Max Current for batteries discrepancy

morpho · January 2016

So, I have read it...have been told it...have seen it on the info sheet for the last set of batteries...

"When Bulk charging a Max Current of 10% of the 20hr Amp...etc etc."

So on my little 230 Amp hour bank 23 Amps...but I just got a new bank of batteries and the info sheet says 30%
But these are the same batteries as the previous set.

So that is a pretty big jump from 23 Amps to 69 Amps. What gives? What's correct? What's prudent? What will give me the longest life?

Thanks.

morpho · January 2016

Image: https://us.v-cdn.net/6024911/uploads/editor/hr/th8ssfqiy7gt.png

morpho · January 2016

It's interesting as well that the float charge from one to the other. One says 2.3 vpc (55.2 v) the other says 2.17 vpc (52.08 v)

What do I believe?

mike95490 · January 2016

morpho said:

It's interesting as well that the float charge from one to the other. One says 2.3 vpc (55.2 v) the other says 2.17 vpc (52.08 v)

What do I believe?

You use the 2.3Vpc, the 2.17 is for unlimited float (standby operation for emergency lighting or power). Since this is a Solar power forum, I'm assuming you are using these daily, and the 2.3Vpc will apply.

The Charging amps, you will have to come to some middle ground, You may not have enough solar hours in your day, to charge at 10% rate, so you need to increase the rate (up to 30%) to get a daily recharge, The higher rates cause more internal cell damage, but undercharging causes more damage.

Mountain Don · January 2016

If the batteries were made years apart the manufacturer could have changed the composition of the plates, changed the electrolyte, or simply had a change of mind. Trojan did something a few years ago that changed their recommendations on some batteries of theirs.

The 30% figure makes specific reference to charging by photovoltaics. Is there a different recommendation for generator charging or grid charging? Is the 10% reference based on the same source of power or possibly is it calculated on being grid powered? PV as a source doesn't have the same duration as being grid powered. ... that could be part of the difference. Just an off the wall guess.

I'd go by the present data chart as long as I was sure it was the correct chart for the product used.

vtmaps · January 2016

morpho said:

So on my little 230 Amp hour bank 23 Amps...but I just got a new bank of batteries and the info sheet says 30%
But these are the same batteries as the previous set.

So that is a pretty big jump from 23 Amps to 69 Amps. What gives? What's correct? What's prudent? What will give me the longest life?

You posted the charging recommendations from two different manufacturers. Follow the recommendations from the manufacturer of your batteries. Your signature indicates that you have US Battery batteries... were those your old batteries or your new batteries?

--vtMaps

morpho · January 2016

VTmaps...oh the mysteries of battery companies. The US batts I had have the US batt label on the top and Deka Solar label on the side.
East Penn apparently makes em for US batt...at least here in Canuckistan (?)

Mike...Thanks...it's interesting...I got called out here for having my last settings exceeding the 10% rule, (not to say I was doing it with any deep knowledge...it was just what the installer had it set at) and now I see the specs call for it...I can't win!

I recently set the limit down to 25 amps and on any sunny-ish day I get to float by 1:30 or 2pm. And this is a part of the world with about as short a day as you are going to get before you start seeing polar bears out your window. But the point is noted for sure
So, if I understand correctly, there isn't any harm in low current charging...unless you are constantly not getting the system into a fully charged state. So maybe the assumption from the manufacturer is these are being pushed to the limit and will need all the current possible to get em charged on a daily basis. Or maybe they are hoping you burn through em and have to buy a new set sooner than later?

Don...Nothing noted about genset charging....though my little yamaha isn't likely to tax the plates!

BB. · January 2016

Many deep cycle battery mfg. recommend 10% as their minimum rate of charge (20 hour rate) for their batteries. To quickly charge and mix the electrolyte.

5% is about the minimum I would suggest.

And, if you are over ~13% rate of charge, you really do want a charge controller with a remote temperature sensor to monitor battery bank. At higher rates of charge (and high charging voltages), it is very possible to over heat a battery bank (fire/explosion/etc.).

Now the details... At below ~80% state of charge, a lead acid battery is very near 100% efficient at charging. And somebody here said that electric car hobbyist will charge at near 100% rate of charge for a deeply cycled lead acid battery.

