Take good care of your CHARGE CONTROLLER because it may become your BEST FRIEND.

A charge controller is a key companion to the RVer. Ideally, battery charging to a full capacity should be done daily.

Let's find out the best battery charger for your needs.

This may differ from one RVer to the other. However, you still need to integrate a charge controller in your system while charging batteries in order to prevent overcharge.

If you are at a campground or a private home and your recreational vehicle is connected to an electrical hookup, it will automatically proceed to battery charging and have sufficient power to run any appliance, having your battery controller protect your house batteries.

On the other hand, if you are dry camping (with no connection to a service), a charge controller is necessary.

Either an engine alternator, a generator (genset or portable), solar panels or wind power will be required as an energy source to your battery bank.

The table below gives you an idea of the state of battery charging of a 12 volt battery according to its voltage.

Level of charge (%) / Battery voltage at 77 F / 25 C

  • [100%/12.63]
  • [90%/12.50]
  • [80%/12.42]
  • [70%/12.30]
  • [60%/12.20]
  • [50%/12.06]
  • [40%/11.90]
  • [30%/11.75]
  • [20%/11.58]
  • [10%/11.31]

A totally discharged battery has a voltage of 10.5 volts. Normally, a panel-style digital meter displays the status of battery charging.

* Note: these figures seem to vary slightly from one source to another. However, to the best of my knowledge, these figures are accurate.

Also these voltages are rated at a temperature of 77 F (25 C), the usual manufacturer's testing battery charging temperature. It seems that for a drop in temperature of 10 F (5.5 F), the voltage would drop by 0.1 volt.

This means for example that a fully charged battery at 50 F (10 C) would have an approximate voltage of 12.36 (12.63 - (2.7 x 0.1)) = 12.36).

This is why we have the option of installing a temperature sensor on the battery bank for monitoring the battery charger. The manufacturer of the battery charger provides specs as well.

If you want to extend the lifespan of your batteries, it is very important NOT to discharge them more than 50% on a regular basis.

The ideal discharge is 25%, replenished to a full charge daily.

To recharge the house batteries, there are different types of battery chargers, categorized according to the battery charging method used.


1) Constant voltage plus tapered battery charging

This is the most common type of charge controller but IT IS NOT the one you want to use to charge the house batteries of your RV. It is used with devices such as:

  • portable generators with DC output
  • regular automotive alternators
  • many converter/chargers
  • inexpensive chargers rated from 5 to 50 amps found at chain stores

When the charging begins, depending on the manufacturer, the charge controller will produce a voltage between 13.8 and 14.5 volts.

The current at that point can be as high as the maximum amperage of the charging unit (50 amps for a 50 amp charger, 100 amps for a 100 amp alternator, and so on).

This high voltage and current lasts a certain time depending on the level of discharge. It quickly charges the battery.

Once the battery voltage reaches about 13.5 volts, the voltage tapers and stabilizes. Then the current tapers off as well only keeping the voltage necessary through a regulator or charge controller until the battery bank is fully charged.

This charging method is fine to charge an automotive battery (SLI) but not suited for deep cycle batteries since the result will be either undercharging or overcharging.

Over time, it will damage the deep cycle batteries and you will have to replace them prematurely.

2 ) Multistage charging - three or four stages

This is also called a 'smart' battery charger because the battery charging stages are controlled by a regulator with microprocessor (a charge controller).

The devices offering multistage battery charging with a regulator are:

  • some alternators
  • some generators
  • most inverter/chargers
  • some converter/chargers
  • most solar power systems

Whatever the device used, a 'smart' charge controller with the right circuitry is the best way to help you recharge your house batteries safely and properly (to its 100% level without overcharge).

This is a more expensive option but IT IS the one you want to use.

Stage 1: called BULK CHARGE

When the battery is deeply discharged (over 50%), the charger regulator will detect this and begin the charging process aggressively. It will raise the voltage to a higher constant bulk charge voltage set for the battery type (between 14.0 and 14.8).With maximum constant current that the device allows (amps), it will charge the batteries more quickly to a nearly full charge level (70-75%).The current would depend on the maximum charge rate set in the inverter/charger, the size and type of batteries plus its temperature measured by a sensor. The inverter/charger is programmed at installation.Note that an inverter/charger is a lot better than a converter/charger which is a poor device to charge a battery bank.

