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Assessing AGM Battery Capacity

I posted here about a year ago from Tahiti where I was attempting to assess whether or not my old wet cell house batteries were dying. Thanks to help from this board I was able to figure out that they were before my boat and I crossed to Hawaii. Luckily I was able to buy a new set of AGM batteries in Raiatea.

Now we are in Sitka, AK, and those AGMs are acting strange. I'm attempting to figure out if THEY are passing into the nether world.

Symptoms:

During the passage here from Hawaii, I began getting unusual voltage readings. The batteries seemed to drain more fully than I was use to, but more worrisome, they began to take higher voltage charge before they were full. One factor may be that it got cold on this trip. From an average temp of 75-80*f in Hawaii down to 40*f mid high latitudes Pacific. I was also using some different equipment...a new chartplotter and radar...and my solar panels got less sun... I was forced to run the engine to charge every other day. While in Hawaii, the top voltage reading when batteries were full (using the engine's alternatore) was about 14.3v. Now to get the same charge amount, the final voltage reading is 14.6 or higher. At night the batts may go as low as 12.5 if used heavily where as expected tropical readings were 12.8 to 12.7. To me this sounds like they've lost capacity.

The Test:

Today I performed a load test using a tester on loan from a local automotive store, the type that measures CCA and is rated up to 100amps. Beginning open cirquit voltage for each of the three batteries was 13.3. This was after charging them and letting them rest over an hour. I applied the load tester and flipped the switch and got very similar results for all three batteries. Each dropped quickly to a certain point and stayed. A) 11.8 volts; B) 11.6 volts; C) 11.7 volts. THE BIG QUESTION: Are these readings that suggest an AGM Battery is healthy? Certainly they would be for a deep cycle wet cell, but I don't know what voltage ranges to expect for AGM.

If they are healthy readings, then the only thing I can think is causing the wider range of voltage readings than what I was use to in Hawaii is the drop of 35 to 40 degrees in temperature. In your experience, is the above consistent with cold weather voltage readings for AGM batteries?

Battery Stats:

I have three 100 amphr AGM deep cycle batteries (group 31) made by YNC, Chinese and all that was available in Tahiti.
The side of the battery lists the following stats--
Valve Regulated
Constant Voltage Charge (really?--not usual for AGM)
Cycle Use: 14.5 to 14.9V
Standby Use: 13.6 to 13.8V
Initial Current: Less than 25amps

I don't know what "Cycle Use" or "Stansby Use" mean.

Randall

Aboard Murre
Sitka Harbor
www.murreandthepacific.wordpress.com

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Re: Assessing AGM Battery Capacity

Originally Posted by rreeves0802
The side of the battery lists the following stats--
Valve Regulated
Constant Voltage Charge (really?--not usual for AGM)
Cycle Use: 14.5 to 14.9V
Standby Use: 13.6 to 13.8V
Initial Current: Less than 25amps

I don't know what "Cycle Use" or "Stansby Use" mean.

Randall

Aboard Murre
Sitka Harbor
www.murreandthepacific.wordpress.com
Hi Randall,

I don't have the AGM experience to tell you whether the differences you are seeing may be temperature related, but I believe that for any Lead Acid battery the charging and other voltages are temperature dependent. I just do not know by how much.

For the battery stats, I think that Standby use refers to batteries that are only ever discharged in an emergency or unusual situation. Once they have been drawn down, there is not an urgent need to recharge them rapidly. So the voltage is essentially the float voltage of a multi-stage charger. A higher constant voltage would be a continuous slow overcharge which should be covered by the recombination mechanism of the sealed cell but may cause problems like dendrite growth or other issues.
Cycle use, on the other hand, is what you are doing and requires that the battery be recharged fairly quickly when power is available. I would, however, doubt whether that constant voltage would really be appropriate for extended use if the batteries are not discharged very deeply. In this situation, I have seen recommendations that the charge controller be kept in the bulk charge phase as long as possible to make the best use of the limited hours of solar input.

I share your doubts about "constant voltage" since they give two different voltages!
Last edited by inetdog; July 17th, 2012 at 19:21 PDT.

3. Re: Assessing AGM Battery Capacity

Do you have battery temperature sensors to tell your chargers what voltage to use? If not, then you are undercharging them when they are cold and overcharging them when they are warm.

