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Author Topic: 12v battery bank  (Read 6003 times)
« on: July 08, 2008, 11:57:44 PM »

We currently have a bank of 12x 2volt single cell batteries which we charge up using 2 solar panels and a wind turbine.  We also have a backup biodiesel generator which is connected to a 50amp battery charger.
We are looking for another solution for a new building that we have at the nursery.  Both buildings are off grid.  The power requirements off this building are as follows :
Lights x 2 - 14w each, 3 hours a day.    Seed radiazer  75w, 3 hours a day,   Seed germinator 30w, 4 hours a day and a fridge 110w, 6 hours a day.
We estimate we will need about 130amps per day to run all these items.
In our existing setup we have old forklift batteries connected to a massive 5000kw invertor.
For the new building, would we need such a big inverter - 14+14+75+30+110 = 223  watts and most of these wouldn't be running at the same time.
We need some form of battery solution -  should we go for 4-5 x 85amp 12v batteries or a battery bank of 6 x 2v batteries?  Not quite sure how to wire it - is it positive to positive, neg to neg.  We need to keep a 12v solution not 24 as existing equipment we have is 12v.
We would then charge it using a 130w kyocera solar panel and another biodiesel generator connected to a 3 step 25/50 amp charger.  Not sure yet which to get.
On our existing building do you think that 5000kw invertor is overkill - we using a variety of low wattage appliances, a maximum of 500 watts at any one time.
We guess a 1000w inverter would be more than enough.
There seem to be lots of companies selling deep cycle batteries etc but I'm struggling to find anywhere that sells bus bars or anything to make a "bank" of batteries.

Advice would be appreciated !  I hope this makes sense
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« Reply #1 on: July 09, 2008, 12:13:21 AM »

If you are using motors, fridge for example,  the inverter should  have four times the current used by  the motor so your old larger inverter is possibly needed.    12V  batteries  are joined in parallel  ie. positive to positive.    Probably best to use value for money to decide on batteries and use enough to avoid discharging too much.   75 amp batteries discharged half way would each give two  hours for everything  less a small percentage for inverter overhead.       

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Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
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« Reply #2 on: July 10, 2008, 10:37:23 PM »

Hi there,

I think your 5kw inverter is a bit OTT for your needs and at 12v it will require some serious cables. Inverters tend to be much less efficient at the lower end of their rated output so whilst it may be 95% efficient at 4kw it could only be 85% at a couple of hundred watts. We use a 4.5kw inverter but it runs washing machine, tumble drier and electric ovens ( though not all at once ) as far as connecting batteries goes I use copper pipe hammered flat.

Good luck, Paul

'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SMA SI6.OH, 800ah Rolls, 8kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
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« Reply #3 on: July 11, 2008, 06:55:33 AM »


I find that my inverter/charger combi unit quite good , and it allows me to use the power from the dieselgenerator in parallel to the inverter ..... So the let me say in your case 1000 watt of inverter  plus xxx watt of the generator (if needed) and charge the battery as well

I sound like you need about a 1kwh per day  and 4-5 x 85amp  batteries store  about 5kwh  at 100 %   so if discharged to advisable 50% you can store power for 2-2.5 days

This battery bank should not be charged with more then 10 % of its capacity = about 40 A  =  500 Watt for a 12 volt battery

So keep that in mind in relation to your biodiesel generator ( to let a diesel generator run for a few hours and only charge   500 watt can be a bit wastefull ....)
If you get a secondhand forklift battery(and lucky that she is in good conditions) all the connections are included and perhaps end up cheaper with more capacity ...


1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
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« Reply #4 on: July 11, 2008, 08:47:24 AM »

It might be worth mentioning that when a battery manufacturer produces a battery with a name plate rating of, say 110 amp hours - that this is under very well controlled, optimised lab conditions and when new.

After a few cycles the battery will have reduced its capacity to something less than the name plate rating. I would suggest that after 3 months (6 months if you are treating the battery very gingerly) that the battery capacity will have a capacity closer to 50% of the name plate rating.

The battery will continue to deteriorate but, (depending upon how harshly it is treated) at a much lesser extent. My guess is that after 3 years, it is unlikely that a well treated battery will have more than 30% of its name plate rating capacity.

This is the reason why large bulk users of lead acid batteries replace them every 2 - 3 years. The SG of the cells will read OK and the discharge test will look fine too, but if you need to rely on their charge carrying capacity for a mission critical job (like running a telephone exchange or emergency lighting in an office block) then you have to replace them after 2 - 3 years.

A battery only ever fails when it is called upon to deliver a significant load and over a fairly significant time. Your 5 or 6 year old battery bank may look OK on the instruments but it is only when you can test and prove that the batteries can deliver as expected that you can be sure that they are capable of doing so.

I hope this helps.