However, when the battery nears 100% state of charge, then the efficiency drops to zero--The two products are hydrogen and oxygen gasses, and heat... At that point, even a 2.5% rate of charge will significantly raise (and even overheat) the temperature of the battery bank.

-Bill

morpho · January 2016

Thanks BB,
Interesting.

Still not really sure where to set the upper limit.
I have two temp sensors on the bank and they never really get much past 78 degrees...if you can trust the sensor as accurate that is. I guess my biggest concern is making this bank last a few years longer than the last one. I'd hate to be slamming amps at the bank and shortening the lifespan.

BB. · January 2016

If the bank is staying cool... you are certainly not over charging (over equalization is a somewhat different question--How much water do you add--every two months, almost never, more often than once per month, etc.)?

A good engineering rule of thumb... For every 10C (or 18F) increase in temperature, you get a 1/2 reduction in life. For batteries, ~25C is considered "nominal" temperature.

And conversely, every 10C reduction in temperature is a 2x increase in life (our Canadian friends get almost 2x longer battery life than down here in the warmer states. I believe the low winter temperatures are a big help there.

So, you end up with two major factors in life... Thermal aging, and cycling aging. If you have a lot of cycling, thermal aging is not going to be as important. If you have only (for example) weekend/seasonal usage, then thermal aging is going to be a larger cause for end of battery life.

-Bill

zoneblue · January 2016

0.1C is the guideline for all FLA batteries. For lots of reasons.

However as pointed out, on solar, as you increase the PV to battery ratio, the damage you might otherwise do from high C rates is somewhat mitigated by the the daylight ramp up, and by the decreasing amount of time it takes to charge at high C rates. For instance, if you were capable of pumping at 0.3C at 12 noon, and your SOC was highish,in practice your battery will be in float prior to noon, before they can ever be hit with 0.3C.

BUT... this can go pear shaped when if it rains in the morning, then suddenly the sun comes out, pouring 0.3C into your batterys at noon. Or, you have heavy loads causing the battery to rebulk. Or you start the day at unusually low SOC. etc. Only you know if or how often those sort of scenarios occur in your application.

However... there are at times benefits in being able to generate higher C rates, say if your sunny weather comes in short bursts. As far as charge controllers go, the classic is theoretically able to seperately limit current to the batterys without necessarily limiting the current to the load... but believe we are still waiting for boB to do the required firmware patch for it.

morpho · January 2016

BB...I probably add about 2 litres over a two month period....so about .08 litres per cell +/- Some cells were larger consumers than others though. Plus I had a battery go bad twice (same spot in the bank) It was constantly spitting electrolyte and making a mess of things. I'll be watching this bank closely for a while I suspect.

The bank is our 24-7/365 power for the house and we typically see a 15-20% reduction through the evening in winter and 10-15% in summer. I typically put the generator to it any winter day I don't expect sun....basically I treat it like there is no such a thing as "X" days of back-up. So it gets cycled, but not typically very deeply and battery temps are in that 77 degree range. Though I would like to reduce it a little. Have been thinking of bringing in cold air from outside (typically well below freezing) and mixing it in with a bit of the utility room air (almost always 77 degrees). I was even thinking about pumping water to remove the heat...(think fancy gaming computers) or maybe even just little freezer packs between the batteries. Probably not worth the effort. Though I am trying for the world record on GC lifespan...cause I am CHEAP!

I have changed a bunch of things on this new bank, so it will probably do different things than the last one. The old box had the batteries in the common four in front and four in back with about an inch between them. The cables were way too long (22") and were snaked all over the place. The new box has the batteries side by side in a single row with 3 inches between them for better airflow. The interconnect cables are 8 inches now. Not sure this will help much, but it sure is neater in there!

Zoneblue...You have described my situation almost spot on. For nine months out of the year I am in float well before noon. On cloudy summer days I typically hit float about 2 or 3 pm. It would have to be a dark and stormy day for me to not at very least not get most of the way through the 3 hour absorb. I have no loads that tap the system in any unexpected way. Its all very predictable.

I should have put the CC and remote in the living room so I could sit on the sofa and not have to stand in the utility room with the cats litter
box and the recycling!

Max Current for batteries discrepancy

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