# Minimum charge current

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Solar Expert Posts: 1,571 ✭✭
Hi all, I keep seeing references to the minimum 5% of C20 charge rate for FLA batteries, but I can't find any hard references from manufacturers to this rule of thumb.

Can anyone provide any hard info of where this rule comes from?

(PS. this is for my own knowledge, I'm not planning on undercharging a system, but I would like to see some data to support this well used rule of thumb)

• Solar Expert Posts: 1,484 ✭✭✭✭
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Re: Minimum charge current

The absolute minimum is current it takes to get battery up to equalize voltage once in a while. This current get greater the older the batteries are.

Absorb level will mix up the electrolyte so you don't get stratification of electrolyte.

5% C20 is about what it takes for an middle aged battery.
• Solar Expert Posts: 1,571 ✭✭
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Re: Minimum charge current

Yep, I've read that many times on this and other forums, what I'm after is the basis for this rule of thumb. I can't find any reference to a minimum charge current in the Rolls battery manual for example.
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Re: Minimum charge current

In the Rolls--we use the 5% rate of charge based on their recommendations for equalization. They actually say "5 amps per 100 ah"... But I cannot find their manual online at the moment (you have to fill out a contact sheet before they will email you a link/copy).

Here is one link on "corrective equalization":
Charge at a low DC current (5 A per 100 AH of battery capacity).

Another is just a practical lower charging limit for an older battery bank... Since you can only charge around 8 hours per day from a solar charger (rough number):
• 5% * 8 hours / 24 hours per day = 1.67% average rate of charge per 24 hours
Flooded cell batteries near the end of their life (like big ol' forklift batteries) can approach 1-2% self discharge per day... So, a 5% charge rate on a solar charge controller is just about using all of its power to make up for self discharge losses.

Obviously, people are probably going to replace their bank at that point (or possibly purchase another set of solar panels to get to 10% rate of charge...

There is one reason we talk about balancing bank size to solar panel capacity... Very large batteries (relatively speaking) just don't charge well on small solar arrays.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 59 ✭✭✭✭
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Re: Minimum charge current

The Crown deep cycle battery maintenance guidelines differentiate charge procedures depending on how the batteries are used.

http://www.crownbattery.com/PDF/Safety.First._Deep%20Cycle%20Batteries.pdf

Renewable energy use is classified as batteries discharged less than 50 percent.
As a general rule, the total input amperes from your RE charging source should be between 10% and 20% of the total ampere-hours (20 Hour Rating) of the battery system capacity.

For Vehicle use discharges are greater and
Crown Battery recommends electronically controlled automatic chargers that are programmed to deliver a high constant current rate of 12 to 18 amperes per 100 ampere-hours (20 Hour Rating) of
battery capacity.

They go on to say
To ensure optimum battery performance, total recharge time should in all cases be limited to 10 hours

With a deeply discahrged battery 5 percent will take significantly longer than ten hours. 5 percent seems to be the minimum charge rate if you are using your batteries with a discharge of significantly less than 50 percent.

Zeuspaul
• Solar Expert Posts: 1,571 ✭✭
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Re: Minimum charge current

Right, so this 5% mininum is partly based on the usage pattern and doesn't represent the absolute minimum current that will still charge a battery. In Spain a common usage pattern for off-grid systems is in weekend homes where the PV has 5 days to charge the battery before it's used. So in these cases, it could be acceptable to charge at a lower rate? As long as you're at least beating the self-discharge rate.
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Re: Minimum charge current

Well, a low current will take longer to recharge the battery above ~75% State of Charge--So now you have your batteries setting longer in the "sulfation zone"...

And, especially, for tall flooded cell batteries, undercharging will:
10. Prevent undercharging the batteries. Undercharging causes stratification.

I guess, if your lower charging current can get the battery voltage high (i.e., upwards of 14.8 volts per Trojan link above)--You will drive the mixing. If your charging current is unable to reach a "gassing voltage" (above 13.7 volts at least every few days???) then it is not enough current for a flooded cell battery.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,783 ✭✭✭✭✭✭
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Re: Minimum charge current
stephendv wrote: »
Hi all, I keep seeing references to the minimum 5% of C20 charge rate for FLA batteries, but I can't find any hard references from manufacturers to this rule of thumb.

Can anyone provide any hard info of where this rule comes from?

(PS. this is for my own knowledge, I'm not planning on undercharging a system, but I would like to see some data to support this well used rule of thumb)

I would say it is the minimum that you want on the winter solstice if you do not want to run a generator! You will learn this soon I believe! Good Luck
"we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
E-mail offgridsolar@sti.net

• Solar Expert Posts: 1,571 ✭✭
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Re: Minimum charge current
You will learn this soon I believe! Good Luck

I'm counting the days Sunny Island will be delivered anytime now, and my 900Ah 48V forklift battery will leave the factory next week. Can't wait to have power without turning the stinky diesel on.
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Re: Minimum charge current

Just to reference another 5% charge current source:
BB. wrote: »
WARNING: This procedure should only be done in a well ventilated area because a significant amount of hydrogen gases may be released from the battery.

1. Stabilize the battery at 68-86°F (20-30°C) for at least 24 hours prior to testing.
2. Charge at a constant current of 5% of rated (20 hour) capacity until the voltage reaches 15.6 volts (7.8 volts for a 6 volt battery), then continue charging at this rate for an additional 4 hours. Note that the charging voltage may get as high as 18.0 volts (9.0volts for a 6 volt battery), so the power supply must be capable of outputting this level to maintain constant current. This constant current charge may take 16 to 20 hours.

CAUTION: If the battery becomes hot (above 130°F/55°C) during this charge, stop the current and allow the battery to cool to room temperature before continuing the charge.

3. Perform a capacity test as described in Section 5.7.
4. If the capacity is less than 80% of the battery’s rating, steps 2-3 may be repeated up to 2 times. Be sure to start the constant current charge with a fully discharged battery.

NOTE: The above procedure should be performed by an experienced battery maintenance facility utilizing the proper charging and test equipment. Concorde recommends the use of our Model CA1550 charger/analyzer. For more information regarding Concorde’s test equipment go to: www.concordebattery.com/accessories.php.
I agree--that sounds like something that would cause more harm than good... But I am not a battery engineer.

And for the other thread "where does the 5% rate of charge rule of thumb come from"--Here is another example.
-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: Minimum charge current

i might add that for some batteries this 5% min may not apply for i have always advocated for one to go by the manufacturer's advice. it is an arbitrary figure and is a minimum that the batteries will see in their net charge for often these batteries will have a load on them at the same time it is to be charging. with the many types and manufacturers out there the range of 5-13% usually works out for the majority of them.
constant current is another method to charge batteries, but isn't encountered as much for it is often easier to regulate the voltage.