Charging AGM 12V at 0.3C instead of the max safe 0.2C?

[Jman]

Eyeing a 12V Fullriver DC AGM and it's specs say 0.2C max, but can I go as high as 0.3C without much issues like shortening of health? I ask as maybe this is just a legal thing & I read some comments say with AGMs in general better to have higher current into AGMs to shorten bulk time (less duration of heat=more life?)

I will be using this battery probably a dozen times a year for a few days each time mostly with a genny, and also understand the importance of getting to true 100% as much as possible.

thanks

[Mike90250]

Generally, when FLA batteries are low in charge, they can be charged at higher than normal rates, and as they "fill up" the voltage difference from source (charger) and battery decreases, and so the charge rate goes down. After about 80% full, charging must be limited to factory specs, or things start to overheat, and that's never good with sealed batteries.

So to answer your question, it's likely OK for the first 80% of charging, but then you should slow charging down to recommended rates

[PNjunction]

High quality conventional agm's should be able to take 0.3C, but stick to the manufacturer's suggestion.

Some internet-info conspiracy sites likes to pawn off being able to charge conventional agm's at ridiculously high rates, as if it was some sort of secret.

The "secret" to vastly going past the manufacturer's recommendation is that even *IF* the battery appears to take a huge fast charge, there is no free lunch. That is, the absorb time to TRULY charge them to full capacity takes much longer. BUT, charlatans pawn off this info by assuming the end user is assuming voltage is the only parameter for identifying a full charge.

Sadly, what they don't mention is that fast charging batteries like these, even if they *seem* to handle it, can't pass the standard C/20 capacity test they were designed for on discharge. Note we are not talking about pure-lead agm's, but the typical conventional types - usually identified by the manufacturer not recommending charging faster than 0.25 to 0.3C.

In your situation, here's the kicker: Unless you want to spend a fortune on gas, you'll never truly fully charge them, as it takes about 16 hours float *after a full absorb down to about 0.05C* to truly get that last 1 percent fully charged. No way to rush this. So perhaps after you use the genny, if you have the chance, put a solar maintenance charge on them for another day - then store.

[Sunking]

Eyeing a 12V Fullriver DC AGM and it's specs say 0.2C max, but can I go as high as 0.3C without much issues like shortening of health?

On AGM never exceed manufactures maximum rate. You are asking of if it is OK to charge at C/3.3 vs C/5. Exceeding the maximum charge rate greatly increases your risk of Thermal Runaway and/or damaging the Valves from excessive pressure.

In your application of infrequent use, there is no reason to even charge as fast as C/5, and absolutely no reason to charge at C/3.3. There is nothing to gain, only lose. As PN points out going from C/5 to C/3 i snot going to shorten charge time. All that does is get you to Absorb phase faster, but as a consequence only lengthens Absorb time which cause the batteries to gas longer and build up more pressure to damage valves.

PN had a good idea. Forget using the genny to charge to 100%, as it is pointless in your application. Purchase a small solar panel to top it off in you absence. Lot cheaper than the fuel and over sized charger required to do it your way. .

[Jman]

In your situation, here's the kicker: Unless you want to spend a fortune on gas, you'll never truly fully charge them, as it takes about 16 hours float *after a full absorb down to about 0.05C* to truly get that last 1 percent fully charged. No way to rush this. So perhaps after you use the genny, if you have the chance, put a solar maintenance charge on them for another day - then store.

Not sure I understand. After my AC charger at 0.3C goes into float the AGM is still taking in current (measured by my watt meter) & after a few hours on float at 13.8V the current going into the battery is down to around 0.05C. This is 4hours+ totalk time from 50%DoD.
So does this mean its 100% full? Or is it really only full if left over night? I agree its a real kicker of a situation

[Jman]

On AGM never exceed manufactures maximum rate. You are asking of if it is OK to charge at C/3.3 vs C/5. Exceeding the maximum charge rate greatly increases your risk of Thermal Runaway and/or damaging the Valves from excessive pressure.

In your application of infrequent use, there is no reason to even charge as fast as C/5, and absolutely no reason to charge at C/3.3. There is nothing to gain, only lose. As PN points out going from C/5 to C/3 i snot going to shorten charge time. All that does is get you to Absorb phase faster, but as a consequence only lengthens Absorb time which cause the batteries to gas longer and build up more pressure to damage valves.

PN had a good idea. Forget using the genny to charge to 100%, as it is pointless in your application. Purchase a small solar panel to top it off in you absence. Lot cheaper than the fuel and over sized charger required to do it your way. .

Ok thanks I will not exceed the factory rating. I thought more current/quicker bulk stage = smaller absorption time or at least getting to the high mid 90s+ quicker. My charger goes into float earlier with higher current. Unless you mean the total true absorption time until current tapers down to 0.05C is longer?

thanks

[Sunking]

Ok thanks I will not exceed the factory rating. I thought more current/quicker bulk stage = smaller absorption time or at least getting to the high mid 90s+ quicker. My charger goes into float earlier with higher current. Unless you mean the total true absorption time until current tapers down to 0.05C is longer?

Very good JMAN, you have discovered something. It is a misconception amound inexperienced users and the get tripped up by the formula:

AH = Amps x Hours

If we factor out time:

Hours = Amps Hours / Amps

A student would conclude If we charge at a higher rate (amps). the shorter the time will be. You have discovered that is not quite true in practice. There is a point in diminishing returns you discovered in practice. Think of it like a speed limit, and if you exceed it, you will be delayed by the traffic cop pulling you over.

The Speed Limit is the manufactures maximum charge rate. Not only does it slow things down, but also causes damage. The faster you charge, the longer you will stay in ABSORB aka Saturation phase. Example for Pb if you charge at C/8 rate you will reach Absorb when the battery reaches about 90% SOC, and from that point will take another 4 hours to saturate as the current tapers off quite quickly. Up the charge current to say C/4, and you reach Absorb at 60% SOC and now spend the next 8 hours in Absorb. Either way still takes 12 hours because the Traffic Cop took you to jail, and you had to post bail to get out and continue to your destination. This applies to all battery types and you stumbled upon it.

Good job, you figured it out.

There is one loop-hole in the law you can exploit and useful with solar. Raise the Absorb Voltage, but I will not go into detail as that is in the Stickies. It comes with negatives is all I will say for now.