Well said Eric.
The point I was trying to make is that those batteries are undersized for your setup and you won't be able to realize the full capability of that motor without abusing the batteries terribly. They won't hold up long to that kind abuse. It's not just the AH rating working against you. The fact that they are FLA (or wet calls as I like to call them, I hate acronyms) also works against you more and more as power draw increases. This is mainly due to their higher internal resistance compared to other popular types.
Because of this internal resistance, the more Amps you try and suck from the batteries, the lower the voltage you'll see at the terminals.
Since power equals volts times amps, this lower voltage results in less power to the motor than expected. So, you increase the power (throttle) setting to compensate, which sucks more amps, which drops the voltage even more, and on it goes. Resistance also creates heat in relation to the square of the amp draw. In the uncontrolled "vicious circle" scenario described above, this heat would toast (or should I say boil) the batteries pretty quickly. To avoid such a scenario, you need to limit your amp draw to a level that the batteries are comfortable with. For 100AH wet cells I like to keep it under 40 or 50 amps at cruise, with max maneuvering power of say 100A, and don't use more than about 75% of their capacity before recharging. I believe that would be considered pretty hard use by most battery experts.
Since AGM's have a lower internal resistance, for any given amp draw they will show a higher voltage than wet cells, and therefore higher power output for any given amp draw. I believe they have other advantages as well but since I don't have any personal experience with them I'll leave it to others to comment. Lithiums are supposedly much better yet, but of course it all comes at a price.
Jim
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Anne,
>
> You are correct, "driving the boat from the charger" is supplying drive energy from a portable generator to the electric drive through your regular battery charger while underway. This is a common strategy for extending the range of your battery bank by members of this group.
>
> I believe that Jim's comment was more related to the 100Ah size of the battery bank rather than the FLA chemistry. If your system can pull 180A, your drive could drain a fully charged 100Ah 48V FLA battery bank to 60% depth of discharge (DoD) in less than 8 minutes under full power (8640W load). Realistically, you wouldn't pull that kind of power for more than tens of seconds or less to get out of a tight spot, your regular cruising will be at much lower power levels.
>
> But still, a 4.8kWh (100Ah x 48V) battery bank is fairly small for a boat your size. Assuming a load of 1500W (31A x 48V) for 4 knots in your boat, the FLA batteries would only last 73 minutes to 60% DoD, that works out to about 5nm on a regular charge. AGMs deliver more power per Ah and can be regularly discharged to 75-80%. The same size (4.8kWh) AGM battery bank would last more than 17 minutes at 180A to 75% DoD and 120 minutes at 31A to 75% DoD. That means that the AGM batteries with the same rated size (100Ah) extend your cruising range from 5 to 8nm.
>
> Before anyone assumes that AGM provide 75% more range for the same size batteries, this math is specifically for a 100Ah bank. As you move into larger battery banks, the difference between chemistries becomes less dramatic. Double the size of the bank and it looks more like this:
>
> 9.6kWh (200Ah x 48V) FLA to 60% DoD = 19 minutes at 180A and 175 minutes at 31A.
>
> 9.6kWh (200Ah x 48V) AGM to 75% DoD = 37 minutes at 180A and 259 minutes at 31A.
>
> This works out to 48% more range from the same size AGM bank at 31A. This is one example of why I recommend AGM for anyone that can afford them.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, amracel@ wrote:
> >
> > Thanks again for the suggestions, folks. A few more questions, however.
> >
> > Capt Mike and Eric, you both mention 'driving the boat from the
> > charger'. I assume you mean running the boat with the generator
> > connected to the charger?
> >
> > Jim, what do you mean the wet cells will get 'toasted in a hurry'?
> > What happens?
> >
> > Mark, I'm definitely going to call about the refurbished units!
> >
> > Anne
> >
>
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