Gardner,

  If you don't mind 24 volts, and you want a plug-and-play solution that includes battery management and waterproofing, I'll echo Roger's vote for the Torqeedo batteries.

  All their batteries are waterproofed to IP67 (30 minutes at one meter depth) and have battery management at the individual cell level.  They'll even shut everything down internally to prevent shorts if water does get inside.

    http://www.torqeedo.com/us/technology/lithium-battery-technology/battery-management-system
    (covers everything you might want in a BMS -- useful even if you don't go with Torqeedo)

  At 2685 watt-hours, their 55 lb 26-104 battery is the equivalent of a 220 Ah 12 volt 4D battery weighting over twice as much.

  They're also introducing a new product meant to drive larger boats called "deep blue", but I haven't seen much on that lately.

  However, four of them would be $10,000, so they aren't cheap.

  If you wanted to piece something together yourself, you could probably do it for half the price.  If you google "Thundersky battery management" or "LiFePO4 battery management", you'll get links to a lot of people who are working with what Torqeedo calls "non-branded" batteries.

  Here's a discussion of cruisers using the lithium-iron-phosphates:

    http://www.cruisersforum.com/forums/f14/lifepo4-batteries-discussion-thread-for-those-using-them-as-house-banks-65069-32.html


        - Mike


sigurd grung wrote:
>
> Hi Gardner,
>
> LiFePo has usually longer life than lead acid. Some have 3000 cycles.
> If you put that number into the cost calculation, it changes,
> obviously. It's been a while since I checked but at that time the
> smaller round cells were the best deal. Look for A123 brand for
> instance (ebay?), used by DeWalt and Milwaukee power tools I think,
> and radio controlled toys. At 2v there is almost no juice left, and
> if you draw them below this, they might be damaged. An 80% charge
> will probably increase life of the cells a lot, so I heard. They
> don't need a constant current charge, just keep it below 1C for
> better lifetime. You should probably have a charging cirquit if your
> solar cell bank is big. Either way a max Volt cutoff cirquit with
> hysteresis (since resting voltage is lower than max charging voltage)
> is needed for charging.
>
> I would see if it is possible to use 24v with all of the equipment,
> then there is less current and copper, and less copper losses. 24v is
> pro standard, trucks and such. Even if most of your equipment is low
> current, perhaps you want a 100w searchlight, coffee maker, autohelm
> or other "high" current things.
>
> From everything I've read, LiFePo is not flammable. But put good
> fuse/cirquit breaker.
>
> ------------------------- From: Gardner Pomper
> <gardner@networknow.org> To: harryproa@yahoogroups.com.au Sent:
> Sunday, March 10, 2013 3:39 AM Subject: Re: [harryproa] Lithium
> batteries?
>
>
> Very informative reply. Thanks!
>
> I have a few more questions... My reading on Li batteries speak of 1C
> and 5C charging and also of constant current charging. Is there a
> requirement to charge these batteries as a constant current? I am
> planning on using solar (perhaps 400w or so) panels as my primary
> charge, with shore power as a backup and possibly a small gasoline
> generator for emergencies.
>
> Another question is about the minimum 2V requirement on the cells. I
> had thought that Li batteries could be drawn all the way down without
> harm, as opposed to lead/acid where you should not discharge them
> more than 50% or so. Is 2v 100% discharged?
>
> I went internet shopping, just to see pricing and availability. I
> found a 100Ah LiFePO4 battery (28lbs, $620) listed as a replacement
> for 12v LA batteries. Here is the link
> http://www.batteryspace.com/LiFePO4-Prismatic-Battery-12.8V-100Ah-1.28-KWh-10C-Rate-Without-Balance.aspx
> I was thinking of it as a replacement for something like this
> http://www.wholesalebatteriesdirect.com/marine-batteries/52236-12-v-200-ah-4d-deep-cycle-agm-marine-battery-ub-4d.html
> (200Ah 4D deep cycle AGM battery 125lbs $385). As a 12v battery
> replacement, it claims to have internal protection for the cells, so
> does that mean I would not need a BMS?
>
> Finally, on a more general note, if you were building a cruising
> boat, without electric motors, would you consider LiFePO4 batteries a
> safe, reliable solution for a house battery bank?
>
> Thanks,
>
> - Gardner
>