Battery Charging and Readings

[Dockhead]

Originally Posted by sstuller

Increasing battery bank size without a corresponding increase in charging capacity will increase charging times versus a smaller bank with a higher rate of charge. This is based on the absorption calculation in the Rolls battery manual. Telling people that increasing bank size so you never get below 80% SOC for the sake of battery life always leaves their bank in the absorption state of charging.

Depends on the the charging capacity; you can't use the word "always" here. If it was marginal before, then yes. If it was not, then probably not. Pretty basic stuff - always be sure the charging system capacity is a decent match for the bank - 10% to 20% C in the case of the battery charger.

Sent from my D6633 using Cruisers Sailing Forum mobile app

[StuM]

Originally Posted by Dockhead

Pretty basic stuff - always be sure the charging system capacity is a decent match for the bank - 10% to 20% C in the case of the battery charger.

Or 25-30% for AGM

[sailinglegend]

Originally Posted by conachair

What criteria are you using to determine fully charged?...

The battery must reach 14.4v (@77F) and it is fully charged when the charge current going into the battery is 0.5% of the bank capacity (C). Staying at 14.4v usually never happens because the charger - solar, alternator or shore power, will have switched down to float long before this and so the charging current will drop by a factor of 4-5 times. As soon as it switches to Float voltage you can force the charger back to 14.4v and will see the charging current is not at 0.5% of C. Float mode doesn't mean fully charged - it could be as low as 90%. When in Float there will never be enough hours left in the day to get to 100%.

Staying at 14.4v until the battery is fully charged is not a good idea - ideally it should switch to a lover Float voltage with a charging current of about 2% of C and slowly top off the battery to 100%.

Getting back to 100% every 2-3 weeks is important otherwise the Lead Sulfate crystals, which always form as part of the discharge cycle, will harden on the plates and reduce capacity. Equalisation after this period may remove some of this Lead Sulfate. If left for several weeks these hardened crystal can never be removed. This is the slow downward cycle of the battery losing capacity. Don't fully charge every month and your capacity may be down to 98%. The next month it will be 98% of 98%..... and so on until the battery dies.

Originally Posted by conachair

And equalising is perfectly feasible with a genny, doesn't have to be all in one hit, you can split it over 2 days if the noise gets too much.

I'm not sure about this - the batteries must be fully charged before EQ starts. Also be aware that Equalising and Battery Conditioning are often confused and interchanged. Battery Conditioning is usually an extreme form of Equalisation designed to remove the Lead Sulfate crystals. Equalisation may only mean slightly raising the voltage for a short time to help remove stratification and mix up the acid evenly. This usually happens with regular and sustained Absorption voltages.

[masonc]

Originally Posted by sailinglegend

Equalisation may only mean slightly raising the voltage for a short time to help remove stratification and mix up the acid evenly.

Absorb and equalization are not the same. Interestingly, one of the purposes of equalization, the mixing of stratified electrolyte, is somewhat redundant in sailing boats, as the batteries get thrown around in choppy water enough to mix up the electrolyte, in fact I believe that is a reason that FLA may last longer on boats than on land. I sometimes bump my land batteries to help release the gas bubbles accumulating on the plates, probably doesn't help but makes for a nice gurgling sound.
Equalization is a bit of a black art, in one way it damages the batteries and in another, it revives them. To be effective, you need to get them bubbling vigorously, so as to mix up the fluids and shake off the sulphate crystals. It is a deliberate overcook, so it needs to be done carefully and with control of the temperature.

[sstuller]

The absorbtion charging time is based on battery capacity divided by charging current. If you increase the battery capacity but leave the charging current the same the charging time will increase. It's easy to increase battery capacity on a boat but charging capacity is usually fixed.
Thanks.

[mitiempo]

Originally Posted by sstuller

Increasing battery bank size without a corresponding increase in charging capacity will increase charging times versus a smaller bank with a higher rate of charge. This is based on the absorption calculation in the Rolls battery manual. Telling people that increasing bank size so you never get below 80% SOC for the sake of battery life always leaves their bank in the absorption state of charging.

Not always if daily usage does not increase.

[mitiempo]

Originally Posted by sailinglegend

.......Staying at 14.4v usually never happens because the charger - solar, alternator or shore power, will have switched down to float long before this and so the charging current will drop by a factor of 4-5 times. As soon as it switches to Float voltage you can force the charger back to 14.4v and will see the charging current is not at 0.5% of C. Float mode doesn't mean fully charged - it could be as low as 90%. When in Float there will never be enough hours left in the day to get to 100%.

Quality solar controllers, Victron MPPT's for example, allow full adjustment of absorption and float voltages, absorption time, and you can turn float off if desired.

