If you have 2 12 volt batteries that are 100ah rated wired in series to make 200ah and you have a drain on them of 18 watts @ 1.5 amps should the batteries give you 120 hours is this correct??? I have one more ??? when you charge lets say a battery that is a 100ah battery that has lost 30ah and you charge it at 1ah will it take 30 hours to charge it??? thanks for any help Jason

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Allow me to give you a little battery 101.

As you can see when batteries are in parallel it adds Amp Hours(AH)

when batteries are in series it adds voltage(V).

I don't really understand this question but I will tell you that the rating of lead acid batteries are normally based on a 20 hour discharge period. The Amp Hour rating tells you how much amperage is available when discharged evenly over that 20 hour period. To know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20.

Example: If a battery has an amp hour rating of 100, dividing by 20 = 5. Such a battery can carry a 5 amp load for 20 hours before dropping to 10.5 volts. 10.5 volts is the fully discharged level, always measured under load, at which point the battery needs to be recharged.

Note: It is important to note you should only discharge the battery to 50% of it's rating for it to have a long life and NEVER NEVER leave the battery in a partially charge state. Always charge it to full charge as soon as possible. One other item of note is, trying to use the voltage as an indicator of the state of charge does NOT work.

Well it really doesn't work like that. There are 3 stages to charging a lead acid battery and each stage charges at a different rate supplying a different percentage of the charge.

Grandpa Chas S.

If you have two 12V batteries that are 100 AH each, you can wire the pack in:

series to get 24V at 100 AH,

parallel for 12V at 200 AH.

You can use 90% of the rated AH of Lithium or NiCD, but for the common and cheap lead/acid batteries, you should only use about 60% of its rated capacity. If you use more than this, the plates erode faster and the pack will die much soomer. If you use half its capacity, it will last 4 times longer.

You have to put the two 12V 100AH batteries in parallel, not series to double the AH rating. Putting them in series doubles the voltage but keeps the AH the same. Your example inplies 12 volts, so I assume you meant parallel.

The total energy capacity of a battery depends on the rate of discharge, the higher the discharge rate the less energy you can get out of them. This is called "Peukerts Effect" after the person who first developed the formula for describing it. This effect is fairly small in lithium batteries, but it is dramatic in lead acid batteries.

To digress a bit, this is one of the biggest drawbacks of lead acid batteries in vehicle applications - and is the reason why changing over to lithium batteries of the same capacity-rating can provide almost twice the range.

The standard AH rating for lead acid batteries is based on being discharged at C/20 which means amps equal to 1/20th of the AH capacity - also called the "20 hour rate". So, from, you example, you will get the full 100AH out of the battery if you discharge it at 5 amps (about 60 watts) or less. Discharge it at 100 amps and you will only get about 60AH (36 minutes) out of the battery before the voltage sags excessively.

All the better brand lead acid batteries publish performance graphs and charts for various rates of discharge. An example is here:

http://www.bb-battery.com/productpages/EB/EB20-12.pdf

HA! Chas, you old sea-dog! you and I must have been typing at the same time!

Listen to Grandpa Chas, he has much more experience than me...

Let's not all jump in and answer at once...

Thanks for the help just real new to the hole batterie scene I did mean to say 2 12v 100amp hour batteries wired in parallel. I still have a final question if a light is running off the batteries and its 14 watts drawing 1.5 amps is it the watts are the amps that affect the power that the light is drawing from the batteries so if I understood correctally I shoul be able to run the light for 10 hours and only draw 15amps is that right and the battery shouldnt be drawn down to a dangerous level based on the 1.5 amps the light draws

hey Spinning,

I have seen that movie before, good thing we were both saying almost the same thing or we would have confused toy21...

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Toy21,

If you draw 1.5 amps that is 1.5 amps per hour or 1.5AH every hour of operation. You asked about using it for 10 hours which should be easy. That is only using 15AH of a 200AH battery or about 7.5% of it's total capacity, no problems there. Are you trying to build a camping light?

Grandpa Chas S.

Jason,

Watts are units of power = volts times amps. I assume you are familiar with this from your example earlier.

What determines the power of the light bulb is the voltage and the resistance of the bulb's filament. More exactly, the resistance of the hot filament. The lower the resistance, the more current it will draw, or vice-versa, in accordance with Ohm's law:

Voltage = current x resistance

or using the conventional symbols:

E = Ir.

So, a bulb with 6 ohm resistance (when hot) connected to a 12 volt battery will draw 2 amps (i.e I=E/r); and 2 amps times 12 volts equals 24 watts of power. So, a 24 watt, 12 volt bulb (say, for a car turn signal light), will have a filament with 6 ohm resistance. A 100 watt bulb for use with typical N. American voltage (120 volts rms) will have a resistance of about 144 ohms when hot. It will be a lot less cold. I just checked a 100 watt bulb and only got 10.5 ohm cold.

Hope this helps.

Thanks for all the info but in answer to the ? about a camping light the answer is no I have some led lights mounted outside powered by a solar pannel and I just wanted to make sure that I was understanding amp hours correctally and the corolation between the battery and the load and or the demand you are aplying to it by running lights off the battery. My solar pannel only puts out 4 amps so I was worried it wouldnt recharge the battery enough each day.