Designing and dimensioning the electric system

For my new to build boat, I will use a dual voltage system. 12V for the engine (starting & controls), 24V for the house bank (with 12 DC/DC convertors for the nav equipment, and few others that need 12V). My boat is being built in South Africa, and most lights, fans, water makers, etc. are all available in 24V.

I made some basic calculations to dimension batteries, chargers etc.. I made use of the calculation tables that can be found on the web, or in books like the ones by Nigel Calder.

Based on everything in 12V I would use about 260 Ah in a 24h period.
For 24V that would be 130 Ah in a 24h period.

To be on the safe side I would take a battery capacity at 24V that is three times the max consumption in 24hrs, so 3 x 130 = 390, so take 400 Ah.

Does this look right so far?

Now for the 24V alternator, I am planning to take a 100 - 150A marine alternator. Would that be the right size for a battery bank of 400 Ah? I would appreciate an experienced opinion on this.

Also could anyone point me into the right direction what I would need in solar panels to effectively keep my batteries charged when I leave the boat in the marina for a while?

Thanks in advance,

Andreas

 

24V should be much more efficient than 12V so you should be using less current from a 24V system.

Most of the electronic equipment works off a lower voltage inside, ie 5V, 9V etc. If the suppliers of these equipments can supply it to you with a ie 12V dc input plug (polarity protected) you would use much less power than with using an inverter to get to mains and then power down to whatever is required, you lose a lot of efficiency up and down powering.

The main concern is the type of battery you use. If you are going to use SLA (sealed Lead Acid) batteries the charging current should be limited as well as the voltage to prevent overcharging. The battery size determine the charging current, ie a 105Ah SLA battery has a typical 10A max charge current requiremen.

Normal LA batteries does not have this charge current limitation and you can let rip at your 100A.

I would recommend you charge the two batteries isn series each with their own independant charger, since the one battery damages by being overcharged in a series charge circuit and shortens the total life expectancy of the pack.

Solar panels have to supply enough power to compensate for the total current used plus losses - you will have to get your sun hour time rating for your part of the world... SA's is 8hrs if I'm not mistaken. To charge 12V 10A you would need a solar panel of at least 150W, allowing for 20% losses.

If the solar's are only to maintain batteries you could get away with a smaller panel, you have to work out what is using current while the boat is at rest. If nothing is using power you could use a 12W panel for each battery to just maintain the batteries.

 

Fanie,

Thanks for your feedback.

Apart from some of the nav equipment which is 12V, all other equipment (lighting, refrigeration, windlass, watermaker etc.) will be 24V. So I will only need a rather small DC/DC converter for 24 to 12V, which also acts as a filter for spikes.

You are right about the battery types. I have not made the final decision yet, which type I am going to use. My thinking in dimensioning the alternator was, that it should be sufficient to provide 25% of the total capacity (400 Ah) as charge current, if needed (like with standard flooded LA batteries). If other batteries require a lower max current, I will have to chose the right type of battery charger.

 

Stranded Mariner,


Think about just stayong with the 12 volt, I know in theory the 24 is sooooo much better etc etc, but in practice it simply is not for the boat you are building.
24 volt stuff is dearer to start with, it is absolutely no more reliable, and the extra system is simply not worth the time and money. If you had a 24 volt starter then maybe the 24 is the way, but with only 12 v starters etc , just stay with the 12 volt, it will be OK

 

I'm using a double system in my current project

Not cheaper? It is on my current project. If you have a hefty electric windlass, you can buy a 135 amp 24 volt alternator with the savings on those two cables alone. I've done the math-It is the amperage rather than the voltage that determines wire size, and good quality tinned, fine stranded 'boat cable' isn't cheap. (You halve your amperage when you double your voltage.) The bigger the boat, the more likely it is to pay, because your wire runs are longer. They don't even make boat cable large enough for a 2500 watt windlass using 12 volts with a 60 foot round trip (30' from the battery and switch to the windlass.)

 

BIG CAT

You need to look at the lengths of your runs mate, batteries located close to the driver are far cheaper than running 0000 cable all over the place, I buy in China at about 1/5th the cost of local, so cables certainly are not a problem anyhow.
When designing an electrical system there are many parameters to consider, cables are of course half the size in theory for 24 volt but not in practice....think about it.