12 volt Electricity

Amps (A)   X   Volts (V)   =   Watts (W)

Watts   X   Hours   =   Watt-hours (wH)

In general you can figure out the power draw of any electrical device on your boat if you check the manual or case for either amps or watts that it uses. For example, a 12-volt reading light has an 8 watt bulb (small fluorescent), then 4 hours of reading uses 8 x 4 = 32 wH (Watt-hours) of battery power. In the same time, a stereo using 40 watts will use 160 wH (more if using a CD drive). If your anchor light is 25 watts and runs for 10 hours, it uses 250 wH of battery power. The total is 342 wH. A 100-amp battery can provide 100A x 12v = 1200W of power if it's fully charged so you could use this amount of power every day for 3.5 days. Check your battery to see what its capacity is, then make a list of the power requirements of every 12-volt device on the boat. To get longer use before you need to recharge, you may need to replace your lights with LED's, turn down the volume, or get more batteries.

Boat Electrical Systems

by Jim Morrison

Question from a boat owners forum:

Would like to upgrade my battery system. I now have two "house batteries" plus a starter. My present stock alternator will not deliver the charging power that I would like to maintain systems on board. I know there was quite a comprehensive dialog regarding this a few years ago. Could somebody point me in the right direction with regard to equipment that I would require.

Reply Oct 2003

Hello all,
This is one of my favorite subjects. I could happily rant for hours, but to save my fingers some stalwarts their eyes and most people use of their delete buttons, perhaps I would be wiser to recommend books by David Smead, such as Living on 12 Volts. I wouldn't say I'm quite a disciple, but I certainly am a fan of his. When I first came across him I had some doubts, but anything I could put a meter on, he was right about. Over the years, my experiences tend to coincide with his comments. Having said that, I will throw out a few comments to ignore.

To my mind, the electrical story mostly goes like this:

  1. First figure out your needs,
  2. then figure out how much to store to supply your needs,

In doing this, don't get lost in the details. Ballparks and basic principals are what you need. Grasping them will get you close enough while ignoring them will get you into trouble.

1. Storage needs = rate of consumption X time

Rate of Consumption: If you have hot and cold aboard (refrigeration & a mechanical heating system) it is hard to get by in less than 100 amp hours a day. (That's the equivalent of) 100 amps for 1 hour, 10 amps for 10 hours, or 1 amp for 100 hours - a very long day!

In practical terms, these are all different things to a battery. You can't run up a mountain, but you can walk up. Batteries get tired in a similar way) If you have simple systems, (ice box & blankets) then you might only draw 10s of amps a day. {I allow about 10 amp hours for my anchor light, and then count up my reading light time and probably don't burn more than 20 on average.)

If you have fancy gear on board, and lots of stuff to plug into inverters, you can slurp up batteries. How Long? If you go from dock to dock & plug in each night, not long. In B.C., the trend seems to be that you begin to itch to change spots after about 3 days. Generally you can count on the wind being from the wrong direction, but you can't always count on having enough ambition to thump and vibrate along long enough to charge your batteries back up.

Calculation: A typical cruising boat asking these questions has systems aboard and wants to cruise, so the most commonly expressed desire is to have 3 days @ 100 amp hours that can be replaced in a reasonable amount of time. That is 300 useable amp hours. (As a note: this is all in 1 bank. If you have a battery in reserve that can crank the engine, then 1 house bank makes the most sense. {Another long story.})

2. Storage choices

This is where reality starts to make you reassess your needs. You don't get 300 usable amp hours by buying 300 amp hours of batteries. Liquid batteries rapidly crap out if you discharge them deeply (in spite of being called deep cycle). Generally discharging down to 50% remaining is reasonably cost effective. (Some liquid batteries claim ability for deeper cycles, but the ones that can actually do that let the voltage drop so low that it is hard on equipment. Another long story.) So you'd think that 600 amp hours of battery would be OK, and it is for the first cycle. But you really have to kick liquid deep cycle batteries to charge them up, and the last 15% goes in quite slowly.

So on a long cruise, your useable window for 600 amp hours is (100% - 50% = 50% & 50% - 15% = 35%) 210 amp hours, or a little better than 2 days. In actual boxes, 600 amp hours is approximately (optimistically) 3 8D batteries or 6 golf cart batteries. Often people can only fit 2 8Ds or 4 golf carts.

You can get around this by using gel batteries (same lead/acid chemistry, but filled with pudding instead of liquid). If thick plate liquid batteries are the plodding dirt-tough and forgiving farm equipment, gels are the sports cars. Incredible performance at a price, and some finicky concerns that you absolutely have to bear in mind. With gels, you can comfortably discharge them down to about 25% remaining, and they don't slow down accepting recharge until the 90%s. 90-25 = 65% useable. So 1 170lb 8D battery (that I've often tested @ 225 amp hours) represents 146 usable amp hours. They are also extremely tough (if you get the ones from the East Penn Factory, whatever their brand name is). There is only one thing you can really do wrong to them, and that is give them too much voltage. The catch is, the temperature of the battery determines what too much voltage is. (This is also true for liquids, but for them, if you boil out some liquid ,you can pour some back in. With gels, you can't.) Gel batteries save lead, & footprint, but not money, and present special charging concerns.

3. Your choice in step 2 tells you what makes sense in step 3 (sort of)

Without question, if you want performance and reliability out of your batteries, you want a 3 step charging regimen in whatever charges your batteries (Yet another long story). This can be manual, if you are cheap and like to diddle, or automatic for not huge amounts more (sometimes.) Alternators that can actually work hard are generally externally regulated, and 3-step regulators now are common. I am a fan of temperature-compensated regulators, and if you go with gel batteries, you would be silly not to have temperature-compensation on everything that feeds the batteries.

For sizing, liquid batteries generally accept about 25% of their capacity That is, if you had 2 8Ds for 400 amp hours, they would probably eat 100 amps if they were hungry. For gels, current is no problem. Published specs suggest a charge rate of 50% of the capacity, but I have seen 4Ds (170 amp hours) happily slurp 150 amps many times. In any case, it is hard to get a hot-rated (stories & stories) alternator smaller than 100 amps, so you will probably find yourself with a100-150 amp alternator and an externally mounted 3-step temperature-compensated regulator.

As far as solar panels go, if you have gels than you want regulation, but if you have liquids, it is possible that if you have a big battery bank and a small panel, you may not need regulation. Usually you think of a 6 hour day (unless you cruise the summer arctic, or want to spend your day pointing your panels directly at the sun.) A 75 watt panel working 6 hours would give you about 35 amp hours of (attempted charge) and occasionally a current approaching 6 amps. If your liquid bank is more than 120 amp hours (real/not advertised), than this current is 5% of the capacity of your bank or less. Translation: even with no regulation at all, unless there is another charge source, such a panel can probably only produce a safe amount of current. You'd want to check your water levels at intervals, particularly during the summer when the batteries might warm up, but by then you should be out cruising anyway, and using up some electricity.

Hope this helps a little. Best of luck. I hope you find (as I did) that messing around with and understanding your boat's electrics is one of life's genuine cruising pleasures.

Best regards, Jim Morrison (Carnaby, 8.2 #126) jim_morrison@telus.net (with permission)

Reply Oct 2003: ... The heart of any system is a high output alternator and a smart regulator followed by as many deep cycle batteries as you can fit in the space available. Install the alternator first. I put a 130 amp unit in my Aloha 32 (from Holland Marine Products) and it was going strong after 10 years. This was the start of a system, that eventually included a large battery bank, a wind generator and a 1500kw inverter. You can spend time adding up all your consumptions if you like but you still that big alternator.

Tony Wright