«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»
For an exhaustive discussion of alternators, alternator regulators, isolators and other related equipment, I recommend reading Nigel Calder’s standard work “Boatowners Mechanical and Electrical Manual” as well as a visit to the websites of Ample Power (amplepower.com), Balmar (balmar.net) and Heart Interface (xantrex.com).
5.2. When the alternator has to charge more than one battery
The bare minimum on a boat is two batteries: one to start the main engine and a house (or accessory or service) battery. To make sure that the engine can always be started, all accessories (navigation equipment, lighting, autopilot, refrigerator, etc.) are supplied by the house battery.
The starter battery (sometimes 2, for 2 engines) should have no other load than the starter motor of the main engine and must never be allowed to discharge, otherwise the engine cannot be started.
When using a standard automotive alternator-regulator to charge several batteries simultaneously, the
following problems arise:
- The house battery will usually be deeply discharged and should really be charged with a high (absorption) voltage. This is particularly the case when the alternator on the main engine is the only source of power and runs briefly every day to charge the batteries.
- In contrast, the starter battery and often also the bow thruster battery are practically always fully charged and do not need any absorption charging.
- Often different battery types are used for starting, for the bow thruster and for house service.
These different batteries all have their own charging recipe.
5.2.3. A wide range of solutions It would be exaggerating to say that there are as many solutions as boats, but there are certainly many ways to, more or less, overcome the above-mentioned problems. Several, but certainly not all, will be
126.96.36.199 Keeping it simple and low cost: the microprocessor controlled battery combiner Let the alternator charge the starter battery, and connect the service battery to the starter battery with a battery combiner (for ex. a Cyrix battery combiner from Victron Energy). When one of the 2 batteries is being charged (the starter battery by the alternator or the service battery by a battery charger), the Cyrix will sense the increasing voltage and connect both batteries in parallel. As soon as the voltage decreases the Cyrix will disconnect the batteries from each other.
The advantage is simplicity and cost: the alternator does not have to be modified or replaced. The drawback is a somewhat longer recharge time of the house battery because bulk charge will stop at approximately 30 % DoD (or worse in case of important voltage drop in cabling or a low alternator voltage due to high temperature) and then be followed by float charge. This means that the battery will be cycled between 30 % and 70 % DoD. The solution is to oversize the house battery by 20 % to 50 % and do a 100 % recharge when shore power is available.
188.8.131.52 Increase alternator voltage
Most alternators with built-in regulators can be modified so as to deliver a higher voltage. Adding a diode in series with the voltage sense input of the regulator increases output voltage by approx.
This is a job for the specialist. We will not dwell on it here, but it is a low cost improvement that, together with 184.108.40.206, will charge batteries quite fast. Severe overcharging is a risk only in case of very intensive motoring every day, and even that problem can be solved by temporarily switching off the alternator (but never disconnect the main output of the alternator from the battery with the engine running, because the resulting voltage spike might damage the rectifier diodes in the alternator).
220.127.116.11 A multi-step regulator with temperature and voltage compensation When choosing a multi-step regulator (bulk-absorption-float, see chapter 4), I would suggest to go
for the best and choose a model with:
- Voltage sensing. This requires additional voltage sensing wires to measure and regulate voltage directly on the terminal posts of the house battery or on the DC bus. Voltage-drop in cabling and isolators is then automatically compensated.
This solution is often used when an additional high output alternator is fitted.
18.104.22.168 The starter battery.
The solutions as suggested in 22.214.171.124 or 126.96.36.199 will improve charging of the house battery, but what about the starter battery?
I hope it became clear from the previous chapters that charging batteries requires careful consideration, especially when conditions of use do change over time.
32 © Victron Energy
- When recharging a battery, the Phoenix charger will automatically adjust absorption time to the preceding DoD. When only shallow discharges occur (a yacht connected to shore power for example) the absorption time is kept short to prevent overcharging. After a deep discharge the absorption time is automatically increased to make sure that the battery is fully recharged.
- If the absorption voltage setting exceeds 14.4 V, the BatterySafe mode is activated: the rate of increase of voltage once 14.4 V has been reached is limited in order to prevent excessive gassing. The BatterySafe feature allows for very high charge rates without risking damage due to excessive gassing.
