Max Charging Current of typical GC2 battery

The usual 10% charging rate recommendations are for longer battery life as measured in number of cycles.

Meanwhile an RVer has to meet camping situations and get it done.

My requirement is to do 50-90s in two hours or a few minutes more (generator hours as set by the campground) if I cheat, and that 40% recharge has to be enough AHs to get me by till the next recharge.

I know I use about 70AH a day in summer plus at least another 70 in winter, so to get by in winter using 140AH where that is 40%, that means I need a bank with 350AH or more. So that is four batteries at 440AH (but that is at 25C and this is winter at 5C, so knock 10-15% off that 440 and you get say 380AH and your 50-90 is now about 152AH, which is enough to get by.

OK so how to replace 152AH in two hours? See ugly graph where the 70amps did it on a 220AH bank. This is a 380AH bank so 380/220 x 70 = 121 amps.

I need a generator that can run a charger doing at least 121 amps and a 121 amp charger. Turns out my 3000w Honda can handle 130amps worth of non-PF corrected chargers before popping a breaker. My collection of Vector chargers could do 130a.

Since then I have gone over to PowerMax adjustable converters, and their 100 amper is PF corrected. The Honda can run it and a PM3-55 amper without popping its breaker for a total of 155 amps initial charge. I crank the voltage to 15.2v at 5C which is the same as 14.8v at 25C. (Good luck with your 14.4v converter :( )

Target has been met and then some. Which is good because the furnace can use over 100AH a night and it is dark longer and so forth, so it can be 200AH a day --BTDT several times.

So how to get by really? Going to need more battery bank and cheat some on the two hours even at that 155 amps. But no problem--nobody else is nearby in the park and the rangers are all hiding inside where it is warm, and don't care about your generator, so you can do it! :)

Just to complete the math on that-- 200AH is 40% so 100% is 500AH needed where that is maybe 85% in the cold from a rated bank of 588AH so you need say four T-1275s at 150 each, or six T-105s at 225 x 3 = 675.

So that all works until you get AGMs rated at 100AH each and it says do not recharge at more than 27 amps per battery!!!! Oops, now what?

600AH minimum needed so that is six of those 100AH AGMs and six x 27 is 162 amps. Saved! 155 is my max with the Honda and the two chargers. But it will take a bit longer than the two hours to complete the 50-90, more like an extra half hour. So cheat! Do it when the rangers are out there with their chain saws going and they won't notice your Honda at all. :)

Also keep the AGMs inside your rig where it is nice and warm and gain 10-15% in AH capacity (but do not overcharge them so badly that they vent!--ie, do not go over that 27a per 100AH limit in the spec--unless your AGMs specify they can do more like some can)

So just get it done and if the batteries only last six years doing that instead of twelve by following the 10% charging rate recommended, too bad. You want to go camping or not?

Different manufacturers give diffrent max charge rates but for the GC-2 with a 110 Amp Hour capacity (C/20 rate) Xantrex recommends 33 amps max or 30% C-20

Lifeline on the other hand.. Suggests more than that for their AGM batteries.. ONLY LIFELINE.. and for GEL types. (not recommended I"d be surprised if you could find one) 20%

Note that manufacturers recommend "limiting the charge current. Trojan says 3-5% of C20 (so about 11A for a 12V pair of GC2). US Battery says 10% of C20, Rolls and Crown say 10-20%"

Fifteen years ago my cardiologist wanted me running a half mile, doing a half hour of calisthenics daily and eating a near vegan diet. Near the same age as me.

Next month I turn 71 - he's dead.

I've gone over this a hundred times. Engineers have demands put upon them by management to make their product look as good as possible. Even if it means having the customer acting stupid to gain 3% in lifespan.

Trojan has changed their GC charging formula three times in the last seven years. Same battery, same plates, same paste, same electrolyte density. Global warming? Lost the Gulf Stream?, Magnetic Pole Shift? Earth rotating backwards? No, merely a new generation of engineers.

I have voltage saturated charging flooded batteries for over forty years, and that's how I life cycled tested over a thousand samples.

The key is to reduce charging voltage drastically if electrolyte temperature reaches 45c and cease charging altogether at 50c. This is damned hard to reach unless outside air temperature is extremely high to begin with.

What DOES hurt batteries sir, is to constantly undercharge them for weeks or months at the stretch. They will sulfate, and few people want to learn how to never mind actually equalize a battery. That ends up killing a battery. Like my cardiologist who was a kind, amiable, misinformed boob. Last week I had a hospital stay full radionucleide, sonogram stress test performed. That cardiologist reported "You have the heart muscle health of a 40-year old". But awful nerve impulse control. To prevent total A/V block they stuffed a dual node pacemaker in me.

