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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.

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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.

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  • \$\begingroup\$ Thanks for the answer -- my charger docs recommended keeping the battery the charger all the time, but I'm glad to hear that's not necessary. Turning it on for several days sounds like a good solution to make sure it reaches full charge. I'm not sure I want to build my own charge controller, but if I really wanted to, I guess I could sense the "float charge" light on the charger to know when to turn it off. Though keeping it on for a couple days sounds easier. \$\endgroup\$ – Johnny May 6 '13 at 14:57

protected by W5VO May 6 '13 at 3:52

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