Is a 12 v battery bank all that bad?

Endurance
Endurance Solar Expert Posts: 40 ✭
in Off Grid Solar & Battery Systems #1
My inverter/charger, a 12 volt 2500 watt Xantrex, died and I am shopping for a replacement. Originally, I was thinking this would be a great time to buy a different inverter/charger to re-configure my battery bank from 12v to 24v. This is on a houseboat with pretty substantial 12v draws. No problem, I thought, I'll just pick up a 24v to 12v converter to run 12v loads from a 24v battery bank. Then I looked at the price tag of converters built to handle the heat of the Utah/Arizona desert at Lake Powell where I keep my boat. For the 50 amps I need to allow, a converter is just under $1,000. There's also the added complexity, heat creation, and inefficiencies of another device. For things that my life might depend on, like radios or an ultraviolet light to kill bacteria in the water I drink, I would just as soon have fewer converters and other things that might break down between the batteries and the lights or radios.

I have also given some thought to my battery bank. Eight T-105 size 6v golf cart batteries series/paralleled with 4/0 cables to operate at 12 volts. Each battery has 190 amp hours at 6 volts, so the total is 760 amp hours at 12 volts. But the battery bank is just over three years old now. I realize that a lot of people only get three years out of a battery bank in off-grid situations. Since I foresee buying batteries sooner rather than later, I will have the chance to re-configure to something like four L-16 batteries in two sets of two or six 2 volt cells in series. I might even consider a pair of lithium batteries. Suffice it to say it seems like I will have a few options when I have to next buy batteries.

So now I have come full circle and I am thinking of sticking with a 12 volt inverter/charger and improving my parallel battery situation the next time I have to buy batteries. I realize that this limits my future solar options and may require another solar controller when and if I up my solar capacity. Right now, my main power source is a Honda eu3000is generator and my only solar panel is a 120 watt 12 volt panel with a Morningstar SunSaver 10 controller. As you might imagine, I view my 120 watt solar panel as more of a battery maintainer than a real power source. but even if I 10 times my solar capacity, I am thinking that I should still be able to fit within a single Outback FM80 or a Midnite Solar Classic 150 at 12 volts.

I am a newcomer here and appreciate the wealth of information. I will value your opinions and would appreciate any thoughts.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Is a 12 v battery bank all that bad?

    To start with take a look at this info on battery system Voltage and equivalent power: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    For your eight 190 Amp hour 6 Volt batteries on a 12 Volt system you have four parallel strings of batteries, and that can be problematic in terms of current sharing.
    A 24 Volt inverter system is somewhat more efficient than 12, uses less current for the same power out, and certainly makes for better battery configuration.

    The questions are:
    1). Do you actually need 9 kW hours of stored power that you have now?
    2). If so, can you switch to larger capacity batteries to eliminate the parallel strings?
    3). Can you have two systems; a 24 Volt for the inverter and a 12 for those DC items that need 12 Volts?

    As far as that last item is concerned, at least one other boat user solved a similar problem by using a Morningstar 15 Amp MPPT controller from the 24 Volt battery to the 12 Volt to keep it charged.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Is a 12 v battery bank all that bad?

    12v is not a no no, but it is more difficult to get it flying right when you are talking huge currents and many parallel battery strings. generally, for every 100a (give or take) of draw we suggest upping the bank to double the voltage. 100a at 12v is 1200w. do you exceed this? if so what is the actual max draw? if not you should be fine at 12v imho.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Is a 12 v battery bank all that bad?
    niel wrote: »
    12v is not a no no, but it is more difficult to get it flying right when you are talking huge currents and many parallel battery strings. generally, for every 100a (give or take) of draw we suggest upping the bank to double the voltage. 100a at 12v is 1200w. do you exceed this? if so what is the actual max draw? if not you should be fine at 12v imho.

    You can also get a "current equalizer" for about $50 which can be used to allow you to connect your 12 volt loads *almost* directly to the midpoint of your 24 volt string, as long as the total current is 20 amps or less. This would allow you to still run the 12 volt equipment directly off the 12 volt point of the series string in an emergency and worry about the battery equalization later. This would be similar in effect to using a 24 volt to 12 volt CC, but without the losses and without the need for a separate battery.

