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Old 07-02-2013, 17:33   #1
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New AGM drained too quickly...?

Hey everyone... first post here. I found the site while looking for a knowledgeable group who might be able to shed some light on a strange problem I just encountered.

I recently built a very low-powered computer designed to be run off of a battery. It draws between 10-20 watts at 19 volts. I purchased a new 18 amp hour AGM deep cycle battery to power it. I estimated that the computer would draw around 1 amp, so the battery should be able to provide at least 9 or 10 hours before I should need to charge it and still not over deplete the battery.

I have a good battery charger, which is specifically set for a mode to charge an AGM battery. Also, the battery is an Odyssey -- which is highly rated. When I initially connected the charger it up it almost immediately read '100 percent' charged... and the battery read about 13.3 volts...

So, I hooked everything up, with the lead to the computer first going through a voltage step-up to convert it from 12 volts to 19 volts. The computer ran perfectly, and after an hour or two I decided to check the battery voltage to make sure everything was running OK. Well, good thing I did, because it had already drained down to about 12.2 volts, which was somewhere in the ballpark of being only at a 25% charge according to the battery spec sheet.

Now I'm thinking... how is this possible? I have everything hooked up correctly, after triple checking. For a fully charged 18AH battery to drop to 25% charge in about an hour would mean the draw is somewhere close to 13 amps... which is obviously way too high for having only a small computer hooked up.

Now the battery is being charged again, and is climbing higher than before -- I'm already at about 13.5v and the charger is still reading below a '75 %' charge.

I can think of a few possibilities but have no clue what could have happened... do AGM batteries behave this way after sitting for a long time, and upon first use? Perhaps somehow they read an OK voltage, but the juice gets sucked out very quickly at first?? Perhaps I have a short somewhere, somehow??? Perhaps my charger is broken???? Perhaps the charger needed to somehow acclimate to the battery?????

Thanks for any light that can be shed --

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Old 07-02-2013, 17:45   #2
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Re: New AGM drained too quickly...?

Go to the web and read a bit about batteries. When the battery is under load its voltage will drop, the more load the bigger the drop. You can't determain the state of charge when the battery is under load. You can take a battery that is discharged 50% and jamb in 14.5 volts but that doesn't mean much as a battery that is lower will accept a higher charge rate. 13.5 volts is probably about right for a battery that is almost fully charged. When you've finished using the battery let it rest and then check the voltage and you'll see that it will have recovered. Keep in mind there is only a little over a volt difference between a fully charged battery and one that is almost dead.

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Old 07-02-2013, 17:54   #3
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Re: New AGM drained too quickly...?

I'd guess that your draw on the battery is going to be almost twice what you figured, i.e., closer to 2A than 1A.

You are stepping up the voltage from about 12 to 19. That involves some conversion loss (in the stepup device itself). Also, 20W @ 12VDC = 20/12 = 1.67A. With the losses involved in the stepup process, call it 2A.

An 18AH AGM battery, fully charged, should give you about 4.5 hours before reaching 50% state-of-charge, whence you'd want to begin recharging.

By the way, the 18AH rating is at the 20-hour discharge rate, i.e., 18/20 = 0.9A. You're drawing amperage at twice that rate, so the AH availability is going to be considerably less than 18.

Those little battery chargers aren't very accurate in their messaging re: state-of-charge. While an AGM battery will take a lot of amperage when it's 50% depleted, it takes at least 4-6 hours to reach a full charge because the charge acceptance rate falls precipitously as the SOC increases.

Since the draw is so small (~2A), why not measure it more precisely either by putting an ammeter in line with the battery -- most multimeters can measure draws up to about 10A -- or by using a good clamp-on ammeter?

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Old 07-02-2013, 18:33   #4
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Re: New AGM drained too quickly...?

New batteries are never fully charged so you need to use and charge them a few times to get them fully charged.

The power draw of the computer can be lowered by setting the screen a little duller.
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Old 07-02-2013, 21:05   #5
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Re: New AGM drained too quickly...?

