AGM battery charging and voltage

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Old 09-20-18, 07:35 AM
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AGM battery charging and voltage

Hello, I opened a thread recently about charging a 75AH lead acid AGM battery, even tho I'm still not sure about what would be the best way to charge, I have another similar question.

I'm using a car battery charger my father is using for to charge the battery, and as far as I know it should stop charging automatically, but when I'm testing the voltage I'm not 100% sure how it works. The charging stops completely at around 13.5V, and it seems to hold the charge, but if for example I'm connecting a voltage tester I have, the voltage drops very fast, the higher the voltage, the faster it drains until it gets to around 12.7-12.8V, then it stops.

As far as I've read about AGM batteries, 12.7-12.8V for a full battery capacity is good and that's how it should be, so my question is if its normal for the AGM battery to charge up to 13.5V, and is it normal for it to drop down like that? Is there a problem with the charger? Or is there a problem with the battery maybe? The battery is a new battery, so it shouldn't have any problems.

Here are some pictures of the battery charger and battery:





The picture of the voltage is while its dropping, it drops at around 0.01V every 15-30 seconds.
Hope I explained it well enough, thanks for the help.
 
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Old 09-21-18, 07:54 AM
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Bump, any answers maybe? ^_^ Thanks.
 
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Old 09-21-18, 12:49 PM
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You always want to measure steady state voltages, so I think you're good at 12.8V on the battery. The 13.5V reading is likely a small residual charge from the charging process that quickly bleeds away through the high impedance digital meter. I bet if you used an analog meter with a higher load it wouldn't even read above 12.8V as the residual would bleed away so quickly.
 
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Old 09-21-18, 12:49 PM
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I'm connecting a voltage tester I have, the voltage drops very fast, the higher the voltage, the faster it drains until it gets to around 12.7-12.8V, then it stops.
That is normal behavior of lead acid battery (AGM is also a type of lead acid).

The voltage drops as charge stabilizes after a full charge.

Here are some articles explaining lead acid battery.

https://batteryuniversity.com/learn/...d_acid_battery

https://www.progressivedyn.com/service/battery-basics/
 
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Old 09-24-18, 01:17 AM
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Originally Posted by ibpooks View Post
You always want to measure steady state voltages, so I think you're good at 12.8V on the battery. The 13.5V reading is likely a small residual charge from the charging process that quickly bleeds away through the high impedance digital meter. I bet if you used an analog meter with a higher load it wouldn't even read above 12.8V as the residual would bleed away so quickly.
Originally Posted by lambition View Post
That is normal behavior of lead acid battery (AGM is also a type of lead acid).

The voltage drops as charge stabilizes after a full charge.

Here are some articles explaining lead acid battery.

https://batteryuniversity.com/learn/...d_acid_battery

https://www.progressivedyn.com/service/battery-basics/
I see, thanks for the replies! So if its normal behavior, both battery and charger are good and its safe to use? And if I'm charging for example and see that the voltage is around 12.7-12.8V, should I just disconnect the charger? Or should I let it "full charge" then drop to the actual voltage?
​​​​​​​Thanks!
 
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Old 09-25-18, 06:37 AM
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If you have a smart charger with storage mode it will automatically disconnect itself and reconnect as needed to maintain full charge. If it's just a regular charger, you need to manually disconnect once the battery is full.
 
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