Does keeping on float charge improve battery?

bgu1982

I was wondering if whenp my mppt controller reaches float mode does it benefit the battery bank to let them to continue in float mode. Or once float mode is achieved it's ok to add a higher load and start using the batteries. My battery bank reaches float mode around 2-3pm and sometimes I think it's  better to keep them in that float mode longer thinking it will improve performance. I've been doing this for the last couple days so any input would be appreciated.
 Thank you

What I mean by improved performance is that the battery will discharge slower since it sat in float mode for a longer period. Probably nonsense but  asking because I've been doing this the last couple day.

BB.

More or less, your batteries "wear out" from two different processes... One is simply age and temperature. A battery may last 7 years at 25C/77F average temperature. If your in a hot area where the battery sits at 35C/95F, then the battery bank will probably "age out" in 1/2 the life or ~3.5 years (heat is "bad").

The other aging is cycling the battery bank. More or less (using made up numbers) say the battery bank has 500 cycle life and is 100 AH capacity. Or a cycle life of 500 cyc * 100 AH = 50,000 AH of battery cycle life (discharge/recharge AH life).

Basically, if your loads are used during the day, then you are not "cycling" the battery bank. So 'no cycle", then the "cycle counter" does not -1 cycle from your "500 cycle" life.

"Float" charging is just the voltage that keeps the battery from "self discharging" over time. But, the solar charge controller" will supply not only the "self discharge current" (which may be 1% for Flooded cell lead acid or less to 0.01% or less for AGM/Li Ion).

But the solar charger will also supply your load currents too (up to the amount of solar power/current available). From the battery point of view--It does not "see" any discharge current, therefore it will not "cycle life" age. The solar charge is happily supplying the load current.

Now--There is no "free lunch" here... You still want to charge your battery bank--So the "day time loads" still need to be less than the amount of energy the battery bank needs to get recharged from your night time loads and supply your daytime loads.

Different battery chemistries do have different "needs" too.

Flooded Cell lead acid batteries tend to "like" 10%-13% when charging. The "higher current" helps keep the plates from sulfating (current and gassing keep the plates "clean"). So, ideally, on average, you want 5% to 13% rate of charge minimum even when the solar array is also powering your daytime loads. Over ~13% rate of charge, Lead Acid batteries tend to get "hot"--Suggest a remote battery temperature sensor to monitor battery bank temperature (hot batteries need slightly lower charging voltage).

LiFePO4 batteries--They do not have a "minimum" charge rate and don't use a temperature/voltage offset--A couple of the reasons they are nice (high or low rate of charge--They are happy).

-Bill

bgu1982

So should I wait till my batteries are in float mode before adding a load so they stay full untill after sundown.

Thank you bill

NANOcontrol

Some have also said that a really actively used solar system seldom gets out of bulk charge with enough time to equalize or balance. That can be a trap for those who use a battery voltage to control a load like a water heater.  I think our batteries are only happy when we go away for a while.  With large systems you can get away with a lot of stuff.  If you are tightly matching energy loads it could become an issue.  Six years later a battery goes bad. Is that procedure or just bad luck. I like to pull excess power directly from the panels and let the controller do its thing.

BB.

Once the battery bank is in the 80-90% State of Charge, the lead acid batteries are accepting less than 100% rate of charge (say 10%) anyway... So, if you are still early afternoon, and charge controller is in "absorb" or float, you can certainly start to use other loads... Ideally, you the Available Solar charging current - loads = Charging current for battery bank.

With solar, you are in the "use it or lose it" operations. If your array can supply 50 amps, and the batteries are using 10 amps at the tail of the tail of the charging (>~90% full)--You have ~40 Amps available for "other work" while the sun is still up.

When the controller is in Bulk Charge mode--That is 100% of the available array current is being sent to the battery bus (whether 100% to battery bank or 50% to battery and 50% to your "day time" loads--Only you know what is running).

Early afternoons, battery getting towards full, you can certainly start up your optional day time loads... Just not so many amps, that you "starve" the battery bank of charging current. Any current that goes from charge controller to loads and "bypasses" battery bank--Is less battery cycling and longer battery life.

Just watch the battery bank state of charge. As long as you are getting >90% on sunny days--The bank should be fine.

You want to avoid over discharging and/or chronic under charging. That will kill the bank.

You can do things too like cycle from 50% to 80% daily, and once a well charge to >90% to 100% SoC. That is actually quite efficient for lead acid batteries and they should not "quickly sulftate" as long as the batteries are cycling.

Letting the bank "sit" for day at less than 75% SoC (again lead acid battery chemistry) is when sulfation starts to accelerate.

Use your system, that is why you have it.

-Bill

Marc Kurth

My own observations have caused me to believe that NANOcontrol has a point. Lead-acid batteries that spend a considerable amount of time in Float, tend to remain in balance better - especially with multiple parallel strings . Most systems that I see are 1-2 strings and sized nicely so that most of year, this happens anyway.