Help converting Amp hours

[48Rob]

I've been doing some tests to find out how long I can run my electrical load until I reach 50% capacity on my batteries.

I have two 6 Volt 105 AH batteries.
Fully charged is 12.6+ Volts.
50% discharged is 12.06 Volts, right?

So, I charged my battery bank, unhooked the charger, then let it sit overnight.

Next morning, I started the test with 12.8 Volts.
Voltage reading 10.25 hrs later was 12.1 Volts.
The constant load on the batteries during that period was 7.46 Amps.

My math, 10.25 hrs x 7.46 Amps = 76.36 Amp hrs.

Trouble is, two 6 Volt, 105 AH batteries should have a 50% capacity of 52.5 AH.

Is this reading just a fluke due to the batteries being new?
Because my charger charges more than most?
Or is this normal, and all the info the experts put out is just an "average"?

Rob

[GeorgeTelford]

Hi Rob

Assuming 12.8 v settled is fully charged, every 10th of volt is 10% ergo 50% discharge would be 12.3 V

you discharged to 12.06v (near enough) and that would be a bit over 70% discharge and lo you discharged (Aprox 76.36 Amps)

If 12.8v is fully charged, then 12.3v should be your absolute max discharge.

[48Rob]

Hello George,

I understand now!

Thanks for taking the time to explain.

Rob

[GeorgeTelford]

BTW Rob

How was the 7.46 A measured?

Was it calculated from the loads rating plate?

Measured and monitored throughout?

The problem that I usually encounter when trying to explian this, is when people charge via a standard alternator and think its fully charged, or even using convertors and again people imagine its fully charged. for example.

Battery is measured at 12.6 after (poor) charging and resting, if you now assume that 12.6 is fully charged and run it down to 12.1 in reality because the battery was not at full charge initially they have discharged to only 30% left in battery.

[48Rob]

Hi George,

Your willingness to go the extra mile when solving problems is appreciated.


Amp draw was measured initially (for 30 minutes) with my Amp meter (through the main fuse terminals).
Since The load consisted of light bulbs only, I didn't feel there would be enough potential variance to effect the test results, so the meter was removed from the circuit.
Voltage was monitored thruout with my on board Volt gauge (after verifying the reading with my volt meter).

The (assumed) fully charged state was the result of using a dedicated Deltran charger.

The voltage stayed steady at 13.6, or thereabouts, for a couple days before the test.
The lights on the charger indicated the battery was fully charged, and the charger was simply maintaining.

Rob

[GeorgeTelford]

Hi Again Rob

I was checking (how you tested) for my own database of tests, I believed yours was fully charged because 12.8 to 12.9 is about the highest you will ever encounter from a 12v (nominal) Battery after resting.

[bdosborn]

George,
According to the Trojan Battery website, a typical deep cycle battery is at 50% charge at 12.1 volts and 80F. This seems to conflict with your info of 10% discharge for every 0.1V. While your values explains Robs battery test results I'm curious about the difference from published values. Its only a 0.2V difference but it would seem to represent a 20% capacity difference.
Thanks,
Bruce

[GeorgeTelford]

Hi Bruce

You will find my explianation will fit both, if the Trojan is at 50% discharge at 12.1 V then fully charged will be 12.6 V

Its a very simplistic way to judge 50% I had avoided mentioning temperature, this can further muddy the waters, but in general 100% equalling 1V overall in 10th of Volt increments still works.

"My" method only requires that the battery is fully charged to find the starting voltage, fully charged by a good 3-4 stage charger.

Later gotta get off to work.

[WarPony]

George, I am really confused about all this talk about 1/2 discharge of a battery.

1) What/who determines a full charge of a battery?
2) Is it by brand or use?

I'm used to using flashlights and headlamps until the batteries are dead (or close to it). I don't understand how, going from 12.8 to 12.1, is half discharged. 12.8V and 12.1V is still 12 volts. Does it depend on what you are powering and for how long as to what the voltage needs to be? I'm sure it does.

All this talk about vehicle chargers/inverters/converters....... REALLY confuses me to the point of not adding ANY electric at all, although the Deltran power tender is something I am interested in if I can get a handle on all this stuff.

Right now, I have a couple of small LED lights and my girlfriend got me a DVD player that uses less than 35W of power. No galley or any appliances to run. Oops, maybe a small fan in the summer time.

