# min charge amps for battery size

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Solar Expert Posts: 242 ✭✭✭
I'm getting new batteries and hoping to double autonomy. I think the standard is that the charging amps need to be 5-10% of battery capacity.

But, which capacity? The standard seems to be the 20hr discharge rate. So, 1000AH at 24v is 800AH at the 20 hr rate. Panel size is 24*80=1920w without factoring in losses, etc. But if you use the 50hr discharge rate, its 50*24=1200.

I ask because in my current bank, I never go below 50% and really never below about 65%. The generator will kick on.

So I guess the question is does it really need to be sized at the 20 hr given that you never discharge down to 20%?

• Super Moderators, Administrators Posts: 33,530 admin
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Re: min charge amps for battery size

We use the 20 hour rate for all of our assumptions/calculations. The 20 hour rate, more or less, approximates the average discharge current for many off-grid users. Also, 20 hour rate = 5% rate of charge.

I believe Rolls/Surrette uses a different discharge rate for their rules of thumbs if you read through their documentation.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,583 ✭✭✭✭✭
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Re: min charge amps for battery size

Keeping in mind, I've recently been called out on my last battery caculation....

Generally, it's the 20 hour rate, the charge rate caculations are base on. But the factory specs are the real answer.

If your batteries are tall cases, you need the higher charge rates, to insure electrolyte does not stratify, the bubbles need to stir the fluid. (not needed for AGM batteries)

Daily, you don't want to go below 75% of full, that's when sulfation begins, and why invite that! Most folks design for 80% daily, and that gives a couple day cushion for clouds.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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• Banned Posts: 17,615 ✭✭✭
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Re: min charge amps for battery size

The standard method used around here is 5-13% of the batteries' total Amp hours at the 20 hour rate.
A battery rated at 100 Amp hours at the 20 hour rate should be charged at 5 to 13 Amps net.
The depth of discharge does not really factor into it, although you could favour the low end of the range if your discharging is minimal providing it is a net rate.
The array size is based on being able to supply this peak charge current losses included.

Nothing is absolute. They are rules-of-thumb designed to give you a short cut to proper system balance. Here's some example numbers:

100 Amp hours @ 12 Volts. DOD 25% (25 Amp hours) = 300 Watt hours DC.
10% charge rate: 10 Amps @ 12 Volts = 120 Watts, less derating: 156 Watts total.
156 Watts derated to 120 * 4 hours equivalent good sun = 480 Watt hours DC.
5% charge rate: 5 Amps @ 12 Volts = 60 Watts, less derating: 78 Watts total.
78 Watts derated to 60 * 4 hours equivalent good sun = 240 Watt hours DC.

As you can see, there is always some margin. The panel sized for 10% rate results in more Watt hours potential than would be used at 25% DOD. The panel sized for 5% rate results in less Watt hours potential than would be used at 25% DOD.

If you switch from pure DC to having to accommodate additional losses through AC conversion the 10% charge rate coupled with 25% DOD maximum works best for all-around functionality.
• Solar Expert Posts: 3,208 ✭✭✭✭
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Re: min charge amps for battery size

Just to beat on that drum again,

High Charge RATE has little to do with electrolyte MIXING. Mixing. primarily, takes place in the later part of the Asorb stage -- when the FLAs are bubbling, and, of course, even more so in EQ.

The higher charge rate -- 10ish percent of 20 hr capacity does help keep active marterial exposed to electrolyte, and helps maintain the Charge Accpetance. Any of the FLAs that I've been responsible for have evidenced very little bubbling during the high-rate part of the charge cycle. This has applied to the Trojans, and Surrette batteires in use here.

The thing that causes the mixing is the part of the charge cycle that consitutes the Charge Inefficency attributed to FLAs -- the breaking down of the H20 of the electrolyte into Hydrogen and Oxygen = BuBbLeS = MIXING

Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: min charge amps for battery size

guys,
bb. said it in post 2 and he has it right that it is a discharge rate. that means a 100ah battery rated for 20hrs will discharge at 5a for 20hrs charging is really just a percentage of the total ah rating of the battery.

broken record time again.:roll: bb. you started this misnomer and i may call upon you to back up this 75% crap as it is either flat out wrong or misinterpreted.

it is a misnomer that sulfation starts only when going below 75% soc as this is not true. if it were true then we wouldn't need to worry about sulfation if we never gave a battery an absorb charge, which is deficit charging, with an soc of about 80%-90% depending on the battery. the remaining parts of the battery that did not charge will sulfate if left to remain uncharged long enough.

another misnomer may arise if you did deficit charge the battery because you never absorb charged that once that 10%-20% of the battery has sulfated that it cannot sulfate any more if you continue to deficit charge it. the sulfation will just pick on a different area of the battery on a percent basis and will continue to do its damage.. only bringing a battery up to 100% soc often enough may stop that sulfation.
• Solar Expert Posts: 242 ✭✭✭
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Re: min charge amps for battery size

ok. thx.

I'm still somewhat confused. My understanding was the high volts of the EQ cycle was what stirred the acid and put the sulfate back into solution. If that's the case, then the panel size should just be sized to return the battery to full while there are system loads. But I guess what I'm hearing is that we also need a certain amperage to do "something" to the battery similar to what the high EQ voltage does.

