I built a portable generator (battery Box) for off road camping, using a 100Ah AGM battery. ( www.rvbprecision.com)

I recently bought a portable refrigerator and wanted to do some testing for an article on my web site. I took the battery down to 75% during one of these tests. Usually I'd use my portable solar panel to recharge the battery, but there has been no sun for days up here in New England

A few years ago, a whole bunch of us RVers were buying and using a Black And Decker "Smart" charger that was the "Cats A**" back then. It offers multiple stages of charge and you can set the bulk amp stage.

To bring the battery back up to 100% for the next phase of testing, I used this battery charger set at 40A. The battery was back up to 95% within 20 minutes! I then set it to 20A and brought it up to 100%....From there it went to trickle charge at 1A

My question is.......Is it bad to dump 40A into a battery at this state of discharge?

My understanding is you should only charge an AGM battery at 20% of capacity (in this case, 20 amps?)

Any comments would be appreciated.....

You're not doing your battery any favours. You've pulled it down really low, and hit it with more charging current than recommended. The most likely outcome is battery replacement sooner than expected. 

Thank you!

Is 75% considered Really Low for an AGM battery?

I thought 50% was the lowest you should go?

Will charging it at 20A do it harm?

Oops, misread your state of charge. 25% discharge is as easy on the battery as you can get. A 20 amp charge current is the maximum recommended for AGM batteries (in your bank size), so shouldn't hurt anything, but lower current is easier on them. Of course, lower current means longer generator run times, if that's how your sourcing your AC.

Thank You! Scared me there for a minute!  Looks like I'm good to go with a three stage smart charger starting off at 20A.

At 75%, the charger has "looked" at the battery and is only delivering a max iAMP input of 11-12A for the bulk charge, it will then taper to absorption charge and then float.................that is what the owners manual states

https://servicenet.blackanddecker.com/documents/English/Instruction Manual/90104835,VEC1093DBD.pdf

  On 12/14/2019 at 11:57 PM, rbertalotto said:

Thank you!

Is 75% considered Really Low for an AGM battery?

I thought 50% was the lowest you should go?

Will charging it at 20A do it harm?

You can't actually "push" 20 or 40 amps into the battery by choosing that setting. The battery's internal resistance controls that without any external input. As the battery gets closer to 100%, the charge current gets smaller and smaller until it reaches equilibrium. It's true that AGM batteries don't generally like more than 20% of capacity in charge current. They also don't like deep discharges. I have 500+ ah of AGM batteries and prefer not to go lower than 75% SOC. If you look at the manufacturers data sheets, you'll see that lower discharges per cycle give you much longer battery life. Also, AGM batteries and lead acid batteries in general, do better when brought to 100% often, at least every few days. Jay

Edited December 15 by Jaydrvr

Typo

My Toy Hauler has two Duracell 110Ah AGM batteries. They are 5 years old and are never brought below 75%.

Using 420w of solar and/or the PD 4-stage on board charger/converter, I always bring them back to 100% by 3pm the next day. Some days I need to run the generator to accomplish this.

After 5 years, these batteries are still working fine.