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Lead acid batteries for backup power

 
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adam2
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PostPosted: Mon Jan 18, 2016 8:44 pm    Post subject: Lead acid batteries for backup power Reply with quote

For various purposes it is desirable to have a backup battery or bank of batteries.
Prices seem to have fallen in recent months, presumably due to a general decline in metal and energy prices.

Many fleabay sellers are offering batteries at less than 50 pence per AH for 12 volt batteries which are by far the most common type.
These batteries are only moderately heavy duty and don't stand up that well to repeated deep cycling, no matter how grand are the claims made.
However at only ABOUT 60 or so for a claimed 120 AH battery, including delivery, they are well worth consideration.

Better quality batteries that should last 20 years or more are available from RE specialists, but at a substantial premium.
My main concern about cheap batteries is that by sods law, they will fail about a week after TSHTF.
Better a cheap battery that you can afford now, rather than a good one that you still saving up for when the world ends.
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biffvernon



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PostPosted: Mon Jan 18, 2016 11:12 pm    Post subject: Reply with quote

Is it not the case that different batteries are good for different purposes? The sort designed for a car is not the same as the sort designed for a caravan. You want to know what you want.
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adam2
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PostPosted: Tue Jan 19, 2016 12:33 am    Post subject: Reply with quote

Yes, but these days only two types of moderately priced lead acid batteries are produced.
Car batteries are optimised for the supply of a large current for a short time to start the engine. Technically known as SLI batteries for Starting, Lighting and Ignition, these are not suited to regular deep discharge and may fail after as few as a dozen deep discharges.

The other mass produced batteries are variously known as marine, leisure, caravan, or deep cycle batteries. They typically withstand ABOUT 100 deep discharges or many hundreds of 50% discharges. No great accuracy can be claimed due to the many variables involved.
These are the type to which I refer as being available for as little as 50 pence per AH. Often about 60 for 120AH or a little cheaper especially if ordering more than 1.

Many other types exist, including sealed lead acid or very long life vented cells, none of these are cheap however.
Trojan, and Rolls are well known brands of medium quality and more costly vented lead acid batteries.
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kenneal - lagger
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PostPosted: Tue Jan 19, 2016 3:23 am    Post subject: Reply with quote

I think that it should be emphasised that deep cycle batteries should never be discharged more than 50% as this will greatly reduce their life. Batteries never discharged below 50% should last for over 5 years. Go over that depth of discharge and they can fail in less than a year.

What this means in practice is that you will need twice the number of batteries that you calculate. If you need 120Ahrs of storage you will need TWO 120Ahr batteries. It is well worth sticking to that limit and many of the better inverters have a setting so that they cut off at a battery voltage of about 11.2 - 11.6V to ensure that limit.
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Pepperman



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PostPosted: Thu Feb 04, 2016 9:25 am    Post subject: Reply with quote

adam2 wrote:
Many other types exist, including sealed lead acid or very long life vented cells, none of these are cheap however.
Trojan, and Rolls are well known brands of medium quality and more costly vented lead acid batteries.


We have Trojan T-105s and they've served us very well for 2 years so far under daily usage and I expect many years more service to come from them.

They were 105 delivered for a while but prices seem to have gone up slightly so now work out at about 1/Ah at 12V which is about 167 per kWh usable (based on 50% DoD).
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kenneal - lagger
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PostPosted: Thu Feb 04, 2016 5:54 pm    Post subject: Reply with quote

I'm dreading having to replace my current 920 Ahr at 12V ex MOD battery set which is getting on for 6 years old now, having been charged twice daily over that time. They only cost us 270 plus the cost of the acid!
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adam2
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PostPosted: Sun Feb 14, 2016 2:54 pm    Post subject: Reply with quote

I doubt that a bargain like that will be repeated, though you might be lucky.

Basic leisure type batteries that are allegedly deep cycle would cost about 400 and probably not last 6 years.

A true deep cycle battery of that capacity would be about 2,000 and should last 20 years or more.

Unless the present battery is very oversized, then bigger is better within reason for a replacement.
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sam_uk



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PostPosted: Thu Feb 18, 2016 10:32 am    Post subject: Reply with quote

Or for true longevity you could remortgate your house and buy some NiFe

http://www.bimblesolar.com/48vnife1200ah

Not cheap, but they do last 100 years..
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adam2
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PostPosted: Sat Feb 20, 2016 4:04 pm    Post subject: Reply with quote

Nickel iron batteries are indeed very durable and might be worth considering if you have the money.
100 years whilst possible is rather optimistic, 30 or 40 years might be more realistic.
Unfortunately nickel iron batteries are NOT a drop in replacement for the much more popular lead acid types.
They need as much as 18* volts to fully charge, that is higher than provided by most chargers or charge controllers and is also too high for most 12 volt loads.
To fully utilise that very expensive capacity they need to be discharged down to about 9* volts which is too low for most 12 volt loads.

Another drawback is that they need to be charged relatively quickly, a low charging current has no effect whatsoever !
A 500AH lead acid battery charged at one amp will eventually charge, though it will take some weeks.
A 500 AH nickel iron battery charged at one amp or even at several amps will never charge.
This means that a relatively small wind turbine or PV module that will eventually charge a lead acid battery, may NEVER charge a nickel iron battery.
Even if the PV module produces say 10 amps, but the current used by the household is 8 amps, then no charging results, the 2 amps is wasted.
A lead acid battery with a charging input of 10 amps and a load of 8 amps will slowly charge.

If the load consists ONLY of multi voltage LED lamps, then the variable voltage will be fine. A 16 cell nickel iron battery would be about 14 volts at end of discharge, and about 28 volts on full charge. LED lamps that work over that voltage range are readily available.

*for a nominal 12 volt system.
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Little John



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PostPosted: Sat Feb 20, 2016 4:27 pm    Post subject: Reply with quote

Does anyone know of suppliers of fully serviceable lead acid batteries?
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adam2
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PostPosted: Sat Feb 20, 2016 6:24 pm    Post subject: Reply with quote

I am not aware of any such, and suspect that they are in effect now banned by elfansafety due to concerns about handling lead compounds and battery acid.

There may not be much advantage anyway in serviceable lead acid batteries.
If individual* 2 volt cells are used, rather than 6 volt or 12 volt batteries, then only failed cells* need be replaced. Thereby avoiding the waste in replacing a 12 volt, 6 cell battery because one cell* has failed.

2 volt cells are readily available in large sizes, but small ones are AFAIK no longer made.

Whilst being able to insert replacement plates into the original container may sound attractive, it should be remembered that the plates represent most of the cost, all that has been re used is a relatively cheap glass or plastic container.

*In practice though, under most conditions, if one cell in a series string has failed then the rest are probably near end of life and it may be a better policy to change them all.
A new cell in series string of older ones is bound to have slightly different characteristics and therefore to be over or under charged to a degree and probably have a shorter life as a result.
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