On a different note, if we are worried about stratification why don't we use bubblers like those in fish tanks..a small low pressure airline that simply bubbles air in each cell?

The problem is that in summer I almost don't need batteries. I dump excess juice by running 2 air conditioners and a dehumidifier. In winter, I'm stuck running the generator. In late fall and early spring i could use about 2x autonomy which I think would cut yearly generator use by 50%.
• Super Moderators, Administrators Posts: 33,530 admin
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Re: min charge amps for battery size

Niel,

The 75% comes from one of the battery faqs that we recommend:

http://www.batterystuff.com/tutorial_battery.html
*Sulfation of Batteries starts when specific gravity falls below 1.225 or voltage measures less than 12.4 for a 12v battery, or 6.2 for a 6 volt battery. Sulfation hardens on the battery plates reducing and eventually destroying the ability of the battery to generate Volts and Amps.

Regarding pumping air to mix electrolyte, I believe that some commercial battery systems are available.

And one member (John P?) did the testing in his lab and posted the results (not worth it, as I recall.

Posting from my phone, so I cannot look for the thread right now.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: min charge amps for battery size

you won't get around charging losses and minimums. some batteries even self discharge at several percent per day. the 5%-13% itself isn't written in stone, but is meant as a rough guideline. you could charge batteries with solar at say a 5% rate and it may need a good kick in the rear with a higher shot of current every few days or even a week depending on the circumstances. eq charging is a way of balancing the charge between batteries or even cells and the stirring of the electrolyte is not the purpose of eqing.

we all suffer from less pv output in the winter unless you're around or below the equator. making winter production right produces quite the excess in summer. it's your call if and how often you'd like to run the genny by design.
• Banned Posts: 17,615 ✭✭✭
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Re: min charge amps for battery size
jtdiesel65 wrote: »
ok. thx.

I'm still somewhat confused. My understanding was the high volts of the EQ cycle was what stirred the acid and put the sulfate back into solution. If that's the case, then the panel size should just be sized to return the battery to full while there are system loads. But I guess what I'm hearing is that we also need a certain amperage to do "something" to the battery similar to what the high EQ voltage does.

With the proper charge rate, Voltage, and Absorb time the need for EQ re-mixing can be kept to a minimum. So battery makers recommend regular EQing, some (like Trojan) recommend doing it only on an "as needed" basis. This is because the higher Voltage levels of the EQ cycle subject the battery to additional heat stress, which is not good.
On a different note, if we are worried about stratification why don't we use bubblers like those in fish tanks..a small low pressure airline that simply bubbles air in each cell?

This has been suggested and tried. Believe it or not, some forklift batteries actually do run this sort of agitation. Forum member john_p here tested out some results on this process. I believe the results were that the advantages were pretty small over the extra power needed for aeration. The larger the battery, the better the return.
The problem is that in summer I almost don't need batteries. I dump excess juice by running 2 air conditioners and a dehumidifier. In winter, I'm stuck running the generator. In late fall and early spring i could use about 2x autonomy which I think would cut yearly generator use by 50%.

Off grid systems always need batteries. They do not work without them. Sounds like your system is bit unbalanced in terms of load/batteries/panels. It's tricky to get it right.

For what it's worth, sulphation occurs on all lead-acid batteries starting at day 1. FLA's sulphate faster than AGM's which sulphate faster than gels. The deeper the depth of discharge and the longer the batteries are left partially discharged the faster the sulphation occurs. Also remember that there's three kinds of sulphation; that which normally occurs in the course of the discharge, that which needs equalization to reverse, and that which is irreversible and turns the battery into scrap metal.

Bill 75% SOC threshold is arbitrary, but you have to have a number somewhere that says "oops, we're in trouble now". Not all batteries are created equal (even the same battery), much less maintained that way.

If we leave out the rules-of-thumb and generalization the replies here will become an very long lecture on electrical theory that would send most newbies running to jump off a bridge. No one wants to be hit in the face with all the data all at once; you have to ease in to it.
• Solar Expert Posts: 3,208 ✭✭✭✭
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Re: min charge amps for battery size

jtdiesel ..

Most of the sulphates are returned to the electrolyte during Bulk and Asorb stages of the charge cycle. However, some cells lag a bit during recharge. These have a bit lower SG, and need a bit more charging when the Asorb stage ends. Often this divergence increases as the bank gets discharged/recharged.

EQing the bank is an attempt to charge ALL cells in the bank, this overcharges the cells with higher SG, in order to fully recharge the lagging cells. Overcharging causes plate erosion (corrosion), and shortening life. It is a tradeoff. Too little EQing and the sulphates harden on the plates, and become more and more difficult to get back into solution over time. Too frequent EQing, and there is that erosion thing. Both of these reduce capacity and shorten battery life.

You are correct, that EQing DOES reduce or elinimate stratification and sulphation. Stratification also causes corrosion of the (lower portion) of plates. We need to be a balance of a number of factors when charging and maintaining batteries.

One very good thing about FLA batteries in general, is, that they are quite tolerant. We do not need to be perfect. Caring about and paying attention to your batteries is the best thing that you can do. Most off-gridders that I know, pay little attention to their banks until the "lights go out'. Suddenly, batteries are important to them!

MHO, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.