What's wrong, help needed....

Cariboocoot wrote: »

Reading from your specs: Eight 530 A/hr 6V batteries? On a 12 V system that's 2120 amp hours total. They would want a minimum charge rate of about 100 amps! 4 * 175 W panels would be 700 watts maximum, @ 12 V = no more than 60 amps charge current available. This doesn't include the usual de-rating for inefficiencies.

I'd say there's no way your panels are able to keep that battery bank charged - which is a different problem, but still a problem. Try disconnecting all but one set of batteries (and charge all the rest) and see if you can't keep the system voltage up above 12.5.

You spent a lot of money on those Surettes - why did you buy the cheap inverter?

Usual caveat: my calculations could be completely wrong. It's happened before and will again!

Cariboocoot wrote: »

May I just ask why you have such a large battery bank?

At 50% DOD that bank would supply 12 kW/hrs of power to your 12 V system. That's about five times what I use at my cabin, with water pump. Ordinarily we try to design systems so that they recharge each day, not over a period of days. That bank would require three charge controllers and about 3600 watts of solar panels to charge up daily. Another 350 watt array isn't going to make much difference.

I suggest you get a handle on your power requirements and see if you can scale back a little. :roll:

Cariboocoot wrote: »

Not quite. If the panels don't produce enough current in respect to the size of the battery bank, the bank will never charge. It just goes down, down, down. Batteries in need of charge are a resistance load on the panels. It's like trying to charge a car battery with one of those 500 mA wall transformers; it won't work. Instead of the supply bringing the batteries up, the batteries drag the supply down. Then it fries. Don't worry: the solar panels won't fry.

The battery bank is sized to supply the load requirements in between charge windows. You can size them large enough to handle a spell of bad weather, but that just means you need all the more panels to bring them up once the sun comes out. Generally speaking, the charge rate should be 5%-10% of the A/hr rating of the batteries. Not enough current = not enough "oomph" to shove the electrons around.

My system consists of 4 175 W Sharp panels feeding 48 V nominal into an Outback MX-60 which charges a bank of (inadequately rated Interstate) 320 A/hr batteries. This powers an Outback FX 3024 true sine inverter, which include a generator input/charge feature in case the sun doesn't shine.

My loads: 240 watts for the 'frige (cycles about 1/3 the time, and hits 500 watts on defrost). Water pump is a 1/3 HP shallow well that charges a very LARGE pressure tank so it doesn't cycle a lot (start up uses more current than run). There is also a 1 HP 'digester pump' for the septic system, which only runs about 1 minute per day. The big user is the computer/satellite/phone set-up which chews a steady 150 watts average when on. There are the usual CFL lights and a small radio/Cd player.

My 'rules of thumb' for system design:

Inverter should be sized to handle the maximum load at any given time (in my case the massive draw of the 1 HP pump plus the 'standard' loads).

The batteries should be able to handle the day's ('over-night', as it were) power usage (mine at 3840 watt/hrs are above usage rating - if they really were as advertised).

The solar panels should be capable of recharging the batteries as quickly as practical (my batteries recharge in less than 3 hours on a good day - PV array handles all power after that 'til the sun starts to go). Over-all the system produces 2.5 kW/hrs on a good day.

Things I will improve: not good enough solar exposure for 'marginal' days means I run the gen more than I like. Batteries that really meet spec would be nice - like some Trojan L16H's.