Of course, for some these facts will enter one eyeball go onto a four lane highway and fall out an ear. I do not offer spuriously contrived advice. Many cyclable batteries survived hundreds of 5% amp hour total, constant discharge until 0.0 volts was reached whereupon full vAbs 14.8 volts was applied for a pre calculated number of minutes whereupon resumption of discharge was re-initiated. A very rude, a most relevant way to separate battery manufacturing quality levels. I tested engine starting batteries the same way. Southern California Edison loved me. Rank & File staff engineering wogs hated and feared me. I reveled in it.

So forgive me if I seem abrupt. Old antagonisms die hard. I dislike BS rote from professionals. I can appreciate an engineer's desire for his pride and joy to reside in battery heaven with 22 virgins and endless bowls of dates and goat stew - but life does not work that way. Generator run time rears it's ugly head. And my Rolls bank is nearing the quarter century mark.

Trojan, up to 20%, when queried no limit but temp and $
http://www.trojanbattery.com/Tech-Support/FAQ/Charging.aspx
"What size charger should I buy?
A properly sized charger takes into account battery capacity and the time interval between charges. In applications where cycling is infrequent, such as weekend RV users, or infrequent or seasonal trolling motor usage, a charger with an output current rating between 10 and 13% of the battery's rated 20-hour capacity will suffice. In applications where battery recharge must be accomplished within 8 to 10 hours, a three stage, automatic charger, rated at 20% of the battery capacity, may be required.

Example: A good charging rate for a battery with a 20-hr capacity of 225 amp-hr is about 22 to 29 amps. You can of course go slightly higher or lower depending on what is available on the market."

Chum lee wrote:
The standard book answer is: the amp-hr rating divided by 10. I know many people will argue with this.

So, if you have a pair of 232 Ah 6 volt deep cycle GC batteries in series that would be:

232 + 232 = 464 / 10 = 46 amps initial charge rate @ +- 14.8 volts or higher.

Nope. When combined in series, the voltage increases, but the capacity does not.

mike-s wrote:
Note that manufacturers recommend limiting the charge current. Trojan says 3-5% of C20 (so about 11A for a 12V pair of GC2). US Battery says 10% of C20, Rolls and Crown say 10-20%.

Good point. Was wondering about temperature issues at the high charge rates mentioned above. But I won't be going anywhere near that.

In my case there are 4 GC2s in series, charging at 28-30V. Solar charging mostly from 690W on the roof. We often boondock.

But I also have a 120VAC-30VDC Meanwell PS which is only 500W, giving me ~16A max charging current. So far it has self-limited to ~16A without any problems when charging the depleted batteries on shorepower.

But wanted the truck's 220A alternator + two truck batteries to help charging the depleted trailer batts as well. And remembered that I still have the old 12VDC-120VAC 1000W PSW inverter. So am thinking to mount it in the Truck's cab or cap with 6awg to the truck batteries, and run an 120VAC extension cord to the Meanwell in the trailer. The max Meanwell load of ~500W might translate to 40A at the 14V alternator.

No need to purchase any new equipment, other than cables.

Also looked at a cheap 12VDC-30VDC step up converter on Ebay that said the load should always be less than the device's 400W rating ... not self-limiting? Anyway, avoided that.

The standard book answer is: the amp-hr rating divided by 10. I know many people will argue with this.

So, if you have a pair of 232 Ah 6 volt deep cycle wet cell GC batteries in series that would be:

232 / 10 = 23.2 amps initial charge rate @ +- 14.8 volts or higher.

That said, they will accept much more than that if you have a charger that can do it. At higher charge rates the batteries will heat up and generate significant amounts of hydrogen and oxygen gas which is explosive. If you do not provide adequate ventilation to the battery compartment, eventually . . . . boom! Just like the picture Mexicowanderer posted the other day except you can expect sulferic acid to be splattered around the blast area too. If you are in a hurry to charge your batteries take adequate precautions unless you want mud (well in this case sulfuric acid) all over your face.

Corrected math. Thinking at the speed of light and writing at the speed of sound or less!

Chum lee

Note that manufacturers recommend limiting the charge current. Trojan says 3-5% of C20 (so about 11A for a 12V pair of GC2). US Battery says 10% of C20, Rolls and Crown say 10-20%.

At 68F it would not be unusual to have an instant 14.8 charge rate consume 108 amps with 6 cells and 216 amps with 12 cells of decent quality GC220 batteries.

Don't feel bad. It takes 670+ amps at 28 volts to get the attention of my 3,250 amp hour 24 volt bank. Stick & mud umbilical connection house.

It's all about slashing generator run time.

I have stuffed 100-110A into a bank of two GC2 batteries on occasion when they were down around 50 percent. they will accept 100+A for around 30 minutes or more or so before the charge started to taper. That was charging from a PD55A and PD70A chargers in parallel.

I've done the same on a bank of four GC2 batteries. they will accept 100A for much longer as one would expect since each bank is getting around 50A in this configuration.