Replace Your Old Battery Charger With A Smart Charger

The next time you pull your Casita out of storage to pack it for a camping trip you may find a dead battery, if it has been sitting for weeks or months without a maintenance charge - or someone played a dirty trick by flipping on the exterior light switch. (Another good reason to install a battery cutoff switch.)

A 12V battery will naturally discharge itself even when fully disconnected (isolated) sitting in storage. The natural discharge rate is about 1% per day at room temperature, 0.25% per day at 10 °C (50 °F) and 1.5% per day at 30 °C (86 °F). This self discharge percentage is the percentage of the remaining capacity, so a flooded lead acid battery will discharge itself to 50% of its capacity after about 6 months.

State of Charge	Flooded battery voltage	Gel battery voltage	AGM battery voltage
100%	12.70+	12.85+	12.90+
75%	12.40	12.65	12.60
50%	12.20	12.35	12.30
25%	12.00	12.00	12.00
0%	11.80-	11.80-	11.80-

State of charge does vary slightly between a sealed lead acid, flooded, gel and AGM deep cycle battery types and also between brands. Even the temperature can play a role. Generally, with no load on a battery, the voltage chart at right gives the state of charge for 12V batteries:

Further, unless your camper has a battery cutoff switch to isolate the battery from the camper’s 12V power center and electrical system, your camper’s 12V components will likely draw a few tenths of an amp or more per day even with all devices switched off.

If your camper has a CO2 and gas sensor alarm, it constantly draws about 0.5A from the connected battery, as will any “instant-on” 12V devices you might have in your camper. I once forgot to switch my battery cutoff switch in our then newish 2017 Casita when we parked it at the RV storage lot and the battery was fully discharged flat when I checked it about six to seven weeks later.

1 amp hour is the amount of power it takes to run a 1 amp electrical draw for 1 hour. For example: An LED lightbulb may use 0.5A. Using 1 LED lightbulb for 6 hours will use 3 Amp Hours (1 x 0.5 x 6 = 3). 6 LED lightbulbs for 1 hour will also use 3 Amp Hours (6 x 1 x .5 = 3).

Battery Amp Hour Ratings are usually measured using the ’20-hour rate’. This is calculated by evenly discharging a battery from fully charged to empty over 20 hours. The total amount of power provided during that time is the 20 hour rate.

Why the 20-hour distinction? Because batteries supply more AH when power is used slowly and less AH when power is used quickly. The 20 hours rate is a good measure for one day’s worth of use, and it gives us good comparative data between batteries.

How Lead Acid Batteries Work

The vast majority of RVs use some type of lead-acid battery. This includes both flooded cell, AGM (Absorbed Glass Mat), and GEL batteries. Even your engine starter batteries are a type of sealed lead-acid battery.

In conventional flooded lead-acid cell batteries the diluted acid is in liquid form. This is why they’re called “flooded” or “wet” cell batteries. AGM batteries have the same lead-acid chemistry, but the acid electrolyte solution is immobilized, either by soaking a fiberglass mat in the acid (absorbed-glass-mat batteries), or by turning the liquid into a paste-like gel by the addition of silica and other gelling agents (gel-cell batteries).

Lead acid batteries contain 2 lead plates (electrodes) suspended in sulphuric acid (electrolyte). To be fully accurate the negative plate is lead and the positive plate is lead dioxide. As power is pulled from the battery (discharge), the lead and acid in the battery undergo a chemical reaction that produces lead sulphate and water.

As the battery is recharged the lead sulphate and water are turned back into lead, lead dioxide, and acid. Every time you drain your battery and then recharge it it goes through one ‘charge cycle’. Batteries have a limit to the number of charge cycles they can go through before they start to degrade. This is why older batteries don’t hold charge as well as new batteries.

12-Volt RV/Marine Flooded Cell Lead Acid Batteries

Most RVs come new with 1 or 2 12V Rv/Marine Flooded Cell Batteries. While manufacturers label these ‘deep cycle’ batteries, they’re really a hybrid starter/deep cycle battery.

True ‘deep cycle’ RV/Marine batteries can be more deeply discharged and then recharged than a hybrid starter/deep cycle battery. Deep cycle batteries also last longer, and usually have higher capacity too.

For people plugged into full hookups most of the time, inexpensive 12-Volt RV/Marine flooded batteries are a good choice. They usually cost less than $100 and can be bought at Walmart and most auto-parts stores.

AGM / GEL Batteries

In regular lead-acid battery cells, the acid is in liquid form. AGM and other VRLA (valve regulate lead acid) batteries keep the acid electrolyte solution immobilized either by soaking a fiberglass mat in it (Absorbed Glass-Mat batteries), or by turning the liquid into a paste-like gel by the addition of silica and other gelling agents (gel batteries).

