Dual DC Voltage Battery Bank

[armadillo]

I'm in the groundbreaking stage of an off the grid log cabin in Montana. I plan to have about 6 Trojan T-105 batteries in the crawl space and have a 24 volt circuit for cieling fan and bathroom fan.

Can I just tap into two of the six batteries for the 24 V circuit yet still have those two batteries participating in the 12 V load as well as get charged by the 12 V battery charger? It seems like there would be backfeed problems.

The cabin will be on a vented crawl space stem wall foundation and I want to put the batteries in an insulated box under the cabin so I won't have to have a fan or deal with the fumes. The cabin will be vacant all winter and I want to avoid any pitfalls associated with this period of non-maintainance.

Are the battery gases lighter than air or heavy like propane?

[JeroenH]

Can I just tap into two of the six batteries for the 24 V circuit yet still have those two batteries participating in the 12 V load as well as get charged by the 12 V battery charger?

No. If you'd want to do that you'd have to disconnect those two batteries and rewire them for 24V. But discharging those two at 24V would create an inbalance with the other batteries, so this is not the preferred way to go.

I'd say if most of your devices are 12V go with a 12V battery bank and use a DC-DC converter to get 24V. If most of your devices are 24V, go the other way around.

Lead-acid batteries produce hydrogen which is much lighter than air, and also explosive over a wide range of concentrations.

[Gary D]

Also, trojans and most true deep cycle batteries are 6 volt. If for a 12 volt system, you'd need 3 strings of 2 batteries in a sieries/parallel connection. Or 2 strings of 4 batteries in a series/parallel for 24 volt. As stated above, keep to one voltage battery configuration for simplisity, and as an alternative, perhaps use a cheap inverter turned on only when needed (can run 110 volts cheaper wiring) for higher drain power needs. Need to vent those gasses away from any sparks, or flames(corrosive too). A small pvc pipe from the sealed battery enclosure top is recommended vented outside, with the feed/ usage lines lower. Just a thought.... Gary D.

[armadillo]

Thanks for the advice, guys. I'll follow it. I love simplicity anyway, just thought 24 volts would be better for fans.

Any thoughts on the feasability of keeping batteries in the crawl space? Since the cabin will be vacant in the winter, putting them inside the house wouldn't keep them any warmer. I was thinking that when I leave for the winter I could remove the vent pipe and put a few extra insulation blankets on them to prevent temperature swings. I don't think the venting would be much of an issue when there is no load on the batteries and thus no charging.

[nothing to lose]

You might think about small 12vdc fans also. I myself am going to use several (or many) of those computer fans in various places. They don't eat alot of power and move fairly good air, not too loud either and pretty cheap. Also wiring should be fairly easy for those too. They run even when the inverter is turned off and that's nice too and saves power at times.

I buy mine at www.bestbyte.net (I buy there, I am not connected with them at all) you can get cheap fans for about $1 each (sometimes less) or fancy fans for alot more. I buy them by the dozen normally and they have better pricing when you do that.

 I also like their cathode lights for small areas or limited use lights when or where you don't need alot of light. Maybe just to see to get outside to the outhouse at night

 If your cabin is gong to be empty durring the winter I would still look for solar gains for heating just to keep it from getting too cold when posible. If you do that I would move the batteries inside before leaving and hopefully they won't get as cold.

 How cold does it normally get in that area you'll be at?

"Are the battery gases lighter than air or heavy like propane?"

 As another said, it's hydrogen, far lighter than air etc..

Also I think you might not know about batteries self discharging when not in use. I myself would leave them connected to the charging source (windmill/solar pannel) so they remain at full charge all winter. You'll need a dump load if you do that and something like a shunt relay to switch to the dump load when the batteries reach full charge to prevent over charging them. Lots of info here for all of this, just some searching needed to find it.

 If your using a dump load then as mentioned, you could use that to heat water in an uninsulated water tank. Since you won't be there using any power, any power produced over what is needed to maintain full charge will heat water. Not insulating the tank will alough the heat to escape and warm the cabin some. You could insulate the crawl space and put everything there and that will help keep the batteries warm.

Good placement of windows for winter sun and the dump load might help alot for the cabin to stay at or above freezing. I know I have left things in an un-heated place like that before to find a big mess later when I return. Stuff like sodas, maybe pickles, bottled water for drinking and filling batteries, and other items I would leave behind to use next year just burst freezing too solid.

[armadillo]

nothing to lose, Thanks for your reply.  I thought I knew a lot about this but now I see I have a lot to learn. I never heard of solar gains or dump loads before, nor did I know that solar panels would overcharge the batteries when the cabin is vacant. I thought the controller would just keep them topped off as needed.

From what little I comprehend, there are two paths to follow; install this shunt relay and dump load to maintain both battery charge and maintain battery temperature, and thus run the risk of a control malfunction that might overcharge or discharge the batteries...or...charge the batteries good, disconnect and place them where they will be least effected by the cold and deal with the premature aging caused by gradual discharge and maybe freeze damage.

