Battery charging below capacity

[peterwilliam]

I have 6 off 210Ah lifeline AGM batteries a little over 2 years old charged by 2 off 200 watt solar panels through a BlueSky Solar Boost 3024iL MPPT Controller and two Cristec 40 amp battery chargers when the generator is running and monitored by a Xantrex light battery monitor and IPN pro remote display for the solar controller.
When the batteries are down 90 Ah and the solar panels are charging them the controller is in acceptance mode and not bulk mode.
Also if the generator is running the the cristec charger only wants to charge at about 7 amps and 13.2 volts and if the fridge cuts in and drags 4 amps it will drop to 3 amps. If the batteries get down 150 Ah then I still only get around 17 Amps.
I would have thought it would charge at a faster rate as it makes it hard for the batteries to recover to full charge once at this level.
Any suggestions or ideas appreciated.

[skipmac]

If I understand correctly you have 6 batteries, 12 volts each, 210 amp hours each so you have a total of 1260 amp hours capacity.

If that is correct then if you have used 90 amp hours from fully charged the batteries are still 93% charged. At that state of charge the batteries resistance to more charge is high and no matter what the capacity of the charging source they will not take a lot of charge. How much charge a battery will take is based on the difference in voltage between the battery and the charger. Charging is not linear. As the batteries' voltage increases that difference gets smaller so the batteries will take fewer amps.

So, would not matter if you had 10 X 200 Watt panels, the batteries will only accept what they can accept. The only way around this is to increase the voltage of the charging source BUT voltage higher than recommended for your batteries will damage them so don't do this.

[smac999]

drop your batteries to 50% and see what the chargers do. 90ah out of a 1200ah bank is nothing. still full

[peterwilliam]

It just worries me that it is detrimental to AGM batteries if they are not fully charged

[sailorboy1]

Originally Posted by peterwilliam

It just worries me that it is detrimental to AGM batteries if they are not fully charged

How do you know they aren't as it sounds that they are?

[peterwilliam]

The battery condition meter shows -90Ah and the solar controller is in acceptance mode and the resting voltmeter shows less than full volts and they are still charging at 7Amps when generator is on.

[sailorboy1]

Originally Posted by peterwilliam

The battery condition meter shows -90Ah and the solar controller is in acceptance mode and the resting voltmeter shows less than full volts and they are still charging at 7Amps when generator is on.

what's "acceptance" mode

7 amps charge from a generator on 1200 AH bank sound pretty charged to me

you have a couple of different charge sources and they both are showing low charge amps, this indicates your batteries are fully charged

[peterwilliam]

Sorry, absorption mode

[skipmac]

Originally Posted by peterwilliam

It just worries me that it is detrimental to AGM batteries if they are not fully charged

Have never used AGMs on a boat so cannot say from personal experience but all the battery experts and manufacturers do seem to agree on this point. This factor and the high expense of AGM batteries compared to regular flooded lead acid (FLA) batteries is why I stick with FLAs.

You are encountering the normal problem of anyone that lives on a boat away from a dock and power. Getting a battery fully charged to 100% is quite difficult unless you have a lot of solar or run a generator or engine for a long, long time.

This example is not exact but gives you an idea of the problem. Assume you have a battery bank discharged to 50% and you have enough charging to charge the batteries as fast as the batteries will accept charge. Then the time might be like this.

50%-60% -- 1 hour
60%-70% -- 1.2 hours
70%-80% -- 1.6 hours
80%-90% -- 2.5 hours
90%-95% -- 3 hours
95%-98% -- 3 hours
98%-100% -- 4 hours

So you see, higher the state of charge, the more slowly the battery is charging and the longer it will take to put in the same number of amps.

All that being said, if you get to the point that your batteries are only accepting 7 amps and the battery bank is 1260 amp hours capacity then that would indicate that the batteries are pretty much fully charged. This assumes that the chargers are set correctly and the output is going into the batteries and not to lights or other loads. If I recall correctly, once a battery bank is only accepting around 1% of the total capacity you can consider the bank fully charged.

[mitiempo]

Originally Posted by peterwilliam

I have 6 off 210Ah lifeline AGM batteries a little over 2 years old charged by 2 off 200 watt solar panels through a BlueSky Solar Boost 3024iL MPPT Controller and two Cristec 40 amp battery chargers when the generator is running and monitored by a Xantrex light battery monitor and IPN pro remote display for the solar controller.
When the batteries are down 90 Ah and the solar panels are charging them the controller is in acceptance mode and not bulk mode.
Also if the generator is running the the cristec charger only wants to charge at about 7 amps and 13.2 volts and if the fridge cuts in and drags 4 amps it will drop to 3 amps. If the batteries get down 150 Ah then I still only get around 17 Amps.
I would have thought it would charge at a faster rate as it makes it hard for the batteries to recover to full charge once at this level.
Any suggestions or ideas appreciated.

