Dc Dc charger or generator and possible safety issues with floating inverter?

jimbuck
jimbuck Registered Users Posts: 46 ✭✭
in Solar Beginners Corner #1
Hello, I had decided to buy a sterling 30amp
Charger for my 115Ah soon to be 225ah battery bank. But then ...

My system is designed to power a caravan (travel trailer) and due to security issues my setup is installed in the back of my panel van, which then feeds the trailer via the inverter. I have been debating wether to buy a dcdc battery charger instead of a mains unit powered from my generator so that I can take advantage of the vehicles alternator whilst driving. The only issue I see is that I may not drive enough or for long enough periods to properly charge my batteries. Is there a simple formula I can use to calculate this? I drive for about 30 minutes per day (weekday) and for about an hour on a Saturday. I tend to draw no more than 30% dod from my battery 110ah and I will take about the same from my new bank 225ah. Is this a viable option or should I stick with the generator and charger to fully charge the bank?
Another question entirely, I know that it would be more efficient to power my devices directly from the battery but I cannot park close enough to negate the voltage drop from the van to the trailer. Having the inverter power the caravan via an extension lead, could this create safety issues due to there being no earth/ground and the inverter floating. E.g. If the lead was cut between the caravan and van?

Thanks again for any help, Jim
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    Assuming the alternator has high enough output, and the RPM is high enough to drive it, you may be able to get some useful charging that way. Assuming flooded lead acid batteries, the time is too short to get a full charge though.

    I usually figure on 1 hour for each 10% state of charge below 90% SOC + 2 hours to go from 90% to full.

    There are always safety issues with shore power. On my boat, there's a "polarity reversed " warning light next to the shore power breaker which is useful for finding some issues. In mobile applications, "ground" isn't really, and some fault conditions can cause the mobile "ground" (frame) to be a significantly different potential than actual ground.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • jimbuck
    jimbuck Registered Users Posts: 46 ✭✭
    #3
    Yes sorry I should have been clearer, the bank is flooded lead acid. I plan on the 30 dod so I guess I'm looking at around 4 hours which is way more than I drive. It looks like the generator may be the only option. Unless I use a standard split charge relay to get upto 80% and finish with the generator and charger. I suppose that will be quite inefficient tough as the amps will have tailed off and the generator will be charging within the absorption stage. Any better ideas? I've only got space for 200w of solar.

    Jim
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  • bsolar
    bsolar Solar Expert Posts: 103 ✭✭✭
    #4
    i'd use a generator and charger  .. 200w of solar should do ok on nice days if your only drawing it down 30% .. but taxing a vehicle alternator like that will wind up frying it before its time and to me getting all greasy having to replace that isnt worth the risk lol ..
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #5
    200w solar might give you ~10a or so at 14.5v charging, depending on angle, etc..  I'd use the generator to get up to about 85%SOC, and the solar should finish the job nicely on a sunny day.  It's not absolutely essential to get to full every day.  Even a couple of sunny days per week getting to full would do.  

    With 225ah and about the same usage, you could probably go 2-3 days without running the generator.  When you got down to 50-60%SOC, fire up the genny for 2-3hrs in the morning.  If sunny later, the solar will finish the job.  If not, try again the next day.  

    At this time of year, even with lots of solar we have to do this.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • jimbuck
    jimbuck Registered Users Posts: 46 ✭✭
    #6
    Thank you for your help,
    I'm a lot clearer on what to do! I will go with the charger and generator option and supplement with solar. My location isn't great for the latter but I will see how It goes. Having the battery monitor really helps to be able to see how each part of the system is performing. I was always under the impression that I had to get the batteries full every day but knowing I can go for longer takes some pressure off me. My ideal setup would be a home made dc generator using a surplus truck alternator (150A) and a Honda gx engine, into the dc dc charger and feeding a 1000w pure sine wave inverter. It would be an efficient charger and a source of clean ac power and probably cheaper to maintain.

    Thanks again, Jim
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