Read This First: Disclaimer: I’ve written what I personally did, and my opinions. Don’t assume what I did was safe, and don’t assume it will work for you. Do more research, and make your own choices. I am not responsible for your outcomes!
Types of Chargers
You can find cheap single-stage bulk chargers rated from 5 Amps to 40 Amps or so in the automotive department in Wal-Mart. These are not what you want for battery charging in an RV. What you need is a “smart” charger with control circuitry that will fully charge your batteries as quickly and as safely as possible.
Batteries don’t charge in a linear fashion. When they are deeply discharged they accept a charge more quickly. Then, as they approach full charge (say 85% or more) they gradually slow. Then after becoming fully charged they require a small charge to maintain at 100%. For this reason you need a 3 stage (or 4 stage) charger:
- Stage 1 is called Bulk Charge. When a battery is 50% to 90% charged it will take a charge very quickly. A smart charger senses the battery voltage and supplies maximum current at an increasing voltage level to attain a nearly full charge.
- Stage 2 is called Absorption (or Acceptance). At this stage voltage is held at a preset maximum level (typically 14.5 volts) while current slowly tapers off until the battery is 100% charged. High-end chargers may also include a temperature sensor that measures battery temperature. Optimum charge voltage varies based on temperature from as low as 13.8 volts to around 15.5 volts.
- Stage 3 is called Float. After a battery is fully charged it requires a lower voltage to maintain its charge while waiting to be used. This is typically 13.5 volts for regular lead/acid flooded cell batteries, and 13.2 volts for AGM batteries. As with stage 2, the optimum voltage varies with temperature.
- A 4th stage that some chargers are capable of is known as Equalizing, something that flooded cell lead acid batteries require periodically. This involves taking the batteries up to a higher voltage level, about 15.5 volts (or 1 volt higher than Stage 2 if you’re using temperature control), for about 2 hours. This ensures all battery cells are equally charged. A more complete discussion of equalizing is included in some of the links at the bottom of the page. Note that AGM batteries should NOT be equalized in normal use, so if you have AGMs and a 4 stage charger be sure to disable the equalizing function.
Q. Don’t I already have a battery charger in my RV?
Yes, it’s likely that you do. Most RV’s at least have a converter. The converter turns 120vac into 12vdc to power your lights and 12 volt appliances. It also charges your battery, but depending on the setup it may charge the batteries slowly with only 10 or 12 amps. In older RV’s, and even some new ones (lower priced) it may not be a “smart charger” as discussed above. It may have a fixed output of 13.7 volts — lower than it should be for fast charging, and higher than it should be for float.
Some chargers sold in the last few years have an optional plug-in module that enables them to function as “smart chargers”. Popular converters with this feature are Progressive Dynamics, Iota and others. If you have one of these in your RV, you might just need to buy the module. If you have an older inverter, you could replace it with one of these advanced models, or add a dedicated battery charger.
If you have a “higher end” RV you likely have an inverter/charger that combines a fairly powerful charger with an inverter that makes AC power from your batteries. In this case your system may already be adequate.
Q. How large does my charger need to be?
For dedicated battery charging (separate from your converter) a 40 or 50 Amp charger is adequate for an RV battery system with Four golf-cart size 6 volt batteries. What about a larger battery bank? And how does battery type effect charging needs?
- Most flooded cell lead-acid batteries should be bulk charged at a C/10 or C/8 rate. “C” stands for “capacity”. The charge rate is Capacity divided by the index number. So a 440 Amp Hour bank of batteries can be safely charged at 440/10 to 440/8 or 44 – 55 amps. Charging them a little faster may be OK, but when you get above C/5 you’ll be heating them up too much, and making them gas excessively. It’s best to stick with what the manufacturer says. Some manufacturers specify a higher maximum than others.
I answered a question about this that was emailed to me, so I’m posting my answer here to further explain…
- A C/4 of C/5 charge rate may not ruin your batteries right away. There are just some issues to be aware of, including safety issues. If you have 440 AH of batteries and a 100 Amp charger I’m not saying you need to toss the charger and buy something smaller.
- First, charging a standard flooded cell battery bank at a C/4 rate (capacity divided by 4) will shorten the life of the batteries. Depending on other factors you might originally have gotten 6 years from the batteries. Now, maybe you’ll get 4 or 5 years.
- Second, maybe more important, you will increase the maintenance required for the batteries. They will bubble and gas more, which means they’ll use more water and, require more frequent monitoring, cleaning posts, etc. Your battery compartment may also need to be cleaned more often, depending on how it’s constructed. All this is because of the excess gassing and the greater corrosion possibility.
- There will also be a greater danger around the batteries, as they’ll be giving off more gas while charging. If the battery compartment isn’t very well vented you may want to install a small vent fan. I’d also be concerned if they’re cheap batteries. Something like a Trojan T-105 will take more abuse than a Sam’s Club Exide. If you severely over charge and over heat them the cases might warp and/or crack and they could leak. Not a good situation, and can lead to a fire.
- Gel Cell batteries can only be charged safely at C/20 — and you can damage them if you push it. This is why they have fallen from favor with most RVers and boaters alike.
- On the other end of the scale are AGM batteries, they can handle a much higher charge current than other batteries. Concord says that its Lifeline batteries have no current limitations in voltage regulated charging! For practical purposes, charging them at C/4 is pretty good.
As mentioned before, some large inverters include a built in charger. Typically a 2000 Watt inverter will include an 85 to 100 Amp charger, and this is adequate for a large battery bank (and overkill for a small one). I’ll talk in more detail about inverter/chargers on a later page.
In summary, you need to consider the SIZE of your battery bank in Amp Hours capacity, and the TYPE of batteries you have (Flooded cell, Gel, or AGM) when choosing a battery charger.
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