Firefly Group 31 Battery Frequently Asked Questions

Firefly Group 31 Battery Frequently Asked Questions

My old AGM batteries had a “resting” voltage of 12.7. What I mean by this is when the battery is left without loads or charging sources for 24 hrs, my meter reads 12.7. Is this what to expect from the Firefly?

Almost, the Firefly G-31 resting voltage is expected to be at 12.8V or sometimes higher. (12.9-13V) but this may take a day or two.

What is the Firefly’s G-31 lowest voltage the battery can withstand?

10.5V is fully discharged, 11.7V is @ 60-70% discharged. You can discharge them down to 80%, @ 11.5V as long as they are brought back up with a charge rate of at least .4C. (.4C is defined as 40% of rated capacity – one G-31 Firefly battery rated at 116 A-Hrs capacity, so periodically it is good to recharge with at least 40-46 amps. Multiply this rate if more than one battery in the bank by the number of batteries).

Charging my existing batteries either with my solar panels or shore power resulted in readings of 13.6 to 13.8 V.

Float voltage on the charging source is too high at 13.6V. Proper care of the Firefly requires resetting your charging sources (shore battery charger, alternator, solar, wind, hydrogenerator or other charging sources) to the following parameters: Bulk 14.4V, Absorption 14.2V, Float - 13.4V. The charging sources need to be reprogrammed to these parameters to properly maintain the batteries and to fulfill the Firefly warranty.

My float setting is 13.4V. I’m told it should be 13.3V. Does that matter?

13.4V is the correct float voltage for the Firefly adjust your charger accordingly.

Firefly literature says the batteries can discharged down to about 80%. How does one know when that occurs?

The most reliable way to monitor battery State of Charge (SOC) is with a battery monitor such as a Victron BMV, Balmar SG200, Xantrex Link Lite or other marine battery monitoring systems. Voltage is an indicator, but unless you are really keeping track of current in and out, number of charge cycles the voltage may not be an accurate read on the actual % of SOC.

Do I need a battery temperature sensor for my charging sources?

To correctly charge the batteries for optimum energy storage and life, battery temperature sensors are required to feedback to the charging source so it can adjust charging accordingly. What is unique about firefly is the manufacturer recommends not to mount the battery temperature sensor on the battery terminal but to have it located so as to measure the ambient temperature in which the batteries are located.

Can my FireFly batteries be mounted side-by-side in a single bank touching each other?

The manufacturer recommends that you maintain a minimum of 1 inch spacing between batteries if they are mounted side-by-side of each other. This allows for airflow between the batteries helping to stabilize/equalize the temperature multiple batteries are having when connected in a physical bank.

Can I use my firefly battery for engine starting?

In many cases the Firefly G31 will work as a starting battery but you should verify, they can supply the required engine cranking current (CCA) necessary to meet your engine starting requirement. Realistically for the price it really isn't the greatest value for a job that a less expensive battery such as a Lifeline could do.

I am confused on how to recharge my firefly batteries with regard to this charge rate of .4C?

This is a unique recharging requirement on these batteries. The manufacturer recommends they are they be recharged rate of at least .4C. (.4C is defined as 40% of rated capacity – so one G-31 Firefly battery is rated at 116 A-Hr. capacity) periodically with at least 40 A ( multiply this rate if more than one battery in the bank by the number of batteries).


NOTE: Resting voltage of all lead based (Pb) batteries will slowly go down over time due to the internal resistance of the battery itself. After a full charge (14.4V until current <.5A) the battery will settle around 13.0V for a day or two, then around 12.9V tapering to 12.8V. After that they will go down slowly depending on temperature, etc.

Correct charging voltage is 14.4. Shouldn't go over that unless in low temps.

Float voltage (if you must leave on float) is 13.4 to 13.5.

Firefly Battery Information