John,
I completely agree with Denny that the Peukert effect is a big consideration for our boats. Let's assume that you boat has a 4kW drive and you prefer to cruise slowly at 50% power, around 2000W (40A @ 50V). Batteries are rated at 20hr capacities, i.e. 5A for a 100Ah battery. You can see that 50% power (40A) is hitting the batteries much harder than the rated loads (5A).
Let's work some numbers for different battery types for your boat.
Here's some numbers that I generated based on the following assumptions:
1) the load is 40A @ 50v (12 volt batteries average 12.5 in use)
2) the Peukert exponents used were 1.3 for wet cell, 1.1 for AGM and 1.02 for Lithium.
3) numbers are for 100Ah @ 50v which is 5000Wh, and 200Ah @ 50V or 10,000Wh.
Here we go...
For 100% depth of discharge (DoD)
Wet cell 5KWh - 80 minutes, 10KWh - 197 minutes
AGM 5Kwh - 121 minutes, 10KWh - 261 minutes
LiFePO4 5KWh - 144 minutes, 10KWh - 292 minutes
The only difference for each calculation is the Peukert exponent, which describes the ability of the battery chemistry to deal with high amp loads. Loads higher than the 40A used in this example will exaggerate the differences. But these calcs used a 100% DoD, which isn't the norm for most of us.
Now let's factor in depth of discharge using 60% for wet cells, 70% for AGM and 80% for LiFePO4 for the same size battery banks used above. The capacities shown now are adjusted to usable capacity with these factors.
Wet cell 3KWh - 48 minutes, 6KWh - 118 minutes
AGM 3.5Kwh - 85 minutes, 7KWh - 182 minutes
LiFePO4 4KWh - 115 minutes, 8KWh - 234 minutes
So, one can see that the same "size" batteries can have more than double the usable capacity depending on the technology. The effects are more pronounced for small battery banks, as illustrated above. Doubling the size of the bank again reduces the advantages of each increase in technology, but there is still a big difference. One should consider this when evaluating the purchase price of batteries; in some cases, more expensive batteries could be cheaper per mile of range.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, John Francis <surv69@...> wrote:
>
> Actually
>
> that's a lot of cycles for a sailboat that normally goes out once a week,
> then sits in dock for the next week or 2.
>
> I never figured flooded cells to 80%(I always figured discharges of about
> 40% max and 30% ideal).
>
> As far as the peukert phenonomon(sp), I have never in the past run my
> electric motors to more than 50% max, except for very, very short periods,
> if at all.
>
> I tend to be a conserver of energy and this is why I think I'm an especially
> good candidate for electric.
>
> Thanks for the info
>
> John Francis
> Pearson 26
> Port Clinton, Ohio
>
> On Fri, Jun 11, 2010 at 8:03 AM, dennis wolfe <dwolfe@...> wrote:
>
> >
> > Also, the peukert exponent of the battery is an important part of the
> > comparison because the capacity at a C20 discharge rate is not very relevant
> > to an application where C5 or even C2 or C3 is typical. Dig into the mfr's
> > data and you can usually find capacity at these high rates.
> >
> > Denny Wolfe
> > www.wolfEboats.com <http://www.wolfeboats.com/>
> >
> >
No comments:
Post a Comment