Do I have to keep an AGM battery on float charge 24x7?

Question

I have a 50 Ah AGM battery that I use rarely (lighting during power failures, off grid camping, etc) - I use it a few times/year.

To prolong the life of the battery, do I need to keep it on a float charger all the time, or is it sufficient to use a timer to turn on the charger for a few hours every month (week?) to top off the charge? The charger I'm using is designed for AGM batteries and is is able to supply a float charge indefinitely.

I'm diligent about minimizing vampire loads by using a hard power switch to turn off unused devices and appliances so would rather not keep the battery charger on 24x7 unless it will make a significant difference in the lifetime of the battery.

Answer 1

The short answer is: No, you don't have to keep a charger connected constantly to keep your battery healthy.

The longer answer is: A naiive timer on the charger may or may not work, depending on a lot of factors. AGM batteries can have a self-discharge of as low as 10% per year (which is quite good). For such cells, a full charge every few months should be adequate to keep your battery charged when you need it. The catch is that you want to make sure that your battery is really fully charged each timer cycle. If the timer turned the charger off prematurely, the battery state of charge could get lower and lower each cycle, leaving you without power when you need it.

For a simple solution (and assuming your charger is appropriately sized), try turning your charger on for several days every 2 or 3 months. This should ensure that the battery is fully charged each cycle, while still allowing you to leave the charger disconnected the majority of the time.

For an EE-centric solution, look for a lead acid battery charge management IC that can give you a reliable indication of the transition from bulk (constant current) to polish (constant voltage) charging, and ideally provide a timer for the duration of constant voltage charging to indicate when the battery can be considered fully charged.

Answer 2

I have found that trickle charging for about a week will "bottom out" the charge rate. However, very shortly after removing an AGM battery from the charger, it begins to self discharge to the point where the SoC (state of Charge) is lessening. A healthy AGM might self discharge 1% to 3% a month so technically, leaving it on the trickle charger 24/7 is the only way to ensure it is at 100% SoC by constantly counteracting self discharge.

Float charging is similar to trickle charging but float will go on and off when the battery drops to some significant level below 100% SoC (perhaps 95%). That is also healthy for a battery but it is debatable which is actually better in the longrun.