T-Pain Is On A Boat: Re: [Electric Boats] Re: Hi ya

Eric one thing that is missing from the equations is comparing going lower on the discharge = less cycles as to cost of extra batteries to stay at the 60% - 80% or so. Compare also to how many times are you going to travel 15 miles at 4 kts. If you are going to do it once every 100 cycles and recharge the batteries very slow and maintain them you should not loose that much in the way of cycles. If you use a flooded battery and take it out slow and put it back slow they will last a very long time. GEL batteries take it out faster and put it back faster but do not last as long that way. On any battery if you take it out slow and do not go below 80%-90% and put it back slow will last a long time. They are almost identical. The advantage come in going lower on the discharge to flooded batteries as long as you put it back slow and maintain it.

So if speed is the limiting factor it is the miles per dollar that will be higher. You need either more batteries or more often.

Range is not the limiting factor because if you slow down your miles per dollar goes up. Less batteries less often.

I can do it either way, it will just cost more to go faster. How much depends on how much you pay for the batteries. Recharging them I think is minimal cost.

Dan

--- On Tue, 8/10/10, Eric <ewdysar@yahoo.com> wrote:

From: Eric <ewdysar@yahoo.com>
Subject: [Electric Boats] Re: Hi ya - battery estimates
To: electricboats@yahoogroups.com
Date: Tuesday, August 10, 2010, 5:01 PM

Ben,

Good catch for a "newbie". 7kW @ 48V means 150A. Upgrading to 72V will drop the max to 100A. But ABYC published TE-30, their guidelines for electric propulsion, and they recommend addditional requirements for high voltage (>50V) systems. Nothing too complicated, just extra planning and some additional expense. While these recommendations are not official code yet, some people have suggested that insurance companies may be looking to these standards when electric drives become more common. Not following these guidelines may make insuring a "high voltage" boat more difficult in the future.

Let's dive into your conversion. Without know much more about your boat, I'll have to make a few broad assumptions. For this exercise, the main assumption is 4kts will take 3000W of power. It could be more, it could be less, but this should be close enough for this discussion.

15nm will take 3.75 hours at 4kts. So that is your target capacity. With flooded batteries, many people limit themselves to a 60% depth of discharge. So 3.75 hours @ 3kW = 11.25kWh. 18.75kWh * 0.60 DoD = 11.25kWh. We're now in the neighborhood. But now you've got to account for Peukert Effect. This is a batteries inability to deliver rated capacity at high loads. Using a Peukert's exponent of 1.25, while aiming for 3.75 hours at 60% DoD and a 3000W load, I end up with 23.7kWh of total rated battery capacity. What does this 23.7kWh bank look like? At 48V, you need 495Ah. Sixteen US125 6V 235Ah batteries will put you close. A bank of 6V batteries that are hooked together as 2 in parallel times 8 in series (2P8S). Going to 72V doesn't change the total energy requirement, in fact Peukert's calculations don't mention voltage at all. You can trade Ah for volts, but the watts are what counts in this scenario.

Remember my comment about battery types? Here's the same scenario in AGM, which cost a little more than 2X FLA per Wh. People regularly run AGM to 70% DoD. Using a Peukert's exponent of 1.1, while aiming for 3.75 hours at 70% DoD and a 3000W load, I end up with 18.25kWh of total rated battery capacity. That is the same range from a battery bank that is 5.5kWh smaller than the golf cart batteries.

Finally, with Lithium batteries (very pricy, relative to the others) people run to 80% DoD. Using a Peukert's exponent of 1.03, while aiming for 3.75 hours at 80% DoD and a 3000W load, I end up with 14.75kWh of total rated battery capacity. Same range, and 9Kwh (37%) "smaller" batteries than the flooded alternative.

Many people find AGM batteries to be the sweet spot between range and cost.

For whatever reason, these numbers are not readily available out there. People mention Peukert's Effect but they rarely quantify the differences. I have eventually learned how to calculate these capacities and I'm always willing to help.

Now you get to research appropriate drive systems and a whole lot of other components, I'll leave that for other threads... :)

Fair winds,
Eric
Marina del Rey, CA

--- In electricboats@yahoogroups.com, "benmckee@..." <mckeeb@...> wrote:
>
> I bet the the first batch of batteries will be basic golf cart 6 volts. Yes 15 nm @ 4 knots would be lovely, the 6 knot max is to help with narrows and the like. For the most part I try to plan my routes to take advantage of the currents.
>
> I am not sure of the "local" conventions for labeling motors, but I assume a 7kW motor @ 48 volts is pulling ~150 amps? I imagine that if I up the voltage I could save some cash on cabling, say to 72 volts which could pull as much as 100 amps (I understand that controllers can be programmed to limit such things).
>
> Eric, I sure appreciate the offer to confirm number and to answer questions. I usually do quite a bit of research before embarking on such projects, and I apologize that you folks get me at the very beginning of that process.
>