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Author Topic: Does 48V give lower battery costs than 12V?  (Read 7106 times)
Paulh_Boats
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« on: January 22, 2008, 03:21:08 PM »

A 12V battery system may have a daily discharge cycle of 40%

But at 48V the discharge cycle for the same power would be 10%


Will the 48V system last more than 4 times the 12V system before the batteries need replacing?   If it does then a 48V system is cheaper in the long term.

-Paul
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odbob
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« Reply #1 on: January 22, 2008, 03:40:58 PM »

Paul, I don't know the finite details of one voltage against another, apart from the obvious such as cable sizes and current probs, specially at the lower voltages and so I can only quote from my experiences.

1; my 12 volt 25 KW battery is now some 28 years old and still serving my systems
2; 12 volt was my original chosen voltage because at the time, many years ago, the battery was used mainly for home lighting and at that time I used halogen capsules throughout (before low energies became readily available). The battery was served either by mains through a large inverter or from a very old genny (long since put into retirement)
3; I am still connected as 12 volt because now I have 600 watt of P.V.'s serving this battery and because of incidental shading from a neighbour's tree these are connected in parallel.

It will be interesting to hear other comments, maybe my battery is due for replacement? I don't know it's present efficiency?
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Paulh_Boats
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« Reply #2 on: January 22, 2008, 05:06:59 PM »

Looks like 48V is cheaper long term:

"the number of cycles yielded by a battery goes up exponentially the lower the depth of discharge (DOD)"

http://www.mpoweruk.com/life.htm#dod

About 500 cycles at 80% DOD, but 2800 at 20% DOD
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billi
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« Reply #3 on: January 22, 2008, 06:26:15 PM »

sorry lads i donot get this one right now ......

my idea is if i have a battery bank with  example 500 ah hours at 48 volts this would be  -to -compare- 2000 ah hours at 12 volt   , isntit?


500 ahh multiplied by 48v   =  24 kwh

2000 ahh multiplied by 12  =  24 kwh


and that has nothing to do with costs 


but thats all theory , i have a 24 volt battery  and glad to have it , next live (if there is one ) at least 48 volts  ( bloody wirelosses and cablecosts)


Perhaps i missed something
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Paulh_Boats
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« Reply #4 on: January 22, 2008, 10:20:35 PM »

Billi,

When batteries are at the end of their life they don't hold much charge and have to be replaced. Their lifetime is often measured in the number of charge then discharge cycles. For example rechargable AA batteries are rated at a minimum of 500 cycles... in other words you can recharge them at least 500 times before they become useless.

Batteries being made of chemicals need special treatment and they don't like a heavy discharge that loses 90% of their power..in fact it can shorten their life. But at a smaller discharge like 20% followed by a recharge they last for more charge cycles before they become useless.

So one battery discharging 80%  is like 4 batteries discharging 20% each. But the single battery has a hard life and fails quickly... and the 4 batteries last much longer..nearly 6 times longer.

So you replace either 1 battery every year, for example ..or 4 batteries every 6 years (thats 1 battery every 18 months, which is cheaper than 1 a year  Smiley)

So a 48V system can have lower running costs than a 12V system.

I hope its clearer now?

cheers
Paul
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« Reply #5 on: January 23, 2008, 01:34:17 AM »

The battery capacity is dependent on the rate of discharge. The C rate would be multiplied by 0.25 by using 4, so although costing 4 times as much will last 4 times longer (in theory) and cheaper in the long run.  However car batteries have a short working life due to the thin plates for starting capacity, so they may not last 4 times as long in any conditions, but thick plates cells do last much longer. BT telephone exchange batteries are often 10,000 A/H or more and they do last a long time!

When you get less then the C rate capacity, I am not sure if it is just the capacity that is affected, or whether the efficiency goes down pro rata - or more!


http://www.kokam.com/english/product/kokam_Lipo_02.html
Quote
Rated capacity of the Lead-Acid Battery, based on 20-hour rate, is indicated by the capacity value when the battery of fully charged state is discharged to specified end of voltage.
The 20-hour rate is equal to 0.05C of SLPB current rate and it is very low current rate condition.
(Rated Current of SLPB : 0.5C)
Generally, Lead-acid battery has poor capability under high discharge current. So it is easy to sacrifice its containing capacity at this higher discharge rate (In other word, Lead-acid battery can not used all rated capacity at higher discharge rate). Because of slow electrolyte diffusion into the electrode, the H2SO4 content is reduced in the micro pore that is occur IR drop rapidly. Consequently, the capacity shows the tendency to be reduced remarkably as the discharge rate increase.

