As we saw in the post about converter amperage ratings that mean little, the unfortunate truth about converters that come preinstalled in RVs is that they are not designed to recharge RV batteries quickly or efficiently. They are in effect overpriced trickle chargers, and take many days on grid power to fully charge RV batteries.
We are making an assumption here that you are using a 12VDC, flooded battery system. If you have a lithium, LiFe-Poly, AGM, cold fusion or other expensive system, your charging will be different from what we are describing here. We are practical people and maintain that for RV systems, which are not weight limited in most cases, flooded lead acid batteries are most cost effective and reliable power stores.
With that said, for the flooded lead acid batteries, what you really want is a real multi-stage charger. Such chargers need to deliver charge to your RV batteries in either 3 or 4 stages:
- Bulk charge: this is where approximately 85% of the battery capacity is restored, using a current capped usually at 10-20% of the battery capacity, which is maintained until the battery voltage reaches 14.4-14.6V. A good charger will let you set that voltage threshold to the exact value recommended by the battery manufacturer. This stage is also called a constant current stage.
- Absorption charge: this is where the magic is in the charge process. Different chargers handle this part differently, but the idea is to maintain the voltage delivered to the battery at the threshold where the bulk charge ended, and keep it at that point until the battery is at 100% charge. The current absorbed by the battery will keep dropping during this stage. The trick is, when should the charger consider the battery 100% charged? There are several ways, such as holding the voltage for a specific time; charging until the amps going into the battery drop to a specific threshold; charging until the absorbed amperage stops dropping; or charging until a calculated amount of amp-hours is restored. This stage is also called a constant voltage stage.
- Some chargers have a higher voltage “finish absorption” stage at this point, which is triggered once the second stage is over. This stage pushes the last bit of charge into the battery and causes additional gassing and stirring of the electrolyte, which is thought to increase battery life. Some chargers don’t have this stage and go directly to the next, which makes them a 3-stage system.
- The last stage is the float charge, where the controller will maintain 13.2-13.4V on the battery to essentially counteract the natural discharge that occurs in normal storage.
- Sometimes, good systems implement a periodic equalizing charge as well. Equalizing is done to provide a relatively short, high voltage boost to the battery to induce short gassing and electrolyte circulation, along with additional plate activation. Equalizing voltages can be 15V and higher, and chargers provide scheduling options for those.
We have a great deal of experience with the Bogart Engineering SC-2030 solar controller, which is capable of both 3 and 4 stage charging and optional equalization, as described above. The controller features a simple setup during installation, which can be implemented quickly and includes factory preset options suitable for many battery brands. It also gives access to every parameter that controls the charging curves, so it is possible to adjust and customize the charge settings if it becomes evident that fine tuning is beneficial. The SC-2030 has been around for quite a long time and faces competition from more modern designs, which feature smartphone interfaces and cool looking diagrams and line charts. However, what the SC-2030 with its companion TM-2030 lack in the visual appeal, they deliver in overabundance in real life features. It is about the only system on the market even in 2022 that includes fully metered battery bank monitor combined with finely controllable, discharge and overcharge based 4-stage charging.
In a nutshell, the Bogart system is a set-and-forget, bulletproof design, with which one doesn’t really need a smartphone display to poke at. It just works, and requires almost no user intervention. The only thing you need to do is check your battery electrolyte level.
Disclaimer: we do not receive compensation or advertisement fees from Bogart Engineering, but we do carry and install their products because we believe they are some of the best. I want to thank Bogart Engineering for discussing the charging process and the possible additional options and ways to use their SC-2030 charger and the TM-2030 battery monitor. These discussions contributed to the writing of this article.
Next: Real life example of using cheap RV batteries with SC-2030 and TM-2030.