smaller batteries/smaller system?

[Great White North]

I have a question maybe you all could help me with.
We have a small cabin in Ontario Canada. Our L16 Trojan (6 volt deep cycle 395Ah) batteries just died after 7 years (8 batteries = 48 volt system). We have been in this cabin for 4 months and I think that the proper maintenance on the batteries was not dine, but that another story.
We have a propane fridge and stove so the system is only needed for cf lights, running a computer , charging cell phones, the water pump(which currently is a direct 12 volt connection ie; not going through the inverter) and the occasional small appliance.
The question we have is 'could we get away with a smaller system and/or smaller batteries?'
We were thinking of replacing all 8 trojan batteries with Crown CR 225 Ah . The dealer has nothing but great things about these batteries and says that we have a system that is bigger than we need.
 I like the price of these batteries but am wondering if he's just trying to sell his stock.


We would appreciate any advice you have as we are new to living off grid on batteries and have inherited a set of old and sulfated batteries.

oh, and we use a generator to charge the batteries which was not the original intent for the system , i believe wind power was but thats not reality right now.

thanks for your time

GWN

[southpaw]

The answer to your question is yes.  Having a smaller ah battery bank means you have to charge more frequently.
Without more info on the size of your generator,  model of your inverter, size of your battery charger, and daily power usage I can't give you any more advice except keep your batteries above 80% SOC.

[OperaHouse]

I have a camp that is used only 5 months a year.  I get by with only two car batteries, one from the vehicle I don't bring and the other from the boat when it isn't being used.  Just use it for lights, TV, kitchen appliences, and a chest fridge that operates only durring the day.  You can get by with very little if careful.

[thirteen]

I suggest take a servey of what you really need for power on a daily use and you will need to plan for a big day of power usage when using your genset. Have you tested the batteries they might be brought back for some use. But if they were abused beyond recovery then replace them. As southpaw suggested do not let the batteries get below 80% below that will destroy the new set of batteries quickly. Useing a generator to charge them is ok but you might also look in your system for a hidden power loss. Sometimes there are hidden power losses if you did not put the system in. Go around and tighten any connections you can get too, they may have been loosened or been corroded by time. You will need to start keeping real good records of the usage of your batteries. Also in planning your power usage do not forget to plan for expantion of you power usage. Pryer planning is a good way to live off grid. Just a few ideas. Best of luck. 
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[Crispy]

My advice would be to purchase as many amp hours as you can afford for a couple reasons. The lower the rating on your bank, the more frequently you'll have to charge it up. There are many factors that determine the life span of a battery bank and the number of charge cycles is a big one. Long term, saving money on less expensive batteries now will mean more frequent replacement leading to higher costs over time. More capacity will allow room to grow or support unusual circumstances or occasional spikes in power consumption.

I agree with Southpaw and Thirteen. It is critical you have specific data on your energy consumption. Once you have that you can do the math to find your storage needs. Example: 2000 watt hours (2kw) per day divided by 48 volts =  42 amp hours. Now add 15% at least for inverter losses and you have 48 to 50 amp hours per day. If you keep your state of charge from going below 80% which is good advice from Southpaw, then the 225 ah batteries would be good for 45 ah per day, so you'd be close.

Now the pump.........Are you somehow pulling 12v off 2 of the batteries in your string of 8? If you are, depending on how much the pump runs and given the fact that 12v will suck out a lot more amps, you may create an imbalance in the bank with 2 batteries being more deeply discharged than the rest. That can cause serious problems for the entire bank.

Do you live in the cabin full time? If not a small solar panel will keep them topped off. Batteries will self discharge over time and lead to sulfation if they sit idle for too long.

[madlabs]

One question that hasn't been asked, what are your charging capabilities? When I first went off grid with a 1200aH bank I quickly found that a 40 amp charger wasn't going to cut it. I had a heck of a time keeping the bank charged ( and ran the genny a LOT) until I increased my charging capability. The bigger the bank, the harder it is to keep charged. And if ya don't, it won't last.

That said, I'm guessing you will be fine with the 8 battery bank you are planning. I live on 10 of the same batteries and have been for almost 4 years. I use more power than you are talking about, my small fridge is electric and I run tools etc.

Jonathan

[birdhouse]

welcome!

a few things:  1st, you mention running the 12v water pump not through the inverter on a 48v bank...  this assumes a center taping scenario??  IE:  just chucking the water pump to two of the eight batteries?? 

if this is the case, it's a really bad idea.  sure, it's a small power draw, but over long term, it is going to throw the whole bank off balance.  equalizing the bank can help this, as well as switching what two batts you connect the water pump to, but all in all, it's a practice that isn't really worth doing.  if you insist on using the water pump without the inverter on, get a dc to dc converter.  it can take 48v to 12v and they can be found on the super cheap from ebay (china). 

also, is there any solar?  can you spend a few hundred bucks on some?  the reason i ask, is the generator charging scenario.  sure, the genset is great for pushing big amps in the "bulk" portion of the charge, but it still has to run for long durations when very little amps are needed for the absorb, and float portions of the charge.  even a few small solar panels could eliminate this time/fuel consuming portion of the charge.  i'll bet even a $1000 investment of solar panels and charge controller would pay off in a year or two, plus it would give you the added benefit of making the bank last longer due to more fully finished float charges per year. 

have you checked the water level in the batts yet?  have you equalized them yet?  i know, dumb questions, but worth asking.  i'd boil the snot out of them for 3-5 hours, and see how they respond.  obviously monitering water level throughout.  depending on the genset charging set up, they may have never been equalized. 

and, to answer the question, i'm sure you'd be fine with eight of the smaller batteries...  just buy some solar, you'd be glad you did!

adam

[thirteen]

I would take everything off of the batteries and clean the posts and all of the wires from battery to battery before I boiled the batteries.( You may have to do it a couple of times if they have been on the low side of charge ). That way you would only be doing the battereis and no ghost circiuts. Plus that way you would have all of the wires checked. You may have  a bad battery drawing the others down. Please use some safety glasses and gloves when working with the batteries. It only takes one mistake and without protection you may be able to see to finish your work. It seems like the pump maybe be on two batteries in your system. If the battereis get some life back in them  then you  might change the batteries you use for the pump to a different set. If you need to get new batteries you might be able to use a pair of the old ones just to run the pump, (until you get your system complete.)if  the pump is not used very much. A set of solar panels could be used to just charge those two batteries charged plus it would be th estart of gettin gmore panels. The people here do not know what you know so we may give you suggestions that you already know so do not take offence .  13