How long should a battery last?

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Originally Posted by crl

I have a brand new 55AH deep cycle battery which I used last night to cool the fridge (40L) in my Disco. It ran all night at 8-10 Deg C and it was left on into the morning. I checked it mid morning and the temp had gone right up to 19 deg C. The night air temp was about 27 Deg C and the fridge would have run for about 19 hours from switching it on till I discovered it nice and warm in the morning. There were also two 15 min drives in there as well which should have boosted the battery a bit. The battery is only new and the altenator has been tested and works OK.

Is this about normal for 55AH batt or is it not up to par?

Thanks in advance,

crl



2000 Discovery II Auto TD5

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Definately something wrong, I have a 60AH second battery that I run the fridge off (40ltr Waeco)in the Disco, along with a camp light and even charging camper, camera and laptop batteries, I have done this for 2 full days (48hrs)and never had a problem, the camper battery was charged through an invertor(it was overcast and usually have a solar panel doing the camper battery)

My dual battery monitor generally looks after things (Traxide), I normally give the car a run for at least 30mins each day, which means I can usually stay anywhere indefinately.

Baz.

My 40 litre Engel draws 2.5 amps when connected to a 12 volt source. The duty cycle, ie how long the compressor is running compared to how long it is resting depends on thermostat setting, outside temp, temp of contents and ventilation of the fridges condensor.

At full tilt on paper this equates to 22 hours of continuous use. ie. 55 AH divided by 2.5 Amps = 22 hours. In practice you don't totally discharge your battery and normal car batterys are not designed for this.

Generally speaking car batterys are designed to give a high current for a short period (starting) wheras what you need is a battery to give you a small amounts of current, comparatively, over a longer period. It also needs to be able to be discharged and then re-charged without damage. Car batterys don't fit this bill. Doing this to a car battery quickly kills it. What you need is a deep cycle battery as this is exactly the scenario they are designed for. If the wallet (and SWMBO) can stand it go for on Optima battery, the ones that look like a six pack as they are extremly robust and fully re-charge in a very short time.

Remember that some dual battery controllers let you draw a certain amount from the main battery, say 50% of capacity, to supplement your auxiluary battery before switching off. This can deceive you into thinking your auxiluary lasts longer than you think.

So the short answer to your question is - 55AH is too small without being suplemented by another source ie. solar panel.

Deano

A couple of points.

1. Current draw is Amps not amp/hr - 1 amp is 1 coulomb/sec. Battery capacity is Ampere hours, i.e current x hours

2. Discharging deep cycle lead acid batteries more than 50% of their rated capacity will substantially shorten their life.

John

Folks, a lot of ppl miss some of the basic principles of lead acid and variants and is always a hot topic. Think of it this way, a glass of milk with cream on top and the battery should always discharge / charge in the cream area and not dip into the lower voltages of milk with cycling otherwise you just reduce the cycle life and whether deep cycle or not, the lower you go the less overall cycles and a battery can fail within say 30 cycles of abuse or hard use.

You can do some basic checks to understand and determine life before you start as you need to know what voltage the car is actually set at to provide the charge as car manufactures are now different to the old guess of say 14.2v I think Prado's are in the high 13.V spec now.

Deep cycles have both float voltage and max voltates and sometimes they are higher than the car can produce so you never really achieve full charge into the cream area. At the other end, as in my van set up, I have 5 deep cycles all with 13.5 float and 14.1 max. I took an educated guess and set the low cycle cut out at 12.25 and I am at the 12 year mark and still going with close to the discharge times (maybe not) with regular use.

I just wanted to make this point given it is just one of the variables of many to so increase the knowledge of matching the load specs to battery type that is often overlooked. A couple of years ago a mate of mine bought an expensive deep cycle for fitout in a hilux but the stated charge rate of the bat (max) at 15.1v to which was never going to happen in most vehicles.

And all the advice in this forum thread is all good information for anyone prior to considering when opening the wallet

Cheers