question

pfk avatar image
pfk asked

AGM battery with charger / inverter.

Hi,


I have a 12v 120ah AGM battery which I’m charging with a Victron 30amp charger.

It takes around 9hrs to reach a charge and tops out at around 12.9v.

Using a Victron 12v 375 watt invertor, I’m powering a 230v 60watt fridge.

The battery only seems to last about 11hrs before the low voltage threshold which I’ve set at 11.75v kicks in and cuts the power to the fridge.

My man maths thought I should get around 19hrs out of the battery.

Am a miss calculating or do I have a battery issue ?

Thanks in advance for your knowledge / reply’s

AGM Battery
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Alexandra avatar image Alexandra ♦ commented ·

@Pfk

I would hazard a guess here and say you do not have the capacity you think you do. This could be caused by many factors, since we dont know what AGMs you have these are the thing to check:-

1) How many cycles does your battery say it can do? and how long have you had it? you can typically count one cycle per day minimum.

2) I do think your charged voltage of 12.9v is not high enough, (but again unsure as I don't know what battery you have) therefore you will not have the full capacity of the battery to use. I would double check with your battery specification sheet. Most deep cycle agms I have used typically charge over 14v and float around 13.8v, which is why I ask. On that note what do you have the solar charger set to?Is it a preset or your own profile?

Hopefully the battery is not heavily sulphated, which will also affect capacity.

3) Also check the C rating of you battery and check if you are charging and discharging within the correct C rating.

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2 Answers
spirou avatar image
spirou answered ·

Also, voltage will sag under load and give you a "false" impression of SOC.

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wild-thing avatar image
wild-thing answered ·

...and 11.75 low voltage cutoff seems rather low to me for an AGM battery. My AGM batteries schould never be discharged below 12.0 V (open circuit; under load the voltage is lower, of course). Life expectancy of AGM batteries sinks exponentially with the depth of discharges.

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