{"id":10,"date":"2007-04-16T07:03:36","date_gmt":"2007-04-15T21:03:36","guid":{"rendered":"https:\/\/currentsunshine.com\/?page_id=10"},"modified":"2010-01-28T14:37:27","modified_gmt":"2010-01-28T04:37:27","slug":"battery-power","status":"publish","type":"page","link":"https:\/\/currentsunshine.com\/?page_id=10","title":{"rendered":"Battery Power"},"content":{"rendered":"

As you know from changing your car battery – they are heavy, real heavy. And for a light ex-racing trimaran like Current Sunshine this makes for a real challenge because it can’t carry much weight.<\/p>\n

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See how wide she is - there's a dinghy sheltering under her wing<\/figcaption><\/figure>\n

Most cruising yachts are keel boats, and use ballast for stability – so in some ways extra weight can help with such a boat’s stability. On the other hand, Current Sunshine uses its great width (36 feet) for stability. And because it was built as a short-handed racing boat it was never designed to carry any cargo, and so I have to be really careful not to overload her with batteries.<\/p>\n

When it was refitted for cruising some extra weight was added for the new cabin arrangements, so she’s already heavier than designed. As part of the refit she was lengthened by a little over a metre in the stern, so that gives her a little more load carrying capacity. This was needed to carry the 25HP outboard motor and its fuel.<\/p>\n

My objective with the new electric motor and batteries was to keep to same the weight as outboard engine and fuel. A 25HP Yamaha outboard weighs about 80kg, and its normal fuel load is a 25 litre main tank and two 20 litre jerry cans. This gives me a total of about 140kg to work with.<\/p>\n

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Torqeedo Cruise 4 from below - showing steering being set up.<\/figcaption><\/figure>\n

The electric drive is an Torqeedo Cruise 4 weighing in about 18kg and that leaves me 122kg for batteries.<\/p>\n

To begin with I explored the possibility of Lithium batteries, seeking lightness, but the price of $60,000 was ridiculously expensive, and so I abandoned that idea for a while.<\/p>\n

And then I discovered ThunderSky lithium batteries. The price for the a suitable battery pack was around $10,000 – amazing reduction from the $60,000 quoted by a French company. ThunderSky batteries are from China and I bought them through the Australian distributor, EV Power.<\/a><\/p>\n

They weigh in at 118kg for a 16 cell battery pack and so they match my weight limits nicely.<\/p>\n

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LFP batteries in what used to be my larder<\/figcaption><\/figure>\n

Each cell has 200Ah of capacity at 3.2 volts, giving a pack voltage of 51.2. These lithium batteries are known as LFP cells which are more thermally stable than earlier varieties, and don’t pose the same fire risk that dogged the early technology.<\/p>\n

They have some substantial advantages over lead-acid batteries – much more than I had first realised…<\/p>\n

The weight advantage is significant when simply comparing amphour capacity against mass.\u00a0 On this simple level, to provide the same nominal amphour capacity using lead-acid batteries of the AGM type would require 8 batteries each of 6 volts and 200 ah.\u00a0 These are around 30 kg each. For example an Absorbed Power<\/a> battery, of about the right size is 33.6 kg. This battery is listed on their website as a 225 ah battery.\u00a0 And eight of these would weigh in at 269kg – over a quarter of a tonne. But when you compare it carefully<\/em> to the LFP batteries they don’t even come close. There’s more to it than meets the eye…<\/p>\n

Lets look at the real capacity of these AGM batteries when under the full load of the 4kw Torqeedo motor. This converts to a current of around 80 amps when the batteries are at 50 volts. The Absorbed Power data sheet shows that for a current of 37.5 amps the capacity is no longer 225 ah, but 187.5 ah. And for a current of 132 amps its capacity is 132 amphours.\u00a0 Somewhere between these two points is the capacity when discharging at 80 amps – I estimate it to be around 160 ah.\u00a0 So we’ll really need more than 8 batteries to get the equivalent capacity of the LFPs.\u00a0 But before we figure this out there’s more to consider.<\/p>\n

On the other hand, the nice thing about the LFP batteries is that at high loads, their capacity is practically unchanged – and they can easily handle loads way more than 80 amps – in fact up to 3C (that is, 3 x capacity) or 600 amps.\u00a0 So we’re nowhere near those levels.<\/p>\n

A critical factor is depth of dicharge, or DOD. The Absorbed Power batteries are listed as having a life of 750 cycles at 50% DOD. The LFP batteries are listed as having a life of 2000 cycles at a DOD of 80%. So lets work on these figures to begin with…<\/p>\n

For the AGMs, I don’t want to go below 50% DOD in regular use, so the regular usable capacity is half of the rated capacity – which at the load I have, is half of 160ah, or 80 ah.\u00a0 For the LFP batteries, at 80% DOD, the regular usable capacity is 80% of 200ah, which is 160ah. Interesting huh – the AGMs real capacity is half of the LFPs.\u00a0 So it looks like we’d need double the capacity…\u00a0 but there’s more to consider yet.\u00a0 (and even that would be over half a tonne of batteries – whew!)<\/p>\n