By solar gains do you mean using the warmth that comes in the south facing windows or some electrical device that runs off the solar panels?(The cabin is going to be solar only)

I don't plan on having more than about 750 watts solar, so any devices that put off heat would have to be minimal. I do not plan to leave anything in the cabin that might be damaged by freezing other than the batteries, as they are too heavy and nasty to take home.

Could I place this dump load device inside an insulated battery box so I won't have to drag the batteries out of the crawl space every fall? Or should I not have them in the crawl space to begin with?

[armadillo]

Forgot to answer your question about how cold it gets. It's near Missoula in the mountains at 4500 feet and usually gets down to about +15 F at night in the winter but two or three times a year it might get down to -20 F or so.

[richhagen]

I agree with most of the above, for your original questions here would be my 2.5 cents worth.

1. Can I just tap into two of the six batteries for the 24 V circuit yet still have those two batteries participating in the 12 V load as well as get charged by the 12 V battery charger?

Trojan T-105's are 6 volt 225 amp hour batteries.  With 6 volt batteries you would need to connect 4 in series to obtain 24 volts.  If you center tap the bank you could pull 12 volts out from one half of the string but I would strongly discourage this for the following reason.  If you pull power from half of your string and then charge the whole string you will fully charge half of the string which was used less before the other half of the string which was depleted further.  As a result, a 24V charger will keep sending current through the batteries causing electrolyte loss and likely shortened battery life in the half of the string which sees the lightest use.  

Here is a previous post with an example of what can go wrong with this type setup:

http://www.fieldlines.com/story/2004/10/13/121519/52

While you could potentially compensate for this with electronics to limit voltage and current to each battery individually, this would complicate the installation and add more components to fail.  My recommendation would be to pick a voltage and use a DC/DC converter to supply different voltages if needed.  12 Volt parts are cheap, but you need twice as thick of conductors to move the same amount of power as with a 24 volt system.

2. Are the battery gases lighter than air or heavy like propane?

While batteries can vent more than one type of gas, largest majority and the most dangerous is the hydrogen because a mixture air and hydrogen will explode if the concentrations are in the right range.  This would be a danger inside a sealed box if their was an arc between a connector.  To prevent this problem you would allow the lighter gases to vent from the box to the outside via a small tube, such as a pvc pipe.  Flooded batteries like your T-105's vent the most gas when exposed to heavy charging currents, especially when overcharged.

Additionally, when batteries are fully charged, they are not likely to freeze until the temperature drops to very low sub zero extremes.  Dead batteries are another story.  As a battery discharges the sulfuric acid is consumed leaving a higher percentage of water in the electrolyte.  As a result, the freezing point depression caused by the sulfuric acid is reduced and the discharged battery will freeze at a much higher temperature.  Batteries left unattended for months will self discharge, leaving you with batteries vulnerable to freezing as well as possibly being damaged and having a shortened life from being discharged.  If you are going to leave the batteries unattended for long periods of time you need to take measures to both prevent them from discharging and prevent them from consistently over charging, electrolysing the electrolyte.  This could probably be acomplished, if their were no draws by a panel in the battery voltage with a rated current of one to two amps per hundred amp hours of battery bank capacity, and a charge controller to prevent over charging.  If the panels were small enough (1 amp per hundred amp hours or so) you could probably even get away without the charge controller, however I would put it in anyway.

Hope this information is of some use to you.  Rich Hagen

[armadillo]

Thanks, Hagen.

I'm now convinced to go with a 12 volt bank and maybe use the DC to DC converters if they are efficient, otherwise I would use 12 volt fans and all other DC loads also 12 volt. Since most runs will be about 50 feet or less I plan to use 10-2 wg romex wire. The heaviest DC load will be a cieling fan. Do you know if the converters are efficient?

I  lost you on the following:  "This could probably be acomplished, if their were no draws by a panel in the battery voltage with a rated current of one to two amps per hundred amp hours of battery bank capacity, and a charge controller to prevent over charging."

Can you please explain it in a way that a novice can understand? What do you mean "draws by a panel"?

[Ungrounded Lightning Rod]

Any thoughts on the feasability of keeping batteries in the crawl space? Since the cabin will be vacant in the winter ...

I wouldn't even consider putting them in the crawl space OR indoors.

 - They'll have to be float-charged - or they'll discharge, sulphate, and die over a few months.  Float-chaging can make them outgass - much more so if something goes wrong with the charge controller and you're not there to catch it.

 - They produce not just explosive gasses but small amounts of corrosive and toxic vapor.  Not only could they blow up your cabin, they could rot out the plumbing and wiring.

- They have to be readily accessable for fluid level checking and maintainence.  There's a risk of spilling acid on yourself or having a short throwing molten metal and maybe starting a fire when doing thi,s even in the best of situations.  You DON'T want that happening when you're in the crawl space.

If I didn't have an enclosed garage to keep 'em in (in box against the wall sealed against building air and vented to the outside top and bottom) I'd be inclined to put them in a small insulated box or building - a shed attached to the outside of the house or (better yet) separated from it by at least eight times its height, so flames won't be likely to spread to the house if it catches fire.  (You don't want it TOO far away because you'll need heavy wire from the batteries to your cabin's load.)

