There are many opinions on this topic and many questions out there. I will talk about how I like to start planning an installation. It will be up to you to read and decide for yourself how this information may help you. This post is about batteries and charging.
First, decide what kind of off-grid energy use you want to get from your RV. If your time away from the grid will be short, consider increasing battery capacity to an amount that will not let you down before you are ready to come home.
If you know you will be out 'boondocking' for any extended period of time, consider adding solar panels and/or using a generator to keep your batteries charged. Several essential things to ensure long battery life include:
- ensure the batteries are always topped up with water (distilled is best)
- try to keep the batteries state of charge above 50% at all times (very occasional discharge below 50% to an absolute maximum discharge of 80% will not cause severe damage)
- ensure the battery is fully charged to 100% at least every 5-7 days
- use a charge controller (or battery charger) that is capable of charging the battery to 100% - this normally means the charge set point is able to be set to the highest charging voltage setting defined by the battery manufacturer and the charge controller (battery charger) is capable of delivering enough current to achieve full charge in the time available
- ensure the cables from the energy source to the battery are sized large enough that voltage drop is less than 2%
A good quality battery monitor that can measure the amount of current flowing in and out of your battery will allow you to understand the impact of the various loads in your RV and help you keep track of the status of your battery. The battery monitor will allow you to see how your existing battery charger is working and help you decide whether it needs to be upgraded.
Many RVs and trailers use 6-volt deep-cycle batteries connected in series. Most ‘golf cart’ batteries, as the most common deep-cycle 6-volt battery is known, have a typical 20 hour rating total capacity of approximately 230 amp hours which provides approximately 115 usable amp hours.
If the only electrical load in the RV is the propane refrigerator with an average draw of 2.5 amperes and the battery is fully charged, the battery will last for approximately 46 hours (115 amp hours available @ 2.5 amp hours load).
When you add other loads, the time taken for the battery to discharge to 50% capacity will be reduced.
By adding two more 6-volt batteries in parallel with the existing batteries giving a new total battery capacity of 460 amp hours, the run time for this example would be doubled to 92 hours.
- Note that the 50% discharge recommendation is a working number, it is possible to discharge your deep-cycle batteries as much as 80% of capacity without significant immediate damage, but the cumulative effect of repeated very deep discharge cycles will result in a shorter life span of the battery. If you know you will need to recharge your batteries while you are camping – you may be staying in an off-grid location for more than a weekend and generator use is not permitted or not desired – then it will be worth it to invest in a quality solar panel system.
Perform an “energy audit”. This is easily done after the installation of a Bogart TriMetric battery monitor, but if you have an appropriate ammeter, you can get results that are just as accurate.
- Make a list of the loads that will be used in your RV (refrigerator, water pump, lights, furnace, inverter, etc.) and measure the amount of current drawn from the battery when the load is active. Many loads are intermittent, so assign a weighting factor to those.
- The refrigerator draws 1.5 amps and runs 24 hours a day. 1.5 amps times 24 hours gives 36 amp hours per day.
- A water pump draws 7 amps when running, but it only runs for 30 minutes per day on average. 7 amps is multiplied by 0.5h to give 3.5 amp hours per day.
- Select batteries that will provide the amount of energy required for the amount of time you wish to be ‘off-grid’. Remember that deep-cycle batteries should not be discharged below 50% of capacity.
- Select the appropriate accessories for your installation. You will need a catastrophic battery fuse and fuse holder, a battery box, battery cables that are sized correctly to connect your batteries together, and a battery monitor with current shunt if not already installed.
- Evaluate the battery charger in your RV or trailer. Earlier battery chargers (known as converters) were not designed to maximize battery charging or battery life. An ideal charger will be capable of charging your battery to 100% capacity in a reasonable amount of time and will not damage your battery if left charging after the battery reaches 100%.
Recommended Charger Operation
For example, the U.S. Battery US2200 that I currently use is a flooded lead/acid 6-volt golf cart style battery 20 hour rated at 232 amp hours.
The manufacturer recommends charging at ~10% of the C/20 rating (or ~23 amps) to 2.40 +/- 0.05 volts per cell (7.20 +/- 0.15 volts per 6 volt battery, 14.4 +/- 0.3 volts per 12 volt series combination) so the charger bulk setting can be 14.1 to 14.7 volts.
I normally choose the higher value of 14.7 volts (see footnote 1).
- The charger should start charging a discharged battery in ‘bulk mode’ at approximately 25 amps.
- When the charger detects 14.7 volts has been reached, it should change to ‘absorption charge’ mode. The battery is now approximately 80% charged (see footnote 2).
- Absorption mode is a constant voltage (14.7 volts for this example) and the charger should now monitor the output current. As the battery gets more and more 'full', the output current drops. The US2200 recommendation is to stay in absorption mode until the current drops to 3% of the C/20 rating, so ~7.0 amps.
- When the battery gets to this stage of charge, approximately 95% full, the charger should change to ‘finish charge’ mode. Now the current is held at a constant of 3% of C/20 and the voltage will rise to 2.55 +/- 0.05 volts per cell (15.30 +/- 0.30 volts for two US2200 6-volt batteries connected in series) for approximately 2 hours.
- Some chargers will now switch to ‘float mode’, which is a long term charge value that will not harm the battery and keep it fully charged. For the US2200, the float charge is specified to be 2.17 volts per cell (13.02 volts for 12 volt battery). This voltage can be applied indefinitely.
- One final mode that many chargers have (and is highly recommended) is ‘equalization mode’ in which the battery voltage is raised to a constant 2.55 +/- 0.05 volts per cell for a period of 1-3 hours every 30 charge/discharge cycles. Equalization mode prevents the battery electrolyte from stratifying. The high voltage causes bubbling inside the battery which stirs the electrolyte and ensures that heavier battery acid does not sink and remain at the bottom of the battery. Equalization charging is most important for lead-acid batteries in fixed locations, such as long term boondocking, or in a cottage or cabin. Nevertheless, equalization is recommended for mobile installations as well.
A ‘three stage charger’ is considered to be one which has a bulk, absorption, and finish mode of charging. Float and equalization charging are not considered to be stages in a charging profile.
(1) Note that a setting of 14.4 volts – at the battery – for a battery being charged from the grid is OK unless there is a limitation on the time available to charge. For a solar charge controller, you would normally set the bulk charge voltage as high as possible within the manufacturer recommendations due to the limited amount of time available to charge each day.
(2) Some chargers use a more sophisticated method of monitoring amount of charge watching for a specific change in voltage (delta V). For simplicity, my discussion will focus on charge voltage setpoint.