    As Niel said, the important thing to know is what your maximum load at 12 volts will be if you stick with a 12 volt inverter, so that you can calculate the effect on wire size and wire losses.
    SMA SB 3000, old BP panels.
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  • Coach Dad
    Coach Dad Solar Expert Posts: 154 ✭✭
    #5
    Re: Is a 12 v battery bank all that bad?
    Cariboocoot wrote: »
    As far as that last item is concerned, at least one other boat user solved a similar problem by using a Morningstar 15 Amp MPPT controller from the 24 Volt battery to the 12 Volt to keep it charged.

    That is a cleaver idea...
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #6
    Re: Is a 12 v battery bank all that bad?

    Charge Controllers ( PWM ) are pretty versitial. You can run a cheesy Battery Charger / Power Converter through one and get a great user controllerable ( Voltage set points ) 3 stage Battery Charger. If the source has a pot inside you can crank up the output and have a true ability to do a Equalizing charge at a reasonable cost.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #7
    Re: Is a 12 v battery bank all that bad?

    This is excellent. These are just the kinds of issues I was hoping to get help with.

    Cariboocoot, I appreciate the questions. Here are the answers I have given my limited understanding.

    1). Do you actually need 9 kW hours of stored power that you have now?

    I had thought so, but maybe I am wrong. I am powering a 36" wide 110v fridge that is capable of going through 300 ah on a hot night in August. If I could always start the night with 100% charge, I could use the 300 ah from a somewhat smaller bank and still stay above a 50% charge. But since my power comes from a generator, I try to stay in the bulk and acceptance charge area as much as I can. It costs a lot to run a generator to trickle juice in at a float rate so I usually just charge to 85% or so when I have to run the generator. Between trips on the boat, my maintainer solar panel builds the bank back to 100% and holds it there.

    2). If so, can you switch to larger capacity batteries to eliminate the parallel strings?

    Yes. The next time I have to buy batteries, I could get about the same capacity as my 8 T-105 size golf cart batteries from four L-16 cells. I realize that this would still involve two parallel sets, but my understanding is that two sets of parallel batteries would be pretty close to no parallel at all. Am I off in this understanding?

    3). Can you have two systems; a 24 Volt for the inverter and a 12 for those DC items that need 12 Volts?

    That is intriguing, especially since you mention the idea of a solar controller as a "battery charger." Depending on how I do it, it might have a hefty price tag, though. If I buy two Concorde PVX 2240T batteries, I am at $658. A Morningstar MPPT controller is another $222. By the time I buy some cabling and DC breakers, I am pushing the $1,000 figure again. I know it seems I have expensive tastes with the AGM batteries, but in the heat of Lake Powell, it is easy to ruin batteries by losing electrolyte.

    neil, you may be onto something about total draw at any given time. In theory, my old inverter was capable of drawing 2500 watts. But 50-amp draws at 12 volts are more typical. Is a 12 volt bank closer to reasonable given typical 600 watt draws?

    inetdog, I would love the idea of using the same bank to supply both 24v and 12v. However, my needs are such that I have to size the 12v system for 50 amps. Of course it doesn't draw that all the time, but I would exceed 20 amps on a regular basis.

    Coach Dad and Blackcherry, Your thoughts might make a second battery bank less expensive than I thought. I might get by with a single 12v battery for my 12v needs if it has ample and constant charging in an efficient way. Please excuse my ignorance, but I have some questions about that:
    1) Let's say I buy a 15a controller and hook it up to act as a battery charger. If the battery for the 12v system gets drawn down so that it could pull 50 amps for a time and the 24v system could supply the 50 amps, would I blow fuses or would the 15a controller act as a limiter that would only allow the 15a to go through?
    2) I had shared your thoughts about a PWM controller as cheesy. My understanding has been that they just waste any power over the voltage of the bank they're charging. If "waste" means they burn it off as heat, that would be bad for my needs as a 24v to 12v battery charger. But if "waste" means they hold back the higher voltage rather than send it through, it would just stay in the 24v bank. In this case, would a cheesy PWM controller work as well for my needs as a MPPT controller?

