Why do my Batteries Keep Dying in my RV? A Case Study, Running a CPAP and Electric Refrigerator from Solar & Batteries.

It is important when RV dry camping that you have enough power storage (batteries) and production from solar and/or a fuel generator to power all your needed appliances, medical devices and equipment. You need to do the math before you go to avoid unexpected problems that can result in your CPAP quitting in the middle of the night or your fridge defrosting and your food spoiling.  Know before you go. 

Background:

Customer has a new travel trailer with a factory installed 100 Watt solar panel (installed flat on the trailer roof), PWM 30A charge controller, 1000W inverter and one 12V 100Ah deep cycle silicon dioxide battery.

They tried taking it out for a weekend without plugging in (off-grid/boondocking/dry camping) and the battery ran out of power the first night. They wanted to be able to camp away from power for up to a week and to use their fridge, CPAP and fan every day. They mostly camped in the summer and were willing to locate the trailer to get good sunlight on the rooftop solar panel.

A quick analysis of the system showed why they were having issues.

The customer was a heavier power user and this system was sized for a light power user, or a user who mostly camped in powered campsites.

What the customer was running:

  • Electric Fridge
  • CPAP
  • Carbon Monoxide detector
  • Lights
  • Bypass Pump (for water)
  • Fan
  • Radio

The two biggest power users in this case are the electric fridge and the customer’s CPAP machine.

Daily Power Requirements:

We listed the components using electricity and figured out the estimated power needed daily.

Estimated Daily Power Requirement – Needed:

Essential
CPAP 70W per hour 9.5 hours 665 Wh
Refrigerator 53W per hour 8 hours run time 424 Wh
Carbon monoxide detector 1.3W per hour 24 hours 31.2 Wh
Lights 20W total per hour 4 hours 80 Wh
Bypass Pump 26.4W per hour 20 min 8.8 Wh
Basic Daily Power Consumption 1209 Wh
Optional
Fan 48W per hour  on high 7 hours 336 Wh
Radio 84W per hour 3 hours 252 Wh
High Use Daily Power Consumption 1797 Wh

 

Current Power Production:

Solar panels rarely produce their factory maximum production. The angle of the sun, weather conditions, and temperature all affect a solar panel’s efficiency. When sizing systems we always use a reduced value as an average production estimate. Usually this reduction is 25% of the rated value. This applies directly to flat rooftop solar, where the panels are downgraded 25% from their rating because the panels cannot move to directly face the sun. So in this case, the 100 Watt solar panel is calculated as 75 Watts per hour.

We also downgraded the production value of these panels because the customer was using a PWM charge controller. PWM charge controllers are lower cost, but generally only 70-85% efficient. MPPT charge controllers are 95-99% efficient, which means that more power from your solar panel actually gets stored in your batteries. (For more info on this, see our upcoming blog on PWM vs. MPPT charge controllers.) When you have limited space, you want the most efficient system possible.

What about sun exposure? This client camped primarily in peak season, so would have long summer days of sunlight. In the shoulder seasons, where the days are shorter, they would have less solar power generation and would need to adjust the amount of time spent dry camping.

Then we looked at how much power production they currently have:

  1. 9 hours of summer sun exposure.
    • Downgraded to 60% efficiency for 25% indirect sun and 15% efficiency loss from PWM controller.
      • 100 Watt solar panel should produce 540 Watt Hours per day.

Current Battery Storage:

The current battery is a 12V 100Ah silicon dioxide battery. This battery has a maximum storage capacity of 1200 Watt hours. At 80% depth of discharge, this battery has usable capacity of 960 Watt Hours.

Summary of Current System:

Power Needed Daily: 1209-1797 Wh

Power in Daily: 540 Wh

Battery Storage Capacity: 1200 Wh

Usable Battery Capacity: 960 Wh

 

Example –Current Scenario:

In from Solar Out to Essential Only Net Stored
Start 1200Wh
Day 1 540 1209 531 Wh
Day 2 540 1071 depleted

 

Or

In from Solar Power Out Net Stored
Start 1200Wh
Day 1 540 1740 Completely Depleted in less than 24 hours

 

This means that the battery has less than one day’s worth of essential power storage. This is not enough storage to avoid power loss overnight when dry camping. This setup does not provide enough power to boondock for more than 1 night under ideal conditions. Any cloudy days, shading, etc. would reduce the solar in and reduce available power.

There is more power going out than going in at a rate that is not sustainable.

Explore Solutions:

This person needs 1797 Wh per day plus a margin for error. Hot days will increase refrigerator and fan run time. Inclement days or shaded parking may result in lower solar production. Additional electronic items or careful power use may increase or decrease daily power requirements. All these scenarios are based on averages and estimates, so actual performance will vary.