An interesting battery http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2003/09/03/ecnalaska03.xml&sSheet=/connected/2003/09/03/ixconn.html


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billi
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« Reply #6 on: January 23, 2008, 08:38:17 AM »

Paul

if you wire 4 batteries to 12 volt    ....is there then a difference to wiring them to  48 volt ?  Wink

all the best
billi
« Last Edit: January 23, 2008, 10:34:25 AM by billi » Logged

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« Reply #7 on: January 23, 2008, 10:23:15 AM »

I think we're getting mixed up here, whatever battery is used, it is made up of cells, each cell with a voltage at or about 2 volts, the life expectancy of this cell is dependant upon a number of factors :-
How often it is cycled
How deeply it is cycled
How often it is left in a state of discharge
How often it is overcharged
Quality of storage
Quality of top up water
The cell's role in life is simply to absorb and to discharge power and it's life expectancy, is as I understand it, not influenced by the internal wiring of the battery ie whether the battery final output voltage is 12 volt or 240 volt or whatever
Incidently I mistakenly said on my last thread that my battery is 28 years old, sorry, these cells are a mere 23 years old
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Paulh_Boats
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« Reply #8 on: January 23, 2008, 10:55:09 AM »

I think I may have started the confusion! Apologies.

I was trying to decide if a 48V system suited me better than a 24V or 12V system. I knew batteries don't last forever and I wanted to know the most cost effective solution for a long term standby/emergency system ............(draws deep breath) as Maggie said once "the lights must not go out".

I have 200 Wp of solar PV, plan to get a Soladin 600 invertor and then add another 400 Wp if I can find a bargain. I want to export to the grid....but it makes sense to keep some batteries charged for an emergency supply.

Odbob - A battery that lasts 23 years! I'll take one  Smiley

-Paul
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LED lighting in every room
NO tumble dryer, +370 kWh per year
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« Reply #9 on: January 23, 2008, 11:03:13 AM »

Paul

if you wire 4 batteries to 12 volt    ....is there then a difference to wiring them to  48 volt ?  Wink

all the best
billi

Yes you have 4 times the current if wired at 12 volts. The voltage dropped across the semiconductor is typically 0.6v then the losses of wiring and fuses makes the 48v system appreciably more efficient.  Also if the cells prematurely age or have slight imbalence, you do not get an identical loading on each, whereas in series the current is the same.

Inverters will cost more simply as they are less common. Maplin 600w was 40 recently.

I suspect the real risk of explosion is greater in parallel connected batteries is higher. Three batteries supplying current to the fourth, probably without a fuse.

I have had a battery buggy battery explode, they are dangerous

http://www.videojug.com/film/how-to-avoid-a-battery-explosion (Lithium polimer)
http://www.whscc.nb.ca/docs/HA-Battery_e.pdf

Useful basic info on
http://www.allaboutcircuits.com/vol_1/chpt_11/5.html

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billi
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« Reply #10 on: January 23, 2008, 08:53:44 PM »

hi paul


 standby batteries have to kept charged  , so some battery types loose ther charge faster or have to be recharged quite often




i donot know if it would work with the solardin  to use it when the grid fails ( so perhaps another inverter is needed , but if it would work then   you need 48 v or higher because of the specs of the solardin  and you could just wire the battery to it  to supply the house (or ?)

and then the higher volts the less cablesize



Actually with the solardin connected would you feed then in the national grid and get something (?)  or is it just your house that is supplied with the solar units ?


thanks

Billi




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Paulh_Boats
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« Reply #11 on: January 23, 2008, 10:19:22 PM »

Billi,

One feature of grid-tie inverters, like the Soladin and SunnyBoy, that seems strange when you first hear it is that they switch off automatically in a power cut! Why switch off in the greatest moment of need? 

Well its done to protect workers on the electric grid who maybe working nearby. They switch off the power to dig the cables up, so the UK G83 certification says all grid-tie inverters must switch off at the same time for safety. If the grid-tie kept running the workers could get a shock.

That means the Soladin is great 99% of the time reducing my electric bill, but in a power cut I will need another inverter, an off-grid inverter run from batteries. 

I'm not sure if you knew all this, but I thought it might be useful to all the new members who have joined recently.

What I'm thinking of is a simple time switch to connect the PV panels to the battery charger an hour a week or whatever to keep the batteries topped up.

-Paul
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2.3kW PV see:
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LED lighting in every room
NO tumble dryer, +370 kWh per year
billi
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« Reply #12 on: January 23, 2008, 10:31:52 PM »

paul  i understand didnot think as far....

But have to ask again just for  information .... hope you have the time...

you plug the solardin in to a normal grid tied socket , isnt it ?  and that then goes out of your house ( if not used in the house )  but does it run your meter then backwards ? 


thanks billi

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