a rough indication of the battery’s state of charge (see preceding table, section 3.1.1).

Important: the battery should be left undisturbed for several hours (no charging or discharging) before a valid voltage measurement is possible.

3.1.3. Amp-hour meter This is the most practical and accurate way to monitor a battery’s state of charge. The product designed for this is the battery monitor. The following sections look in more detail at the use of the battery monitor.

3.2. The battery monitor is an amp-hour meter The battery monitor’s main function is to follow and indicate the DoD of a battery, in particular to prevent unexpected total discharge.

A battery monitor keeps track of the current flowing in and out of the battery. Integration of this current over time (which if the current would be a fixed amount of amps, boils down to multiplying current and time) gives the amount of amp-hours flowing in or out of the battery.

For example: a discharge current of 10 A for 2 hours means that the battery has been discharged by 10 x 2 = 20 Ah.

3.3. Energy efficiency of a battery

When a battery is charged or discharged losses occur. The total quantity of electric energy that the battery takes up during charging is approx. 25 % greater than the energy given out during discharging, which means an efficiency of 75 %. High charge and discharge rates will further reduce efficiency. The greatest loss occurs because the voltage is higher during charging than during discharging, and this occurs in particular during absorption. Batteries that do not gas much (low antimony batteries) and that have a low internal resistance are the most efficient.

When a battery is used in the partial state-of-charge mode (see the example in section 2.5.6.), its energy efficiency will be quite high: approx. 89 %.

To calculate Ah charge or discharge of a battery, a battery monitor only makes use of current and time, so compensation for the overall efficiency is not needed.

3.4. Charge efficiency of a battery When a battery is charged, more Ah has to be “pumped” in the battery than can be retrieved during the next discharge. This is called charge efficiency, or Ah or Coulomb efficiency (1 Ah = 3600 C).

The charge efficiency of a battery is almost 100 %, as long as no gas generation takes place. Gassing means that part of the charging current is not transformed into chemical energy that is stored in the plates, but used to decompose water into oxygen and hydrogen gas (this is also true for the “oxygen only” end of charge phase of a sealed battery, see section 2.3.2.). The “amp-hours” stored in the plates can be retrieved during the next discharge whereas the “amp-hours” used to decompose water are lost.

The extent of the losses, and therefore the charge efficiency depends on:

A. The type of battery: low gassing = high charge efficiency.

B. The way in which the battery is charged. If a battery is mainly used in partial state of charge (see the example in section 2.5.6.) and only charged up to 100 % now and again, the average charge efficiency will be higher than if a battery is recharged to 100 % after each discharge.

C. Charge current and voltage. When charging with a high current and therefore also a high voltage and a high temperature, gassing will start earlier and will be more intensive. This will reduce charge efficiency (and also the overall energy efficiency).

In practice charge efficiency will range in between 80 % and 95 %. A battery monitor must take the charge efficiency into account, otherwise its reading will tend to be too optimistic. If the charge efficiency has to be preset manually it is advisable to initially choose a low value, for example 85 %, and adjust later to suit practice and experience.

© Victron Energy

3.5. Effect on capacity of rapid discharging As discussed in sect. 2.5.3. The capacity of a battery is dependent on the rate of discharge. The faster the rate of discharge, the less Ah capacity will be available.

Back in 1897, a scientist named Peukert discovered that the relationship between the discharge current I and

the discharge time T (from fully charged to fully discharged) may be described approximately as follows:

n Cp = I x T where Cp is a constant (the Peukert capacity) and n is the Peukert exponent. The Peukert exponent is always greater than 1. The greater n is, the poorer the battery performs under high rates of discharge.

Peukert’s exponent may be calculated as follows from measurements on a battery or using discharge tables or graphs.

If we read (from a discharge table) or measure discharge time T1 and T2 for two different discharge currents (I1

and I2), then:

–  –  –

As shown in the tables of section 2.5.3, increasing the discharge current from C / 20 to C / 1 (= increasing the discharge current of a 200 Ah battery from 200 / 20 = 10 A to 200 / 1 = 200 A) can reduce effective capacity by as much as 50 % for a mono block gel battery.

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