    This is very helpful. I wish I'd known about you all when I first bought things three years ago.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #8
    Re: Is a 12 v battery bank all that bad?

    My Cheesy comment was about a low end BC or PC ""You can run a cheesy Battery Charger / Power Converter "". A Xantrex C35 - C60 controllers do a excellent job controlling output a a reasonable price.

    You need two sets of batteries, one you charge off your generator and a second set you use at night to power your Refrigerator and charge off solar or a small Honda type during the day.

    My boat has a 20 KW generator, I use it maybe once a day for a hour or a little more to cool off the boat and to charge my main bank of 10, GC-2 batteries in the evening. I switch over at night to a 4 battery bank and run the Refrigerator ( 25 cf ) by itself on them. It's about power management, In the morning I switch the Refrigerator back to the main bank for the day cycle. I'v been doing it 30 years, it works. Everybody leaves the dock 100 % charged, it's that 35% that you have to play with after you run them down that counts.

    Solar for me is a non-starter, my boat is undercover, unless I am out and that is 3 days a week. A boat moves and space is hard to get enough panels to make much of a difference in the scheme of power use. I can throw 200 amps at my big bank and it charges fast enough to cook and cool. My little Honda can be used to top off either bank.

    I don't have 10 and 4 batteries in the banks by accident, they are sized by what I use and what I can charge back within reason.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Is a 12 v battery bank all that bad?

    Two parallel battery strings is no place near the trouble that four can be. Simple diagonal wiring connection on the strings will keep the current fairly even throughout, so that would solve one potential problem.

    Adding more panel so there's at least enough to do the 'finish charge' after bulking with the generator would be good. Batteries like to be fully charged a few times per week. And it's a good idea to do this all in one go, rather than relying on a small amount of panel to accomplish it over days. If you could complete Absorb most every day your batteries will last longer. If you could then achieve Float and supply at least some of the power need "directly from the panels" you'd be better off. I think you already know this.

    Your 50 Amp 12 VDC draw indicates a need for a substantial battery for that alone. Even though it's only occasional, it could be enough to severely drain a small bank in a hurry. Possibly fast enough to instantly drop the Voltage and cause low Voltage problems. From that POV, the two parallel strings of L16's at around 640 to 780 Amp hours would be a good idea.

    If you do use the separate charge controller idea, the Amp rating of the controller is its limit; the current draw should be on the battery with the controller putting out whatever it can to help out when possible. It should not cause a failure in any fuse/breaker you have on the controller output.

    From what I've read so far it would seem the most cost-effective solution is to go with the larger capacity batteries and stay with 12 Volts.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #10
    Re: Is a 12 v battery bank all that bad?

    Cariboocoot, I appreciate the thoughts from those of you in the know. I was leaning toward keeping a 12v bank, but had some misgivings after looking at some of the battery articles on the NAWS site. Three years ago, L16s were a lot more than GC2s on a $/ah basis. Maybe the off-grid business is causing the battery manufacturers to make more L16s; they seem to have $/ah costs similar with GC2s these days.

    Blackcherry, it's great to get help directly from someone doing this on a boat. Many thanks. At what voltage do you operate your bank of ten GC2 batteries?
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #11
    Re: Is a 12 v battery bank all that bad?
    Endurance wrote: »
    Blackcherry, it's great to get help directly from someone doing this on a boat. Many thanks. At what voltage do you operate your bank of ten GC2 batteries?
    The 10 battery bank is 12 V and the 4 battery bank is 24 V. The only reason for the 24 V is I got a good deal on a 1500 w inverter and a bow thruster is hooked the it. I have since changed to TSW and just left it @ 24V. On the 10 battery bank I run the rest of the boat, TV, Satellite, lights,etc and a 5000 BTU A/C. Generally that bank will be down to about 12.2 v in the morning.