--vtMaps

4. Re: Assessing AGM Battery Capacity

you didn't exactly say what the load current was that it loaded them down with. if that is a 100a load then it is quite normal to see a strong downward pull on the voltage. most load tests are at a prescribed current draw and to be done over a specified period of time.

cold will also drag any battery down in capacity and that's normal. to what extent those particular batteries will degrade can vary from manufacturer to manufacturer and even battery model to battery model at times.

we also don't know how much quality was put into those batteries as some do well for many years and some could crap out after a few years.

i'll take a stab at what they meant.
cycle use = bulk
standby use = float

of note- those charge voltages seem quite high for any of the agms i'm familiar with.

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Re: Assessing AGM Battery Capacity

I guess my basic question is ... That the batts drop quickly to a certain point and STAY is a good sign, but are load test readings of 11.7 indicative of an AGM in good shape or not. If these batteries were deep cycle wet cells and my results were 10.5, I'd be pretty sure they were ok. I'm presuming that because these readings are higher than a typical wet cell that im probably in good shape, but I don't know what readings to expect for AGM.

you didn't exactly say what the load current was that it loaded them down with
True. And good point. Don't know and cannot find any stats re this load tester online. Assume when it says is rated for 100 amp batt, it knows. It's the basic load tester one buys for \$40 at an automotive store, carquest cpe30370, in this case.

For the battery stats, I think that Standby use refers to batteries that are only ever discharged in an emergency or unusual situation. Once they have been drawn down, there is not an urgent need to recharge them rapidly. So the voltage is essentially the float voltage of a multi-stage charger. A higher constant voltage would be a continuous slow overcharge which should be covered by the recombination mechanism of the sealed cell but may cause problems like dendrite growth or other issues.
Cycle use, on the other hand, is what you are doing and requires that the battery be recharged fairly quickly when power is available. I would, however, doubt whether that constant voltage would really be appropriate for extended use if the batteries are not discharged very deeply. In this situation, I have seen recommendations that the charge controller be kept in the bulk charge phase as long as possible to make the best use of the limited hours of solar input.
In like it! Makes sense. Thanks.

Re: Assessing AGM Battery Capacity

Do you have battery temperature sensors to tell your chargers what voltage to use? If not, then you are undercharging them when they are cold and overcharging them when they are warm.

--vtMaps
For solar, I do. Not for the alternator. When in the tropics I lived entirely off solar. Here, not so much. So I need to update my alternator. But not if I need to buy new batteries as AGMs aren't very available up here ... Etc.

Thanks for all the feedback.

RR

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Re: Assessing AGM Battery Capacity

Originally Posted by rreeves0802
I guess my basic question is ... That the batts drop quickly to a certain point and STAY is a good sign, but are load test readings of 11.7 indicative of an AGM in good shape or not. If these batteries were deep cycle wet cells and my results were 10.5, I'd be pretty sure they were ok. I'm presuming that because these readings are higher than a typical wet cell that im probably in good shape, but I don't know what readings to expect for AGM.

Thanks for all the feedback.

RR
Since the drop in voltage from the "resting" voltage is primarily the result of the internal resistance of the battery (with some help from Peukert's Law), I would expect the voltage drop to be lower in a AGM battery of the same AH rating for the same load. (One of the advantages of AGM is lower internal resistance.) I agree with your presumption, but can't say for sure whether this voltage is right for those particular AGM batteries with that particular (unknown) load. If all of the batteries behave the same way when tested individually it is also a good sign.

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Re: Assessing AGM Battery Capacity

Originally Posted by rreeves0802
... One factor may be that it got cold on this trip. From an average temp of 75-80*f in Hawaii down to 40*f mid high latitudes Pacific...
I hope your solar charge controller has temperature-compensation.

At night the batts may go as low as 12.5 if used heavily where as expected tropical readings were 12.8 to 12.7. To me this sounds like they've lost capacity.
These voltage readings were taken with at least 4 hours or more of rest, with no charging or discharging taking place right?

My experience with AGM's, is as follows - and PLEASE doublecheck / correct me if wrong - I'm a fledgling solar user myself.

12.8v or more on an AGM is 100% charged. Typically, I see up to 13.3 volts for very fresh, quality-maintained ones.
12.6v = 75% SOC (preferred max discharge for regular cyclic)
12.2v = 50% SOC (the lowest I'd ever want to go in regular cyclic use)
11.8v = 25% SOC Try not to go here unless you have no choice.

These seem to differ just a bit for the voltages I see for FLA batteries in the faqs.

This has seemed to be pretty much the case on my measured sustained loads and pretty close with UPG, Powersonic, and Deka AGM's in practice.

I don't see anything too wrong, but would definitely make sure you have some sort of temperature-compensation going on. Also, how are those three batteries wired? Are they all in parallel, two on standby/offline etc?

Remember, this is coming from a rank beginner, so I BEG for any corrections to make sure that I'm somewhat on track too.

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