When is a battery bank full?

[Mike Crothers]

My Magnum Battery Charger and Balmar Regulator have different ideas of when a battery bank is considered full. I have 830Amp Hours of AGM batteries by Rolls and I don't want to waste hours charging them at anchor. I killed my last batteries in 26 weeks and don't want a repeat.



I hear terms like taper voltage and tail voltage but I am unsure of there definitions if you use terms in your reply please define them for me.



Is charging the batteries ton when they are accepting 3% of there capacity (.03*830amphrs) a until they accept a continuous 25 amps a reasonable proxy for a full battery? This only takes 60 to 90 minutes each morning.



Or should I run my Honda 2000 for several hours once a week until the Magnum Charger decides the batteries are full? The magnum O&M says it will do an ABSORB cycle for two hours then FLOAT cycle for four hours and finally it will call them FULL. This seems arbitrary and a lot of hours (+6) of running a generator.



Thanks in advance for your insights into the Black Arts.

[rslifkin]

I'd consider them full enough for most purposes when you're down to 0.01C in absorption (unless the manufacturer has a different spec for when to end absorption). Truly full is that plus at least a few hours in float, but that's pretty hard to achieve without shore power or a combination of solar and generator (or a ton of solar).

[Sailmonkey]

See the charging parameters published here by rolls

https://www.rollsbattery.com/wp-cont...ser-Manual.pdf

You are mixing up voltage and current, you’re concerned with tail current at the absorption voltage.

Float isn’t charging the battery, it’s just maintaining it at 100% soc, you’ll need to program your charger to stay at adsorption voltage long enough to fully charge your battery. And that also depends on how much power you use from the battery before charging it, as well as the size of the charger.

[rslifkin]

Originally Posted by Sailmonkey

Float isn’t charging the battery, it’s just maintaining it at 100% soc, you’ll need to program your charger to stay at adsorption voltage long enough to fully charge your battery. And that also depends on how much power you use from the battery before charging it, as well as the size of the charger.

In theory float is just to maintain. But even after hitting the end absorb target, most batteries will still take a little bit of power in float, which eventually tapers down to near zero. As an example, Fullriver says their batteries are considered truly full after running absorption to their specs, plus an additional 8 hours in float.

[Mike Crothers]

Originally Posted by rslifkin

I'd consider them full enough for most purposes when you're down to 0.01C in absorption .

Can you help me understand what you mean exactly by 0.01C in absorption?

[StuM]

Originally Posted by Mike Crothers

Can you help me understand what you mean exactly by 0.01C in absorption?

A current in Amps equal to 0.01 or 1% of the banks capacity in Amp Hours.
In your case, that would be 8.3A


Your "charging the batteries ton when they are accepting 3% of there capacity (.03*830amphrs)" is 0.03C at which point they still have a fair way to go to reach full charge.

[Sailmonkey]

Originally Posted by rslifkin

In theory float is just to maintain. But even after hitting the end absorb target, most batteries will still take a little bit of power in float, which eventually tapers down to near zero. As an example, Fullriver says their batteries are considered truly full after running absorption to their specs, plus an additional 8 hours in float.

that's true, and all well and good if you're charging off of shore power, but off grid you need to essentially ignore float unless you want to run a generator all day (or install sufficient solar to do this).

Either way, I've a hunch the OP's charger is dropping out of bulk and absorption way too soon.

[Stu Jackson]

Mike, I see you contributed to the Balmar and SG-200 threads, too. IIRC, I posted my "Gotcha" article link in one or both of those. It also explains it, in addition to the tail current at absorption voltage. As does Maine Sail's programming a battery monitor article.

[Mike Crothers]

If possible please provide here a summary of your relevant points regarding "fully charged".

[Sailmonkey]

Originally Posted by Mike Crothers

If possible please provide here a summary of your relevant points regarding "fully charged".

According to the manual that I referenced above, your Agm bank should be charged at the voltage by temperature reference here



Until the charger output is within 3% of C.

So at a battery temperature of

50 deg F
And a charger voltage of 15.06 volts
You should be considered to be fully charged when there is no more than 25 amps of current.

However also not from the reading rolls suggests a charge efficiency of 80%. And a big charger.

[smac999]

have you adjusted the balmar settings? the deafault absorb time is only 15 mins. you'll need to greatly increase that for an 800ah bank. probably 2-3 hours. depends on the size of the alt.

to answer the question. batteries are generally full when they are taking less then 2% current at absorb voltage. so when the batteries are taking less then 16a at ~14.7v they are full. the problem is many chargers go to float to early so batteries never get full if it's already gone to float. and you see under 16a that does not mean they are full. you need to get the voltage back up to know for sure. shutting off the engine and restarting will restart the balmar timers.

you are never going to get that bank full from the engine or the gen. you'll be running for 5-6 hours to do that. it takes 2-3 hours to get from 95% to 100% you need lots of solar or a dock to get a battery to full. or a lot of fuel and noise patience.

do you have a battery monitor? to see battery net amps and AH. that's how you know it's full. charger amps does not tell you anything either. because some of that power is being used by the boat. you need to see battery net amps.

this is one of the reasons a battery charger can never know if a battery is full or not. you need to know when based on the battery monnitor

if you have a 100a magnum charger and 800ah battery bank. and you draw to 50% that is -400ah.

likly it should bulk charge for around 2.5hours at 100a that would give you 250ah back into the bank and put you to -150ah or 81%. now from -150ah to 0 it'll take another 3-4 hours in absorb to get to 100%. as durring that time the current will drop from 100a to 10. it should not go into float untill the batteries are already 100%. so yes it's around 6 hours to charge your batteries if you drop them to 50%

if you had lithuim batteries it would take 4 hours instead of 6 as they will take 100a the whole time.

[Stu Jackson]

Originally Posted by Mike Crothers

If possible please provide here a summary of your relevant points regarding "fully charged".

Overnight on a shorepower charger.


That's the simple, but true, answer.


You're a tough crowd, Mike. You want it simple?!?



smac discussed the shortcomings of shorepower chargers which in many cases drop to float altogether too soon, lengthening charging time by reducing voltage too soon.


As mentioned, tail current at absorbtion voltage can only be done with a shunt which measures the current. In many cases, egg timer chargers (which almost all shorepower chargers are) need to be restarted to keep voltage up.


Mike, that's all I got from 23 years at this stuff. It comes from reading a lot. Maine Sail covers it all but in a lot of different places. Most books simply do not. My ES101 has a lot of links.

Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

[Mike Crothers]

I put in a ticket with the manufacturer Rolls and they did a super job answering my questions which differed some from both Magnum and Balmar but at the end of the day I guess their opinion is the one that counts. I had not idea they would be so responsive and a lot less confusing than the forum approach.



Thanks everyone who weighed in for your input.

[Stu Jackson]

Originally Posted by Mike Crothers

I put in a ticket with the manufacturer Rolls and they did a super job answering my questions which differed some from both Magnum and Balmar but at the end of the day I guess their opinion is the one that counts. I had not idea they would be so responsive and a lot less confusing than the forum approach.



Thanks everyone who weighed in for your input.

You're welcome.


Would you care to share the responsive reply they provided you? I'm sure we'd all be appreciative to continue learning.

[Sailmonkey]

Originally Posted by Stu Jackson

You're welcome.


Would you care to share the responsive reply they provided you? I'm sure we'd all be appreciative to continue learning.

I’d like to see if their reply mirrored their published charging specs.