This one is easy, check the engine manufacturers recommendation, typically it will be around 80 Amp Hour, the higher the "Cold Cranking Amps" it will supply the better. If at all possible it should be the same technology as the service battery and certainly needs to have the same charging profile so don't mix flooded with Flooded with AGM.
Service / Domestic
This one is tricky there are many "rules of thumb" and it is often not clear if they refer to available capacity (e.g. allowing for normal maximum depth of discharge which can be from 50% - 80% and 80% - 90% with TPPL or Lithium) or total capacity and if the latter which technology.
Some heuristics relate to alternator size which ignores any significant solar power, wind generation or boosting technologies.
RYA training books say to do a power budget and work from that, a good starting point but often not as easy or as useful as it first appears. The rule of thumb is then to install battery capacity 3 or 4 times the daily requirement, again which capacity is not defined.
The Power Budget
The power budget can be done in amps with an adjustment for intermittent use then the total converted to amps / hours or in amp / hours from the start, I did the former:
|Part of Sancerre's power budget in amps.|
The autopilot usage will depend very much on the weather, point of sail, sea conditions etc., sometimes it will be minimal but in bad conditions usage goes through the roof dwarfing all of the others combined and if you have wind vane steering when that is in use there is no power requirement for the ram.
The usage from the cool box (or fridge) also varies dramatically, in cold weather, set for low power and opened infrequently the usage is quite low but on a hot day with it set to nicely chill some beer and opened frequently it will use a lot of power. Then there is a heater if installed, mine uses a lot of power to start then about 1.6 Amps when running but how often will it be used and for how long?
OK, guess some of those, divide the usable capacity in amp hours by the average amp hours required will give a rough estimate of how long you can go without charging the batteries, for Sancerre with three 120 Amp hour AGM batteries with a sustainable 80% "DoD" that comes out to 101 hours or 4.2 days. But in real life if you are costal sailing the engine will be run going in or out of anchorages, motoring through calms to make a tidal gate etc. and perhaps having shore power charging at times it is unlikely that the engine will not need to be run just for charging (unless stuck in an anchorage for days).
For long distance sailing however, if you don't have solar and wind generation you would probably run the engine every day to keep batteries well charged, strategies for this are covered on the page "Battery Management".
Alternator Size v Battery Capacity
Ideally the alternator should be sized to produce the highest amount of current your batteries can accept (their charge acceptance rate, which over shortish time spans when reasonably discharged ranges from 25 to 40% of the battery bank’s total amp hour capacity). This gets the bank recharged as quickly as possible, minimizing the engine running time and the fuel needed to replenish the batteries. Thus, a boat with a 200Ah battery bank is best be paired with an alternator that maxes-out at 50–80A but this ignores solar and wind generation.
But changing an alternator is not likely to be cheap and may not be practical, a bigger one may not fit and a small engine may not like it, my Yanmar 2GM20, rated at 16 HP continuous, noticeably slows down when the external alternator controller cuts in and will quickly trash a standard alternator belt, even the premium toothed belt starts to smell of burning rubber if the engine is run at high revs with low batteries and the alternator is pushing out 60 amps. Sterling Power report that some low powered boats have been known to slow down by 1 - 1.5 knots when their external alternator controller is charging low batteries and they explain how to set up the controller so that it can be switched off if extra speed is needed (if the alternator has its own internal controller to take over).
An easier method of improving charging is to fit an external regulator or a device to "fool" the alternators internal controller into producing more, these are covered in a separate section.
An appropriate charging strategy will also help, again covered later.
Solar, Wind and other sources.
These technologies introduce more variables and, it can be argued, makes the usage budget a waste of time. Solar only produces power when there is sun and the power generated will vary greatly depending on the angle of the sun w.r.t. the panels - panels that tilt in any direction are a big advantage if you remember to adjust them and sea conditions permit. Tilting in one directions as on Sancerre is easier and better than fixed but in some conditions can cause problems for the wind vane steering.
The wind generator is as problematical, it does work 24/7 but requires a reasonable amount of wind over the deck so may generate power when sailing up wind but none down wind. The power generated also varies with output rising steeply with wind speed so how can you estimate it?
|The performance curve of my mid range generator.|
If you are not installing renewable sources then just put in as much as you sensibly can and look at boosting alternator output. 240 AHr of AGM or better is likely to suit most people with 360 or 480 not unreasonable for long distance sailors with no secondary sources.
With wind and solar 240 AHr is likely to be adequate for most and my 360 AHr has not caused me any problems when the wind generator is working and only minor inconvenience when it failed - and I was stuck in an anchorage for days with minimal sun and even then with occasional use of the diesel heater the domestic batteries did not go below c 60%.