    Some Tips :

    Parasitic loads are a Battery killer, anything with a Remote, put on a power bar and cut it off when not in use. Up-grade the refrigerator to a energy star, you'll cut the power use by 1/3. ( You may have to have TSW inverter to run it. ) Swap your lights to CFL's or led's. As careful as I am , I still have a 10-12 amp draw, over 10 hrs that over 100 amp hrs.
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  • Bigwooo
    Bigwooo Solar Expert Posts: 62 ✭✭
    #12
    Re: Is a 12 v battery bank all that bad?

    I live in an off-grid home that was built over a period of about 7 years. Started as a vacation home, now it's full time. When I first started, I had no idea solar would progress as it has, and unfortunately I bought most of my equipment up front anticipating how large my system would eventually become, and like you, without buying new inverters, I'm stuck with 12 volts. I have a lot of outdoor lighting that it 12 volts. Even though I have a 12 volt system, my house is quite a distance from my solar room, it would be very costly to run the necessary cable to the house for the 12 volt lighting. So I'm using this for the 12 volt power. If you switch to 24 volts, it may be a good option to supply your 12 volt power. Although I have not tested mine yet, the manufacturer says it has zero draw beyond what the lights use: http://www.landscapelightingworld.com/Low-Voltage-Transformer-Outdoor-Lighting-Landscape-Lighting-s/31.htm
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #13
    Re: Is a 12 v battery bank all that bad?

    There are all kinds of 24 to 12 V DC to DC converters out there relatively cheap. This company has a 20 amp for $90.00. They also make a 60 amp, but they do not carry it, I am sure someone does.

    http://www.jamestowndistributors.com/userportal/show_product.do?pid=51844&familyName=Victron+Orion+Non-Isolated+DC%2FDC+Converters
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #14
    Re: Is a 12 v battery bank all that bad?

    I appreciate all the good thoughts. It sounds like the short answer is that while there ways to get to a 24v bank, a 12v battery bank will give me greater simplicity and that there are things I can do to minimize the down-sides of the 12v bank:

    1. Conserve, conserve, conserve. My fridge is likely first on the chopping block. It's a 1995 model that I took out when I remodeled at home. I had always thought of it as pretty modern, but it sounds like energy savings have come a long way in 17 years. I will also vigilantly look for "always on" parasitic losses.

    2. I'll make sure the new inverter I buy is a true sine wave and has enough capacity that it is working no where near its rated output. Those steps have to add to efficiency. For the sake of not hijacking my own thread, I'll post that discussion in another thread.

    3. When (not if) I next buy batteries, I will be shopping for something like four L-16s rather than the eight GC2s I have now. A bank should be much happier with only two sets in parallel rather than four.

    Again, I appreciate all your knowledge.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #15
    Re: Is a 12 v battery bank all that bad?

    Pick up a Kill-a-Watt type meter and measure the amount of kWatt*Hours (1.0 kWH = 1,000 watt*hours) of your AC loads... That will quickly tell you what is using how much power (measure power 1-7 days per major appliance to get average kWH per day).

    Don't guess--You can still be way off (between cycling, age, standby loads, equipment problems, dust on condensers, "unhelpful" name plate ratings--Guesses are almost useless).

    If you have significant DC loads--Getting an inexpensive DC Current Clamp meter can be a very handy tool. And, I always like to recommend a battery monitor for long term operation of a battery bank (not perfect, but does make it much easier to see what state the battery bank is in at any time, and makes it much easier for spouse, kids, guests to operate your off grid system without killing your battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #16
    Re: Is a 12 v battery bank all that bad?

    I was hanging on a old 1996 Refrigerator with Solid Cherry panels on the doors that matched the cabinetry, I know I paid $800 extra for when it was installed, because we just didn't want to give them up. The new Refrigerators were a different size and the handles were different and they wouldn't fit. In the end I bought a new one, best move I ever made. If I was on-grid all the time I might have kept it longer, but it would have been foolish as little power as the new one uses.

    Yeah, most new refrigerators do require cleaner power than MSW inverters produce. Of course that doubles the cost of replacement.
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  • TheBackRoads
    TheBackRoads Solar Expert Posts: 274 ✭✭
    #17
    Re: Is a 12 v battery bank all that bad?

    I would also recommend a battery monitor.

    NAWS sells some good ones.

    And my favorite... 600s Victron
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