- The charging recipes for flooded batteries include two float charge levels. If only very shallow discharges occur, a float level of 2.3 V / cell (13.8 V respectively 27.6 V) is maintained, with regular short absorption charges. In case of no discharge at all, after a time which depends on the intensity of previous use, the charger switches to the Storage mode: the float level is decreased to 2.17 V / cell (13 V respectively 26 V), with a regular short absorption charge. The Storage mode will carry flooded batteries through their winter rest without any additional care needed (except for topping up, if needed, with demineralised water before the winter rest starts!).
5.3.3. Charging more than one bank The problem has been discussed under section 5.2. There are 2 solutions to the problem. The second best solution is the multiple output battery charger 188.8.131.52 The multiple output battery charger In its simplest and most common configuration a multiple output battery charger has 2 or 3 outputs, which each can supply the full rated output current and are isolated from each other by diodes. The charge voltage is regulated on the primary side of the diodes and is slightly increased to compensate for the average voltage drop over the diodes. Including the cable to the battery terminals the voltage drop at full output current can exceed 1.5 Volt. At close to no load the voltage drop will reduce to less than 0.5 Volt. This means that a charge voltage of for ex. 14.4 V will drop to 13.4 V at the full output current. This is OK as long as during charging DC loads on the system are small or nonexistent: at the end of the charge cycle the current will drop off and the 14.4 V absorption voltage will eventually be reached.
Temperature compensation Temperature compensation will not be accurate because the different banks will also have different temperatures. Temperature compensation is especially important in case of sealed VRLA batteries, see section 4.4.
Voltage sensing Compensation of the voltage drop by measuring the charge voltage directly on the terminals of one of the batteries will result in a perfect charge of one bank, and possibly overcharging others, see for ex.
note 4 of section 4.5.
184.108.40.206 A dedicated charger for each battery This is the best solution, at a price. A compromise can be to take good care of the expensive house bank, if needed including temperature compensation and voltage sensing, and to use a smaller multi output charger for the other batteries.
220.127.116.11 The microprocessor controlled battery combiner Charge the expensive house bank with a good charger, including temperature compensation and voltage sensing. And connect other batteries to the house battery with microprocessor controlled battery combiners, for ex. the Cyrix battery combiners from Victron Energy.
The Cyrix will also make sure that all batteries are parallel connected to the alternator when the main engine is running, see 18.104.22.168.
6.1. Introduction Now that we know, more or less, how to charge batteries, it is time to discuss the consumers, which will discharge the batteries.
In order to better understand the impact on energy consumption of the different consumers on board, it is
advisable to think in 3 categories:
Long duration consumers (navigation lights, autopilot, cabin lighting, water maker, air conditioning) that need power from between one hour to several hours a day.
Short duration consumers (pumps, electric winches, bow thruster, microwave, washing machine, dishwasher, electric stove) that need power for between a few seconds up to, say, one hour per day.
In my experience everybody, myself included, tends to underestimate the daily energy consumption of continuous and long duration consumers and to overestimate energy consumption of short duration consumers.
Especially when the source of electricity is a battery, it is important to differentiate between power and energy.
Power is instantaneous, it is energy per second, and is measured in Watts (W) or Kilowatts (1 kW = 1000 W).
Energy is power multiplied by time. A battery stores energy, not power.
Low power but consumed over a long period can result in a lot of energy consumed and drain a battery. Power is measured in Watt-hours (Watts x hours, or Wh) or Kilowatt-hours (1 kWh = 1000 Wh).
As a preparation for the chapters to come, some examples of power and energy consumption of household appliances and other equipment are discussed in the next sections.
6.3.1. Introduction More often than not, refrigeration on board is a nightmare, or at least a headache.
On small yachts the refrigerator often takes more energy from the battery than all other equipment together.
On medium sized yachts it is the refrigerator plus freezer that will drain the battery.
And on larger yachts it is because of the air conditioning that a generator has to run day and night.
In order to understand why, and see whether anything can be done about it, some theoretical background is needed. This is the subject of the next section.
