Need help getting correct battery AH for off-grid

[chuck3]

Hello,

I will be making a few off-grid one-day trips and wanted to make sure I am buying the correct battery for my needs. The battery will be a deep cycle AGM. The Amp Hours (AH) I'll need per day will be 20.57 (see chart below for how I arrived at this number).

Does this look correct to you? If so, is the rule of thumb to multiply the needed AH by 2 so as not to damage the battery? That would mean a 50 AH battery would work...assuming my calculations and other assumptions are correct.

The battery would be charged with house current, and the pure sine inverter (600w) would be less than 3ft from the battery, connected with 6-gage wire.

So, will a 50 AH deep cycle AGM battery meet my needs? Thanks!

EDIT: I can't space out the chart. I measure the Watts with a Kill-A-Watt, calculated the Amps (assuming 120V), added 20% to the Amps for each item to account for inefficiency which gives Adjusted Amps, and multiplied the Adjusted Amps by Hours of use per day to give Amp Hours (AH).

Item Watts Amps Waste Factor Adjusted Amps Hours AH
--------------------------------------------------------------------------------
Water Pump 9 0.08 1.2 0.09 22 1.98
6” Fan 15 0.13 1.2 0.15 16 2.4
Laptop 1 80 0.67 1.2 0.8 9 7.2
Laptop 2 80 0.67 1.2 0.8 4 3.2
Hotspot 10 0.08 1.2 0.1 16 1.6
Cell Phone 5 0.04 1.2 0.05 3 0.15
Tablet 11 0.09 1.2 0.11 4 0.44
Lights 30 0.25 1.2 0.3 12 3.6
-------------------------------------------------------------------------------
TOTAL AH: 20.57

[jrnymn7]

So, times Amp Hours by 12v to get Watt Hours... then divide Watt Hours by 12v to get Amp Hours? Seems like going in circles to me.

[scrubjaysnest]

This is a starting point based on the 100AH battery.

100AH * 12 volt * 0.85 inverter eff *1/1 days storage * 0.50 maximum discharge = 510 WH.
Or your inverter at full output for less then 1 hour. You will most likely draw less but this is worst case Without your other loads.
1 LED @ 0.1 amp for an hour adds 5.1 WH.
Loads for less then one hour; an example 2 amps for 15 minutes: 2 amps * 12 volts * 15 minutes/ 60 minutes per hour = 6 WH

You can estimate all your loads that way. Total them all up and work the first equation backwards to get battery bank size; Or a quick and dirty estimate: total WH/12 volts

[time2roll]

Group 27 Lifeline and 100 watts solar would be sweet.

[pianotuna]

Hi,

Your battery bank should be the maximum size possible if you plan on boondocking. I'd suggest adding a solar system, too.

[tpi]

Glad to hear the good about the Lifelines. They were OEM in my motorhome, and I opted for 300 AH option. Can't say I'm poster child of deep discharges (frequently 30-40 AH daily due to efficient loads and no cooking loads). Have high hopes for longevity. I have manual two stage solar charger 14.4 volt and 13.2 volt. I charge them at 14.4 until they're drawing about an amp, then float them at 13.2 This seems to be in accordance with Lifeline documentation, perhaps the float could be a tenth volt higher. Lifeline says to equalize as necessary, which I haven't needed for my usage. If Mex thinks I should further tweak this care regimen, I'm all ears.

[MEXICOWANDERER]

Cycle lifespan and positive plate thickness go hand-in-hand. Same for float lifespan. This is why I chose Lifeline by Concorde. .100" positive plates are as thick as flooded golf car battery plates. Beware of AGM deep-cycle batteries with .045" thick plates - such are a walking talking oxymoron.

[chuck3]

jrnymn7 wrote:

Here's but one example of Depth of Discharge vs. Life Expectancy:

DOD vs Cycles Chart

Thank you for the chart. That looks "significant" to me 🙂

I will go with 125AH since the price is not much more than 100AH. Beyond 125AH, price seems to go up quickly.

As for why I chose a 600w inverter: I may need more power in the future, and, just like with batteries and generators, I like for my load to be a fair amount below the max rating of the equipment to help extend the life of the equipment.

[jrnymn7]

chuck3 wrote:
Yes, the battery will be charged using a 120v to 12v converter charger. The trips will be for one day each, and if for some reason I need 2 days, I can make a 10-mile overnight trip to recharge the battery.

"But keep in mind, the deeper the cycles, the lower the battery's life expectancy." - Given my use, would a 75AH battery as opposed to 50AH make a significant difference in the battery's expected life?

Here's but one example of Depth of Discharge vs. Life Expectancy:

DOD vs Cycles Chart

[tpi]

You'll not be pulling anywhere near the max amps from inverter with this load. But do think that maximum inverter load should be calculated into battery capacity just in case you need to plug something in which does take close to 600 watts. I don't know if you're size restricted, but if not why not just go with something like 100-120 amp hours? Sometimes more commonly available stuff has better bang for buck.

[chuck3]

Thanks all! No fridge or CO detector will be used, nor will anything be used that needs propane or gas.

That was a good catch on the 600w inverter drawing amps from the battery. I had not thought of that 🙂

The only items connected to the battery (via inverter, and not all at the same time) will be:

2 laptops
Cell phone charger
Tablet computer
6" Fan
Hotspot
LED lights
Water pump (9 watt, 120v)

Now I'm leaning toward a 100AH AGM deep cycle.

[RJsfishin]

Why don't you just get a 100-120 AH group 27 battery typical RV DC battery, AGM or otherwise.

Or at minimum, a group 24 75-85 AH.
And any reason for a 600 watt inv ?

A 150 will most likely do all you want it to do. Otherwise get a 1500 watt that willrun the micro,......off 1 battery ?? I don't think so !

[tpi]

If in RV check draw of fridge and CO detector. Depending can be 10-20+ AH per day. Planning on using furnace?

Also the 600 watt inverter can draw 70 ish amps from battery. Doesn't look like you're planning on that from your load list.

Personally I'd plan for worst case scenarios. There's many common battery sizes in the 100 AH range.

[chuck3]

Yes, the battery will be charged using a 120v to 12v converter charger. The trips will be for one day each, and if for some reason I need 2 days, I can make a 10-mile overnight trip to recharge the battery.

"But keep in mind, the deeper the cycles, the lower the battery's life expectancy." - Given my use, would a 75AH battery as opposed to 50AH make a significant difference in the battery's expected life?

[jrnymn7]

"The battery would be charged with house current" ???

May I assume you mean the battery will charged using a 120v to 12v converter charger?

Yes, it is best to not take the battery below 50% of its 20 hour capacity rating. But keep in mind, the deeper the cycles, the lower the battery's life expectancy. Plus, you're limiting yourself to one days use.