This is a good example of where LiFePO batteries offer advantages over lead-acid.
They are quite content sitting at low state of charge, and don't suffer like lead-acids do. You can drop them down to 20% charge and still get around 2000 cycles out of them (which is about three times as much as lead-acids cycling to about 50%).
I have Thundersky LFP batteries in my boat http://currentsunshine.com and initially installed them because of the weight advantage. But since then I've gotten to learn more about them and it turns out that they have quite a few advantages over lead-acid batteries for powering a solar boat.
As well as being able to deeply discharge them, and leave them at low state of charge without damage, the other advantages are:
virtually no peukert effect
low voltage drop under load
low loss of energy when charging
With the high amps that you can draw with an electric motor, the peukert effect reduces the capacity of a lead-acid battery from its nominal value, which is figured out at a low rate of over 5 hrs or 20 hours, depending on the rating. So the usable power in a lead-acid bank is less than the nominal rating so you have to figure this into your calculations, and get more batteries to counter this effect.
Under load the voltage of lead-acids sag noticeably, and also sag as the battery discharges. As the voltage sags the motor will draw more current to get the same power, so the amps get sucked out of lead-acid battery even faster as the capacity diminishes or as you give it more throttle. And this is on top of the peukert effect. The LFP batteries show little voltage drop under high current and also the discharge curve is very flat until the battery reaches about 5% capacity when its voltage drops off very quickly.
When recharging a lead-acid battery it typically has a charge efficiency factor of about 90% - that is, only about 90% of the power you put in is actually stored in the battery and available for later use. With LFPs is very close to 100%. This is especially important with solar recharge - it means you need 10% less solar panels for the same energy storage, or for the same solar capability, you can recharge quicker.
When you consider the peukert effect, which could be about 20%, and the charge efficiency factor or about 10%, the total loss of energy in the energy-in/energy-out equation is about 30%. So the total effect on solar panel requirements is that you need 30% less when working with LFP batteries. (or you recover the charge 30% faster)
When you take a look at the prices of LFP batteries, it can be daunting. But when you consider available energy, as opposed to nominal capacity, the price is not so bad. And if you consider lifecycle costs they are actually cheaper than lead-acids. For details, take a look at my analysis here http://currentsunshine.com/?page_id=10
There is a downside to this. The technology is new, and the some of the figures, such as number if cycles, are manufacturer's claims rather than well proven information.
Charging from solar sources is not easily handled with off-the-shelf items so you need to be handy to be able to cobble together the right charge controller and battery management system. I'd be happy to guide you though from my experience so far.
On my boat I have just added a 1kva portable genset and 10 amp charger to use as emergency backup - in my case I'm using the boat in coastal waters, and there could be safety issues with having enough power for crossing barred river entrances and so on. So I've added this backup system to cover those possibilities. But for your case of protected inland waterways, and leisurely cruising, such a backup is hardly necessary.
Cheers
Chris
On 29/01/2010, at 11:44 PM, hardy71uk wrote:
theres an extensive canal network here in UK. I've often thought it would be great to do a leuisurely trip . stopping for a while to explore an area whilst charging batteries from solar panels and then moving on. The problem would seem to be that if charging over say a week or two that the batteries would most of the time be only partly charged . Is this a fast way to kill lead acid batteries or would a slow charge resist sulphation?
I'm guessing that a better regime would be to just do a few miles per day using a small depth of charge. Any comments?
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