Lets talk about amp hours and cheap batteries

[kevschofield]

I am no electrical engineer so I may need some schooling here on amp hours and how that all works..

If I was to use this type of deep cycle battery (12V 55ah) for $100..

www.amazon.com/Powerstar-Group-22NF-Sealed-Battery/dp/B000W6Z2X2

What would that get me in terms of 'power' through an inverter ?

I'll be using LED lighting inside and I've already done a test using 12V 12ah batteries from my kids powerwheels toys, and those batteries will run 24 LED lights for more than 3 days, 24/7 and there's still charge left.

I figure the LED stuff will be negligible as far as electrical draw goes as they would only be on a few hours a night anyways.

What about a small coffee maker? or a USB charger to charge my itouch or cell phone? a 12V DVD player? or laptop charger?

I guess I'm trying to figure out how much 'battery power' a small inverter will need to make cofee once a day, and charge a couple accessories.. I dont need to run lights, an electric griddle, or watch a big screen TV LOL..


What are your thoughts on that $100 battery? deal? or no...
What would be the best inexpensive inverter for the very rare times I'd be camping away from electric hookups?


Mostly asking as I've booked a trackside spot at Lime Rock Park for the historic racing weekend over LaborDay and there are no hookups.. want to be prepared for 4 days and nights.
(I can run my truck for a while each day if need be to charge the battery if thats an option)

Wish I was smarter at this stuff~

[clayton]

The coffee maker should be avoided - no matter how small it is. Creating heat from a battery is a quick way to suck the life out it.

Everything else sounds fine.

You didn't mention a fan. Anything motorized will probably be your limiting factor.

Clayton

[kevschofield]

I suppose we could do instant coffee from water heated on the burners.. so no big problem there.

Forgot to mention the fan.. Will have a Fan-Tastic 4000r installed as well. Here's the electrical draw they state on the website

High = 3.00 amps
Medium = 2.29 amps
Low = 1.86 amps


When a battery has an 'amp hour' rating does that mean it can operate something rated at that emps for 1 hour?

(meaning a 12V 30AH battery would run the fan at 3amps for 10 hours??)

[clayton]

Your math to get to 10 hours is how I understand it. I'm sure that's also under 'ideal' conditions - which means actual results will be something less than 10 hours.

Like you experienced, the LED lights don't use much juice. Also, I think most modern battery operated gadgets are pretty efficient and won't draw much power. Your cell phone and ipad/touch won't use much either because there aren't any moving parts. The 'motor' that spins the DVD, your laptop hard drive, and their fans will be what uses up the battery (albeit efficiently) - - so just be sure to unplug them when you're not using them.

I look forward to hearing other members' input.

Clayton

[kevschofield]

Something like this at $27.99 shipped should work fine for plugging in a charger or portable DVD player etc.. and it has 2 USB charging jacks as well which is great for the ipod / cellphone thing.


Maybe this won't be too hard afterall to do some light boondocking for under $200 to set up..

Need to research battery charging off the cars alternator as well..

[cowcharge]

What's wrong with a percolator on the stove?

Only you can know if that battery's big enough for what you'll need camping. I know batteries are expensive, but cheap batteries force you to adapt your lifestyle to your power supply, rather than letting your lifestyle determine what battery to get. This can get really unsatisfying, especially if you have to tell your wife she must choose between running the microwave or the AC... Ideally the battery is the answer at the end of the equation, determined by variables in your life, and not the constant in the equation which determines your lifestyle...

I've been doing a lot of reading on amp hours during my rebuild. You're right, a 30ah battery will power a 3 amp fan for 10 hours. But you don't ever want to drain your battery that low, never below 20% IIRC, and 50%'s better. Every time you drain it below 20% or so you damage it, and it only has so many "lives". That goes double for "cheap" batteries like car batteries or those "marine/deep cycle" poser batteries. Car batteries have thinner plates with more surface area so they can generate high amps for starting, "giving their all" when you turn the key. Deep cycling a car battery will kill it dead very soon. Real deep cycle batteries have thicker plates and aren't meant put all their power out at once, and they can take discharges much better. "Marine/deep cycle" batteries are in between, but not that much better than car batteries. Of course budget determines all, so we use what we can afford. I have two huge 6v golf cart batteries from an old, old solar installation that were given to me, and they work pretty good, but if they hadn't been given to me, I'd probably have car batteries from Walmart.

Amps times volts equals watts is the main equation you need to know. The tedious, accurate, mathematical way to calculate how big a battery bank you need is to figure out as closely as possible what your actual needs are during the trip. Imaginary 12v coffee maker, 5 amps, used 20 mins/day uses 5/3, or 1.67 ah each day, or 5ah over three days. 12v LED lights, .5 amps, 6 hours/day uses 3ah/day. And so on. The amp or watt draw will be on every appliance, if it's in watts just divide by 12 to get the amps. That's it for 12v appliances. Total them all up for one trip and that will tell you how many ah you'll use. Divide that by .8 to allow for never discharging below 20%, and that's the size of the battery you need, if you plan to run off battery for the whole trip with no charging. If you have a generator then you need to keep your total ah usage below what your generator can deliver in the hours allowed for its use, if you want to maintain the batteries at full charge. You can also charge some lesser amount, enough to keep from going below 20% charge but not enough to keep the battery full. Like an airplane on one engine that can fly but can't maintain altitude, you lose ground the whole time, but hopefully it stays up long enough so you can land at the right place, lol.

So normally, you would determine your electrical needs first, and those would determine the size battery to get. If you start backwards with a fixed battery size, then you have to tailor your lifestyle to match your battery... If you must start with the battery, then 80% of 55ah is 44ah, your available power without charging. You have to dole those 44ah out like rations in a lifeboat in order to get a week out of them. Trade off a shower for a coffee brew session, etc. Your appliances and your lifestyle will tell you if that battery's big enough.

It's the 120v appliances that'll bite ya in the amp hours. When you use an inverter and 120v appliances, it takes 10-12 amps DC from your battery to create each amp of AC out of the inverter. 120v appliances are usually rated in watts, so a 1200 watt microwave draws 10 amps AC, or 100-120 amps DC off your battery. So your 55ah battery would run that microwave for about half an hour and it would be drained flat.

As for using your alternator, you need what's called an isolator switch, that automatically switches the alternator over to charging the house battery after the car battery is charged. They're rated in amps to match with the amp rating of your alternator. I'm uneducated when it comes to how well an alternator will charge a deep cycle battery, whether the voltages are the same, or whether it would cause gassing or overcharging etc.

Hope this helps a little!

[kevschofield]

Actually, thats very helpful, thanks

I've got some calculating to do

[vikx]

Great advice above. Anything with a 110 heating element or motor will eat the battery for lunch. Likewise for 12 volt "heating" appliances. The Fantastic fan is 12 volt and will draw the battery down but not as fast.

The laptop shouldn't draw much because it runs on it's own battery. 110 power would be needed to recharge, but it wouldn't be like running a motor, just recharging.

I would consider a 2000 watt quiet generator for recharging off the grid. Also, the portable "jump start" batteries are very useful.