There is limited space for batteries. The battery size recommendations are based on getting the largest battery possible in the space available. Different sized batteries may work better in your space.

1. Solution: add 100 Watt solar panel

In from Solar
200W x 9 x .60
Power out Net Stored
(12V x 100Ah = Wh)
Start 1200Wh
Day 1 * 1080 Wh 1797 Wh 483 Wh
Day 2 1080 Wh 1563 Wh Depleted

* Note: a regular lead acid battery of the same capacity would be over-discharge with one day’s use. Max discharge for a lead acid battery is 50% Depth of Discharge.

Lower rows represent the point where the battery has 20% battery charge left (80% Depth of Discharge). Although Silicon Dioxide Batteries can recover from 100% depletion, doing this regularly will shorten battery cycle life.

2. Solution: add a 100 Watt solar panel and upgrade charge controller to MPPT, increasing efficiency from 85% to 98%.

(A PWM controller creates a downgraded efficiency rating of 73%. 25% indirect sun and 2% efficiency loss from MPPT.)

In from Solar
200W x 9 x .73
Power out Net Stored
12V x 100Ah = Wh
Start 1200Wh
Day 1 * 1314 1797 717 Wh
Day 2 1314 1797 234 Wh 80% DoD
Day 3 1314 1548 depleted

* max depth of discharge for equivalent lead acid battery. (600Wh)

3. Solution: add 150 Watt solar panel, keep the PWM charge controller.

In from Solar
250W x 9 x .60
Power out Net Stored
12V x 100Ah=Wh
Start 1200Wh
Day 1 * 1350 1797 753 Wh
Day 2 1350 1797 306 Wh 80% DoD
Day 3 1350. 1656 Battery depleted.

* max depth of discharge for equivalent lead acid battery. (600Wh)

4. Solution: upgrade battery to 2 x 6V 260Ah batteries, and add 100 Watt solar panel, keep the PWM charge controller.

In from Solar

200W x 9 x .60

Power out Net Stored
Start 3120Wh
Day 1 1080 1797 2403 Wh
Day 2 1080 1797 1686 Wh
Day 3 * 1080 1797 969 Wh
Day 4 1080 1797 252 Wh
Day 5 1080 1332 Depleted

* max depth of discharge for equivalent lead acid battery (1560Wh)

5. Solution: upgrade battery to 2 x 6V 260Ah batteries, and add 150 Watt solar panel, keep the PWM charge controller.

In from Solar

250W x 9 x .60

Power out Net Stored
Start 3120Wh
Day 1 1350 1797 2673 Wh
Day 2 1350 1797 2226 Wh
Day 3 * 1350 1797 1779 Wh
Day 4 1350 1797 1332 Wh
Day 5 1350 1797 885 Wh
Day 6 1350 1797 438 Wh 80% DoD
Day 7 1350 1788 depleted

* max depth of discharge for equivalent lead acid battery (1560Wh)

This solution will give a full week off-grid and mean that a return to an external charging source would be needed between day 6 and 8. It would take 17 hours to charge these batteries from a 15A AC Charger from 100% discharge.

6. Solution: upgrade battery to 2 x 6V 260Ah batteries, and add 150 Watt solar panel (total solar 250W) and upgrade the charge controller to MPPT. (98% efficiency vs 85% efficiency).

We’ve downgraded the panel input to 73% efficiency (25% indirect sun and 2% efficiency loss from MPPT controller). This is an increase of 293 Wh per day solar power production.

 

In from Solar

250W x 9 x .73

Power out Net Stored
Start 3120 Wh
Day 1 1643 1797 2966 Wh
Day 2 1643 1797 2812 Wh
Day 3 1643 1797 2658 Wh
Day 4 1643 1797 2504 Wh
Day 5 1643 1797 2350 Wh
Day 6 1643 1797 2196 Wh
Day 7 1643 1797 2042 Wh
Day 8 1643 1797 1888 Wh
Day 9 1643 1797 1734 Wh
Day 9 * 1643 1797 1580 Wh
Day 10 1643 1797 1426 Wh
Day 11 1643 1797 1272 Wh
Day 12 1643 1797 1118 Wh
Day 13 1643 1797 964 Wh
Day 14 1643 1797 810 Wh
Day 15 1643 1797 656 Wh  ** 80% DoD
Day 16 1643 1797 502 Wh
Day 17 1643 1797 348 Wh
Day 18 1643 1797 194 Wh
Day 19 1643 1797 40 Wh
Day 20 1643 1683 Depleted

* max depth of discharge for equivalent lead acid battery (1560Wh)

b. Same as #6, but based on 5.5 hours of insolence (usable sunlight) per day.