...what Bill said, and, when charging, if the charger is truly set up for AGM charging, it should provide an absorption voltage > 14.3VDC before tapering to a float voltage of about 13.6VDC. These voltages are given assuming that the system is in an environment of about 75 to 80F.
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"The Devil is in the details and so is salvation."
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Old 12-02-2013, 00:12   #6
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Re: New AGM drained too quickly...?

Wow, you guys know your stuff... thanks everyone -- each response offered a good bit of knowledge. From here I went and did quite a bit more reading on the subject... here is what I found.

While there are many small variables to take into account, I found the bulk reason why my estimation was so off initially. Battery manufacturers rate their total amp hours (AH) using 10.02 as their base voltage... Why such a seemingly obscure number, I'm not sure. What is more confusing is that the total voltage drop, as Robert mentioned earlier, is only about one full volt before the battery is at the bottom of its full depth of discharge (DOD). So, using my battery as an example, the voltage rating of a 100% state of charge (SOC) is about 12.8. The voltage at 0% SOC is around 11.6, BUT it is recommended not to drop below a 20% SOC, or 11.8v, else the battery will suffer a serious reduction in its rated cycle life.

Ok, so realistically we are left with a one volt drop of useable headroom before the battery must be charged, yet the low voltage of the rated AH figure is another 1.8 volts below that... so, even though the discharge rate probably isn't linear, I'm still going to make a very rough estimate that we would then need to divide the usable voltage range (1v) by the manufacturers AH voltage range (2.8v)... which leaves us with roughly .36, or just over a third... So, I estimate as a very rough guideline to take the manufacturers AH rating and divide it by 3 as a practically usable figure.

This would mean, in the example of my battery that is rated as 18AH, I should expect (18)x(.36), or roughly 6.5 amp hours.

So, how does this estimation play out in reality? Well, my motherboard has a total wattage draw of about 10 watts (very efficient), then one SSD will draw another 2w at most. I also have a spinning drive which draws about 6w, but is powered up for no more than a sixth of the time the computer is running, so we will say that adds another watt average. On top of that, I am using a step-up converter to boost voltage from 12 to 19 on the feed to the computer. Lets say that there is a little bit of loss there due to resistance, but I'm not sure how much... but I would imagine an over estimation would be to add another 2 watts to our figure... which leaves us with a total draw of about 15 watts. Well, amperage equals wattage (15) divided by voltage (19), which rounding up gives us about .8 amps. Our usable 6.5 amp hours divided by our .8 amp draw tells us that the computer should run for about 8 hours before the battery is at its bottom 'safe' limit of 11.8v, or 20% SOC.

So, do these calculations prove themselves in practice? Based on running my computer and monitoring the battery levels as it goes down a few times -- yes, it consistently looks like I will get about 7 or 8 hours if I run the battery all the way down to a 20% SOC.

Now, if you were in an emergency and needed to have power regardless of whether you were going to destroy the life of your battery, I bet you could get the manufacturers full AH (from a battery that still holds a full charge). However, I don't see how it is a realistically helpful figure to rate batteries at an inflated figure from what they will actually provide in a long term cyclical application. This isn't a product that should have inflated figures -- leave that to amplifier manufacturers. Batteries are too critical in certain situations and could mean the difference between life and death... if someone plans on having a healthy battery providing the manufacturers rated amp hours AND holding up for the rated healthy life cycle (about 400 full discharges to 20% SOC on my AGM), they may be left stranded after depleting their battery too far a few times, or find themselves where I was wondering why I'm only getting a third of the rated amp hours... the only way to know is to if they didn't go out of their way to read the small print about how these inflated figures are generated.

I accept that there is a high chance that I may be missing something here in regards to why 10.02 volts are used as the basis for measurement... but I've become too distrustful of corporate advertising schemes to not suspect that this is yet another... even though Odyssey does put out a quality product.

Anyways, I hope this info helps somebody somewhere sometime who may find themselves in a similar situation of bewilderment.

Cheers everyone and happy sailing... can't wait to get out on the sunfish this spring (yup, I'm a lightweight).

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