When Is a Battery Fully Charged?

RZRBUG said:

I have been hooking my 100W solar suitcase to the battery a couple times a month to keep it charged.  Today I decided to connect it to my regular 120V charger.  I have a 150ah group 31 sealed battery.  I set the charger on AGM at 20A.  At the beginning, the charger display said my battery was at 95%.  It went up to 98% fairly quickly, but two hours later it was still at 98%.  I understand that the last little bit to fully charge takes longer, but how long would I need to leave it to reach 100%...or will it never reach that point?

Are you using a solar controller when you charge via solar, Larry?

RZ, assuming your solar panels have an output of 17.8-18V and they are working at optimal capacity, that 2% you are missing on your battery could take several hrs to get to 100%.  Your charger is in that slow absorption stage An AGM supposedly should take less time to get to 100% vs a flooded lead-acid battery, but it'll still take a couple hours to reach 100%. Charging any battery creates heat. That last little bit of voltage needed to reach 100% charge needs to be done slowly in all types of batteries for different reasons.  AGMs don't off-gas during normal charging conditions like lead acid batteries, but the chemicals and internal pressure that drives the chemical reaction will expand due to heat buildup - leading to bulging of the battery, cracked case or worse.

Outback - AGMs accept a charge nearly 5x more efficiently than a lead flooded battery, so yes, it should charge faster just based on the battery type, regardless of how you discharged the battery.  Now...in real life, how many charge days it'll take to restore your lead-acid battery vs your AGM battery to a full charge depends on their initial amp/hr capacities, how low you discharged them, how efficient your solar panels are (winter sun, summer sun, ambient heat decreases efficiency, etc). and assuming you are getting 4.5 hrs of direct sun with panels angled towards the south. The AGM will still charge faster.

Edited to add: St_Bob and I posted about the same time lol.

Diehard battery gurus will tell you that you will decrease the life of any battery by not giving it a full charge periodically. On lead acid batteries, it is due (in part) to sulfurization of the lead plates even if kept on a float maintainer. Equalizing lead acid batteries improve their life and it is supposed to help de-sulfurize the conductor plates.

As far as AGMs, people who use them under real-life conditions and keep discharge of the deep cycle/hybrid ones to no lower than ~50% (not 80% as some people may suggest); the life of the battery doesn't seem to be as affected or seems to need to reach 100% charge state. This could be due to the battery quality (you get what you pay for), number of charge cycles a battery endures, or how much heat or abuse the AGM is exposed to during its lifetime. There are many factors that could come into play. 

In the end, if you have an AGM, I wouldn't worry about that 2%.

If you have a lead-acid deep cycle type, check the water (add diH20 as needed) and know lead acid types discharge about 3% sitting around without a power load over a month or so (AGMs lose a tiny fraction of that in the same amount of time - nice to know if you have a cut off switch and let the T@B sit for a couple of very cold months). Lead acid batteries periodically like to be taken off a float maintainer and be fully charged, and take at least 4-5x longer to charge than their AGM counterparts.

Battery age is more important to folks with high power needs. People with complex high capacity battery banks sometimes set up 2 systems where one bank is being used full-time, while the 2nd set is being solar charged to 100% (which can take a couple days depending on bank capacity). Batteries have to be matched to type, cabling is important, and it's not a good idea to mix different age batteries as the efficiency drops to the least efficient battery in the bank. For the majority of T@B users, this isn't a problem.

For reference... Maybe it's the smaller battery (Group-24) but with fridge, lights, Alde running I see 12.2 when I go to bed (12.4 without Alde). With sun rising earlier now, I'm fully charged by 9:00am off my 120 watt Zamps. Note, I got my T@B without the media package so am not running any of that at night.

It is definitely the time of year. My 50w struggled to keep up with battery drain in December and January and is fully charged by mid morning now.

I like soft lights at night, unless I'm writing. Trust me, I don't deny myself much.....remember the back of my truck!!!

I figured if I ever met Jenn, she'd have the most stuff - Then I met Verna :-)