Moderator Note: James only gets Special Notices emailed from this list, so there’s no guarantee he’ll see any of our responses.
Nevertheless, here goes:
James-
First, as Kevin suggested, we don’t have a complete picture of what you are intending to do. More info would help. However, based on what we read so far from you on this, it appears that a discussion of power, energy and efficiency is in order---although, unfortunately, not soon enough to keep you from spending money on components that may give you performance that seriously disappoints.
Component components:
· Marine 12v Deep Cycle Batteries: Not the best choice for an electric boat---you will likely find that they are not rated in amp-hours, but in cold cranking amps and reserve capacity. Reserve capacity is the number of minutes that the battery can stay above 10.5v at 25-amps of constant drain. As example, a Group 27 Marine Deep Cycle battery might have 180-minutes of reserve capacity. This equates to 3hrs at 25amps, for 75amp-hours capacity to 100%DOD. You plan a single string of these. At 25-amps drain, your string (if Grp 27) would have 75amp-hours max capacity. At 100-amps drain, that capacity would be significantly less---find the specs on your battery (including Peukert coefficient) and do some calculations.
· Twin 15-20HP motors: Why---do you need 30-40HP? Even 20HP equates to 15kw. And 15kw from a 72v battery bank will require over 200amps of current. And 200-amps of current drawn for just 15-minutes would pull 50-amp-hours out of your batteries. It’s likely that Peukert effect would cause each battery to reach 10.5v well before 15-minutes. Hence, you might only have 5- to 10-minutes of use at 200-amps.
· PMA’s: Don’t get me started! Assuming you bought these from Thermodyne Systems, unfortunately, if you even test them, you are not likely to be able to send them back for a refund…this, despite the fact that their online performance claims are misleading or false and their specs seriously incomplete. Contrary to the claims, these PMA’s do NOT “like high amps”. The internal resistance of the one we tested a few years ago was so high that we barely got 20-amps of charge current into 12v from an SC12 PMA being driven by a steam engine. And at just 20-amps, the PMA was running HOT! After testing the PMA for 5-minutes, the manufacturer would not accept us returning it for our money back despite our pointing out that their claims online were false, misleading and specs inadequate. The online claims and documentation to this date haven’t changed. While these PMA’s can be operated at efficient operating conditions, running at slower speeds and high currents will cause them to be highly inefficient and dissipate heat.
While it is certainly doable and reasonable to consider a balancing strategy for your batteries that involves DC/DC or chargers connected to individual batteries while underway, based on what you have described, you seem to expect lossless energy conversion or better. If so, you will be seriously disappointed.
Even a very efficient generator (90%), coupled with an efficient regulator and a pair of blocking diodes (1.5v min. drop together è 10% or more loss) would be at best, 80% efficient. With a PM motor & controller combined efficiency of perhaps 85%, overall efficiency from battery bank to controller => motor => PMA => single battery would be AT BEST, 0.8*0.85, or 68%. This means that for every Joule of energy you pull from the battery bank, you’ll only get 0.68 Joule delivered to the battery of your choice. Most likely, your efficiency would be FAR LESS than these estimates.
What this means is that it is a losing proposition to do what you are investing your money into doing. It seems that you expect that with the motors driving the boat at a given speed that attaching generators to those motors will somehow not draw extra power from the motors or the battery bank. If so, again, you will be disappointed.
Now, assuming your boat is a sailboat, what you describe may have some benefit in that you could use the PMA’s for regenerative power delivery to batteries while the prop is being pushed by the relative current. But a good regen controller and motor would do that without adding the PMA’s, which at low RPMs won’t generate high enough voltage for charging unless you have a regen controller for each PMA---a waste.
Finally, if you were mostly just interested in balancing the battery loads in the string such that a weak battery’s voltage could be picked up by the rest of its brothers, that’s doable and can be efficient: Use one or more 72vDC-12vDC DC/DC converters.
Best of luck…it appears you need to do some thinking and figuring.
-MT
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of james4078
Sent: Monday, April 11, 2011 6:08 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Generator and recharging
I was wondering who knows the answers to these questions?
I am putting together an array of 6 marine deep cycle batteries to make a 72 volts battery bank.
I ordered 2 PMA's (permanent magnet alternators that charges up to 300amps) to recharge the batteries (each PMA to charge 3 batteries)
There is one 72volt/300amp motor controller running two 15/20hp Permanent Magnet electric motors as the primary drive and the two PMA's will be driven off these to recharge the battery bank.
I want to recharge the batteries individually using 2-12v regulators to regulate the PMA's down to 12volts(voltage usually increases with the rpm's). 12 blocking diodes to go each battery (6-negative terminals, 6-positive terminals) to allow each battery to recharge without any drawback form the battery bank.
As the consumption of the drive motors increases with the rpm's so will the output of amps from the alternators thus trying to get as close to a 1-1 drain to charge ratio as possible.
So here are the problematic questions:
#1 What size voltage regulators do I use to handle these 300 amp PMA's
#2 Where do I get them?
#3 What size blocking diodes are suggested as well?
No comments:
Post a Comment