One of the most common complaints during the RV storage season is that the batteries quickly die. The typical scenario is when the RV is under wraps only for even a couple of weeks, the owner attempts to start the coach and discovers both the engine and house batteries are dead.
There are two reasons for loss of charge in a lead-acid battery; self-discharge, and residual loads on the batteries. The battery self-discharge is a normal by-product of storage and nothing can be done to prevent it; other than changing the battery from a standard wet-cell lead-acid to an AGM (Absorbed or Absorption Glass Mat) battery. AGM batteries do self discharge, but at a far lower rate.
However, there are issues with AGM batteries as well; namely initial cost and compatible charger. The scope of this discussion will be limited to standard wet cell batteries as these are far more common in the RV environment.
Allowing a wet-cell battery to fully discharge is not only hard on the battery, but it can also increase the likelyhood of it becoming permanently damaged due to freezing. The sulphuric acid electrolyte in a fully charged battery is not generally prone to freezing. However, as the battery discharges, the electrolyte becomes less sulphuric acid and more water. And when fully discharged, the electroltye is mostly water - which will crack a battery if allowed to freeze.
The typical discharge rate for a wet-cell lead-acid battery depends on many factors; battery design and manufacture, storage temperature, and others. However, you can typically expect a battery to be fully discharged in a 3 to 6 month time period; which is also the typical storage season for many RV'ers.
The only real solution to keeping your battery from self-discharging is to place a charge on the battery. This can be difficult to do if the RV is stored off-site as a source of power is requried. Of course, if AC power is available, the batteries can be charged via the on-board charger. In some cases, the only solution is to run the generator periodically (if available) to keep the battery charged. Other solutions might be a solar charger; providing the unit is not stored in a dark building, and you keep the snow off it.
As well, the charger you use may cause more damage than prevent. You want a charger that maintains a "top off" or trickle charge, but not too high of a charge or you risk boiling off electrolyte. Many automotive chargers generate too much current for the health of the battery if left on for a significant period of time. Many RVs have a smart-charger as part of their DC distribution system, and these chargers are generally capable of safe trickle charging. Still, you need to check the electrolyte level (monthly) during storage to ensure the charger does not boil off any of the electrolyte.
The second, and perhaps more insidious reason batteries discharge during storage is residual loads (sometimes called parasitic loads). A residual load can completely discharge a battery within a few days or weeks. If you are finding your batteries quickly discharge, it is likely due to this cause rather than the battery's self-discharge rate.
Almost all RV's have some sort of a battery disconnect mechanism, whether it be a mechanical switch or remote contactor. While this switch is supposed to disconnect the batteries from the RV, we know from actual experience that this not always so. The RV manufacturer might hook up a circuit, such as a radio or other device that bypasses the switch, or a dealer or previous owner of the RV may have connected something directly to the battery.
Let's do a case study. On my RV, I had an aftermarket hydraulic lift system installed. The installer wired the system directly to the house batteries. When I asked about the system draining the battery, they responded that in sleep mode, the drain on the battery is very low, so it will not sufficiently discharge the batteries.
Yea, OK... whatever.
I decided to see just how much drain the lift system puts on the battery when it is in sleep mode, which kind of became the catalyst for this article and video. I discovered the sleep mode the load is indeed very low; only about about 27mA, or 0.027Amps.
So how significant is this load?
Considering I have two 100AH batteries, and notwithstanding any Peukert's Law issues, this load should drain the batteries fully in about a year (200Ah / 0.027AH = 7,407 hours/308 days), so in reality, the battery's self-discharge rate will probably be higher than this load.
While this load is indeed insignificant, I still don't like anything putting a drain on the batteries as these things tend to be additive. A few mA here, and a few mA there, and soon you are talking about a significant load. One future project will be to put a switch on that circuit.
To diagnose and correct any residual loads, you need to invest in a DC Clamp meter. These meters can be expensive (mine range from $40 to $150), but it is the best way to trace these pesky problems down.
Clamp ammeters work by determining how much current is flowing through a wire, so it is a handy way to check each lead of the battery for activity. Simply put the meter on each positive lead, and measure if any current is flowing out of the battery under storage conditions. That way, you will know if the battery is being discharged or not - and at what rate.
Voltage testing batteries. You can determine the state of charge when voltage-testing batteries but only under certain conditions... namely, the battery is disconnected (a no-load or open-circuit condition where all wires have been removed) and it has been resting for 24 hours. While 24 hours seems an excessive amount of time to wait, a reasonably accurate measurement can be made after an hour at rest.
The table below shows the voltage that will be produced by a battery when tested in an open-circuit condition:
You cannot accurately measure the battery's voltage under load conditions, especially in a RV as you may have a charger attached (which produces a higher voltage). As well, the load on the battery can cause it's voltage to drop while it is supplying the load.
Charging voltage. While you cannot measure the battery voltage under load, you can determine if the charger is working by measuring it's voltage. A battery is a voltage-charged device, meaning the charge rate is determined by the voltage applied to the battery. The higher the voltage, the faster the battery will charge.
Modern "smart" chargers (called Three Stage Chargers) found on RV's typically have three charge modes; Bulk, Absorption, and Float. The state of charge of the battery will determine which charge state the charger will be in:
Note: that this table is only for chargers supporting wet-cell batteries. AGM chargers will be slightly different. As well, different brand/model chargers use slightly different charge voltages for their various stages, which is reflected in the table. Check the technical specifications of your charger to determine the voltages of each state in your situation.
In Bulk mode, the voltage is the highest, which recharges the battery at the fastest rate. This is typically only used when the battery is deeply discharged. Application of a Bulk charge to a fully charged battery will result in electrolyte gassing (boiling off water).
In Absorption mode, the charger, in conjunction with the battery, powers the RV. The charge rate is just high enough to replentish the amount of current demanded of the battery in operation. This is the mode the charger will typically be in during the operation of the RV.
In Float mode, the charger maintains a "Trickle" charge, which maintains the battery's charge during storage.
Some chargers are called 4-Stage chargers, with an additional maintenance charge called an "Equalization Stage". This is essentially a periodic timed Bulk charge which supposedly "shakes off" any residual sulfate on the battery plates to prevent stratification. Still, other chargers that call themselves 4-Stage chargers refer to the Float and Trickle modes separately (which are essentially the same thing), so this appears to be more of a marketing ploy to me.
In conclusion, reducing or eliminating the residual loads (if any) on your RV, with the periodic application of a charger during storage (coach charger, external charger, or solar charger) will maximize the life of your expensive deep-cycle batteries.