Given that I've learnt an awful lot form the kind folks at canal world, I thought I'd give something back. That something is a collection of articles detailing basic level information about a wide variety of narrowboat related topics. Now I could have picked something less contentious than batteries, lets say choice of toilet, but I thought I'd get it over and done with. These are simple guides for simple folks but I'd like them to be acurate in the main, so those that actually know about batteries, feel free to correct anything I've written and I'll update the article accordingly!

What kind of batteries have I got?

A boat has two types of batteries, a starter battery and a domestic (leisure) battery bank. The starter battery is like a car starter battery and serves the same purpose. It fires up your engine. The domestic battery bank supplies the electricity used by your lights, fridge, pumps and via an inverter any 240v appliances you may have.

Batteries have a number of qualities that define them:

Firstly they have voltage. Most narrowboat systems are 12v, some are 24v or even 48v, but the most are 12v and supplied by 12v batteries. Secondly they have capacity, measured in amp hours. Thirdly they have a type, such as lead acid or gel. Some are maintenance free, others require you to top them up with distilled water from time to time. I am not going to go into the advantages of each here, because I'd be typing all day and probably get it wrong.

My boat's got 6 12v lead acid110ah batteries and that's fairly typical of a liveaboard boat.

How do I charge them up?

The most common ways to charge your batteries are either by plugging in the shore power or using your engine. Shore power will be routed through some sort of charger, perhaps one side of a combined inverter/charger unit. The charger part will be rated in amps. This is the amount of juice they can pump into the batteries in an hour. Engines have one or more altenators on them. These belt driven gizmos generate electricity and pump it into your batteries while the engine is running, and like the charger used by the shorepower, they're rated in amps. My boat's got two, a 50amp that charges the starter battery and a 100amp one that charges the domestics. Other boats may be set up similarly or may have a split relay charging system where one alternator serves both functions. Whichever way it is, that's how the engine pumps the juice into your bank, Some engines require you to rev em up to get the altenator going. Mine does and it took me a fair old while to work it out. The third option is a built in generator that some boats have. It works pretty much the same way as the engine, I think.

So on the face of it, a 50amp charger will stick 50amp hours into your batteries, right? Er, no. Wrong actually. Due to a variety of long and technical reasons it'll put in just over half that. The charge rate falls as the battery fills up. So at first it'll do the full 50, but by the end it'll be doing more like 10, The last 10% of charging takes the longest.

There are different stages to charging (one, two, three or four depending on how fancy your charger is) and during them the charger will behave differently. As far as I can tell they vary the voltage depending on the state of charge. I'm sure this is for a very important reason, but the details are fuzzy. You can tell you batteries are charging by the voltage at the terminals (or on the display of your charger). If it's 13.6 or so or above then it's working. If it's less that 13 then it's not. If it's 15+ it's doing something called an equalisation charge and if it's above 15.5 or so it's ruining your batteries and should be stopped immediately.

Looking after your batteries

"But they're maintenance free" you cry. Well, yes and no. You can knacker out your batteries really really quickly if you abuse them. And by abuse I mean draining them flat. The advice I've been given is to keep them at least half full at all times, which is easier said than done. I wouldn't worry too much if you can't, but it's something to aim for if you want your batteries to last a reasonable amount of time. To measure how full your batteries are, the voltage at the terminals should be 12.7v when full, 11.8 or less when nearly empty and somewhere inbetween for half full. I think it's 12.2 or 12.3 for 50%. If you've only got one of those 3 or four LEDs to indicate state of charge, just try not to let it go down to 1 bar.

The more knackered you batteries the less charge they'll hold. I'm fairly certain I can't fit 660ah in my bank any more and I'd guess they're closer to 440ah now, simply because I abused the hell out of them before I knew the above. So take care.

Some people view batteries as disposables, lasting a few years, others spend more and look after them and have them for years. It's horses for courses and you'll find some, er, vigorous debate about the merits of each on the forums.

Oh teh noes, my engine is bust and shorepower not available!

Yeah. This happened to me last week, so I spent a while working out 1) where to get a generator at short notice 2) how to plug it in to my charger and 3) how to get it to actually &%%^& work! My advice to you is that if you're living aboard, knowing where to get hold of a generator at shortish notice is a useful thing. You'd be suprised at the number of places that don't stock them.

Once you've got the generator going (you need to fill new ones with oil and petrol/deisel) you plug it in the same place your shorepower would go - you'll need an adapter for this. One end has a normal 240v domestic plug, the other the round blue connector your shore power expects. Hopefully it's plain sailing after that and your charger will take over, but if it doesn't and it keeps triping fuses/rcds or whatnot a simplpe solution that seems to work is to plug something else into the generator alongside the shorepower. I used my tv. No idea why this works, but it worked for me and it worked for others too. In terms of how big a genny you need, my 1000w genny will happily charge at around 30-35amps.

How much leccy will I use then? All these numbers don't help

If you're powering a firdge, pumps, lights and a bit of tv you'll use between 100 and 200ah a day. Nobody will be able to tell you for sure without a power consumption audit. Suffice to say as a livaboard with a reasonable altenator you'll be running your engine a couple of hours a day to keep up. If you use more your choices are to run the engine more or cut down the amount you use. Simple eh?

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Thanks to forum member ymu we now have another section! (05/08/2010)

Battery Management Systems.

A battery management system does several things, depending on what you have installed. Adverc, Sterling and Victron are the best known, and they all do similar things in slightly different ways. The following description is based on Adverc's system but the principles will be similar for all of them.

The primary component is, surprise surprise, a battery management system. This tricks the alternator into giving more charge than it was designed to; ensures that the starter battery is given priority whilst sending all the excess to the domestic bank; prevents overcharging of batteries (which ruins them very quickly).

A battery isolator prevents the starter and domestic banks from discharging into each other in a costly suicide pact.

Shunts can be fitted to measure the demand from 12v and 240v electrical systems (together or separately, depending on how many shunts you have fitted and how they are connected). The information is sent to a visual monitor which can tell you what's going in when charging, what's going out when appliances are in use, as well as the current voltage of the bank. The monitor will have a low voltage alarm - ideally audible as well as visual - to warn you when the batteries are at 50% (on the default setting). Discharging below 50% too often will reduce battery capacity over time.

It takes more and more energy to charge a battery as it gets close to full. Ideally the batteries will be charged to full, or near full in reality, at least once a month but need not be charged up beyond 80% (when it starts getting expensive) on a daily basis. Ideally you will have a battery bank more than 3 times bigger than your maximum daily requirement of them, so that you can charge to 80% only and not need to discharge below 50%.

The cables connecting the batteries in the bank must be beefy enough to take the input provided by the alternator with minimal losses - bearing in mind that you might wish to upgrade your alternator to reduce charging times. The cables connecting the batteries must all be the same size and length to put the same load on all batteries in the bank and should be as short as possible to minimise losses. The input and outputs should be connected at diagonally opposite ends of the battery bank, rather than having them both connected to the same battery.

3000 is about optimal for revs at the alternator - more than this will give more charge but at a substantial additional cost in fuel. Less than this will give suboptimal amps for the fuel used. The revs at the alternator may be calculated from the engine rev counter by considering the ratio of the outside diameters of the engine and alternator belts. If it's 2:1, you need to run the engine at around 1500 revs for optimal charging.