Battery space in 400?

qhumberd

In my 2019 the 2 6V Harris batteries provide 224 amp hours Total. this is important for planning usage for boondocking since you typically do not want to discharge the battery below about 50% before recharging or you will shorten the life of the battery. That means I have 112 amp hours of usable power in my batteries. Some have posted that you can discharge lower but no one recommends using the full 224 amp hours at one go. You monitor your use with a battery shunt monitor or via the battery voltage (but not the one the camper comes with). See the attached.

kbdigitiqe

@Ksublett both are correct, the “rating” is for 220ah but the problem is AGM batteries are only good practically speaking until they are about 40% - 50% drained so the 220ah thing isn’t really a helpful number. 

Also, for what it’s worth I find the whole battery amp hours rating thing to be really odd and extremely misleading. Imagine if the gas gauge on your car told you the correct amount of gas in the tank but then if you went below half full your car just stopped working. It’s not very helpful number I guess that’s what it comes down to.

NovaTab

kbdigitiqe said:

@Ksublett both are correct, the “rating” is for 220ah but the problem is AGM batteries are only good practically speaking until they are about 40% - 50% drained so the 220ah thing isn’t really a helpful number. 

The AGM can go to 20%

kbdigitiqe

yeah, except that damages the battery and shortens it’s life right?

NovaTab

it doesn't do that, it results in less cycles so it's actually longer life google it

kbdigitiqe

Ok bye internet stranger

SweetlyHome

Am I correct?  The batteries are rated for 220aH.  This provides about 110 to 140aH usable aH.

Sharon_is_SAM

@SweetlyHome - AGM batteries are more forgiving than regular lead acid batteries.  Here is a great thread that adds some perspective about discharging AGM batteries.  You can discharge lower with limited loss to the battery life.  

https://tab-rv.vanillacommunity.com/discussion/9940/agm-battery-depth-of-discharge-myth-busted

TomCanada

I suspect the 50% number came about as a safety factor because most people are using their rudimentary See Level battery level indicator (or even those plug-in readers you stick into a 12V outlet) to determine their battery's condition, which as many have pointed out are inaccurate due to voltages varying depending on load.  Using those meters you would want to tell people to stick to 50% (or more accurately, the voltage corresponding to 50% capacity at no load) since the 'real' discharge could vary wildly above/below.  In addition, the AGM battery voltages can vary a bit between manufacturers, so those SOC charge tables you see on the internet should always be taken with a grain of salt.  For these 'typical users', sticking to 50% therefore is a safer place and adds lots of margin since they are usually taking the measurements with some unknown load on the batteries.  There *are* ways to make these plug-in voltage readers provide you a better idea of SOC, but who really wants to shut down their whole RV and let their batteries 'settle' just to get this understanding?  I wager most RV'ers would rather just stick to the 50% number they've been given and be on their merry way and not care about loads or any other mumbo-jumbo technicaleze that we nerds here obviously enjoy just to enable us to safely take our batteries lower

Anyways, anyone with a proper battery monitor like the Victron should have no concerns taking their batteries down to 20% since the Victron measures actual Watts (voltage*current) consumed over time (Watt-hours, ie. Wh) and can compare to the battery's full capacity (12V*224Ah = 2700Wh).  Discharging deeper, you'll reduce cycle life, as the link Sharon provided above explains very well, but the total Wh you'll get from the battery stays the same during the battery's life.

kbdigitiqe

Very interesting indeed! I guess my question would be even if it doesn’t “harm” the battery, most of the charts show the battery providing less than 12v at or below 50% - so are the systems (12v fridge for example) going to keep running properly when the battery has depleted that far? 

TomCanada

Most circuit logic does not run off 12V.  Appliances take the 12V input and then internally regulate it down to a solid/stable 5V, 3.3V, or even lower, depending on the technology used.  The input regulators of these appliances can work down to quite a low voltage, and up to quite a high one - they're very versatile.  Here's the spec from a random 3-way Norcold fridge I google'd:

DC OPERATION [3-WAY MODELS) — 12 volts D.C. (15.4 volts max., 11 volts min.)

And here's a device (granted, not an RV appliance specifically - this is a 12V mini-USB charger) with an even wider range (input range 10.8V - 33V, output 4.75-5.25V):

SweetlyHome

@Sharon_is_SAM,  thanks for that link!  I missed that thread somehow.  I try to catch all those involving batteries.

Sharon_is_SAM

@TomCanada - I think you mean discharge down to 20% SOC.  That may be fine if you have a long drive planned or optimal solar conditions to recharge.  I think you are correct that the 50% discharge gives casual users a generous margin of error.  

TomCanada

Sharon_is_SAM said:

@TomCanada - I think you mean discharge down to 20% SOC.  That may be fine if you have a long drive planned or optimal solar conditions to recharge.  I think you are correct that the 50% discharge gives casual users a generous margin of error.  

Good catch - edited/corrected.