Everyone… I get questions like this one every week, and it’s a great opportunity to show you the basic math that goes into figuring out how much battery capacity is needed for common electrical devices.
Now, since I’m laid up with COVID and bronchitis this week (even after being double-vaccinated and double-boosted) and a little fuzzy on the math, I’ve asked Diane to double-check my numbers… [No pressure here. ? Just get better, Mike. —Diane]
Dear Mike,
Yesterday I ordered a new Lance 1985 trailer with the 190-watt solar panel on the roof and 1500-watt inverter. I plan to install lithium batteries so my wife and I can use our CPAP devices all night if we are boondocking for a few days. If we are staying in an RV park hooked up to 30-amp service, will the manufacturer’s supplied onboard converter charge the lithium batteries to 14.4 volts or just 13.6 volts? Does it matter, since the solar panel will top them off during the day? Also, can you recommend a preferred lithium battery? Thank you in advance. —Johnny
Dear Johnny,
The only way to know for sure is to look at the spec sheet for the converter. Do you know the name and model? And be aware that you’ll probably have to change a switch on the converter to set it to lithium mode.
Also, a CPAP machine can use up to 500 watt-hrs overnight if you use the humidifier. A 100 amp-hr lithium battery only has 1,200 watt-hrs of storage, so you could drain the battery overnight running two CPAP machines. Do you have any data on your CPAP units?
I’m guessing you’ll want 200 amp-hrs of lithium, which is 2,400 watt-hrs. That 190-watt solar panel can only provide around 700 watt-hrs per day. So that’s around four days to completely recharge the 200 amp-hrs of batteries from 0% to 100%, if everything is off.
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First, thank you for your timely reply. Our CPAP machines’ combined draw is 11 amps. So, for a 7-hour use period, that’s 77 amp-hours or 77 X 12 for 924 watt-hours drain, if my calculations are correct. So, yes, a pair of 200 ah batteries are most likely the best choice for me.
Our converter is a Progressive Dynamics 4060, which charges lithium batteries at 60 amps. UGH. Too much for a number of batteries like the SOK and Enduro 100 ah ones, as well. The Enduro Baja Series 12 V 200 ah accepts a 20-100 amp charge, has over/under charge, plus high/low temperature protection as well. So far, this is on the top of my list, but my research isn’t over. Any suggestions are welcome… Regards, Johnny
Everyone,
Did you see how the math worked? Johnny was able to measure that his two CPAP machines used a combined energy of 77 amp-hrs over a 7-hour period. That suggests a 100 amp-hr lithium battery would be nearly drained overnight, or at least down to 23 amp-hours. So he really needs at least 200 amp-hrs of lithium batteries to do this comfortably and still have enough battery charge to make coffee in the morning.
You should be aware that all modern lithium batteries are rated to be discharged down to 0% SoC (State of Charge). But that’s not the case with FLA (Flooded Lead Acid) or AGM (Absorbed Glass Mat) batteries, which should only be discharged down to 50% for best life.
However, since I often have to combine 12-volt DC and 120-volt AC loads in my calculations, I like to get all values into watt-hours (Wh or Watt-Hrs) of energy.
To do that, simply multiply the voltage times the amps. So a 12-volt battery rated for 100 amp-hrs of storage is able to provide 12 x 100 or 1,200 watt-hrs of energy. Now we can use that number to figure out all sorts of things.
For example, a 1,200 watt-hr battery connected to a 1,000-watt generator would need 1.2 hours to charge. See that? 1,200 / 1,000 = 1.2 hours. And a 600-watt space heater would drain a 1,200 watt-hr battery in 2 hours. That’s because 1,200 watt-hrs / 600 watts = 2 hours.
This same formula can tell you things like how long a 2 kW generator would take to recharge an EV that had a 75 kWh (kilo Watt hour) battery. And the answer is 75 kWh / 2 kW = 37.5 hours.
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Armed with this simple math, you can also determine how much this would cost if you were plugged into a home outlet. So, if you pay 20 cents per kHw of electricity at your home, and you’re running a 1,000-watt space heater, that’s 1 kWh of electricity (or 20 cents) per hour of operation. If it runs continuously for 24 hours, that’s $4.80 per day, or $144 per month (because 30 days x $4.80 per day = $144).
For the EV example, you just multiply the $0.20 per kWh rate times the 75 kWh of storage and see that it equals $15 to fully recharge it at home. Not too bad, when that $15 charge can provide 275 miles of range. Compare that to the current price of gasoline.
Now, don’t think that means you can tow a trailer for that same amount, because you’ll likely lose at least 2/3 of your range while towing. But I am working on a few possible solutions (really, I am…).
Let’s play safe out there….
Send your questions to me at my RVelectricity forum here.
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.
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I went with solar and lithium two years ago and wish that I had done three batteries instead of two. Now I’m told that to add a third battery I have to change out the other two at the same time even though they haven’t been through more than a half dozen deep cycles. Moral of the story: Get more battery than you think you will need to begin with….kind of like tow vehicle!
Lithium batteries are not quickly damaged by discharge below 50%.
But lifespan is affected by deeper discharges over many cycles:
“Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses.”
https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
Somewhat related – If you use a lithium battery, discover you need more, can you add a non matching battery – as in different value, age, manufacturer, etc? Typically with wet batteries we are told to have them all be identical in these regards.
Great question. I don’t know this for sure but as long as the lithium battery has its own BMS (battery management system), you should be okay since each battery will be managing itself without respect for any other batteries in the connection. I hope Mike Sokol can add his 2 cents here…
When I travel I use a small, older CPAP that can run directly on 12 volt. It uses almost no power. It does not have a heater or humidifier. I have a friend with small Self contained CPAP, that has its own battery and he can tent camp with it.
My new home CPAP has all the electronics and humidifier but sucks a lot of energy. I’ve learned to live without the humidifier when camping.
You are correct, it’s the humidifier in a CPAP machine that draws all the extra power.
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