AGM batteries hold a charge much better than traditional flooded cell batteries, the acid won’t evaporate away. I recommend AGM batteries if you store your RV for months at a time. AGM batteries “naturally” discharge more slowly during storage.

That means you can leave the batteries in your RV between trips without having to worry about checking the acid level and keeping them charged. I’d still remove them from the campet and trickle charge them over the winter months when camper sits at the storage lot.

The only negative is that AGM batteries cost up to double compared to traditional flooded cell batteries. A comparable AGM also weighs more than a flooded cell battery – in both cases assuming similar Amp-Hour capacity.

Lastly, it’s not possible to open, inspect, or fill AGM batteries. This means they won’t last as long as well-cared for flooded cell batteries – which is because AGM batteries still outgas. This is also why they’re called ‘Valve Regulated Lead Acid’ batteries. They have a valve that allows outgassing when pressure builds up in the battery due to overcharging or rapid discharge (electrolysis). As the batteries are sealed any gases that are vented can’t be replaced.

Resuscitating A Fully Discharged Battery

If you find your battery has flat lined sometime over the storage months, it may be possible to bring it back from the dead, particularly if you have an AGM-style battery. The Casita power center battery charger isn’t likely to bring your flat lined battery back to life. In fact, check the battery condition before plugging your trailer into that shore power outlet. If you find it flat lined, remove your battery to the garage to resuscitate it.

I still have the circa 1965 analog battery charger I borrowed from my dad in 1974, and never returned. Those old analog battery chargers won’t bring flat lined batteries back to life, and they are likely to fry AGM-style batteries.

Here’s the problem: Most older battery chargers have built-in safety features which prevent them from recharging deeply discharged batteries. If a 12V battery is at 10.5 volts or less, an analog charger will remain off because the battery is seen a defective. The charger will remain off because charging a “bad” battery could create an unsafe scenario. Modern lead acid batteries, particularly the AGM type, that have not remained in a flatlined condition for a long period of time, may be just fine, if slowly recharged, but an older analog charger just won’t charge them.

Additionally, as with all gelled and sealed battery types, AGM batteries are more sensitive to overcharging. This kind of battery has a different charging requirement based on its specific design. My dad’s old battery charger would grossly overcharge my AGM Casita battery and pretty quickly fry it.

The best way to charge, or resuscitate a dead 12V lead acid battery, is to purchase a modern smart charger. If you have a AGM-style battery, select a smart charger that also includes an AGM charge mode. Smart chargers with a battery recovery/repair mode are able to charge deeply discharged batteries and also run desulfation steps that help recondition flat lined batteries.

When a smart charger automatically senses that a battery is deeply discharged, it runs a low energy charge cycle until the battery charge reaches 25 percent. It then increases the charge rate until the battery is fully charged. After the battery is fully charged, it may be necessary to run repair desulfation steps, if your battery has remained in a discharged state for any length of time. Mind you, fully charging and repairing a flatlined battery can take several days.

Remember, sulfation occurs each time a battery is discharged and is a normal part of battery operation. The process of sulfation is critical to converting chemical energy into electrical energy, without sulfation there is no electrical energy release from the battery. This sulfation reaction is reversed when the battery is normally recharged.

Even when stored fully charged, sulfate will form without a frequently applied maintenance charge. Batteries must be charged frequently enough to maintain their charge level above 12.4 Volts.

There are two types of sulfation: soft sulfation, and hard sulfation. If a battery is serviced early, soft sulfation can be reversed by applying a regulated current at a low value with respect to the battery capacity, for an extended period of time.

During long period of discharge and disuse, the lead sulfate will convert into stable (hard) crystalline deposits on the negative plates. These stable crystals will not dissolve during a normal charge cycle, reducing the battery’s active chemical material, thus decreasing the battery’s capacity and performance.

The only way to recover a hard sulfated battery that has been in a flat line state for a length of time is to redissolve the hard sulfate crystals. This can sometimes be accomplished by applying a regulated low current for an extended period of time. Some modern smart chargers include this mode of highly regulated low currents combined with high voltages. Some include a high frequency pulse charge method of dissolving the sulfate.

These advanced desulfation techniques dissolve the crystallized sulfation and turns it back into active materials again. Unfortunately, long term hard sulfation, from long term flat line discharge, is typically permanent. At this point, recovery of the battery may be minimal, even when utilizing the advanced smart charger desulfation methods.

The other easy way to kill a flooded lead acid battery is by not adding water. Lead Acid batteries have vents so that they won’t explode during outgassing (electrolysis). Outgassing can happen either due to excessive discharge (high-draw 12V battery usage) or overcharging with a bad converter/charger. Lead acid batteries have vents to release excess gases and water vapor from outgassing. This lost water needs to be regularly replaced. The lead plates must be submerged to function. Exposing the plates to air will kill your batteries quickly.

I hope you find this article helpful the next time you think about shopping for RV house batteries!