I agree with the others that a battery bank that large that is only accepting 7 amps of current can certainly be considered full.

Do you ever synchronize your LinkLite monitor when the batteries are accepting this small current? You should as all battery monitors of this type will accumulate errors over time.

This link may be informative.
Programming A Battery Monitor Photo Gallery by Compass Marine How To at pbase.com

[btrayfors]

The OP said "7 amps @ 13.2VDC", but still hasn't told us whether the AGMs are 12V @ 210AH each making a total of 1260AH capacity OR whether they are 6V @ 210AH each in series/parallel making a total of 630AH capacity.

Please clarify.

Now, 13.2VDC is way to low to be a charging voltage. It must be a float voltage (and even then is too low for my liking). Lifeline AGM batteries need 14.4-14.6 or even 14.8VDC during the absorption cycle. When significantly discharged, they'll accept a whale of a lot of current -- way, way more that even the two battery chargers together can provide.

First thing I'd do is check the voltages, both charging and resting, with a good multimeter of known accuracy. Don't believe the battery monitor.

Then, I'd check the actual current flow using a good clamp-on DC ammeter to be absolutely sure how much current the battery charger(s) are putting out as well as the load current on the batteries.

Next, if there's a way to get the voltages up to 14.4-14.6VDC do so, and check current flows at that voltage.

It could be that nothing is amiss, merely that the batteries are very near full, but you need to verify that with some reliable metering.

Bill

[peterwilliam]

Thanks Bill, This is what concerns me. The question is why are these voltages low, on solar charging I assume it is just available sunlight but under generator/battery charging??? Do you think the chargers might be faulty?
and yes they are 12V for 1260 Ah,
The solar controller equalizes automatically once a month (subject to available sunlight) and the monitor usually synchronizes itself at this stage.
And another question. Are they close enough to full (when down 100 odd Ah) not to damage an AGM battery.

[btrayfors]

Originally Posted by peterwilliam

Thanks Bill, This is what concerns me. The question is why are these voltages low, on solar charging I assume it is just available sunlight but under generator/battery charging??? Do you think the chargers might be faulty?
and yes they are 12V for 1260 Ah,
The solar controller equalizes automatically once a month (subject to available sunlight) and the monitor usually synchronizes itself at this stage.
And another question. Are they close enough to full (when down 100 odd Ah) not to damage an AGM battery.

Again, don't trust the battery monitor. There are all kinds of reasons not to, which MaineSail has recently elucidated nicely.

Well, a 100AH draw on a 1260AH capacity battery bank is NOTHING -- that's only about 7% of the total capacity. And, if that's all you usually draw, then you have a battery bank which is WAY too large for your vessel.

Why don't you try putting a good load on the batteries, and draw them down by 300-400AH? Or even more. How to do this? An easy way if you have an inverter is to put a heavy AC load on the inverter....using, e.g., a space heater or a water heater or other appliance which draws a lot of current.

When you get down by 25-30%, then see what kind of current the chargers are putting out.

One thing I'd do: cut the solar panels out of the picture. They may be providing a sufficiently high voltage to "trick" the battery chargers into believing the battery voltage is higher than it really is.

Do the tests with the charger(s) only. And, again, there's no substitute for good meters: a good multimeter and a good clamp-on DC ammeter. Fluke is a good brand..almost any models. There are a few Chinese meters which are very good also, e.g., the Mastech MS-2108 (not the 2108A) clamp meter now selling for under $60 online. IMHO, every boat should be so equipped.

Bill

[peterwilliam]

Thanks mietiempo, I read the article very technical (I like that) but from a practical point of view it is far easier to have some excess battery capacity and not discharge below 70% generally and you don't have to spend most of your life checking the battery.
Thanks Bill I can work with your advice and will do some experimenting. I think I am getting the idea that I am worrying about nothing. Better safe than sorry. I have a good Fluke clamp on meter and I will get it to earn its keep. I inherited the battery bank and am working on how far to push it considering the investment there.
I really appreciate the help and knowledge from all on Cruisers Forum. What a great bunch of guys to part with their hard earned valuable Knowledge so freely. We live in a great age even though times are tough outside the boating world.