UKC

Hi,

Ok, geeky question, but how efficient is a 12v leisure battery system in a van? e.g. if I have a 100 amp hour leisure battery and use it for 20 hours drawing 5 amps, at the end of the 20 hours the battery would be drained. But, that assumes a 100% efficient system. Is there any kind of agreed amount that I should divide the amp hours by to represent the fact the system isnt 100% efficient?

Any advice appreciated!

Tom

In reply to tnj:

You're best avoiding really deep discharge so I'd design for maybe 70-80% depth of discharge in normal use. Also, with lead the higher the discharge rate the lower the capacity (Peukert effect) so you should build in surplus capacity if you're using high power devices. I suspect your 100Ah battery will be specified at a discharge of 0.1C (10A, 10Hr) or less, if you're going to draw more current expect less capacity. Other losses like I^2R heating of wires will be pretty insignificant.

jk

In reply to jkarran:

Thanks for the info. My calculation is that the maximum I will use is:
4 amp for a cool-box,
1-2 amp for MP4,
1-2 amp for portable speakers
1.6 amp for a strip light.


What does the 0.1C mean?

Cheers

Tom

In reply to tnj:

0.1C = Discharge at 0.1 x Capacity

0.1C from a 100Ah battery will give you 0.1x100Amps for 1/0.1Hr

www.wikipedia.org/wiki/Peukert's_law

So if you're looking to draw 7A from a 100Ah battery that's 7/100=0.07C and a pretty low discharge rate, your battery should last ~14 hours (to totally flat which isn't good for it) giving something pretty near its rated capacity.

jk

In reply to jkarran:

Ah, I see (sort of!) So, if I do my calculations for how much battery life I should have left and try not to go above 80 amp hours used I should be protecting my battery from being fully discharged.

How much does temperature (ie very cold at a ski resort for a few nights) make to how much charge is in the battery?

Tom

In reply to tnj:
For lead acid batteries, you can get a good idea of the state of charge just by measuring the voltage across the terminals. I'm sure you can find a nice neat panel mounting meter with green, yellow and red areas or LEDs on it to put in your van.
A fully charged battery should read about 12.8V, and for low discharge rates, stop once it gets down to about 11.5V. Deep discharging of the battery will significantly shorten its life. If you leave the battery standing it will slowly discharge by itself; again don't let it drop too low.

In reply to tnj:
I´ve worked on hundreds of these systems in boats, the normal rule of thumb is you can use 60% of the rated power.
Your standard alternator regulator is designed for a large number of moderate duty cycles while connected to relatively high running loads and will normally only charge the battery to about 80% of it´s capacity (and is designed for maintenance free batteries). There are other charging systems available which override the standard system but generally you have to start fitting heat sensors on the batteries and be prepared for considerable water consumption (and change from maintenance free to deep-cycle batteries).

In reply to jimtitt: Surely if it's a leisure system the OP is asking about, it will already be deep cycle batteries?

In reply to Oceanrower:
If it´s a sealed battery it isn´t really a deep-cycle battery since one of the needs is to continuously replace the water lost, or you buy AGM or gel batteries.
I´ve never seen a leisure battery that anyone would realistically call deep cycle apart from the advertising department of the manufacturers.
Since unless you modify the alternator you aren´t going to fully charge the batteries it´s fairly irrelevant and there is no point in a deep-cycle battery, if you do mod the regulator you then have to make provision for charging the normal starter battery differently or change it as well. Any simple add-on leisure system is going to use a starter battery with reinforced grid plates as that is the cheapest and lightest solution and maintenance free, a deep-cycle battery of the size talked about (100Ah) weighs in at around 30-35kg if not considerably more and will be charging at over 16V.

In reply to richard_hopkins:

Thanks for that Rich. Are the volt meters thatI could plug into the 12v outlet for my leisure battery less accurate than those that go directly across the terminals? I have now put the van back together and would prefer to use this simple "plug-in" solution than start running wires through the panels again!

Tom

In reply to tnj:

The voltage should be exactly the same if you measure it on the terminals or just plug it in to the socket. The advantage of wiring it directly onto the battery terminals is that it will display continuously, including when your gadgets are drawing current. The current drawn by a simple tester will be negligible and you can leave it running continuously.
If the voltage you measure across the battery terminals is significantly different to the voltage you measure across the socket whilst it is powering your equipment then your wiring is too thin.
I'm sure a caravan shop will have something simple suited to this job.

In reply to tnj:
i have 88 Ah leisure in use on my boat

i use the same draw as you propose approximately for echosounder, nav and anchor lights, VHF radio, bilge pump, sat nav

fully charged im at 13.1 V and after a week of use every day it was at 12.4 and on the bring of a charge , i have to take it off the boat for that .

this info is boat related but its juts another vehicle using a similar battery type so is relevant perhaps

http://www.yachtsurvey.com/boat_battery_basics.htm

http://www.vonwentzel.net/Battery/00.Glossary/

http://www.marxrv.com/12volt/12volt.htm