#### sailaboutvic

##### Well-known member

EX ….say 100 amps = 1 volts …. Is the sum 10 amp over 1 hour =100 amps

100=1 volts which means

battery will be down to 11 v from 12 v in an hour

If this is not the case can someone show me how it is done please …

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- Thread starter sailaboutvic
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EX ….say 100 amps = 1 volts …. Is the sum 10 amp over 1 hour =100 amps

100=1 volts which means

battery will be down to 11 v from 12 v in an hour

If this is not the case can someone show me how it is done please …

Therfore 500W (inverter say) to work out the current is Power/Voltage = Current = 41.6A for a 12v system..

Hope that is of some use!!

ML

Batteries are rated in amp hours which means it will deliver the number of amps stated for one hour , or twice the amps for half an hour and so on.

Voltage is a product of the number of cells a battery has, wet cell batteries are 1.5 volts per cell, and the durability of that cell is a product of its physical size.

I dont know off the top of my head how this relates to voltage drop off and will have to look it up.

I hope this is of some help.

However, it does bring back memories that I asked a very similar question when about 8yrs old of a schoolfriend who's father was a Professsor of Electrical Engineering, so perhaps should not be too hasty!.

Don't you read the forum regularly though? .. I see you've been registered over a year, and would certainly have picked up on this?

Vic

You are confusing several different measurements.

You can think of Volts as a measure of the 'pressure' that the battery exerts.

Amps is how much electricity if flowing.

Think water and pressure. The pressure is like the voltage and when you open the tap how much flows through the pipe is the current.

Amp-hours is a measurement of how much electricity is being used, how much a battery can give etc.

Power is measured by measuring the voltage and the current and multiplying the two together. 5 amps at 12 volts into a device is 60 Watts. If you carry on allowing this to happen for an hour you will have used five amp hours.

Your problems are exacerbated and confused by the fact that lead acid batteries don't maintain a constant 12 volts over their charge discharge cycle. In other words, a fully charged battery will (when its rested for a while) settle at about 12.6 volts. As power is drawn off the battery the voltage gradually reduces until it will eventually be 11 volts or so and then collapse rapidly to 0 volts. (Doing this to a battery and leaving it without recharging it is a quick way to destroy a battery!) Some people use these voltages to assess the state of their batteries. How quickly a battery will reduce its nominally fully charged voltage by one volt depends on lots of factors - how old the battery is? How charged up it was to begin with?. What capacity the battery is?

Hope that this helps.

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To use the water flow analogy volts can be likened to the water pressure suplying a tap for instance.

Amps can be likened to the flow rate when the tap is turned on. The higher the pressure (volts) the greater the flow rate (amps)

Amp-hours can be likened to the total volume of water that flows out of the tap in any period of time. The longer the tap is turned on the greater the volume of water that will have been drawn.

Amp-hours when applied to a battery is the total "volume" that the battery can hold. In the same way that a water tank might hold 100 gallons. If the tap were taking 5 gallons per hour from the tank then in 20 hours the tank would be empty.

So, If you had a fully carged 100ahr battery it would be flat after supplying 5 amps for 20 hours.

Before 50 or 60 people start objecting I must say this is a pretty rough analogy but it is one that is commonly used to explain basic electricity.

I deduce that you are interested in relating a voltage reading to the state of charge of a battery.

Now a good 12volt battery that is fully charged and then rested for 24 hours will show about 12.7 volts on a high resistance (eg digital) voltmeter.

As a rough guide you can say that every 0.1 volt drop from that figure represents 10% of the battery capacity. So when the reading falls by 0.5 volts to 12.2volts the battery is about half discharged. Again these voltage readings apply to a rested battery. While the battery is under load the voltage readings will be lower and will depend on the size (amps) of the load.

I hope this helps. You may find the electrical section Reading College Boat Maintenance Course notes helpful.

http://www.reading-college.ac.uk/marine/

The capacity of batteries is measured in 'amp hours' (normally referred to as a/h) and represents the theoretical amount of electricity the battery can store. So your 100 amp battery could supply 1 amp for a 100 hours, or 100 amps for 1 hour, 50 amps for 2 hours and so on. However because the voltage falls away as the battery discharges, most lead acid batteries will only maintain a useable voltage for about 2/3rds that time. Once it reaches around 50% discharge, the lead acid 'car starter' battery will be unable to maintain a high enough working voltage to be useful, and also chemical changes take place that eventually destroy its ability to hold electricity.

Most appliances power requirements are measured in watts. To work out how many amps, divide the watts by volts, and the result is the number of amp hours the appliance will use. So at 12 volts a 25 watt bulb is taking a fraction over 2 amps. At 230 volts mains voltage a 70 watt appliance is taking just over a third of an amp, and so on.