[madjack]

WP, I am not an expert and was very confused by the terminology for a while, so let me see if I can explain it...
1) a fully charged battery at rest(no load or trickle charge going to or from it) is around 12.6 to 12.8 volts depending on environmental factors(temp, age of battery, etc)
2) as you use the battery it discharges(duh)...every tenth of a volt drop is considered 10% of USABLE battery power
3) so if you have used 5 tenths of a volt(measured with battery at rest) you have used 50% of usable battery power and you are at the point of where battery manufacturers say you should not pass before recharging for best battery life
4) you can continue to use the battery and at 11.6 or .8 volts the battery is considered DEADthere is still some juice in the battery but it is not really usable
5) if you discharge past that 50% mark on a regular basis, you will shorten the battery's life and efficiency greatly

there is a lot physics involved in the reaction betwenn the lead and the acid that is of no use here but may be the above helped a little(or not)
madjack

[GeorgeTelford]

Hi Warpony

Jack pretty much covered everything. but here is a little more

What determines battery voltage at full charge? the type of battery and how its made also as an effect, ie even the acid water ratio, but sealed and unsealed batteries, type and amount of materials etc.

To find full charge when you get a new battery, fully charge it with a good 3-4 stage charger. The reason I always say that it must be charged by a 3 or 4 stage charger is this, convertor may charge the battery to 90 % if you leave it running for 4 weeks or so and do not use anything on board. So called automatic chargers may do very slightly better in less time. The only easy way to fully charge a battery (and rapidly) is to use 3 to 4 stage charger.

You mentioned flashlight batteries, if they are disposable they are intended for one time use and damaging the internals by over discharging is not important. If they are rechargeable taking them right down shortens their life dramatically ( but everyone does it so we dont notice how much longer they would last )

The DVD player, I have measured many many appliances, TV's that are plated at 55 w generally use less than 40w in normal use ( the plated rating is hard to achieve even if you switch volume colour and everything way up) so if you ever measure its consumption you will likely find it is less than 35w.


Lets say for example that the only item you ever run is 2 amps consumption and your battery is a 100 Ah battery. if the battery is fully charged, you could reasonably run the item for 25 Hours (25 hrs X 2 Amps = 50 Ah), so you are getting maximum use from the battery and the battery will remain in active and happy service for many many years.

If you were to charge via the vehicle alternator, you would be extremely lucky to get 70% of fully charged, therefore max "safe" useable power is 20 Ah ie you couls use the item for 10 Hrs Max. If this is the only charging that the battery ever gets, you would be lucky if the battery lasted 2 years even though you have not over discharged, the reason is sulphation, battery power is a chemical reaction as you use power the plates form a layer of lead sulphate, if the battery is fully recharged this lead sulphate turns back into sulphuric acid, if its not fully recharged then the lead sulphate eventually hardens and becomes charge resistant.

The above leads to 3 simple rules

1. Always use a good 3 or 4 stage charger.
2. do not over discharge the batteries.
3. Always fully recharge as soon as possible.

Apply the above rules and you will get much greater use from the battery between charges, much longer years of service from the battery before replacement, you will save the cost of the decent battery charger over and over.

There is a 4 th rule avoid "charging via an Alternator" this one is much harder to explain, but say you fully charge your battery by Shore power at home, if you drive to your destination with the battery connected to the alternator, the battery will be at 65 to 70% by the time you arrive.

There is a simple experiment that proves this, say you have 2 batteries one fully charged and one discharged to 50% connect them togethor in parrallel for a while and they will try to level out, when you come back say a day later the 50% will have gained charge and the 100% will have lost charge. Even if you add a trickle charger the 100% will still lose charge.

I did the following experiment and had the following results.

from fully charged at 12.93v (measured 12 hours after coming off the Sterling charger) connected bulb, it took 6 hours and 17 mins to reach 12.2v (ie flicking over backwards and forwards from 12.3v)

It was then fully recharged on the Sterling, allowed 13 hours to settle the voltage, reading was 12.94v flash 12.93v, the engine on the iveco was started, then I connected the battery using a good solid cable as before and drove this morning for 1 hour 14 mins, as soon as I got back it was connected to the same bulb (55w halogen) connected at 12 : 12 today, it started flashing between 12.3 and 12.2 at 5 : 37 PM today

Same battery, same full charge by Sterling, only difference 1 hour 14 mins of extra "charging" as knocked off 52 mins off usable power !!

Note Sterling is my good 4 stage charger.