Having recently looked at our house battery bank and our energy budget, I thought it was a good time to look at how we ‘pay’ for that budget. In times past, it was common to run the engine to charge the batteries, but his has always been a bad idea. Running a diesel engine without load can kill a motor in only a few hundred hours. Running under load is fine, as long as we are using around 70+% of the rated output. In reality this mean running at least one of our motors at around 2800 RPM in gear just to provide battery power. not very efficient. Our alternators are 60 amp units, so a single unit should be capable or recharging the house bank in around 4 hours, and both engines in around 2 hours. however due to the way that lead acid batteries absorb charge, this is not the case.
Lead acid will charge in the so called Bulk phase until around 80% charged, before the charge rate falls significantly. For our lead acid house bank, rated at 420A/hr, assuming a 50% depth of discharge this means that the 30% (126A/hr) will take a little over 2 hrs to bring the battery back to 80%. As the battery charges it becomes progressively more difficult to charge. At 13.8v (typical alternator voltage) A typical 125-AH Marine battery will take approximately 80 hours to recharge at 13.6 volts. Increasing the charge voltage to 14.4-volts will reduce battery recharge time for a 125-AH battery to 3-4 hours. The problem is that there is no way to simply increase the charge voltage without altering the alternator. Some intelligent alternator controllers are available, which have this function, but they are far from perfect. One disadvantage of recharging a lead acid battery using the engine alternator at a fixed voltage of 13.8-volts is the recharge time is very long. So batteries are rarely fully charged under engine alone unless we do a ‘lot’ of motoring. Another disadvantage of recharging a lead acid battery at a fixed voltage of 13.8-volts is that once it is fully charged, 13.8 volts will cause considerable gassing and water loss, meaning more maintenance.
Another problem which besets lead acid technology is Peukert’s constant or the Peukert effect. Essentially lead acid batteries are not very efficient. They take far more power to charge than they hold. They also decrease in capacity with an increase in load. This is best illustrated by the battery rated capacity. Our flooded lead plate design house bank has batteries rated at 260A/hr at the 20 hr rate. This means they are rated at 260 amp/hrs with a discharge rate of only 13 amps. Increase the discharge rate and the capacity falls. At the 5 hr rate (around 39 amps) they provide only 215 A/hrs, or around 82% of the capacity when discharged at the 20 hr rate. Give them some real stick, for instance running the inverter or perhaps the anchor winch and these values go through the floor.
The situation is similar when it comes time to charge them, the faster you charge the battery, the more it resists being charged, thus the more total power that needs to be supplied. Add to the fact that lead acid really needs to be charged to 100% as often as possible, and we begin to see real issues with lead acid as a house bank long term.
All that having been said, lead acid batteries have been supplying reliable house loads since it’s invention in 1859 (French physicist Gaston Planté). It is the oldest rechargeable battery technology as is by far, the most widely available. No matter what storage is used, the issue is always how to get the power back into the batteries. Where does the power come from?
In a marina, plugging in the shore power and using a decent battery charger will keep the batteries topped up, and indeed if day sailing or short cruising, then that is fine as you will have plenty of hours plugged in, in which to get the batteries to 100%. If offshore or on longer passages, that option is not available, so what else will do the job?
Solar power comes from the conversion of sunlight into usable electricity, usually via a photo voltaic panel. These come in a number of different constructions with varying efficiency, weight and cost, but all perform the same function in much the same way. A catamaran is an ideal platform for solar power as it affords plenty of space for the panels. For Elijah I will be going for around 400 watts of solar power. This would appear to be rather a lot for our power budget of 2500 watts per day, but of course solar is only available when the sun is out.
At first glance, assuming 6 hrs of sunlight per day and 400 watts of solar, you would expect around 2400 watts of power. This however is not the full story. Solar panels will only produce their maximum output when illuminated at 90 degrees to the light source. This means that in reality, even using inclined panels, we are unlikely ever to seen maximum output for any length of time and the true output is more likely to be 50% on average, given bright sunlight. Ideally then we would perhaps be looking for 800+ watts of solar in order to meet our budget. This however means we would need to be able to potentially charge the batteries at close to 60 amps – fine for LiFePo4, but not really for Lead Acid, as the charge rate tails off fast above 80% charge. It would be better to augment the charge with a lower current over a longer period.
Small wind turbines have been around for years. Originally of very low output, units of 3-400 watts and higher are readily available, if not exactly cheap. We will go for a moderately priced Rutland 914i. This is a 300 watt unit with a published output of around 143 watts at 20Kn. While hardly earth shattering, the ability to pump out power 24*7 really does add up. 140 watts over 24 hours = 3360 watts – well in excess of our target daily load.