Trying to understand amp hours!

Thepipdoc

Thepipdoc

Well-Known Member
Joined
13 Jun 2009
Messages
937
Location
Torquay, Devon/River Dart
Visit site
Can I ask what might at first sight appear to be a simple question? (Sorry but I'm going to anyway and I'll try to keep it simple so that any answers will be equally simple!)
It’s concerning battery charging and for the purpose of this question let assume that the batteries were almost flat, at say 11.8 volts when the battery charging process started, and when it ended they were both reading 12 volts.
If I charge my batteries using a wind generator and the wind gen regulator says it's produced 40ah does that mean that if I have an appliance that uses 4ah, I can run it for 10 hours before they return to 11.8 volts?

........If that is the case then does it follow that if the batteries are showing a charge of 13.5 volts and I run the appliance for 10 hours (10 x 4ah =40ah) they will then show a charge 13.3 volts?
I heard an analogy recently which sort of made sense which went something like this – Charging a battery is like feeding a baby, the first few spoonfuls go in quite easily but the last few are almost force fed. If this true then does it follow that once the baby is full it takes a long time for the baby to become hungry again?
I hope this makes sense!
Thanks
 
You must distinguish between amperes (or amps) and Amperehours (amphours)

Amps is the rate of flow of current ( like say gallons per minute , to use a water flow analogy)

Amphours is the quantity of electricity. Number of gallons

amphours = amps x hours

So a rate of flow of 3 amps will in 4 hours give 3 x 4 = 12 amphours

just as 3 gallons per minute will in 4 minutes give 12 gallons

(Volts in the water analogy is equivalent to pressure)

If you have a battery at some state of charge and you put 40Ah into it yes, for the purposes of this explanation, it can supply 4 amps for 10 hours before it is back down to where it started,
Or 8 amps for 5 hours, or 10 amps for 4 hours, or 40 amps for one hour. Anything where amps X hours = 40 !

( not quite that simple in practice, but what is ? )


State of charge of batteries is best related to the voltage reading after they have rested ( no charge and no load) for a period of say 12 hours

OK you have a battery at 11.8 volts, more or less flat. You put 40 Ah into it and read the volts after is has rested and get 12 volts ... it is only about 20% charged at that so it must be a battery with a capacity of 200 Ah ( 5 x 40)

If you were to put 200 Ah into it it would be fully charged ( well not in practice .. it isnt that simple) and would read 12.8 ish after resting.

If you ran your appliance taking 4 amps for 10 hours it'll take 40 Ah out. Thats 20% of the capacity and the rested volts reading volts will drop by approx 0.2 volts ie down to 12.6.volts.

All over simplified because you cant get back quite as much as you think you are putting in and the actual current in amps affects it all as well.
 
Last edited:
Amp Hours

As Vic says Amp Hours is quite a simple and precise value of amps as accumulated over a time. It can measure amp hours in or amp hours out.
However talking about a lead acid battery gets very complicated and variable.
Firstly a battery is rated at Amp Hours capacity as a measure of its storage capability. So 100AH battery is pretty big.
However 100AH can only be removed from a fully charged battery by reducing it to a very low voltage (dangerously low for the batteries life) and usually at a very slow rate. Even then if you got out 80% of the capacity claimed from a new battery you would be doing well. As a battery gets older 60% might be good. Most consider 50% is useable in practice.

Roughly speaking if a battery is around half charged then perhaps 10% of total capacity can be added and subsequently that amount can be taken out (with perhaps 10% loss due to inefficiencies) to bring you back to near where you started. However if the battery is fully charged then adding more will only produce heat and boil the water such that subsequent taking out will leave the battery less charged.

A battery is charged by putting amps into it. However most simple charging is done by what is known as a constant voltage charging. This is typical of a car alternator regulator. Or a simple boat system.
Here a carefully controlled voltage 14v is presented to the battery terminals. An amount of current will flow in to charge the battery. This current will depend on what we might call the inherent voltage of the battery. This voltage can vary from around 11v or less to perhaps 13v or more depending on charge state. The difference between the inherent voltage of the battery and the charge voltage gives current. So a flat battery will take lots of current which then tapers off as the battery is charged. This is a great system it means lots of charge for a flat battery but if you continue to run the engine the charge will taper off so the battery is never damaged by too much charge. it does mean however that a battery in practice is never quite fully charged. (just very close after a long time)
Modern electronics have produced the stepped charger concept. Where the charge voltage is a lot more than 14v so more current is pushed in for a longer time. The charger eventually decides the battery is charged and reduces the voltage down to what is known as trickle or maintenance charge of a small current which can not harm the battery even though it is fully charged. The advantage of this system is that with a limited running time for the engine the total charge AH put into the battery can be greater because the charge current is not allowed to taper off. The rise of battery voltage is not linear but rises rather quickly in the charge process hence the analogy of feeding a baby. olewill
 
It's a good analogy. If you overcharge the battery and boil the acid away it's like making the baby sick.

A further point is that if a battery is very flat, and especially if it has been left in that state, it may be reluctant to take any charge at all. You may have to leave it on trickle charge for hours before it shows any sign of responding and taking a higher charge.
 
To further complicate the battery charging minefield, the actual capacity of your batteries may well be a lot less than you think since the capacity reduces with age and also is seriously reduced if the batteries have not been looked after. (are often heavily discharged or are not charged up promptly)

You'll only get a rough idea* of the capacity by charging up fully and then connecting say a 55watt bulb and seeing how long it takes for the voltage to drop to 11.5v

* the capacity is also affected by the size of the current being drawn - more current = less apparent capacity.
 
cliffordpope said:
It's a good analogy. If you overcharge the battery and boil the acid away it's like making the baby sick.
Click to expand...

And what the baby chucks-up is pretty corrosive!

And if you starve the baby it will get to the point where no matter how much energy you stuff down its throat, it will not recover! It's dead, get a new one!

And like a baby, a battery should be firmly tied down when under way.
 
onenyala said:
Is the polarity of the charger the reverse of the battery ?
Click to expand...

No.

The battery charger is the same polarity as the battery, but in order to charge it has to put out a voltage higher than that of the battery, so that there is a voltage difference to cause the current to flow.
 
All good stuff but 1 vital ingredient missing. A battery is specified by the manufacturer as, say, 200AmpHrs @ a 20hr Rate. So the battery will be discharged in 10hrs if that is the constant electrical load on it. If the load is higher, say 40A, then you will get less than 5hrs, maybe 3 or 4. If the load is lower, you will get proportionately more time.

The charge time will depend on the level of discharge and a good charger should never overcharge a battery.
 
dehlerman said:
All good stuff but 1 vital ingredient missing. A battery is specified by the manufacturer as, say, 200AmpHrs @ a 20hr Rate. So the battery will be discharged in 10hrs if that is the constant electrical load on it. If the load is higher, say 40A, then you will get less than 5hrs, maybe 3 or 4. If the load is lower, you will get proportionately more time.

The charge time will depend on the level of discharge and a good charger should never overcharge a battery.
Click to expand...

That is why I qualified my original explanation with "not quite that simple in practice, but what is ?"
What was required I felt was simple explanation of what was meant by Amp-hours.

For a more detailed approach see http://www.smartgauge.co.uk/peukert2.html
 
You must log in or register to reply here.

Other threads that may be of interest

webcraft
Resetting SmartShunt to 100%
Replies
7
Views
340
webcraft
webcraft
R
Leisure batteries, how to fit? Help!
Replies
10
Views
605
Skipper Felice
Skipper Felice

Share this page

Top