# Leisure battery - Volts/AH's



## alshymer (Nov 4, 2007)

We like to wild camp and I am used to checking our volts/amps and using our inverter.
I believe that the volts should not drop below 12.4, that amps are the amount of electricity stored and that volts are the pressure.
My husband has now changed our 220 ah battery for 3 at 125 ah giving 375 ah.
What I don't understand is that, although I now have more amps, do I also have more volts so can use the inverter more often before reaching 12.4 and needing to recharge? In reality we use Efoy, solar etc. to keep the battery topped up.
Could someone kindly explain this to me please?
Many thanks and Merry Christmas and a Happy New Year to all.
Wife of Alshymer


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## Penquin (Oct 15, 2007)

The volts will remain the same (unless he wired it up wrong which I am sure he was careful not to do....) but the amps is effectively how much electricity you have stored in the batteries.

So by having 375 ah you have more that can be used - quite a lot more, but it remains the same that you should try to not drain the batteries below about 12.4 volts.....

If you have Efoy, solar panels etc it is a bit like holding your trousers up using a belt, braces AND a piece of string so you should be able to wild camp without too many problems - particularly if it is sunny and the panels are in the sun (i.e. not hidden under trees......)

The Charente should better than the UK for sun, but the Lot et Garonne has not been overwhelmingly sunny of late..... but as Tescos say (often) "every little helps" and by being careful with the amount you use you should be OK. 

The inverter effectively drains the batteries out 20 times as fast as using 12v - so care is certainly needed using that; if the appliance uses 3 amps (not a lot) at 230v, you are effectively taking out about 60 amps from the battery assuming the inverter is superbly efficient (bit they rarely are that efficient so you may use more......  ) - so your 375 amps will not last as long.

You also only ever take about half the charge out in order not to damage the batteries.......

So care is essential with the inverter in particular.

I am sure that if I have not explained something clearly or correctly someone will be along to correct me - but AFAIK what I have said has a large grain of truth......  

Dave


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## listerdiesel (Aug 3, 2012)

AH or ampere-hours is the storage capacity of the battery, defined on every battery by a figure of XX AH, BUT it is also given at a particular rate of discharge.

So, 120AH at the 20hour rate (C/20 in battery terms) will allow you to take out 6Amps for 20 hours.

If you discharge the battery faster, by using an inverter, the rate of discharge is much higher, and the available capacity in the battery is lower at the higher discharge rate, so your:

120AH at the 20hour rate will become:
100AH at the 10hour rate
80AH at the 5 hour rate

(all figures approximate)

so with your new, larger battery, you can sustain much higher discharge figures for longer, but the faster you drain the batteries, the less capacity will be available.










The chart is a typical discharge curve set for a lead-acid battery, and although these are for a different battery to yours, the principle remains the same:

The faster you pull the current out, the less capacity will be returned from the battery.

The figures on the dotted line are the discharge rate as a proportion of the capacity. 0.05CA is C/20 or the 20 hour rate. The rate that the battery will discharge over 20 hours.

1CA or C/1 won't give you a 20th of the time, in fact it is less as the capacity is shrinking at the higher discharge level, so 1CA gives you markedly less capacity, about 35 minutes instead of the expected hour, and so on.

The Yuasa curves shown are typical for lead-acid batteries.

Peter


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## peribro (Sep 6, 2009)

Penquin said:


> The inverter effectively drains the batteries out 20 times as fast as using 12v


I don't think that's the case. Take my mobile phone that can be recharged using a 12V cigarette lighter plug or a mains plug through my inverter. My mobile phone needs the same amount of charge to go into it regardless of which way it is being recharged. Therefore the amps that are being drawn from the battery are the same (except for inverter inefficiency of say 10%) whether it's drawing the charge through the 12v socket or the inverter.


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## alshymer (Nov 4, 2007)

*Leisure Battery*

Thanks for your replies. Very complicated, I wish I had paid more attention at school!!!
The inverter I am using is a Sterling Inverter/Charger which appears to consume 3 AH itself before anything else is connected.
Regards
Wife of Alshymer


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## listerdiesel (Aug 3, 2012)

There is a sort of inverse effect on the battery recharge needs as well.

At lower rates of discharge, the battery will run almost to zero capacity and will need immediate recharge, while at very high rates of discharge, the battery reaches its cut-off point while still leaving a significant amount of capacity behind in the battery.

There is a chart for this as well, showing the returned capacity after various rates of discharge down to a normal cut-off voltage.

Peter

Capacity table NP batteries, doesn't come out too well:

Capacity 20hr 10hr 5hr 3hr 1hr
24.0ah 24.0 22.3 20.4 18.5 14.4 (14.4ah at the 1hr rate)
26.0ah 26.0 24.2 22.1 20.0 15.6 (15.6ah at the 1hr rate)
38.0ah 38.0 35.0 32.3 29.3 22.8 (22.8ah at the 1hr rate)
65.0ah 65.0 60.5 55.2 50.1 39.0 (39ah at the 1hr rate)

So the 65ah battery gives you 39ah at the 1 hour rate, that's 60% of its design capacity.

Lots of good data here:

http://www.enersysreservepower.com/documents/NPAppManual(Rev0500).pdf


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## rayc (Jun 3, 2008)

peribro said:


> Penquin said:
> 
> 
> > The inverter effectively drains the batteries out 20 times as fast as using 12v
> ...


But that is not what Dave was explaining.
Taking Dave's example: 240v @3 amps gives power of 720w

The Power rating is of course the same if it is being powered via the inverter. 720w at 12v gives a current of 60A, possibly a little more due to power loss. That is indeed 20 times more current.

If your mains charger is providing a nominal 12v at, for argument sake 0.5a then the DC power is 6w. This is indeed the same as it would be from a direct battery connection but it is not the mains current being drawn at the ac O/P of your inverter by the mains charger . I doubt it is drawing much more than 20ma ac.
Therefore I think it is true to say that your mains charger draws 20 times the DC current from the batteries as it does from the ac mains.


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## BillCreer (Jan 23, 2010)

Hi W of A,

Yes, in simple terms, your conclusion is correct in that you have more than a third more stored energy to play with and you can use your inverter a lot more now before the voltage drops below 12.4.
Voltage is only a crude indicator of how much energy is left in your combined batteries.


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## peribro (Sep 6, 2009)

rayc said:


> peribro said:
> 
> 
> > Penquin said:
> ...


Yes, I agree that the mains charger draws 20 times the DC current as it does from the AC supply but what I was trying to correct was any misapprehension that you will get 20 times less energy from a battery by using an inverter rather than a 12V connection.


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## Scattycat (Mar 29, 2011)

Reading another posting some months ago I was under the impression that if you took the battery down to 12.4 volts there was still around 80% of the charge left, in fact it only started to become a problem if you regularly took the charge below 11.9 or 40% on a regular basis

Or did I mis-read it?


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## 747 (Oct 2, 2009)

It is not much use checking a battery voltage if something 12 volt is operating as it will give a false idea of the battery state. If the battery is at rest and has recovered from a recent drain then you should get a true indication.


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## alshymer (Nov 4, 2007)

*Battery*

Hi 
I have been told by a highly regarded motorhome equipment installation company that a battery is flat at 12.4v
Regards
Alshymer


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## peribro (Sep 6, 2009)

You are correct about 12.4v equating to 80%. This diagram sums it up pretty well, I think.


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## Philippft (Feb 16, 2008)

The attached information may prove helpful.


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