Battery Charging and Readings

[StuM]

Originally Posted by 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.

Yep, which was also the days when batteries had to be replaced much more frequently.

[sstuller]

Paraphrasing from a Sandia lab report referring to a large battery bank relative to the load - "Charge efficiences at 90% and greater were less than 50% requiring a PV array that supplies more than twice the energy for a full recovery charge." These were tests run on a Trojan FLA battery. Thanks.

[StuM]

Originally Posted by sstuller

Paraphrasing from a Sandia lab report referring to a large battery bank relative to the load - "Charge efficiences at 90% and greater were less than 50% requiring a PV array that supplies more than twice the energy for a full recovery charge." These were tests run on a Trojan FLA battery. Thanks.

The actual wording is:

"Charge efficiencies at 90% SOC and greater were measured at less than 50% for the battery tested here, requiring a PV array that supplies more than twice the energy that the load consumes for a full recovery charge.

Did you read this part:
"Battery charge efficiency is also a function of charge rate, with lower
rates resulting in higher efficiencies. The larger battery will be operating with a lower charge rate, which will result in higher charge efficiency"

No one disputes that you need to size your charging source appropriately for the size of your battery bank.

[masonc]

Originally Posted by sstuller

Paraphrasing from a Sandia lab report referring to a large battery bank relative to the load - "Charge efficiences at 90% and greater were less than 50% requiring a PV array that supplies more than twice the energy for a full recovery charge." These were tests run on a Trojan FLA battery. Thanks.

It's academically interesting that there are different efficiencies at different charges levels, but you have to charge all the way or face sulphation, so in the real world it makes little difference.

My home system has 7KW of PV charging 49KWh, 7KW x 5 hours full sun at 18 latitude = 35KWh, so when we use 30% of capacity we are fully recovered.
49KWh x 0.30 = 14.7KWh used per day.

Our system fully charges every day so the expected battery life is long.
If you look at most cruisers, I expect the number of days to fully recover with adequate time to absorb charge is pretty small, most complete a full charge cycle when on shore power or using the engine.

I expect LiFePo4 to exterminate FLA in boating, as it wins on every count. It is much more suitable to boating than FLA and will be cost effective very soon.

[sstuller]

StuM and I are in agreement. We are both saying the same thing in our own idiosyncratic ways. Thanks.

[sailinglegend]

Originally Posted by masonc

...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....

As already pointed out these two sentences are nonsense.
It would seem you've got your information from the Battery University, and stole their graph without crediting them. They are the only people I can find who call Absorption a Topping Charge - except all the other sites who have directly copied Battery University's errors.

Originally Posted by masonc

...Float charge plays no significant part, it's just a state of readiness...

This again is a load of nonsense from the Battery University site. Their terminology of "Topping Charge" suggests that this tops up the battery to 100% and Float just compensates for self-discharge. This is in fact what they say but couldn't be further from the truth. Float can happen at 90% SoC or lower and depends on the settings of the Charge Controller. On good chargers Absorption can be extended a few hours till the batteries are at 95%, but it should not be much longer. Battery University say don't stay at "Topping Charge" for more than 48 hours!!!! They are talking about Automotive batteries and charging - nowhere do they mention deep cycle batteries!

Too many "experts" post on here who don't fully understand what they are posting. Electrickery is hard enough for the newbie to understand anyway.

The first graph is the Battery University's Charge Graph. Mine is the second, and many other sites will show this true Voltage/current relationship that Battery Charger manufacturers don't really want to talk about. The key point is the difference at the transition to Float mode where there is still considerable charge current going into the battery which drops dramatically as the voltage drops to a lower Float voltage. This is why in Float mode it takes a very long time to fully "Top Off" the battery to 100% fully charged.

[Cheechako]

Typically a 90% full battery is said to be 12.5 volts. 50% is slightly above 12 volts.
A couple years back I put new batteries in my runabout. After charging all day with a commercial style service station charger, they typically were at 12.45/12.46 volts after resting overnight, disconnected from any drain. They read this same reading after running the boat all day as a charge source too.