Calculating useful watt hours from 6 GC-2's

[Frank B]

We have our trailer up for sale, and a retired couple where the husband needs an oxygen concentrator is interested. The unit they want to run uses 3.4 amps at 120v AC, and he needs to run it 10 hours at a time. I make that 3.4 x 120 x 10, or about 4,000 watt hours

Our trailer has 6 GC-2 batteries in a series parallel setup where the output voltage to our 2500 watt pure sine inverter is 12 volts.

I looked up the calculation on Google, and got this:

The total watt-hours in six GC-2 batteries depend on the specific battery model's capacity, as GC-2 is a standard group size, not a specific capacity rating. However, most common GC-2 batteries have a capacity of approximately 210 to 232 Amp-hours (Ah) at 6 volts (V). Assuming you are using a typical 225 Ah, 6V GC-2 battery, the total energy for all six batteries is approximately 8,100 Wh (8.1 kWh).
Calculation To calculate the total watt-hours (Wh), you use the formula:\(\text{Watt-hours}=\text{Amp-hours}\times \text{Volts}\) Calculate the Watt-hours per battery:Using the common 225 Ah capacity and nominal 6V:\(225\,\text{Ah}\times 6\,\text{V}=1,350\,\text{Wh}\) per battery.Calculate the total Watt-hours for six batteries:\(1,350\,\text{Wh/battery}\times 6\,\text{batteries}=8,100\,\text{Wh}\) (or 8.1 kWh). Key Considerations Usable Capacity: For traditional lead-acid GC-2 batteries, it is recommended to only use 50% of the total capacity to avoid premature damage and extend the battery's lifespan. The usable capacity in the above example would be approximately 4,050 Wh (4.05 kWh).

If my calculations are correct, then the 6 GC-2's (when new) would just barely give the capacity needed. Is this correct?

The batteries in the trailer at the moment are nearing end of life, so I am going to suggest that if they buy the trailer, that they have the lead-acid batteries replaced with lithium.

And yes, I know, that on cloudy days they may not get a full charge before the sun goes down. They will be staying in some powered campgrounds when traveling, and they do have a 3Kw gas portable generator as well.

All I need to know is if my calculations are correct.

Thanks!

 

[Gary RV_Wizard]

4000 watt-hours at 12v = 333 amp-hours. When new, your GC2 batteries have a total capacity somewhere between 210 & 230 AH for each 12v pair, so about 630-690 AH altogether. Despite popular internet wisdom about 50%, he could reliably use 70-80% of the stored amp-hours (see a battery manufacturers capacity graphs).

So, when new that's plenty to run the CPAP for one 10 hour shift. But if nearing end of life (how do you know?), maybe just barely enough.

A new set of batteries wold be a wise choice. Whether another set of GC2 or some nice LiFePO4 with 500-600AH capacity.

 

[Frank B]

Thanks, Gary. As long as my calculations are not way out in left field. I don't want to make false claims.

Batteries are 7 years old. I have cared for them well, and they worked fine last summer with no noticeable degradation. But 7 years is a long time for lead-acid. Or so I am led to believe.

>So, when new that's plenty to run the CPAP for one 10 hour shift.<

This is actually an oxygen concentrator, not a CPAP. The interested people are confirming with the manufacturer of the device as to what the AVERAGE draw is. It says 3.4 amps on the label on the machine, but that may be full-load.

Thanks again.

Frank.

 

[DonTom]

Batteries are 7 years old.

It's time to replace them. They can crap out with no advance warning at this point, such as a shorted cell that happened to me in Death Valley several years ago. They were fine one night before they became unusable for most things. No advance warning.

-Don- Douglas, AZ

 

[ferfer]

3.4 amps on the label

The 3.4A should be max. power when concentrator set at the max. liter oxygen flow setting. However, I doubt there is stationary home style oxygen concentrator that will run on 12Vs. When only have 12V available are the batteries and 2500 watt pure sine inverter capable to supply 120V required.

 

[John From Detroit]

Google made the common mistake of using the TOTAL capacity of the batteries.
With Lead acid and an inverter you can only use around 25-50% depending on the inverter.

I suspect you will be good. If the batteries are good.
But if it's a trailer Upgrading to LiFePO4 with heater for low temp use.. You'd be much better.

One way to find out is to plug in lights. Good old fashion incandascent lights in an amount equalling the Concentrator and come back in 10 hours.

 

[Mark_K5LXP]

For 3 pairs of GC2's, a 340 watt load is easily within their rated performance spec (nominally 10A per pair, C/20). So you could get all they have to give provided the inverter works over that input range (some do, some don't). The elephant in the room is their age. They may still work OK for what you need which checks that box but you really have no idea of actual capacity. The suggestion to put a test load on and see how long it goes is a sound one. But even if they pass the load test we're back to their age. 7 year old batteries will not have much longevity with deep and frequent cycles (in this case, daily). For something like medical equipment I wouldn't mess with it. A review of the actual loads and run times is in order, and a storage solution implemented to satisfy that.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

If the buyer needs to have an oxygen concentrator, gambling on older batteries is foolishness. This is about survival rather than comfort. He undoubtedly carries a portable oxygen bottle for back-up as well.

 

[Frank B]

Thanks all. As I mentioned, all I needed was to know if my calulations were correct. My original post noted that I was recommending replacement of the current batteries due to their age.

With that said, I think my prospective buyer is just "kicking tires" at this point anyway.

 

[Adventurous Traveler]

Frank, not many folks selling "used anything, no less an RV" are as honest and straightforward as you. Nice job looking out for strangers and taking pride in your honesty.

 

[Frank B]

What a kind comment! Thanks! I do try.

Wanna buy my trailer?