If they're receiving a non-trivial amount of charging and discharging they'll be putting out heat.  Insulate the box and they'll stay warm.  Just use ordinary siding and household fiberglass insulation - and I'd put a layer of drywall on it if the insulation is paperbacked, to protect it from fire if there's a spark.

You can put in a thermostat to turn on a couple small automotive lightbulbs - two or more for redundancy from burnout - to keep the batteries from freezing up in a DEEP cold snap.  But if they're kept charged the acid also serves as an antifreeze.

Put the batteries on a small rack above the dirt/slab floor to allow air to circulate all around them and prevent temperature gradients between the top and bottom of the battery.

(Please note that this is from my understanding of the theory of how this should be done.  I don't have direct experience with a battery bank of my own.  But I'm sure someone else on the board will correct me if I'm giving your wrong, or excessively elaborate and expensive, advice.)

[Aelric]

Just a lil suggestion, on the enclosed battery box, its probably a good idea to have your wires come out close to the bottom, I read this somewhere that there is a chance that Hydrogen gas (HIGHLY flammable potentially explosive if under pressure) could leak out with the battery cables, soo keep them low, keep venthole at the top and give the gas an easy way out that will prevent it from getting anywhere near spark or open flame :-)  Also when servicing batteries its probably a good idea to wear rubber gloves/facesheild/plastic or rubber apron (I'm just paranoid hehehehe) thats how we service our forklift batteries at work and I believe that proper PPE is always a good thing.  

[richhagen]

sorry about the English there, I was trying to state that if there were no loads drawing energy from the bank when you were not at the cabin, a solar panel or two ought to be sufficient to keep the batteries charged.  Rich Hagen

[richhagen]

Also, I would tend to agree with ungrounded in that it might not be a good idea to keep the batteries inside unattended for long periods.  If you had a small secure ventilated compartment outside it would likely be safer for your cabin, all things considered.  Rich Hagen

[richhagen]

DC/DC converters are usually 75 to 95% efficient, depending on their design and the load attached.  Some are phantom loads when not in use, in that depending on the design they may draw a few milliamps even if no load is attached.  If you use a small one for a single device, put the switch for the device between the converter and the battery and you will eliminate that load when the device is not in use.  Rich Hagen

[richhagen]

One more thing, (I wish I'd thought this all out and put it in one post instead of so many) it is better to avoid the dc/dc converters all together if possible as they are one more thing that can and ultimately will fail, as well as an additional expense.  It is preferable, when feasible, to have devices you can power directly from the battery bank voltage.  You have to weigh the penalties in efficiency and reliability against the differences in cost and come up with the answer that works for your situation.  Rich Hagen

[nothing to lose]

At 15F down to -20 only a few days you r not really talking that cold to me, I thought it got alot colder around there. Heck in a normal winter here we nromally get down to 0 afew times ans below 20 fairly often, but this year was pretty warm winter most the time.

 Probably not needed if your not leaving anything up there. Solar gains I was just meaning capture whatever solar heat you could and let it even out night and day best it would. I used to leave things behind alot, often thinking I would be back sooner when I had place. Sometimes I just left things because I forgot about them too.

I think everyone has covered pretty good on the battery stuff. I didn't know what you'd be using for chargeing, thought maybe a wind gennie.

[armadillo]

I just typed out a long reply and posted it and it just vanished! I read the "how to use this board" section and it said something about using spellcheck that can cause that. I don't think I will be using spellcheck again as it is apparently not programmed for forgetful people like me, LOL.

After reading this thread again I realize I have asked questions that have already been answered (duh). You guys are great! I have learned more here than I have from countless hours of my hurried, misdirected, addled research. It's hard to beat hands-on experience backed up with comprehension of all the theories of operation.

If keeping batteries under the crawl space is a bad idea I need to put them somewhere that they will stay until I get back in the spring. An old freezer in the yard would be subject to bears, vandals and thieves. I could put them in a 20 foot ocean-going shipping container that will be located 50 feet from the house, and would use about 65 feet of 2/0 wire to hook it up. Would that work or be too inefficient?

When I install and hook up my solar panels should I make arrangements in the wiring so that I can eliminate all but one panel when I vacate in the fall? Would one 80-120 watt panel be about right to keep six 225 amp hour batteries maintained all winter if I keep them in an insulated, thermostatically controlled (light bulb)vented box? Do they have to discharge occasionally to maintain? Should I worry that the light bulbs used to warm the box could provide an ignition source to ignite flammable gases?

[DanG]

Your 65 foot run = 130 feet of 2/0 wire, $250 at least to purchase then you've really limited yourself due to 12V losses, 24V or higher over longer wire runs is the way to go. Also the cable run should be buried in conduit if you're concerned about security from petty theft while you're away, that much copper is an inviting target all on its own.

thanks to taosgreen

http://www.taosgreensolar.com/PANELS/mod_wiring%20folder/loss_tables.html

run at 5% loss that's 40 amps of 12 volt = 480 watt power delivered

run at 2% loss that's 40 amps of 24 volt = 960 watt power delivered