So to work out how long your on board battery will last before going flat, you need to know the wattage rating of all electrical items that are likely to be switched on at any given time. Add all the watts together, divide by 12, and the answer will be the number of amp hours taken out of your battery. Divide about 2/3rds of your battery's amp hour rating by this figure and you will get the approximate time it will take before your battery goes flat.

so say your various electrical bits require 48 watts, divide that by 12 (volts) = 4 a/h. Say you have a 60 a/h/ battery, you can safely use around 40a/h before voltage falls off too far. 40 / 4 = 10. So you can expect your battery to go 'flat' after around 10 hours.

Hope that helps a bit?

There is no direct correlation between Current(Amps) and Voltage in the way you mean unless you bring in Ohms Law, which states that "a current of one amp will flow at a voltage of one volt into a resistance of 1 ohm.

Current is the amount of electricity "drawn" from the supply.

Voltage is the "pressure" at which it flows.

Resistance in Ohms is the load which is applied to it.

Watts (power) is the amount of work done or equivalent energy in heat.

This is a very simple analagy...not entirely technically correct...but near enough for explanations.

From this one can use equations:

Volts = Amps X Ohms

Amps = Volts/Ohms

Ohms = Volts/Amps

You may see this equation written as

_V_

I R

To give it it's correct SI units.

I = Amps

R =Resistance (ohms)

V = Volts.

Cover the unit that you want to solve for.

Also:

Watts = Volts X Amps.

For your batteries, in simple terms, your battery is rated in AH or Amp Hours.

If you have a 100AH battery, this means that it is ""theoretically"" capable of supplying a discharge rate of 1amp for 100 hours, or 2 amps for 50hours, 10amps for 10hours...etc.

However things are not that simple..and many factors come into play, losses in the circuit, battery plate deficiencies..etc, and it is highly unlikely that even a new battery will give anything like it's correct AH perfomance.

I usually rate mine at about 75% as new, and then take off 10 percent for each year of use...but this is very much rule of thumb and based on my own observations over the years. I have had batteries that have done better than that, but also some that have gone belly up in under a year.

A lot is to do with the way they are charged. Trickle of float charging is to be preferred over short sharp blasts with a rough old car battery charger.

A good battery fully charged should show between 12.5 and 12.7 volts after charging, with about half an hour's rest period before taking the measurement.

Any battery that does not show this is usually U/S or deficient, and should be investigated with a Hygrometer, to measure the specific gravity of each cell, which is the only sure way of testing a battery properly.

Confused???

Sorry...

Steve.

I appreciate that the english language is a very flexible thing but here is a case of short cut causing confusion.

It is great to see all the school teachers coming out of hiding, it must be a cold night in UK with nothing on telly.

Chippie be careful of the water analogy. Volts is better described as the water pressure rather than speed. So as an analogy you can get an old fashioned water wheel with thousands of gallons per hour flowing (current) with a head (pressure)(voltage) of 6 ft giving say 50 HP ( 50 Kilowatt aprox) or you can have high pressure water flow with a head of 500 ft with very small flow giving similar power. The size of the reservoir is similar to the capacity of the battery. The first is like a low voltage system the second like a high voltage system. regards will

Thanks for all the info guys and no it wasn’t a wind up …volts and me don’t mix….

This is the problem I have , battery fully charges say 13.2v as soon as I put say a light on it goes down to 12.9 ,…..lights draw 1.2 amps …. with in say hour volts meter is now reading 12.5 turn another light on now the amps is 2.5 draw…..and with in say another hour battery read s 12.v so it seem to me that if you have a 100AH battery and you can only use 50% would made that say 50AH so drawing 2.5 amp should that not mean that the battery last for 20 hours , if that is the case why is it that my battery after 2 hour is down to 12v or is the battery …. US …

Sorry everyone who disagrees with me - but they are!!

The only meanigful voltage reading of a dry cell battery is the 'quiescent' voltage - that is the voltage measured when nothing has been connected for at least an hour. In practice, the voltage ten minutes after switching off is near enough.

Quiescent battery voltage for a fully charged battery should be around 12.8 volts, and should return after running the lights for a couple of hours to at least 12.6 or more.

Fully charge the battery and take it to a garage and get them to do a heavy discharge test - it only takes a couple of minutes. This will tell you if the capacity is reduced.

What is the lowers you can let a battery go down to , as I been told to day 10v by a garage

and 100ah battery is that really only got 50ah

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Once the reading on a rested battery is down to about 11.7volts it is effectively dead flat.

Totally discharging a battery once or twice should not have a disasterous effect but the more frequently you do it the more serious the consequences. What is important though is not to leave a battery in a discharged or even partially discharged state. It should be recharged again without delay.

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