AGM VRLA Batteries and Constant-Voltage charging.

[paulca]

So I got a new battery for my motorbike.

It's an 8.6Ah AGM VRLA battery.  The manual that came with it was rubbish.  It just said to charge at 10% capacity for 8-10hours before use.

Reading on the Yusa website (my battery is actually a Lucas though), they say you should not use a constant current charger with this battery type.

The charge type matters less to me as the battery has gone straight to constant voltage (absorbsion) anyway.  I'm just curious as to how exactly a charger is meant to charge them from flat if they can take a huge amount of current at, say 10.5V and retain a constant voltage.

Anyway, I put it on my car battery charger for an hour and it went from 2 Amps to 1 Amp in under a minute.  I moved it to my bench supply to check on it and so far the current at 14.40V has gone UP from 275mA to 285mA and is still slowly rising.

Should I worry, just leave it for another few hours or back it off to float at 13.8V for a while before fitting to the bike on Saturday?

[Benta]

For AGM batteries, you need constant voltage charge with current limiting. For an 8.6 Ah battery, I'd set the current limit at 1/10 capacity (~1 A). The battery will probably accept more, but unless you're in a hurry there's no need to.

[paulca]

I'm not sure how you can have constant current AND constant voltage, but anyway.

On 14.40V the current fell back to 265mA and didn't move for an hour, so I took that (and a quick read on battery university) to mean it was fully charged.  I dropped it back to a float voltage of 13.5V (based on BU) and it seemed to stablise at 50mA after about 15 minutes.

I have taken it off the charger and will now let it rest.

I'll check it's fully charged; by checking it's voltage after 24 hours is still above 13V, ideally 13.1V and 13V after 48hours.

Of course this poor thing is about to be stuck in a bike outside, sit idle for weeks at a time, often in sub zero temps and then have 14.40V applied to it while getting cooked by the engine next door for an hour, for the rest of it's life!  I might as well baby it now.

[Benta]

I'm not sure how you can have constant current AND constant voltage, but anyway.

I never said "constant current", I said "current limiting". The point is, that if you just apply constant voltage, you'll have a heck of a lot of current going into the battery if it is discharged. Limiting current until it's properly formated is just a precaution.

[paulca]

I'm not sure how you can have constant current AND constant voltage, but anyway.

I never said "constant current", I said "current limiting". The point is, that if you just apply constant voltage, you'll have a heck of a lot of current going into the battery if it is discharged. Limiting current until it's properly formated is just a precaution.

But how do you limit current without changing the voltage?

[Gyro]

Of course the voltage drops when you hit the current limit - it doesn't stop it being a current limited constant voltage supply though.

[metrologist]

but that charge curve will look exactly like CC/CV for lithium, just different voltages.

[Gyro]

Yes, the profiles are very similar - the difference being that you need to terminate the charge on a Lithium, you don't float charge those!

[paulca]

But it's an AGM lead acid.

[BradC]

but that charge curve will look exactly like CC/CV for lithium, just different voltages.

^^ What he said. If you are struggling with this, think of it this way :

You have constant current up until it hits the absorption voltage. So from 0->13.8V you get a constant current. As soon as it hits 13.8V you switch to constant voltage mode until the current drops to sfa.

I use a cheap CC/CV bench supply to charge lead acids all the time. Set the current limit at whatever the maximum charge current is, and set the voltage limit to the absorption voltage. The battery will sit on the current limit until it hits the crossover point, where the voltage will the remain constant as the current drops away. You can do the same thing with Lithium chemistry, it's just less forgiving of errors.

[CJay]

As soon as it hits 13.8V you switch to constant voltage mode until the current drops to sfa.

SFA being what as a percentage of max charge current?

I use a cheap CC/CV bench supply to charge lead acids all the time. Set the current limit at whatever the maximum charge current is, and set the voltage limit to the absorption voltage. The battery will sit on the current limit until it hits the crossover point, where the voltage will the remain constant as the current drops away. You can do the same thing with Lithium chemistry, it's just less forgiving of errors.

That's my SLA charge methodology too but with a rather expensive bench PSU, I will occasionally 'spike' a battery with a higher voltage if it refuses to take a charge.

[Lightages]

AGM batteries should be charged at a rate of a minimum of 5% of their rated current until they reach their asorbtion voltage, 14.4v nominally. Then the voltage should be held at that voltage until the current drops to less than 2% of the rated current. You should also limit the time spent in asorbtion to somewhere around 2 hours even if the current doesn't drop. Once that occurs the voltage should be dropped to the float voltage, somewhere around 13.6v.

The actual voltages and times should be taken from the manufacturer's specifications.

This is my experience after running and installing solar energy systems for over 8 years.

[metrologist]

paulca, did you consider a lithium battery, given your use profile? I think I will try one of those to save some weight next time, but I ride just about every day.

[Lightages]

I should have also said that the manufacturer will or should specify the maximum charge current.

[paulca]

What I couldn't understand was how "Constant Potential" differed from CC/CV charging. 

It seems that constant potential IS CC/CV as opposed to constant current charging which is more like a NiCAD/NiMh charger which pumps a fixed current at whatever voltage the battery will take until you stop it.  Consider chargers for cordless drills with NiCAD packs.  The instructions are to plug it in and leave it for 10-16 hours.  The charger pumps 250mA into the battery constantly, allowing the voltage to rise to a point the pack starts to put out 250mA of heat and charge no more.  This is fine, even to leave it charging over a weekend.

Not fine for lead acids, especially AGMs.

Doesn't need said, but highly dangerous with Lithium.

[paulca]

paulca, did you consider a lithium battery, given your use profile? I think I will try one of those to save some weight next time, but I ride just about every day.

I have considered them, but they seem much more expensive.   I'm also fairly sure they have to have some form of charging circuit within them as 14.40V will overcharge 3 cells and undercharge 4 cells.

Also, lithium batteries do not like to be stored full charged and in occasional automotive use the battery will be kept fully charged most of the time.

[Benta]

What I couldn't understand was how "Constant Potential" differed from CC/CV charging. 

It seems that constant potential IS CC/CV as opposed to constant current charging which is more like a NiCAD/NiMh charger which pumps a fixed current at whatever voltage the battery will take until you stop it.

Yes, you can look at it that way. But you mentioned your bike. In an automotive or motorcycle environment, you shouldn't have deep discharge scenarios. Normally the battery will be at 80% or more, which is the CV range.
If it IS in deep discharge (not healthy, most lead batteries don't like this), you need to make certain the battery doesn't absorp excessive amounts of current. Thus the current limiting.