How Much Charge Current When Battery Full?

[btrayfors]

Ever wonder how much charging current a full battery will accept? Obviously, the answer depends on a number of factors, but here's one useful benchmark:

A pair of brand-new (manufactured this month) Trojan T-105's when fully charged and floating at 13.8VDC is accepting less than 0.5A charging current.

That's about 2/10's of one percent of the bank's rated capacity of 225AH.

Why is this useful?

One often hears statements like, "When your batteries are accepting less than about 5% of their rated capacity, they are full."

Well, that's not really true. They need to be accepting a lot less then 5% of their rated capacity, especially at typical float voltages of 13.2-13.4 (which are, IMHO, too low for flooded batteries).

So, when you calibrate your battery monitor be careful to be sure your batteries are really full, not just nearly full.

Bill

[Andina Marie]

Originally Posted by btrayfors

<SNKP>
Why is this useful?

One often hears statements like, "When your batteries are accepting less than about 5% of their rated capacity, they are full."

Well, that's not really true. They need to be accepting a lot less then 5% of their rated capacity, especially at typical float voltages of 13.2-13.4 (which are, IMHO, too low for flooded batteries).

So, when you calibrate your battery monitor be careful to be sure your batteries are really full, not just nearly full.

Bill

The 5% level for terminating chargng is common for cruising boats that are running an engine at anchor to charge their batteries. It can take as much engine running time to get that last 5% into the batteries as it can to get to the 95% level so it is just not economical.

[Maine Sail]

Originally Posted by btrayfors

Ever wonder how much charging current a full battery will accept? Obviously, the answer depends on a number of factors, but here's one useful benchmark:

A pair of brand-new (manufactured this month) Trojan T-105's when fully charged and floating at 13.8VDC is accepting less than 0.5A charging current.

That's about 2/10's of one percent of the bank's rated capacity of 225AH.

Why is this useful?

One often hears statements like, "When your batteries are accepting less than about 5% of their rated capacity, they are full."

Well, that's not really true. They need to be accepting a lot less then 5% of their rated capacity, especially at typical float voltages of 13.2-13.4 (which are, IMHO, too low for flooded batteries).

So, when you calibrate your battery monitor be careful to be sure your batteries are really full, not just nearly full.

Bill

Bill,

I just finished equalizing some batteries for a customer, I do them in my shop one at a time, and the 12V 105Ah Deka battery at 13.6V is accepting about 0.04A @ 70f.

This is not even enough current to register on a battery monitor and the one I have in the shop reads 0A. Often folks think the charger shuts off but many don't and the bank just accepts so little that it can appear that way..

For battery monitor re-set I often use less than 2% acceptance as "full" or "full enough".... This is if the boat has solar, wind or is connected to a dock charger regularly. If they only have an alternator I will sometimes bump the % acceptance higher for the monitor "re-set" even thought he bank is really not really "full" but I also adjust down the banks Ah's slightly when doing this...

As Yandina mentioned above that last few % takes a VERY, VERY long time. With an alternator getting to "full, full" is not likely going to happen very often.

When my customer brought me his Deka's they were by most means, "full", and reading 12.72V, after having been rested for days. They were taking about 3.5 - 4A when first plugged in at 14.6V, higher "pressure", but it took over a day for them to be taking less than what the battery monitors resolution can display, at 13.6V. That last little bit takes a looooong time.

Lifeline Battery suggests "full" as less than 0.5% acceptance at 77F and 14.3V.. At a float level voltage you'd see less than .5% acceptance....

[btrayfors]

Yep, yep, and yep. That's why I posted my findings.

No question reaching true 100% takes a long time. That's what big solar panels, wind generators, and shorepower-ed battery chargers are for :-)

Just sayin...

Bill

[goboatingnow]

Originally Posted by btrayfors

Yep, yep, and yep. That's why I posted my findings.

No question reaching true 100% takes a long time. That's what big solar panels, wind generators, and shorepower-ed battery chargers are for :-)

Just sayin...

Bill

nope you don't need "big" anything to finish off batteries.

[btrayfors]

Originally Posted by goboatingnow

nope you don't need &quot;big&quot; anything to finish off batteries.