6.3.2. Theory of the heat pump Nearly all refrigeration systems are of the compressor heatpump type.
Operation is as follows:
The compressor, driven by a DC or AC electric motor compresses a gas (freon, until this was forbidden because it destroys the ozone layer in the upper atmosphere) which is cooled down in what is called the condenser. The condenser often is a small radiator with a fan in the cupboard under the sink, or it is a much larger naturally ventilated radiator at the back of the refrigerator (normal household type refrigerator), or it can be water-cooled. In the condenser the gas condenses to liquid and in that process a lot of heat is taken from it. The liquid then moves to the evaporator, which is the cold plate in the refrigerator or freezer. There the pressure is reduced and the liquid evaporates. To evaporate a lot of heat has to be absorbed; this heat is removed from the refrigerator or freezer. The gas then goes to the compressor, and so on.
The amount of energy needed for drawing a certain quantity of heat from the surroundings with a heat pump may be calculated with the formula;
This means that for every kWh of heat that leaks in through the refrigerator’s insulation, or is drawn away from food or drink put into the refrigerator while still warm, 1 / 1.34 = 0.75 kWh of electric energy is needed to “pump” this heat out again.
6.3.3. The refrigerator and freezer in practice
And then the efficiency of the compressor and motor could be improved. This is a difficult one, as all small compressors have similar specifications.
If permanent AC power from an inverter is available anyway (see chapter 8) it is certainly advisable to install a standard household refrigerator and freezer.
Air conditioning requires enormous amounts of electric energy. Especially small airco sets, with 1 kW to 5 kW cooling power (3.400 to 17.000 Btu) in general have a low efficiency. If a generator is running anyway, no problem, except perhaps for fuel consumption. But as soon as soon as air conditioning also has to run on battery power, efficiency becomes extremely important.
Just like the refrigerator and freezer, an air conditioner is a heat pump with a compressor-motor, a condenser (on a boat always water-cooled because of the high power involved) and an evaporator.
6.4. Electric winches, windlass and bow thruster More and more common, even on smaller boats, these products will draw very high currents, but for a short period.
- An electric winch or windlass on a 15 m boat is in general powered by a 1 horsepower motor (1 HP =
0.736 kW) and will draw at nominal load 736 / 12 = 61 A from a 12 V battery (current draw can increase to several hundreds Amps if the winch is under a near stalling load!). If operated for 1 minute, the Ah consumption will be 61 / 60 = 1 Ah (see sect. 6.2). So energy consumption is not the issue, but it is very important to properly dimension the fuse, contactors, cabling, and batteries to withstand the high currents and eliminate the risk of fire due to overheating.
- A bow thruster will often take even more power, for example 300 A from a 24 V battery if fitted with a 10 HP motor. Current draw will be 10 x 736 / 24 = 300 A. One minute of operation will result in 300 / 60 = 5 Ah taken from the battery.
6.5. A battery powered washing machine and dishwasher?
Most of the energy goes into heating the water (hence the large difference in energy consumption between a 60 ° cycle and a 40 ° cycle), and using hot fill (supplying the washing machine and dishwasher with water at C C the right temperature instead of cold water) would further reduce energy consumption to a few hundred Wh!
A standard household dryer, though, takes 3 kWh, which means 3000 / 24 = 125 Ah from a 24 V battery. This is because preheated air is used to evaporate all the remaining moisture. And I do not know of any dryer heating the air with a hot water heat exchanger instead of an electric heater… A wash-dry cycle of a small washer-dryer as is often used on boats will take approx. 2.7 kWh.
6.6. Ever thought that electric cooking on battery power was feasible?
I didn’t, until I made the calculations and verified in practice.
And since that time I have a two-hob electric induction stove on my trimaran, powered by a 24 V 200 Ah house battery and a 2.5 kW Multi.
When compared to other electric stove, my preference goes to induction. With electric induction it is not the hob that is heated, but the bottom of the pan directly. The heating is therefore extremely fast and the hob does not become hotter than the bottom of the pan, which increases safety.
For that reason electric induction is also 20 % more efficient than other electric stoves (this is not just theory, I have measured it).
For the spaghetti we bring 4 litres of water to the boil, add the spaghetti, bring the pan to the boil again and leave it boiling slowly for 8 minutes. Power consumption: 400 Wh to boil the water, 100 Wh to boil it once more, and 400 W for 8 minutes to keep the spaghetti boiling, total 400 + 100 + 400 x 8 / 60 = 550 Wh.