FLA and AGM batteries require different voltages for charging, with the FLA needing upwards and above 15 volts, AGM’s are 14.4 volts max. These two batteries have different charging profiles with all of the smart chargers on the market, all the profiles work to charge the batteries fully with minimal gassing. So, if you are charging FLA and AGM batteries with a two bank charger with the profiles set to each battery and the dvsr is active you are over voltaging the AGM battery.
Lead acid batteries require 10-13C of charging current, that means 10-13% of the Ah rating of a battery, so for a 75Ah battery that is 7.5-9.75 A’s. AGM batteries can use 25A’s as long as the voltage doesn’t exceed 14.4 volts. I corresponded with Interstate Batteries lab on that last bit, still have the emails if you need proof, and that was for a group 24 battery.
A battery tender may keep a battery charged but it is not designed to properly recharge a deep cycle battery.
If left in OEM configuration, the DVSR light will come on as soon as the voltage reaches 13.2 Volts on either battery, and will parallel them.
You can use a single bank charger on either the start or house battery with the dvsr in the OEM configuration and both batteries will charge, but as
@MrBubbaGump points out, you eliminate the smart chargers ability to charge a deep cycle battery properly both the FLA and the AGM.
If you have FLA batteries you need to do a manual equalizing charge at least monthly on deep cycle batteries to assure the cells are balanced and are at the correct specific gravity to get the most performance out of them, to check this you need a hydrometer, I’d say the vast majority of people do not do these steps and get poor performance out of their FLA batteries and then spend wayyyy too much money on replacing them with same or agm batteries. Other things the equalizing charge does is to clean the plates thoroughly and it mixes the electrolyte fully as it can get stratified over time. The price for going with a FLA battery is just a couple of maintenance steps that will take about 10 mins a month.
Modifying the dvsr is super easy. Cut the red wire and connect the proper end to a ring terminal and attached it to the switched side of the house or start battery. If you don’t want to do that, simply remove the ground wire, thin black wire attached to the batteries, or just put a switch in line and it has the same affect. Using the battery switch method makes it a bit of a no brainer.
If you the reader follow these steps with FLA batteries I guarantee you there will be a noticeable difference in battery performance if you have been using these batteries for a while. In fact when I talked with Trojan some time ago they told me how their deep cycle batteries take several discharge / charge cycles to gain their full potential.
Note:
Since my handle was mentioned above and also mentioned LiFePO4 batteries, if you go to a LiFePO4 house battery you will need to mod the dvsr to either storage or ignition mode as the resting voltage of the dvsr is 13.6 V when fully charged and will not drop to 12.7 V until almost completely discharged, which means the dvsr will stay closed and the LiFePO4 battery will dump power into the lead acid start battery until it is depleted, I cover this in my upgraded battery thread. So, I have my dvsr hooked up to a rocker switch on the dash so it is fully manual.