« Reply #5 on: July 13, 2008, 09:43:29 AM »

Hi there

On the existing offgrid solution which was supplied by a third-party and I'm not altogether sure that
it is still 100% working.
The solution comprises of 12 x 2v batteries which seem to be wired up positive to negative.  I was told this is
wired up for 12v but from what I've read positive to negative, surely thats 24v?

This is attached to a Ardent 5000w inverter which has two outputs.  Half the batteries power the top plug and the
other half power the bottom plug.
We charge the batteries using 2 x 130w solar panels, a wind turbine and a backup generator which is attached
to a 50amp 3-stage powermaster charger.
What i don't understand is that according to the installer the batteries have a capacity of 1000amps.
We tend to use low wattage appliances - we have a fridge 110w (on for 6 hours a day), a freezer 120w (on for 6
hours a day), a computer (not sure), on for 2 hours a day max, we also have lighting (100w in total - on for about
5 hours a day).
We don't use high wattage appliances but the batteries seem to only have enough power for a day, before the
inverter beeps and all the power goes off. That's if we don't get a sunny day AND we don't run the charger for at
least 2-3 hours per day.  There seems to be no way of saying "that battery has ... amps left", but according to the
voltmeter - the maximum I can charge the batteries to is 13.5v before the charger goes off and the inverter
beeps and goes off at 11.0v.   

As the batteries seem pretty ancient, I am guessing that they are not charging 100% to maximum.  I am
also not sure what sort of drain the 5000w inverter has to the battery.

I am seriously thinking of replacing the 12 x 2v batteries with a bank of deep cycle 110amp batteries. I was thinking of getting about 6. 
I also thought about getting a smaller inverter.  If we didn't get a invertor straight away, would our huge 5000w invertor work with such
a battery bank.   We would have it wired up for 12v - as the solar panels, charger and invertor are 12v.

I am seriously confused!


Phil : )
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« Reply #6 on: July 13, 2008, 10:35:27 AM »

Hi Phil (rosebarn)

You say you're confused. I guess (and hope it's not insulting!) that the physics of amps/volts/watts/wiring isn't your top subject, maybe? Certainly I'd say to get a professional in if you can't work out what kind of battery bank you need, and whether it ought to be connected in series or parallel.

You said you have 12 x 2V batteries wired 'positive to negative' (that's 'in series'). You correctly say that this would make 24V, but then you say that half the batteries power one part of the inverter, and half the other. If the batteries are in a 'string' of 12, then this sounds impossible. Are you sure it's not two strings of 6, which would then indeed be 12V each?

You say the installer says the batteries "have a capacity of 1000amps". Batteries don't have capacities in 'amps' - that's a current, an instantaneous rate of flow of electricity. The capacity is often measured in 'ampere hours' (or 'amp hours' if you like, Ah). That's the current in amps multiplied by the number of hours that the battery can sustain it for. If you multiply it by the voltage, then you more or less get a 'proper' capacity in 'watt hours' (Wh), which can then be compared with your daily needs.

So we'd really need to know whether this '1000amps' should have read 1000Ah, whether it was one battery or the whole lot or what, and whether the voltage which goes with it is 2, 12, or 24??

Let's look at the daily requirement in Wh (from the appliances you mention). Let's assume the computer is 200W. So that would be 110*6 + 120*6 + 200*2 + 100*5 = 2280 Wh (or 2.28 kWh).

I can't really work out what capacity of batteries you might have given the uncertainties above.

What about the power input devices. 260Wp of solar panels might produce a (very optimistic) 260kWh over a whole year, or an average of 0.7kWh per day. So that's only about a third of your average requirement, and of course it will produce far more in summer than winter. No idea what the wind turbine might generate.

Think I'd better stop there for now. Perhaps some of the others will be able to make a better guess about what's meant about the batteries - not familiar with typical forklift batteries myself.
« Last Edit: July 13, 2008, 10:37:05 AM by CeeBee » Logged

« Reply #7 on: July 14, 2008, 12:59:24 PM »

Thanks for the reply.  Yep, physics was never my strong point! 
I've decided to go for 4 x 110ah batteries for the new building.
Attached to this bank will be a solar panel (still to get), a 30 amp 3 step charger connected to a generator and a 1000w/2000w invertor.
Hopefully this will be enough power to run a standard fridge (110w) for a few hours a day, low wattage lighting and a few other small things (no more than 30w each, not at the same time).

Looking at our existing battery bank, it is wired up postive to negative and there are 2 lots of 6 x 2v batteries.
The batteries on the existing building are very old and came out of a submarine apparently!

When wiring up for 12v the new batteries have standard terminals (like a car battery). The other batteries (the 2v ones) have screw connectors.
Anyway, when wiring up the new batteries. What is the best way of connecting them together?  Someone suggested copper pipe hammered flat. I suppose if i drilled a hole it would fit over the terminals?  Is there a better way.   I will have a bank of 4 x 110ah batteries. 


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