Stage 2: called ABSORPTION (or acceptance)

At this stage, the charger applies a fixed voltage (normally 14.5 volts), and decreases or tapers the current flow (amps) until it is fully charged.

On high-end units, you may have a temperature sensor that will measure battery temperature and achieve more precision in the charging process, yet never overcharging, varying the voltage accordingly (13.8 to 15.5 volts).

The battery then accepts the charge at its own internal diffusion rate until it is 100 % charged.

Stage 3: called FLOAT (lower maintenance voltage)

Since we already achieved the 100% level, the charger applies in stage 3 a lower constant voltage just to maintain the full charge without causing gassing or overcharging while waiting for the battery to be used.

It varies from one battery type to another and also its temperature as detected by the sensor (you remember): wet cell (13.5 volts) gel (13.4 volts) AGM (13.2 volts) but always check with the battery manufacturer.

Current is held at a low level just to maintain the voltage required. Higher current is available to supply DC loads.

Stage 4: a 4th stage called EQUALIZING (or conditioning)

This stage is to ensure that all the battery cells are charged equally. Its purpose is also to remove sulfates from the battery plates and break up the stratification of water and acid in each cell.

It is normally the application of a higher voltage (one volt higher than the FLOAT stage) at low amperage (5% or lower than the battery Ah capacity) for a short period of time (up to 4 hours).

This stage is only used for 12 volt wet cell deep cycle batteries. RVers charging golf cart wet cell 6 volt batteries with a multistage charge controller are told by some manufacturers to terminate charging after the absorption phase. (verify with the manufacturer.)

AGM and gel cell batteries should not be equalized.

This would damage them. The multistage charger offers you the alternative of not applying the equalizing stage according to your type of batteries.

During equalization, the voltage can rise up to 17 volts and there is a danger of damaging sensitive electronics. For this reason, the batteries should be disconnected from the RV loads.

In order to get a good charge, it is important to have a battery temperature sensor on the battery bank as the voltage varies depending on the temperature. The regulator's microprocessor will process the information.

Again, a 'smart' charger may be more expensive but is the best option.


If you use a multistage charge controller for your house batteries like you would often find in inverter/charger and/or converter/charger units, here is a formula that will help you estimate the time involved to get back to full charge.

(CAP X DOD) / (CC X 80) = CHARGING TIME (in hours)

CAP = the amp-hour capacity of your battery bank

DOD = depth of discharge in percentage (remember the 25% rule)

CC = the maximum charge current of the charger (rating in amp)

For example, using a 600 watt inverter/charger of 40 amps capacity with a 160 Ah battery bank:

(160 X 25) divided by (40 X 80) = 1.25 hours of charging time

Notice that this formula uses the maximum charge current (CC) of the charge controller (i.e its amperage). It is important to understand that the amperage is the rate of the current flow, in this case from the charger to the batteries, thus affecting the charging time.

A 100 amp charger will recharge faster than a 40 amp model. However, the amperage is not charging the battery, it is the voltage which charges the battery.

Knowing that a battery has a voltage of 12.63 volts at full charge, you have to apply a higher voltage (normally around 1 volt more or 13.63 volts) in order to recharge.

You can connect a 100 amp charge controller to a discharged battery but it will not charge unless it has the minimum voltage of 13.63 volts. There are maximum voltages that can be applied, as well, depending on the battery type: wet-cell (14.4), gel cell (14.2-14.3), and AGM (14.2-14.3).

Each manufacturer has instructions to follow and provides specs about their batteries in regard to recharging.


Here are some leading manufacturers of battery charging devices.

  1. Balmar
  2. Dualpro
  3. Iota Engineering
  4. Xantrex Technology

OTHER TOPICS ( 5 total ) directly related to RV BATTERIES

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