Right! But I said "big" because cruising boats don't just sit there with no load on them waiting for the batteries to charge. You need something big enough to provide for both the charging and the ambient load, and that's usually more than just a small solar panel :-) Bill

[Mark Zarley]

Bill's original statement was about "fully" charged batteries and the percent of charge vs amp hrs. The discussion then deteriorated to what can be called "the economy full charge". It's like sayin an apple is juicy, but an orange is juicer. Well it may be but we are talkin about apples. Thanks for the observation Bill.

[Nicholson58]

Originally Posted by btrayfors

Ever wonder how much charging current a full battery will accept? Obviously, the answer depends on a number of factors, but here's one useful benchmark:

A pair of brand-new (manufactured this month) Trojan T-105's when fully charged and floating at 13.8VDC is accepting less than 0.5A charging current.

Bill

You really really really need to read about Trojan (and other) battery charging. 14.8 is the magic number. Your batteries are nearly discharged according to Bob and Trojan. The short of it is the charger people don't talk to the battery makers. The added problem can be that the wire gauge is too small so the charger voltage is measured at the charger, not at the battery where the voltage will be less due to drop in the wires.

https://handybobsolar.wordpress.com/


Bob's blog is very long. It took me most of an evening to go through it. He hasn't much nice to say about the usual folks who sell us this stuff.

[btrayfors]

Originally Posted by Nicholson58

You really really really need to read about Trojan (and other) battery charging. 14.8 is the magic number. Your batteries are nearly discharged according to Bob and Trojan. The short of it is the charger people don't talk to the battery makers. The added problem can be that the wire gauge is too small so the charger voltage is measured at the charger, not at the battery where the voltage will be less due to drop in the wires.

https://handybobsolar.wordpress.com/


Bob's blog is very long. It took me most of an evening to go through it. He hasn't much nice to say about the usual folks who sell us this stuff.

Well, my friend, you REALLY REALLY need to read my post more carefully. What I didn't say, and perhaps should have, is that these new batteries were charged at 14.8 volts during the absorption phase and later reduced to a float charge of 13.8 volts overnite. They were very nearly fully charged by that time. You're right, of course, T-105's need to be charged at 14.8 volts initially and during the absorption/acceptance phase. I like to float them at not less than 13.8 volts, which is a bit higher than most folks do. Bill

[Cheechako]

I often read these technical "charge your battery bank" posts and come away thinking "so what?" You use electricity each day, you recharge as much as you can without letting it dominate your "reasons for cruising". Agonizing over your battery monitor is like trying to determine if the fuel tank on your car has the last .2% in it for a trip that will require you to stop many times along the way! JMHO

[sailinglegend]

Originally Posted by Maine Sail

......................
Lifeline Battery suggests "full" as less than 0.5% acceptance at 77F and 14.3V.. At a float level voltage you'd see less than .5% acceptance....

I agree - but haven't found this info from other manufacturers!

It is also worth noting that chargers don’t actually measure the current going into the batteries, and they don’t know what your battery capacity is, so they are set to drop to a float voltage well before they can overcharge the batteries. This might happen after a set time and/or at a set current, but it will usually not be when the batteries are 100% fully charged. They may well take another 24 hours on a low float voltage to fully charge. Not very helpful if you go into a marina at night to recharge and leave in the morning.

One way to check their state of charge is when you think they are charged – or when they show a low charge current of say 1-2 amps in float mode, switch the charger off and then back on. If it goes back up to 14volts+ and the charge current increases considerably - well over 1% of their Ah capacity - then the batteries were not fully charged.

[goboatingnow]

It is also worth noting that chargers don’t actually measure the current going into the batteries,

all smart ones do. Otherwise they don't know when to terminate absorption.

dave

[Maine Sail]

Originally Posted by goboatingnow

all smart ones do. Otherwise they don't know when to terminate absorption.

dave

I know of at least two manufacturers of smart marine battery chargers that do not do this and who do not operate in-line with the "all smart ones do" statement.

[sailinglegend]

Originally Posted by goboatingnow

all smart ones do. Otherwise they don't know when to terminate absorption.

dave

Chargers know what current they are delivering BUT half of this could be going to run lights, nav equipment, auto pilots - so my point is how do they know when a battery is fully charged because they don't know what is actually going into the battery unless they come with an expensive shunt and complex installation instructions. I don't know any that do this.

[Nicholson58]

https://handybobsolar.wordpress.com/

Great observations. You may find Handy Bob's posts useful. Its a long read but may give you insite and help you pick your next toy purchases. He has a lot to say about battery charging.