12 volt battery bank

[horsefly76]

what would be the general max amp-hour size a person would go for a 12 volt battery storage bank befor going to a 24 volt setup?

[rmchambers]

It all depends on your usage.  The higher the volts the lower the amps for the same watts.  You can use 12V batteries in a 24 volt setup, or 6V (thats what I have) or even 2V if you can get hold of them.

Do you have the inverter yet?

If you haven't bought the inverter yet I'd say go for the 24V system which should give you flexibility with battery choices and by adding enough battery capacity you can ensure you have enough power when you need it.

[GuyFawkes]

what would be the general max amp-hour size a person would go for a 12 volt battery storage bank befor going to a 24 volt setup?

Speaking personally I wouldn't want to be handling much over 100 amperes on average, maybe a peak of 150.

100 A x 12 VDC = 1.2 kW

100 A x 24 VDC = 2.4 kW

100 A x 48 VDC = 4.8 kW

100 A x 72 VDC = 7.2 kW

100 A x 96 VDC = 9.6 kW

Large battery banks, particularly when you get above 24 VDC, are BLOODY DANGEROUS, as it they are just waiting to maim or kill you, 96/110 VDC used to be very common on boats, but everyone got out of it for safety reasons, and safety reasons alone, for all other reasons it beat all the other options hands down, but for safety it was a nightmare.

A decent sized 96/110 VDC bank could produce tens of thousands of amperes, and not instantaneously, but keep that current going, while producing copious quantities of gas, I've seen inch and a half spanners you could beat a gorilla to death with flare up and literally blow apart like a fuse in about a second or two.....

The other thing about why DC is dangerous, anything much over 24 VDC, apart from literally cooking whatever flesh the current is flowing through, is enough to spasm any muscles near the current path, and that is spasm as in lock on tendon ripping hard and stay that way, too bad if that muscle contraction puts you in more harms way, or simply clamps your hand around the power.....

A 100 Ah 12 VDC car / pickup battery can chuck out 500 amperes without too much sweat.

500 x 12 = 6 kW, it can sustain that for some time, 6 kW is / 0.75 approx 8 brake horse power, so the analogy here is deliberately stick your arm in a running CS 6/1 flywheel, you're messing with the same sort of available power as a single battery, never mind a bank of them.

Working on such batteries, which includes switching loads on and off them, is therefore directly akin to working on your Lister WHILE IT IS RUNNING.... just because the elektriktrikkery is invisible and silent (most of the time) don't mean it ain't there.

Juggling with sticks of nitro is the closest thing I can think of to large battery banks, if you know what you're doing you'll be ok and will know which ones you can throw on the fire and which ones to treat with an inhibitor, but since you asked the question you DON'T know what you're doing, so the best advice is do some studying and if at all possible pay good money for some hands on tuition.

HTH etc

[ronmar]

Well idealy, for efficiency and battery longevity, you want to draw from the battery at the 20 hour rate at which the amp hour rateing was established(some are rated on a 10 hour schedule). Drawing at a greater rate would cause losses in heat due to internal battery and wire resistances.  So a 500AH bank divided by 20 hours would be most efficient delivering 25A.  This of course limits your available wattage 12V@25A is only 300W, minus conversion losses in the inverter. In theory you could have a huge AH 12V bank to deliver the required amperage, but that is a lot of batteries, connections and all their associated maintenance.  It is far easier to double or quadruple the voltage, and available wattage for a given current resulting in fewer batteries to maintain.  As Guy pointed out, battery banks can store some tremendously destructive energy so the fewer you have to deal with the safer you can be.   I would say if you are trying to drive more than a kilowatt in AC load, go with a 24 or 48 volt bank. 

Lets do a 1KW comparison.  If the inverter efficiency is say 85% then that 1KW of load would actualy be, from the batterys perspective, just under 1200W of load.  1200W divided by the 48VDC supply is 25Amps   25Amps times a 20 hour rate is about 500AH of required capacity at 48V to feed that 1KW load.  That same load and efficiency at 12VDC would require a 100A feed(1/4 the voltage, 4X the current) and at a 20 hour rate would require a 2000AH battery bank...

Ron       

[listerdiesel]

Bear in mind that the higher the discharge current in relation to the battery capacity, the lower the returned capacity will be.

A 100AH battery at the 20hr rate (means that it will deliver 100AH over a 20 hour period, approximately 5A discharge)
A 100AH battery discharged at the 5 hour rate would return about 75AH capacity (25A for 5 hours)
A 100AH battery discharged at the 1 hour rate would return about 65AH capacity.

The use of higher voltages for reserve batteries is quite common, we make a living out of designing and building specialised chargers for all sorts of batteries, so far up to 620V at 150A.

High voltage DC is pretty lethal, so anything above 50V comes under the Low Voltage Directive in the EEC, and appropriate safety measures have to be taken to protect users.

For anything much over 7kW I'd go for 48V batteries, much higher output than that and you may as well look at 110V.

It's a specialist subject.

Typical 110V battery installations are on our company website:

http://www.prepair.co.uk
http://www.prepair.eu (Spanish version)

Peter
 

[Tom]

I've just purchased 4 - 1055 amp hour 12 VDC batteries for our off-grid solar home. They will be wired in series for 48 VDC. I will be very careful around them!!! They weigh 815 lbs each!

