Mark and Richard,
I'd like to chime in about the notion of charging one bank while using the other for propulsion.
With lead acid batteries there is a significant amount of power lost in the charging process and in the discharge process, especially with the amperages that we are involved with here with electric drives. This means that much of the power that you are pushing into the charging bank will not be available for propelling the boat later when you switch banks.
Just for the purpose of this discussion lets assume you're motoring at 1500 watts and the charger can deliver 1000 watts. As well lets say you can draw 1500 watthrs from the bank before wanting to switch banks. (1 hr) And say both banks are full when you start.
For the first hour you don't have the genset turned on, cos there's no advantage in charging a full pack. So after an hour you switch banks, and start charging the off-duty bank.
For the second hour, with an hours running of the genset you'd be delivering 1000 watthrs of power. But only about 900 watthrs of this will be stored for later use. And then when you go to use it, because of the peukert effect that we hear about on this blog, its not all going to be available. Maybe you can get back say 750 of the 900 watthrs you put in. So after an hours running of the generator, you'd have added enough power for half hours battery use. And at the end of the second hour its time switch banks again.
But on this round you only have 750 watthrs available from the first bank. So on this cycle you'd need to switch after half an hour. That is, at two and a half hours from start. And because the off duty bank only got half an hours charging its only got 375 watthrs of power available for us. So you'd need switch again after 15 minutes of use. So pretty quickly its gotten unmanagable and we've only gone for about three hours having switched banks four times already.
On the other hand if you connect your charger to the bank being used for propulsion, you'd have 1000 watthrs of power delivered to the motor. That leaves only 500 watts to be provided by the bank that you are drawing from. And it looks like you'd be able to go three times the distance because you're only drawing 1/3 of the power from the batteries, before having to switch to the other bank.
But because of the peukert effect, you're lowering the current needed from the propulsion bank, and so even more of its available power can be delivered for use. And so you'd be able to go more than three times the distance you could if running from the battery alone, that is, more than three hours running before switching banks.
Then when you do switch banks you'd have another three hours running before the second bank is empty.
I've tried to simplify this description with the numbers I've used but they are about right. The main point is that you lose so much energy by the conversion to the battery chemistry.
And by the way, lithium batteries (specifically LiFePO4 cells) lose virtually no power in the storage and discharge process, and you could use the method of charging the off duty bank without penalty. (well, not quite - you'd still have the penalty of not using the energy available from the genset during the first cycle.) But I digress huh.
Cheers
Chris
--- In electricboats@yahoogroups.com, "acsarfkram" <acsarfkram@...> wrote:
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> I do like the separate bank concept for other reasons. If using a portable generator to extend range you can charge one bank while using the other. It keeps the set-up simple (same as shore power) and while I have no technical reasoning to back this, it seems that the charging bank will be easier to charge fully if it is not being used for propulsion. Ideally, you should bring the bank to a fully charged state before using it for propulsion. My guess is a smaller bank is easier to charge? Again this is just my intuition :-).
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