Building an off grid cabin - need power answers

[Maine Man]

Hello all,

I am hoping that there is either somewhere on this site that you can direct me to or help provide a few answers.

I am building a small cabin in Maine, it is a one bedroom, one bath and kitchen / living room comb. It is heated by wood and propane and I am wiring it the same as a house, except I will use a 10k propane generator that I already own. I have plans to contract a local company in the future to set up a real solar system and then the generator will be just the back up, but i can not afford to do that right now. In the mean time I want to set up a simple battery / inverter system that can be charged during the day using the generator, then use it at night to power a TV, refrigerator and a few lights. The maximum light that would be on at any one time would be two 60w max, the refrigerator uses 465 kWh (not sure to convert that) but I already have it in the garage and it works so I would like to use it and a led TV.

I currently have a few batteries that a friend gave me that came out of rotation from a UPS system, they are UPS12-400MR's and have a 102AH each. I also have a 2000 watt inverter. Are these capable of getting me through the night and can I then recharge them during the day with a generator until I get the solar system installed?

Any guidance is appreciated.

Thanks

[Mike90250]

Welcome. Please read some of the "stickie" threads at the top of the list at

http://www.solarpaneltalk.com/forumdisplay.php?28-Off-grid-electric-systems

There are 3 steps:
Define your loads -
Loads define the battery size - (for 1 thru 5 days of backup) -
Battery size defines the charging source size.

if your batteries are GEL cells, you will need to limit the recharge to very slow rates [ 10% of
battery capacity (C) or less ] If they are flooded wet cells, recharge is 10-20% of C,
But if those are AGM cells, you can recharge really fast, 50% of C, as long as you stay within the
voltage limits. So your battery charger needs to have settings that match the chemistry of the batteries. If they are GEL batteries, you should consider a small inverter generator that can throttle itself back and save fuel.

[Maine Man]

Mike, Thanks for your comments. i will research the sticky notes.

The short answer to your question is that I have AGM cells. THe charger that i currently have has a selection for 12v with options for 6amp (Medium Charge) and 2amp (Slow Charge) selection only. Based on your comments I would guess to use the 6amp. Thanks again and I will be reading in preparation for my future system as well.

MM

[Mike90250]

based on what you say, a basic 2 or 6 amp charger, will not be capable of producing the right voltage for AGM cells. There is both a voltage spec, and an amps spec to look for. You want a 3 or 4 stage charger, not an automatic automotive charger.

[Maine Man]

Thank you. I will get the spec sheet from the manufacture and find a new charger that is acceptable. Do you have any recommendations on one that won't break the bank? Thank for all your time and effort.

[Mike90250]

still 2 issues.
1) Your 10Kw generator. Way too much power for what you need. For charging 3 or 4 of your 100ah 12V batteries, 50 -60 amps @ 15V is sufficient. That's about 1,000 watts for 2 or 3 hours. If your batteries are really low, you could double the charge rate, and that's only 2,000 watts pulled from the generator. Still wasting a lot of fuel.
2) battery requirements. Here's the link to the mfgs manual. http://www.cdtechno.com/pdf/ref/41_7525_0712.pdf These have about a 3 year life, maybe up to 5. There is a reason they get pulled and swapped out at regular intervals. With some weekend use, a few will die right away, and you can keep weeding them out till you have a couple that last a weekend. The harder you use them, the faster they will fail. The manual page 17 & 18 covers the voltage needed for recharging. I'm guessing you need a smart charger that can be set to 14V for a couple hours of charge, then shut down or float at 13.5V till recharge is needed again. The proper chargers are not cheap, but you could manage with a "smart automotive" charger, and carefully watch it after a couple hours of charging, and make sure it's not wandering too far from what you want.

The caution with sealed AGM batteries, is that any overcharge boils off electrolyte through the one way valves on the safety vent. When you loose electrolyte, you loose a bit of capacity. A couple overcharges, and you don't have much battery left. Some vents can be manhandled and opened, and a bit of distilled water added, but there is no guideline of how much to add. Add too much, and you weaken the acid and loose capacity ....

I use a Vector VEC1093 12V Charger for my 12V batteries, it can pump 40amps, but you have to manually set it to AGM configuration at each power up cycle.