[PUP] Energy resource management ( was PPM in general - what should it be ?)

[John Marshall]

There is a downside to long, slow charging, at least when it comes to  
AGM batteries, which would be a big issue with solar. Lifeline (and I  
assume this applies to all low-resistance AGM's) is very specific that  
for repetitive charging, you should charge at a rate of 0.2 x battery  
bank capacity in amp/hours, otherwise you'll lose capacity over time.

So you get clobbered on both ends if you aren't careful. Here's the  
link:

http://www.lifelinebatteries.com/manual.pdf

In my case, when I parallel all 12 batteries, I've got 1530 amp/hours  
capacity at 24v (charge relays automatically connect thruster  
batteries to house bank during charge). Per Lifeline, I should have an  
initial NET (after house loads) charge current of AT LEAST 306 amps,  
with a maximum of 5x capacity -- a staggering 7650amps (you really  
can't charge these batteries too fast). Between my Xantrex 4KW  
inverter/charger and two additional Mastervolt chargers (each rated at  
50amps at 24v), I can't get above 220 amps at 24v.

Guess I need to add yet another 100 amp at 24v charger. Good news is  
that its more load for the genset (makes it more efficient) and  
shortens bulk recharge time.

But that said, I've never met anyone who could achieve a 320amp charge  
rate at 24v on their boat (640 amps at the usual 12v). That's ~8kw,  
net of house loads and charger efficiency losses -- probably need 10KW  
with only light house loads on the DC system just to charge.

It also says that bringing the house bank back up with my engine- 
driven 170amp at 24v alternator is a recipe for short battery life and  
short alternator life. Which says I have to do the bulk charge with  
the Genset even if I'm underway. Not many people think of doing that  
when pulling out of an anchorage with a half-depleted house bank.

And clearly, if you go to solar charging, stick with old-fashioned  
flooded cell batteries. You can't get enough amperage for a large AGM  
bank unless your whole boat is covered in solar cells. Even then...

Arild is right... energy management gets to be a huge deal if you need  
a lot of power. And the idea of split battery banks sounds better all  
the time.

John Marshall


On Dec 7, 2008, at 1:46 PM, 2elnav at netbistro.com wrote:

> John Marshall wrote
>> We cook with propane, and rarely fire up the electric oven, although
>> we do microwave a bit.
>>
>> In warmish weather, our major current drain is refrigeration. In
>> winter, in the PNW with short days and heating needs, lights and
>> heating dominate.
>
>
> REPLY
> Once we get past the obvious issues of sea keeping ability, hull  
> strength
> and integrity; the biggest ongoing issue facing all cruising boats  
> is how
> to manage the available energy resources. This ranges from the most  
> fuel
> efficient cruising speed to getting the maximum return on genset run  
> time.
>
> When fuel was cheap and refueling stations plentiful, it was not as  
> much
> of an issue. The new reality seems to be, fuel is getting much more
> expensive and many of the smaller fuel docks may not continue to  
> stay in
> business.
>
> It has for a long time been considered a given that you stop the  
> generator
> when the battery is recharged to about 80% of full charge. Once you  
> get
> into the later stages of absorbtion and float charge, the  genset is  
> no
> longer efficient in terms of amp hour output for fuel consumed input.
>
> I do not propose to question thi, but the one thing about this  
> practice
> that is a downside is the fact not all of the lead sulfate is reversed
> back to lead and lead oxide during each charge cycle. The remaining  
> 15% -
> 20% remains as lead sulfate and the longer it remains so, the harder  
> it is
> to reverse. The accumulation of lead sulfate will eventually lead to
> battery failure. Even periodic equalization will not totally remove  
> the
> most stubborn remnants. And frequent equalization has its own issues  
> in
> terms of damage to the battery.
> Boats that frequently tie up to shore power can overcome this  
> because the
> duration on shore power will enable the charger to complete the float
> stage and hopefully reverse all of the accumulated lead sulfate. But  
> what
> about cruisers who routinely stay at anchor for days on end, then  
> make a
> short hop to another anchorage?
>
> The float stage must be maintained for several hours to remove the  
> last
> vestiges of lead sulfate. Battery engineers make the assumption that  
> the
> charging regime is done without simultaneously depleting the same  
> battery
> bank with an ongoing load. Unfortunately this is the norm for cruising
> boats.
> One solution might be to split the house bank and bring one half up to
> float charge with wind or solar panels while the other half is used to
> support hotel load on board. This may or may not be workable,  
> depending on
> how big the hotel load is.
> Has anyone developed a good routine?
>
> best regards
> Arild
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