[a64pilot]

Originally Posted by sstuller

The absorbtion charging time is based on battery capacity divided by charging current. If you increase the battery capacity but leave the charging current the same the charging time will increase. It's easy to increase battery capacity on a boat but charging capacity is usually fixed.
Thanks.

Yes, but. Most boats I'd think have excess charging capacity, if you count the alternator.
I may be a little unusual but in theory I have 390 amps of charging capacity with my charger, the inverter / charger, Solar and alternator.

But yes your electrical system, is a system and it needs to be matched or you will not get full use of it, but excess charging capacity does not hurt, cause the batteries will only accept what they can

[Stu Jackson]

Originally Posted by sstuller

The absorbtion charging time is based on battery capacity divided by charging current. If you increase the battery capacity but leave the charging current the same the charging time will increase. It's easy to increase battery capacity on a boat but charging capacity is usually fixed.
Thanks.

This is wrong based on the words being used.

Originally Posted by mitiempo

Not always if daily usage does not increase.

This corrects the upper quote, thanks, mitiempo.

Why do you think folks don't get this? We see it all the time. Failure of high school math?

If one increases bank size/capacity WITH THE SAME DAILY LOAD and the same charging input, then nothing changes as far as charging is concerned. (Battery acceptance has a small impact)

What does happen is that the battery bank lasts longer.

Of course, if you increase the bank capacity and then, with the same daily load/draw go out for two days, then, of course, charging will take longer, because you taken out twice as much.

[StuM]

Originally Posted by sstuller

The absorbtion charging time is based on battery capacity divided by charging current. If you increase the battery capacity but leave the charging current the same the charging time will increase. It's easy to increase battery capacity on a boat but charging capacity is usually fixed.
Thanks.

If you had enough Amps for bulk charging a smaller bank, you've got enough Amps for absorption charging a larger bank at higher Amps than you could achieve with the smaller bank. Sorry, I'm still not seeing your figures indicating that it will take longer.

[masonc]

"The absorption charging time is based on battery capacity divided by charging current"
Bulk charging current is calculated as a ratio of the maximum discharge rate, commonly the C/5 rate. My Trojan IND17 can be charged at a maximum of 130A as long as the charge current is temperature controlled.
Absorption is different. Absorption, also known as topping charge, only begins when the battery has accepted almost of the of the energy is can hold. From then on, the voltage is held constant high level and the current is limited to allow saturation of the plates. Absorbtion takes a long time and cannot be rushed. Its essential to the long life of FLA to allow an absorption charge every week or so if possible. My IND17 absorb at 14.5.
Float charge plays no significant part, it's just a state of readiness. However, it is important that float is not set to high so the battery does not continue to charge after its absorb stage.
Equalization is a destructive charge. It raises the voltage to about 15.5-16v, causes the battery to bubble, and causes a little damage. Because it is vigorously charging, it mixes the electrolyte to prevent stratification and sheds hardened sulphur from the plates. The more often your battery completes the full charge cycle, the less often you need to equalize.

[sstuller]

If you have to replace 40 AH with 20 AH during the bulk phase and 20 AH in absorb the absorb phase will take about twice as long as the bulk phase. This is the reason for the old saw about keeping charging and discharge between 50% and 80% SOC. The absorb phase just takes too much time. Thanks.

[StuM]

Originally Posted by masonc

Absorption is different. Absorption, also known as topping charge, only begins when the battery has accepted almost of the of the energy is can hold. From then on, the voltage is held constant high level and the current is limited to allow saturation of the plates.

A couple of misconceptions there. Absorption begins when the battery can no longer physically accept the desired/available amount of charge (Amps) at the set peak voltage. At this point, the battery is generally at around 80% SOC and can still accept a lot of energy.

The current is not externally "limited to allow saturation of the plates", it is only "limited" by internal resistance in the battery and the amount of charge that the battery can physically accept at the set voltage.

[StuM]

Originally Posted by sstuller

If you have to replace 40 AH with 20 AH during the bulk phase and 20 AH in absorb the absorb phase will take about twice as long as the bulk phase. This is the reason for the old saw about keeping charging and discharge between 50% and 80% SOC. The absorb phase just takes too much time. Thanks.

Acceptance rate doesn't just fall off the cliff when you go into absorption mode, it tapers down linearly. If you double your battery size, your batteries will be accepting twice as much current at every stage of absorption and will accept the full 40AH deficit in the same time as the original battery accepted the second 20AH. In other words, when fully charging your battery bank, you will save that initial 20Ah bulk charging time so your batteries will be charged quicker.


That old saw, like many, is just an "old wive's tale" and is damaging to your batteries.

[DeepFrz]

I don't believe that old saw was an old saw. I think it was a description of how most people kept their batteries charged in the days before solar.