[Tom]

I'm getting them from here.

http://www.dcpower-systems.com/product_detail.aspx?gid=23&pid=12

Hawker PV1 Solar Industrial Batteries

A top-of-the-line industrial battery that's optimized for deep-cycle photovoltaic service. Rated for 2,100 cycles to 80% depth of discharge. Batteries just don't come any more robust than this. In an off-grid renewable energy homestead with reasonable attention and care, these batteries can be expected to deliver trouble-free service for 15 to 20 years. Packaged with six propylene-clad cells in a steel tray. Features include heat sealed cell covers, the thickest plate grids in the industry, maximum density paste, with a multi-layer retention system that's wrapped horizontally and vertically to help hold active paste material onto the grids. We offer removable, or non-removable cells. The non-removable cells, with soldered, molded, plastic-encased cell interconnects are recommended, and are less expensive, so long as you can deal with the weight of these 6-packs. Interconnect cables are only needed between 12-volt trays with either cell version. Order multiple trays as needed for voltage and amp-hour capacity.

These are only the "medium" sized ones.    The cost is about $2k each.  so I'm paying $8k for the pack of 4.

I have a backup inverter system with 4 L-16 batteries wired in two strings of 12 VDC ea for a total of 720 amp hours so I have some experience with larger packs.

The PV1's are not your "first timer" set of batteries because if they are abused they can be turned into junk in no time. I will be picking these up my self and delivering them to the home site to save some $.

[snail]

That type of battery is a must for serious off gridding. I use something similar, the 1320AH ones at:http://www.centuryyuasa.com/data/portal/00005057/content/31840001135066989345.pdf and they really are the way to go.
Whilst there is a potential for disaster from shorting these monsters, it's really not that bad if you use insulated tools and only remove one of the plastic terminal covers at a time. There's more risk of a hernia from shifting the buggers!
The 24v set = 800kg!
I also have a set of the 535Ah ones on a separate system. Typically, topping up the two sets requires 14 litres of deionised water

Cheers,

brian

[horsefly76]

 My set up now is 7 C&D tech 12v AGM 100 amp hr (20hr rate) wired parallel to get 700 amp hr.
 
 My thinking is to drive a 1000 to 1500 watt sign wave inverter & power the not so important items in the home.  Lights, outlets, computer, entertainment center and the like and leave the grid for the A/C fridge, washer, dryer. A hybrid if you will.
 
The solar panels & the metro 6/1 would take care of the charging end of the deal.

 I'm a mechanic for a rental company's battery operated equipment and diesel generators 25kw to 300kw. I have spent a lot of time playing with T105's and L16's. Next door is an Interstate battery warehouse. You guy's getting a picture yet?

[Tom]

Sounds like a good start. In my location power is over a mile away and I believe it would cost more $ to get a power bill than buy the $35K solar PV system.

[Chaz]

My set up now is 7 C&D tech 12v AGM 100 amp hr (20hr rate) wired parallel to get 700 amp hr.
 
 My thinking is to drive a 1000 to 1500 watt sign wave inverter & power the not so important items in the home.  Lights, outlets, computer, entertainment center and the like and leave the grid for the A/C fridge, washer, dryer. A hybrid if you will.
 
The solar panels & the metro 6/1 would take care of the charging end of the deal.

 I'm a mechanic for a rental company's battery operated equipment and diesel generators 25kw to 300kw. I have spent a lot of time playing with T105's and L16's. Next door is an Interstate battery warehouse. You guy's getting a picture yet?

SEVEN IN PARALLEL?? 
A serious no-no, I'm sorry to inform you....

[rcavictim]

My set up now is 7 C&D tech 12v AGM 100 amp hr (20hr rate) wired parallel to get 700 amp hr.
 
 My thinking is to drive a 1000 to 1500 watt sign wave inverter & power the not so important items in the home.  Lights, outlets, computer, entertainment center and the like and leave the grid for the A/C fridge, washer, dryer. A hybrid if you will.
 
The solar panels & the metro 6/1 would take care of the charging end of the deal.

 I'm a mechanic for a rental company's battery operated equipment and diesel generators 25kw to 300kw. I have spent a lot of time playing with T105's and L16's. Next door is an Interstate battery warehouse. You guy's getting a picture yet?

SEVEN IN PARALLEL?? 
A serious no-no, I'm sorry to inform you....

I see no problem with placing a number of batteries in parallel on a common bus PROVIDED THAT each separate battery has a fuse between it and the bus sized at say 2X the normal expected max charge current for that battery (max charge current as high as 1C is often higher than discharge current of 1/10th C).  If a battery develops a short fault it can cause huge fault current to flow from all the other batteries on the bus to the faulty battery causing an explosion, fire or worse.  The fuse will limit the fault current to a non-event, taking the shorted battery out of the circuit.

[Doug]

I did the same on my UPS.

I made the mistake of reversing a battery once. Thankfuly the CBs caught it.

Doug

[listerdiesel]

There are good reasons NOT to parallel batteries or cells, as you can get all sorts of problems with balancing capacities and internal current flows between them, however it is done commercially, albeit with less than 7 in parallel, and seriously big banks have steering diodes in the chains to prevent the current flow between banks during both discharge and charging.

Fusing is a consideration also, our last installation of 2 sets of 108V 200AH batteries had fuses in both conductors and blocking (not steering) diodes, as they had individual chargers for each battery.

It is quite possible to see batteries 'walking' their capacities down when in parallel as they discharge into each other.

Peter

 

[rcavictim]

Peter,

Are you aware of any resources on the internet that would be a good tutorial on this subject of parallelling batteries which you could share?