Hi William,
You made a mistake on your Li pack size and you're still carrying some of the old "rules of thumb" based on flooded cells with your plans. First of all is acceptable DoD. AGM can safely be discharged to 70% DoD without reducing cycle life significantly. So you can adjust your calcs accordingly. With Lithium cells, most users stop at 80% DoD, discharging to 100% DoD is flirting with danger. A single event of undervoltage an a cell can kill it dead. So stopping at 80%, gives you a buffer for safety. And 4 100Ah Li cells in series gives 100Ah at 12V, hooking 4 sets up in parallel gives 400Ah at 12V, the terminology for this configuration is 4P4S (P = parallel, S = series).
The old cruiser's rule of only charge to 80% full was based on lead acid battery's slow down in current acceptance around 80-85% charged. With AGM, this is still a consideration. With Lithium, the batteries will accept full charging current (bulk phase) past 95% full. So Lithium batteries will charge just as fast from 90 to 95% full as they did at 50% full, there's no need to stop at 80%.
So your 800Ah of AGM have a cruising capacity of 400Ah (20% DoD to 70% DoD). Anytime you charge with shore power, your charger should take the bank to 0% DoD giving you 560Ah as you pull away from the dock.
Doing the same with your 400Ah of Li cells (4 x 100Ah at 12V), you've got a cruising capacity of 300Ah (5% DoD to 80% DoD) and 320Ah as you pull away from the dock.
A 100A power source is no problem for either battery bank. Most AGM batteries can take charging current up to C/2 so your 800Ah bank could take 400A of charging. Most AGM controllers recommend a bulk phase of 0.3C for safety, that's still 240A for your big bank. The Li cells can take 3C or 1200A for your 400Ah bank. My Elcon charger has a profile that limits the bulk phase to 1C or 400A on your bank. The important part is that Li and AGM have different charging profiles so your charger and voltage regulator need to be programmed to the right kind of battery. You can see a number of lithium charging profiles on Evolve Electric's website on the Elcon Charger page. My charging profile is 512a.
Yes lithium cells have a cycle life greater than 3000 when kept above 80% DoD, but that is a complete deep cycle everyday for more than 8 years in. Chances are that the Lithium cells will die of old age before they succumb to cycle life. The AGM batteries should have a cycle life around 800 (I couldn't find expected lifespan for US battery AGMs) and using 3 deep cycles a week, they should last more than 5 years. So it depends on how often you expect to really cycle your batteries. Personally I've never had a battery die from cycle life, I generally kill them throgh mismanagement, though my first set of Li cells are still going strong.
Keep asking questions, we're here to help.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, william munger <wmunger@...> wrote:
>
> After more reading I of course have more questions.
>
> For 800AH in AGM I am looking at 4 US AGM 185 batteries for a total of
> $2000 and a weight of 600lbs. That would give me 280Ah of usable
> battery at 50%DOD and charging back up to 20% while away from dock with
> alternator on inboard diesel. I think it would take me about 3 hrs of
> charging with a 100amp alternator to get from 50% dod to 20% dod (is
> that correct?).
>
> Looking at LIFEPO4 Sinopoly 100Ah batteries it would take 4 in series to
> make 12V, then could use 16 for four banks of 12V for a total of $2160
> and weight of 112lbs. That would produce 640 usable amps at 0% to 80%
> discharge. Or 480 usable amps at 20% to 80% discharge.
>
> My questions are:
>
> What rate of charge will the LIFEPO4 batteries take? (do they require
> float charging as AGM's do)
>
> Can I use a normal 12V 100amp alternator to charge these?
>
> Would it be better to series all 16 for a 48V bank, get a 48V alternator
> for the motor and then use a DC/DC converter to get back down to 12V for
> my house loads?
>
> I have a 4 bank charger designed for AGMs, What type of charger would I
> need to handle LIFEPO4?
>
> My understanding is the LIFEPO4 batteries give you at least 2 times the
> cycle life, way less weight, more usable amps via deeper discharge, no
> self draining, no issues with sitting unused at less then full charge,
> but way more cost. (although in this case less then $200 more for the
> same capacity)
>
> Am I missing something important/costly?
>
> William
>
No comments:
Post a Comment