Do I really need a separate starter battery

[Zzmeyer]

Originally Posted by Mickeyrouse

Nah, you don’t need no stinkin’ separate start battery. And you could probably do without a bilge pump…or fire extinguisher….or first aid kit…or spares…or tools…or VHF…Just keep an eye on things.

I think you nailed it. I have a list of about 75 systems on the boat for maintenance scheduling, but also used it to determine spare parts or replacements. I use the following to determine what spares, if any, I carry. What is the probability of failure? What does a replacement or spare cost? What is the result of a failure without a spare? I started the thread as I believe the probability of failure of my house bank has gone way down (due to size, technology and monitoring improvements), the cost went up to have a starter battery (two battery types requiring more complex integration) and the result of failure remaining fairly low (if the engine doesn't start I become a sailboat). On the other hand, because my offshore crew and I are in our 60s an 70s I carry a defibrillator. I suspect most don't. I do carry a VHF, fire extinguisher and have a bilge pump (multiples of all) because of the result of the above three factor analysis (and the law). The probability of failure is moderately low, but the cost is low and the result of failure is high. Unless you are inclined to bring along a spare identical boat, we will all need to make similar decisions.

[donradcliffe]

The spares you carry are a function of the way you are going to use the boat. I sailed for 15 years in remote areas without a separate starter battery. I had room for 3 batteries, and made them all into one big gel bank, because Peukert's equation said it would be more efficient and the batteries would last longer.

I still deliver boats across oceans, mostly race boats. These boats have been prepped for the race by professionals with high budgets, and they are well maintained. They have no alternative power sources beyond the main engine, and the design philosophy is to charge batteries at least twice a day. Most of them have a separate large alternator for the house bank. One boat has recently put in a Mastervolt lithium system, which is not only lighter but has cut their charging time in half.

The tendency among race crews is to put the boat on a severe diet before the race. Sometimes they go too far, and it ruins their race. I make sure that spare parts and tools are shipped to the destination along with the delivery sails, and bring a duffle with me on the plane. I consider the electrical system critical, and its performance during the race is always one of the questions I ask of the race crew. If they report problems, I fix them prior to departure. I also check voltages, start the engine, and check charging current, then do an overnight load test.

In spite of all this, I have experienced electrical issues on these deliveries. One boat's Balmar regulator died on the second day out of Hawaii. My solution was to hotwire the alternator field to the batteries with one or two 12v incandescent nav light bulbs in series to limit the current. After a bit of experimentation, the boat would work with 4 hours a day of motor charging. Once I got into the Pacific High and was motoring 24 hours a day, I switched the house load to the stock alternator, which held up fine. Was I happy that the boat had two independent alternators and a separate starting battery? You bet your sweet bippy.

As I was preparing to deliver the boat with the new Lithium battery system this year, I gave some thought to what problems I might encounter. My biggest concern was BMS failure, but the house bank has three drop in batteries in parallel and I figured that I could disconnect a bad battery and carry on. In case things got worse, also had a spare 200 amp lithium BMS that to use with the cells, but that would have required destructive disassembly of the batteries. If the lithium BMS cut out under load and destroyed the diodes in the big alternator, I planned to switch all loads to the starting battery, but that would have meant idling the motor for 2 weeks.

Turns out the boat dropped out of the race, but I have since put 800 miles on it since and the lithium batteries are indeed a big upgrade. I may bring a multitool on the next offshore delivery in case I have to cut them apart.