A/B switch for new batteries

offgridoffgrid Registered Users Posts: 20
Hello All,

I am in the process of purchasing new L16's (trojans) for my off-grid setup. The exisiting batteries are coming up on the two year mark, doing well I might add. I am now financially ready to purchase 8 more, and a couple more panels. My question, would it be wiser to buy an A/B marine switch due to age difference ?

This is the switch I am considering..350 Amp Battery Selector Switch - Battery #1, #2, or BOTH

http://www.solar-electric.com/basw1300amp.html

Hope the link works :)

Thank you for any feedback

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: A/B switch for new batteries

    I am not sure that using a 1/2 (A/B) type switch will help you extend the life of a mixed age battery bank.

    If it was a choice of where to spend money... I would suggest a Battery Monitor (just a general recommendation to make monitoring the battery bank easier) and an inexpensive DC Clamp Current Meter so you can check each parallel string and see how much current is flowing during heavy load/charging to ensure that each is processing current appropriately.

    If/when you see a change in current flow by string--you can then further investigate the problem before it cause more problems for the bank (open cell, shorted cell, wiring issues, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A/B switch for new batteries

    It's a good switch. It's a good plan. You just have to remember to use it. :D
    Also make sure the charge source and the load are both on the "output" of the switch. Essentially one bank will be "idle" while the other is "in use".

    Even at two years old you my not need it. Check the Specific Gravity of the old and new batteries and see how close they are. Then check the resting Voltage. If they're very close and the old bank has not been cycled deeply (more than 25%) or misused in any other way you could probably tie the two banks together directly without problems. However, if you do that you need an array and charge controller capable of charging both sets.

    Whatcha need all that power for? :p
  • offgridoffgrid Registered Users Posts: 20
    Re: A/B switch for new batteries

    Thanks Guys for the quick reply.

    Bill, I agree I need a clamp meter & monitor, that is in the budget as of now, thanks ! :)

    I will also need more battery to supply the power used without exceeding the 50% d.o.d. with existing loads, which they have on occasion but not very often. I was worried that the age difference would drag down the new batteries to their level. Currently the SG levels remain around 1.26 when fully charged. I have been very lucky with my first set of batteries after hearing all the horror stories.

    Cariboocoot, I was planning to bring on more lights (ext. flood lights) to the system as well as having additional power in case we have a couple days of no sun, or adding other devices. At some point, this system is going to my mountain property where there is NO power. My reasoning for the switch, other than age of batteries, was not to have too many batteries on a string, due to many stories I have read...not sure how true that is. I am still a "newb" to this which is why I come to you guys for advice :)
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: A/B switch for new batteries

    I like to work at a "balanced" system design... The right amount of batteries to support the needed loads... The right amount of solar panels to support the size of battery bank/loads.

    Conservation and choosing the most efficient appliances/lighting for your needs is important (i.e., LED/CFL outdoor flood lights running on motion detectors, etc.)...

    So backing up a moment... Watt*Hours per day (by season), location (amount of sun), supported loads, days of no sun support (typically ~2 days, usually 3 days maximum), etc.. And size the genset/AC charger for the bank for optimum fuel usage.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A/B switch for new batteries

    So you're going to have three strings of 370 Amp hour batteries @ 24 Volts? That's 1110 Amp hours @ 24 Volts = roughly 13 kW hours of power maximum. More than four times what we use. Usually when you go off-grid you try to get the loads down as much as possible.

    As a monolith battery bank, that would require about a 4 kW array - and two charge controllers. Remember these batteries really want that 10% charge current and their life will be shortened with less.

    Also, with one bank of four and the other of eight the A/B will be unbalanced; one will be 370 and the other 740. Not good for charging/usage. Maybe just two strings of 370 each?

    BTW, there's no problem hooking two strings in parallel and keeping the current flow even. It gets trickier with more than two, though.
  • offgridoffgrid Registered Users Posts: 20
    Re: A/B switch for new batteries

    Again guys Thank you !

    Bill, I have, for the past couple years, tried to maintain a balanced system. I believe I have accomplished this with only a couple hick-ups but nothing too drastic, yet. You have provided, in the past, with excellent advice as well as a few links to determine local, sun, etc... I can't thank you enough for all you do.
    More than four times what we use. Usually when you go off-grid you try to get the loads down as much as possible.

    Cariboocoot, Thank you for pointing out the unbalanced load of banks. Obviously I did not do enough reading. I have been reducing the load as much as possible. I would be grateful if you could provide me with any advice as to your current load. How many of you live there ? How are you making it work with only 320 AH of batteries ? If I can make it work with what I have...woah ! I would be in hog...uhh solar heaven :)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A/B switch for new batteries

    There's just the two of us, unless you count the dog. But he rarely stays up late at night reading. :p

    We use propane for cooking & hot water, wood for heat. The electric runs the refrigerator and the office set-up mainly. Even that is on two power strips so when it's off, it's off; no milliamp loads secretly draining away.

    But we also have electric water pump (1/3 HP - runs around 800 Watts) and digester pump (1 HP - over 1200 Watts running). The digester only take 30 seconds per day. The water pump can be run when the sun is shining and the batteries full, as there is a large pressure tank which holds a day's worth of water for most uses. Load shifting plays a major role in power management. We can even run the microwave (large power, but short time). Note the lack of big screen TV! :p

    If I had it my way I'd up the batteries and panels just a bit. This year the kids were there on their own - and killed the batteries every night for a week running the pump. Technically we use the batteries to store enough power for overnight. The panels run a bit more efficient than typical due to high elevation. Most days we can harvest/use 2.4 kW hours. Some days are better, others worse. Overnight use is more long the lines of half that as the 'frige has proven to be quite efficient.

    I would like to up the bank to 390 Amp hours and double the panel capacity to 1400, plus a couple of small "morning" panels facing East to start charging earlier. Maybe this year once I'm out from under the megaproject. If I still have my right arm by then!
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