Something else I notice about the LFPs is that the voltage doesn’t sag under load – well, not much anyway.\u00a0 But the AGMs do – and even more as the battery is discharged. I don’t have the figures for this.\u00a0 But what it means is that as the voltage drops, to get the same power, that is watts, more current is needed, which reduces capacity even more.<\/p>\n

And what about price? At $10,000 it seems these LFPs are expensive huh.\u00a0 But if I bought them today the cost would be around $7000, so already they’ve dropped.\u00a0 The price of a 6v 200 ah AGM is around $350 and we need at least 16 of them to get close to the capacity of the LFPs, which comes to $5600.00\u00a0 So even with AGMs the first cost<\/em> is significant and only 20% cheaper.\u00a0 But when we consider that in the lifetime of the LFPs (2000 cycles) I’d have to buy two other sets of AGMs, (750 cycles) the truth is that the LFPs are far less expensive than AGMs.<\/p>\n

But wait, there’s more…<\/p>\n

Another important aspect for a solar boat is the charging conditions. First there’s the charge recovery or the ability to get the electrons back into the battery for more use.\u00a0 The LFPs have the nice property of virtually no losses when charging.\u00a0 That is, if I put 100 amphours into the battery, that means I have 100 amphours available for use from the battery.<\/p>\n

But the AGMs have about a 90% charge efficiency factor.\u00a0 That is, to get 100 amphours of usable power in the battery, I have to put back about 110 amphours.\u00a0 So to achieve the same usable power I’d need a 10% increase in my solar panels if I choose AGM batteries.\u00a0 And this would increase the cost of an AGM system and this will brings its first cost even closer to the cost of the LFPs.<\/p>\n

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Mainsail being made at Gary Saxby's loft<\/figcaption><\/figure>\n

And secondly, and perhaps more important, is the low rate of charge recovery on a solar boat.\u00a0 Because the power comes from the sun or wind at a fairly slow rate, it may take a week or two to recover the power used from a burst of motor use. With LFP batteries it doesn’t matter if they sit at a low state of charge.\u00a0 But for AGMs it can be damaging if they are left at 50% state of charge for any length of time.\u00a0 Do you leave the batteries in this low state? Run the genset? Have an even bigger pack, say double again, so that the discharge is only 25%.\u00a0 Its becoming ridiculous – doubling the pack again would mean the cost is now more than LFPs and the mass is over one tonne.\u00a0 Or the other alternative of running a genset every time the charge drops erodes the advantages of a solar boat.\u00a0 Or leaving the batteries to recover slowly reduces their life even more from the assumed 750 cycles.<\/p>\n

The bottom line is, that on every consideration so far, weight, price, power available, charge retention and longevity, the LFPs are the best choice.<\/p>\n

The only disadvantage I’ve found is learning to manage them.\u00a0 They’re different to lead-acids and they need different management.\u00a0 I’m confident that I’ll soon have them set up in a way that requires no more from me than is needed for my lead-acid house batteries. In fact I’m already planning to change my house pack to LFPs as well.\u00a0 But, hey, that’s another story.<\/p>\n

To see how this is actually working in practise see the chart tracking motor use and battery status here<\/a>.<\/p>\n

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Jib in about 8 knots of wind<\/figcaption><\/figure>\n

Oh, another thing to keep in mind…\u00a0 she is after all, a sail boat, and the electric motor is an auxiliary propulsion system.\u00a0 The new sails are much lighter than the old dacron and cruise-lam sails. Not sure how much much, but I probably saved around 30kg.\u00a0 Being lighter the new sails are really good in light air. In sailing from Sydney Harbour to Pittwater in winds of around 5 knots we came out of the heads with two monohulls, both of which were running their engines.<\/p>\n

We stayed in touch with them just about all the way to Pittwater, and even came close to overtaking one when the wind freshened to around 8 knots.\u00a0 (We think he cranked up the diesel a little).\u00a0 We lost contact only when the wind became a 2 to 3 knot zephyr. So she suffers no real penalty even in the lightest conditions.<\/p>\n","protected":false},"excerpt":{"rendered":"

As you know from changing your car battery – they are heavy, real heavy. And for a light ex-racing trimaran like Current Sunshine this makes for a real challenge because it can’t carry much weight. Most cruising yachts are keel boats, and use ballast for stability – so in some ways extra weight can help … <\/p>\n

Continue reading “Battery Power”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"spay_email":"","footnotes":""},"class_list":["post-10","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/pages\/10","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/currentsunshine.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10"}],"version-history":[{"count":17,"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/pages\/10\/revisions"}],"predecessor-version":[{"id":299,"href":"https:\/\/currentsunshine.com\/index.php?rest_route=\/wp\/v2\/pages\/10\/revisions\/299"}],"wp:attachment":[{"href":"https:\/\/currentsunshine.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}