When Is a Battery Fully Charged?

RZRBUG

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?

jkjenn

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?

RZRBUG

Thanks, Bob.  It is an AGM battery.  I'll hook it back up and leave it.  I'm curious to see if it will actually reach 100%.

RZRBUG

jkjenn said:

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

Yes, Jenn.  It is the Renogy suitcase with the built-in controller.  I have it set to AGM as well.

Tabaz

Bob - RZRBUG brings up an interesting point.  Let's say I'm boondocking using only my Zamp 200W solar array and run my "600W" microwave for five minutes.  Will it take longer (I.e. more sun time) for an AGM battery to recharge than a lead-acid?

Ratkity

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.

Tabaz

Bob and Ratkity - Wow! Thanks for the useful information.  People like you make this forum a wealth of knowledge.

RZRBUG

OutbackAZ said:

Bob and Ratkity - Wow! Thanks for the useful information.  People like you make this forum a wealth of knowledge.

Ditto!!!

So, is that final 2% important?  Is it true that batteries have a charging memory and that eventually the original 100% will never again be achievable; and therefore, the 98% becomes the new 100%, etc.?

RZRBUG

Thanks!

Ratkity

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.

RZRBUG

More good info.  Thanks!

RZRBUG

UPDATE:  Today I decided to put the charger on the battery again to see how long it would take to reach 100%.  I plugged the charger into AC, set it to 20A and started the charge with no load on the trailer.  It began at 95%.  I checked it about two hours later and it was at 100%.  I disconnected the charger from the battery, checked the voltage, and it read 13.4V.

I don't know why it never reached 100% before when I left the charger on for several hours (3-4).  98% was the highest level it reached before.  One thing I made sure of today was that the alligator clips were firmly seated on the terminals.  They may not have been before, if that makes any difference.

Just to recap here, I have a 150ah Group 30 12V AGM battery.

Verna

My two 6V golf cart batteries have just starting to being 100% charged (according to the solar panel controller) this week. I've been using them 95% of the time since January 10th. I don't know why the change unless they just needed more hours of sun, or the charging built up over the past 2+ months., or maybe it has something to do with 4,000 ft in elevation.  

I'm very happy to see that "FUL" at about 2 pm in the afternoon. 

Nomad

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.

Verna

The TV and satellite receiver take very little energy.  I haven't used the entertainment center.  I tend to use solar or battery operated lights in the evening. I normally only use .1 or .2 volts overnight. That's even with the Alde running and the water pump running once or twice. I haven't really changed any habits over the time I've been out. 

jkjenn

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.

Nomad

Verna - With your dual battery, 200 watt solar setup you should be able do whatever you like. I rarely use my battery operated lights (except if I want a dim setting) - Quit being a miser. Quit denying yourself. Live it up or I'm taking your batteries away ;-)

Verna

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

mash2

Verna.  Between February and march, the solar panel output should have increased by about a third (my panels yield 35 amp hours in mid february and 45 by mid march.  Should go up through most of the summer (more than double the output of February levels).  Output should have been rising since early in the year.  PxLated is right, with the increasing yield through spring and summer, you should have plenty to quit denying yourself.  You should be in land of plenty for the next 7 months.  Time to splurge is upon you.  Seize the day.  

Nomad

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