Deep Discharge 12-Volt Battery Enough?

[JohnF]

I don't expect to use camp sites with commercial power so I thought to install only a fair sized deep discharge 12-volt battery to power a few lights and perhaps a small vent fan. Outings for only 4 or 5 days at most.

Any experience that indicates something else would be better? (Cubby in planning stages)

[SteveH]

John,

First, welcome to the forum.

I have the same setup as you are discribing, but I use a marine deep cycle/starting battery. It will not go for more than a couple of nights. Maybe a good deepcycle battery would.

[goldcoop]

John F-

Welcome.

I have used my Optima D31A Yellow Top Deep Cycle dry cell battery for 5 days & nights with no prob. Pricey at around $170.00 but well worth it for those on extended outings.

Be conservative, let your companions (particularly kids if you have any) know that the power is FINITE.

Also get yourself a digital volt gage (that plugs into a 12V cigarette outlet).

The one I got at Wal-Mart was around $13 and has green, yellow and red warning lights.

Monitor it a couple times a day & don't sweat it untill it goes RED, even then you will find reserve for a while.

Another consideration is around 30 W (min) worth of solar panels/controller to charge your batteries, Northern Tool is a good source...

Cheers,

Coop

[Q]

I use a group 27 RV battery in my tear along with a 43 watt solar module mounted to the galley lid. Longest backcountry campout so far without plugging in to power was 3 months. I run a couple lights, fan, forced air heater fan, laptop, TV, camera battery charger, etc..

Without any charging source at all, you'll just have to try it and see if it has enough capacity for your purposes. As someone else said, pick up a volt meter so you can monitor the batter voltage.

Q

I don't expect to use camp sites with commercial power so I thought to install only a fair sized deep discharge 12-volt battery to power a few lights and perhaps a small vent fan. Outings for only 4 or 5 days at most.

Any experience that indicates something else would be better? (Cubby in planning stages)

[jimqpublic]

Go with a true "deep cycle" and not a combination starting/marine type battery. You will get a longer service life.

All of these batteries are lead-acid, no matter whether they're the standard wet cell, AGM, or Gel Cell. The usable capacity is pretty much directly proportional to their weight. It's generally suggested to only plan to use 1/2 to 2/3 of the capacity before recharging for best battery life. Using that factor- You basically get about one amp*hour per pound of battery.

In my trailer I have two 6v golf cart batteries that weigh 65 pounds each (130 combined). Their rating is 220 amp*hours so using my formula of planning on a service goal of 130 amp hours that means 60% of the full capacity. My main reason for the two batteries is running our furnace blower when winter camping- we've gone four nights in snowy conditions and still been above 50% state of charge. (The capacity numbers I'm using are the industry standard capacity for a 20 hour discharge to 10.5 volts)

Cold weather reduces the output but also reduces damage from deep discharges so my 1 amp*hour / pound still works.

Back to the question- For a simple teardrop I would go with a single group 27 deep cycle battery. This weighs about 55 pounds so it will give you about 55 amp*hours of usable capacity using my formula. If weight is a big concern then go with the slightly smaller group 24.

For lighting either flourescent or LED's are much more efficient than incandescents. Figure about 1 amp draw for a bright incandescent bulb or 1/2 amp for a typical small RV flourescent.

A Fantastic Fan draws about 1 amp on low, 2 on medium, and 3 on high.

If you don't have a forced-air furnace or other big loads- Let's do some math:

Lighting- 1 amp x 4 hours/day (some outside, some inside, can't be two places at once!)=4 ah/day
Fan- let's say it's hot and you run it on low for 8 hours/day= 8 ah/day

So here we are at 12 ah/day. That group 27 battery will give you five nights and still have almost 40% reserve.

In the winter you'll run the lights more but the fan less.

For an electrical system- I'd personally go with a simple fuse block and plan to use a regular battery charger when you get home. Installing an RV power converter costs more money, takes up space, and generally doesn't charge as well as a separate charger. If you'll be out longer and will have electricity available on the trip just bring the charger along.

[GeorgeTelford]

Hi

If fully charged is 12.8 Volts, then when the battery voltage is down to 12.3 the battery is 50% discharged. The problem with this kind of measuring is that the battery need to be measured after resting, if you are only using low wattage items (single light etc) 12.2 will in use is a good enough rule of thumb

10.5 Volts is way too low. No battery manufacturer agrees to this level of discharge. I have seen this quoted on Pro battery sites too, but it is easily disproved.

A 110 Ah battery weighs 33 KG or 73 lbs useable capacity is 55 ah weight per Amp Hour useable is 1.33

[jimqpublic]

"No battery manufacturer agrees to this level of discharge"- Actually they all agree that it is the baseline for completely discharged. They also all state that shallower discharges dramatically increase the life of the battery and that 80% is about as deep as you should ever discharge.

I'll stand by my one pound per usable AH rule at around 2/3 discharge.

Here's a chart from US Battery: http://www.usbattery.com/pdfFiles/Avg%2 ... Cycles.pdf

Prompt recharging and proper care is probably more the key to battery life in an RV.

Jim

[GeorgeTelford]

Hi Jim


Quote

No battery manufacturer agrees to this level of discharge"- Actually they all agree that it is the baseline for completely discharged.

Endquote

Completly discharged, No battery manufacturer is going to agree that that is a good place to discharge to, I repeat no battery manufacturer will agree to that level of discharge for their product (ie none will be happy for you to use that amount from the battery)

They quote the full Ah figure of say 105 Ah for a group 27 Battery, but in reality you wont get many cycles at all if you ever use the whole 105 Ah


Looking at your quoted chart 50% is the sweet spot.

I agree prompt recharging and care are crucial to battery longevity