I need some enlightenment or illumination.

Des,

A battery is not quite like a bucket of water. When the level in a bucket has fallen to half the bucket's height it is half full. When a battery has discharged to half its voltage it is empty, or flat. That is a rough explanation.

I will assume that we are talking about a deep-cycle type battery. It should only be discharged to about 50-60% of its rated capacity at which time its voltage will be about 11.8 Volts. If it is rated at 100 Amp hours capacity then you will have used about 40-50 Amp hours.

A 2 Amp light will consume 50 Amp hours in 25 hours.

Discharging a battery below its 50% capacity (11.8v) will cause it harm.

G'day Des,

There are a lot of variables, but I would say you wouldn't get 50 hours in the scenario you outlined. The type of battery is important too. AGM batteries are the best for this type of usage as they will best handle being deep discharged.

If the light was an incandescent type, it would simply get progressively dimmer as the battery discharged. By the time it got down to 10.6 volts it would be barely visible as being "on".

If it was a flouro-type light, then it would simply go "OFF" altogether at some point when the available battery charge became too low to run the electronics inside the light; not sure what the actual voltage would get down to for that to occur though.

I like to work on the basis that a reading of 12 volts on a battery is FLAT and I do my damnedest to try to ensure that that never happens. Batteries are too bloody expensive to kill, so I keep them over 12.6 at all times (if possible).

Having said that, we used to often carry around a 7.2 amp/hour mini 12v battery pack and run a 1w flouro lamp off it during the evenings around the camp fire. It would run for several hours (5 maybe??) and then the light would "die".....that was our indication that it was bedtime....time to tell Pesty to shut the F*** up and go to sleep!!! hahaha

That little battery pack finally packed-up after about 7 years of being deep discharged.

Cheers mate

Roachie

PS: The above is just my opinion.....I am not an electro-mechanical engineer (as you know).

This may help you as well
http://www.aussiebatteries.com.au/deep-cycle-battery-info/

I gather that as a battery ages as well it has less capacity. It may read 12.8 V eg. after charging but with a drain get much quicker to 12 V. A capacity test is needed to check battery health. Turf it if it has under 70%.

Hi Des

Your 2amp light will run longer , for between 60 & 70 hours.

Manufacturers of batteries publish the capacity of their batteries.

For example, here are some of the published specifications for a 100Ah GEL battery. (Jaycar SB1695)

Capacity (25 degrees C)
20 hour rate (5.1A) 102AH
10 hour rate (10A) 100AH
5 hour rate (16.2A) 81AH
1 hour rate (58.5A) 58.5AH

The 10 hour rate is what is manufacturers should publish so comparisons can be made.

If you divide the total AH by 10 you get the load that the battery should be able to supply for 10 hours.

If the load is higher than 1/10 the capacity of the battery is lower than expected.
If the load is lower than 1/10 the capacity of the battery is higher than expected.

In your case with a 2A load you should get a great deal more than 50 hours.

The link to the specification page is:
Jaycar SB1695 Spec sheet

I have used this Jaycar battery as an example only. I have no connection with Jaycar and do not endorse any type of battery.

Hi Des Lexic,

Most manufacturers publish a 20 hour capacity - C20. Some provide others as well or instead of, but the most commonly advertised capacity is C20.

If the C20 capacity is 100 amp-hours, then a new fully charged battery will provide 5 amps for 20 hours before being fully discharged (under test conditions).

If you put a 2 amp load on the battery, then because of a thing called Peukerts effect it will last longer than 50 hours (100 divided by 2). The extra time is determined by a complicated formula called, unsurprisingly, Peukert's constant.

These figures apply to a new battery. As a battery ages and is cycled its capacity is reduced and the Peukert's constant changes, so any testing you do with a used battery may not give the results you expect.

Allan B's advice re avoiding discharge below 50% is sound. Even if a battery is specified as "deep cycle", deep discharges reduce its life. The commonly accepted "sweet spot" between practical useage and battery life is to discharge only to 50%. Yes, with a deep cycle battery you can go further and it will recover, and you can do that quite a number of times, but you will reduce its life.

Cheers

With your typical caravan installation using 2x (or whatever number) of 105 AH AGM DC (which seem to be a pretty common arrangement) batteries charged by the vehicle through an Anderson plug while on the move and plugged into CP power usually when parked for the night(s), what do you do when at home for 4, 6 or whatever months?

Leave the batteries on charge (float)?

Or just fully charge, disconnect or turn off the charger, remove any loads (disconnect) and monitor the voltage over time. Recharging as required?

The charger I have is a smart charger which has the usual bulk, absorbsion and float stages. The selector switches provide selection of wet (flooded) or GEL/AGM and float voltages of 13.2, 13.5 and 13.8 which seem to be pretty typical.

I have heard various opinions about which float voltage is best and which practice is best. Charge and then disconnect or leave connected with appropriate float selected.

The batteries are about 4 years old and seem to do what I need but maybe there is a better way??

Cheers
Pop

So Mr Lexic - is there light at the end of the tunnel?
Batteries in essence are a simple device but crickey why are they so damn difficult to understand?