This would be the sunlight available in early spring or fall. This would reduce potential dry camping days to a weekend.

In from Solar
250W x 5.5 x .73
Power out Net Stored
Start 3120 Wh
Day 1 1004 1797 2327 Wh
Day 2 * 1004 1797 1534 Wh
Day 3 1004 1797 741Wh
Day 4 1004 1745 depleted

* max depth of discharge for equivalent lead acid battery (1560Wh)

7. Solution: Change fridge to propane

Alternatively, this client could swap the refrigerator out from an electric only to a 3-way system, so that propane could be used when off-grid. This would drop their daily power consumption to 1373 Wh/day. Paired with 200 Watts of solar panels and their existing 12V 100Ah battery, this client could dry camp for a week or two in mid-summer, depending on the conditions and if they were careful with their power consumption on rainy days.

In from Solar
200W x 9 x.73
Power out Net Stored
Start 1200 Wh
Day 1 1314 1373 1141 Wh
Day 2 1314 1373 1082 Wh
Day 3 1314 1373 1023 Wh
Day 4 1314 1373 964 Wh
Day 5 1314 1373 905 Wh
Day 6 1314 1373 846 Wh
Day 7 1314 1373 787 Wh
Day 8 1314 1373 728 Wh
Day 9 1314 1373 669 Wh
Day 9 * 1314 1373 610 Wh
Day 10 1314 1373 551 Wh
Day 11 1314 1373 492 Wh
Day 12 1314 1373 433Wh
Day 13 1314 1373 374 Wh
Day 14 1314 1373 315 Wh
Day 15 1314 1373 256 Wh  ** 80% DoD
Day 16 1314 1373 197 Wh
Day 17 1314 1373 138Wh
Day 18 1314 1373 79 Wh
Day 19 1314 1373 20 Wh
Day 20 1314 1334 Depleted

* max depth of discharge for equivalent lead acid battery (600Wh)

Recommendations and Implementation:

Solution 6 is our recommended solution that provides the best safety margin and versatility for dry camping in the summer for up to 2 weeks without AC charging. Dual 6V batteries are the most common battery system for RVs and many battery boxes are built to house GC2 size batteries.

The package upgrade components for solution #6 include:

Clients can extend their dry camping time by recharging the batteries from a fuel generator when the power gets low, or by returning to a powered campsite to plug in to AC for the day. Some people use the alternator on their car to charge their camper batteries, so if they are driving between locations, this will top up the batteries and extend their dry camping time.

To recap:

  • You should have enough solar generation ability to replace the power taken out each day, or over time your batteries will slowly discharge.
  • Your batteries are like your fuel tank. A larger fuel tank can store more gasoline, but if you don’t refill the gas (or power), then eventually you will run out of fuel, no matter how much storage you have.
  • A full battery acts as a buffer between the power generated by solar and when you actually draw the power out. Often we use most of our power at night after the sun goes down, so we need to store the entire days’ worth of solar production to keep our electrical devices like lights, fridge and heater running through the night.
  • Your battery storage should be at least enough to store 2x your daily power needs.
  • Most lead acid batteries can only be discharged to 50%, so if you have a 100Ah battery, you only have 50Ah of usable storage. SiO2 batteries can be discharged 60-80%, which means a 100Ah battery gives you 80Ah of usable storage.
  • What equipment you run and how long it runs for will determine how much battery storage you need and how long you can boondock.
  • If you want to dry camp (boondock, off-grid), then you need to be very aware of your power consumption vs. how much power you are generating each day, especially if you have critical equipment like your fridge or medical devices running on electricity.
  • The shorter days in spring and fall have major implications on your power generation ability.
  • Your solar panels will only produce a percentage of the power they are rated for in actual use.
  • Panels on the roof of your RV or camper may be shaded and not at the optimum angle for solar collection. This will affect the amount of power generated.
  • Portable folding panels can be angled and placed for optimum power generation and as a result often perform better than equivalent classic glass rooftop panels.
  • The solar panel Watts are the maximum power they can produce in an hour of usable daylight.
  • In Southern Canada, usable daylight (insolence) ranges from 2 hours in the winter to 9 hours in summer. The average daylight is 5.5 hours.
  • MPPT charge controllers are very efficient. PWM charge controllers are not as efficient and you can lose up to 30% of your solar panel’s power if the PWM charge controller is not matched well to your panels.
  • You can supplement solar charging with AC charging from a generator, alternator or grid power.
  • Estimates are based on averages and actual energy out and energy in will vary. It is important to monitor battery levels daily to make sure that they are within expectations.
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