solar panels coming

[CaptainBirdseye]

I have 500 w of solar coming soon, which will be on an mppt controller, and I need a whole new battery bank anyway, so I'm thinking of Trojans, someone has said I should get 6v ones ? but my main question is what size bank would be suitable for all this solar ?

 

My guess is four x 110 ah .

 

I also want an inexpensive cut off or alarm that stops me draining my batteries below 50% soc, so I can get a long life out of them.

[Kae]

Battery bank size has little to do with your solar panels, what you need to do is a "power audit".

Lots of info on the forum :-)

[Paul C]

I have 500 w of solar coming soon, which will be on an mppt controller, and I need a whole new battery bank anyway, so I'm thinking of Trojans, someone has said I should get 6v ones ? but my main question is what size bank would be suitable for all this solar ?

 

My guess is four x 110 ah .

 

I also want an inexpensive cut off or alarm that stops me draining my batteries below 50% soc, so I can get a long life out of them.

 

The alarm would need to accurately measure SoC - so that rules out simple voltage based devices. Both smartgauge and Victron BMV have configurable (ie from a SoC amount) alarms which could in theory be wired up to switch devices off at a certain SoC, but neither are what I'd classify as "cheap".

[MyLady]

I have 500 w of solar coming soon, which will be on an mppt controller, and I need a whole new battery bank anyway, so I'm thinking of Trojans, someone has said I should get 6v ones ? but my main question is what size bank would be suitable for all this solar ?

 

My guess is four x 110 ah .

 

I also want an inexpensive cut off or alarm that stops me draining my batteries below 50% soc, so I can get a long life out of them.

 

Remember that 500W of panels will hardly ever produce 500W - first the output depends on alignment to the sun; second the intput to the batteries depends on the battery voltage the controller senses, and the time since dawn (I think), because the MPPT controller may well have a multi-stage charging regime - when the controller thinks it has gone into maintenance (?) stage, it will only provide a trickle charge, no matter how bright the sun and how well the panels are aligned.

 

We've got 330W of panels, soon to be increased to 495W, and only twice during the last few months has our Tracer 30A MPPT controller has reported 20A into our bank of 4x110A (nominal) batteries - and 20A into 14V (charging voltage) is 280W - 6 or 7A @ 13V is much more typical: ~90W. 80+V has frequently been reported from the 2 panels.

 

That's been enough to keep our batteries charged, but we only have a 12V fridge (2-3A, probably less than 70AHrs per day) occasional water pumps and up to 8 LED lights for a few hours a day, oh yes and 12v chargers for 2 phones & a laptop - note no TV, washing m/c, microwave or vacuum cleaner, either 12 or 230V.

 

As far as capacity goes, watt hours might be a better criterion than amp hours to judge the effects of 6V batteries versus 12V ones, 1 AHr from 6V gives 6Whrs, but from 12 V gives 12 WHrs. Trojans seem, from what I've read, to give better deep cycle performance, but half the voltage means half the power if all else is equal - it never is! You need to know the difference between amps, watts, amp hours and watt hours. With domestic mains things are much simpler, because the supply is maintained at a fairly reliable 230V at 50 Hertz. On a boat, it's much more complicated: the difference between 12V - the nominal voltage - and 14+V - the charging voltage (and the 15+V of an equalisation charge) is more than just significant! Then add in the complication of the difference between 'surface charge' - when there is a charging current - and 'real" voltage - when there isn't - and your mind, if it's like mine, starts to buckle under the strain...

 

As has already been suggested you need to do a power audit - at least an informed guess like ours, but preferably measured with a meter. Like last year, we plan to turn off the fridge at night when the fore deck gets cold enough - below 4 degrees C.

 

A lower battery voltage will 'pull' more current from the solar controller during its bulk charge phase, but a bigger bank will be less stressed by whatever current you draw.

 

Finally, how often and how long do you expect to run your engine/alternator and any other charger? We expected to run our engine 4+ hours per day, but hanging around waiting for work to be done means that we've often just run it for half to one hour per day, just to heat the water, often not at all. Our 'best' times have been when we weren't on the boat, just running the fridge and nothing else with the 2 solar panels keeping the batteries charged when we went home, pissed off by just hanging about waiting for boatyards, engineers, etc. to do some job or other. Mind you, we got through last winter OK, with no solar, because we were, mostly, doing our best to get somewhere - and the fridge was off. We're Continuous Cruisers (genuinely, by choice) and had hoped to get to the Lancaster, or at least the Llangollen, this summer, but, "the best laid plans..." The moral of which is, expect the unexpected, unless you've been live-aboards for years and know exactly how to cope!

 

More capacity seems better, but less drain seems best...

 

Roger

[Grace and Favour]

 

I also want an inexpensive cut off or alarm that stops me draining my batteries below 50% soc, so I can get a long life out of them.

I suggest you get a (cheapish) LED volt meter from Ebay, connect it to your leisure batteries and mount it inside your boat where you are most likely to see it frequently.

 

When it reads 12.24v - you know your batteries are at 60% SoC, when 12.1, they are 50% SoC

 

WHEN IT READS 12.24