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laurapakora91 avatar image
laurapakora91 asked

SOC, used amp hours, volts

I need some really clear help please.

I have 3 AGM batteries each with a capacity of 95ah at C20.

I charge using solar and DC2DC.

As the volts of the battery can be iffy for measuring state of charge due to loads always changing on it. I usually use used amp hours as my guide of what I have left in the energy piggy bank.

Recently I have noticed when driving that the state of charge percentage and the used amp hours just jump to fully charged even with around -20 amp hours to go, then I go into absorption and it trickle charges for ages.

In the past I have noticed my voltage (I know not very accurate) can drop to 12.2 with only around 10 amp hours used. I guess this jump to fully charged has happened a few times and cumulatively has made my used amp hours untrustworthy as a guide.

Can anyone give me guidance on the settings I need to put in. I've worked out the peukert, watched all the videos and twiddled the tail current but it seems it is not working. I need an adult with some idea of what I'm doing wrong please.

Thank you!


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3 Answers
pwfarnell avatar image
pwfarnell answered ·

The tail current setting in the shunt is too high at 5%, with a 285Ah battery this means that the SOC will reset to 100% when the charge current falls below 0.05 x 285 = 14.25Amps, at this point the batteries will still be at 90 to 95% SOC. On my AGM bank I have set the tail current to 1% and my SOC goes up to 99.5% before jumping up to 100%. Just be careful, if the batteries are ageing then the tail current may not go below 1%, mine are 18 months old and still go below this, so you may need to tweak this setting. Note then once the tail current is at 5%, the charge rate continues to drop off and the absorption time could be 2-3 hours to get to full, that is the way lead acid and AGM batteries work.

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laurapakora91 avatar image laurapakora91 commented ·
Thank you!

I feel I understand this slightly more.

I will try lowering my tail current setting and see how it goes. My batteries are around a year old so I guess 1% should be fine?

Thanks again

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pwfarnell avatar image
pwfarnell answered ·

As a follow up, if you reduce the tail current and find that there is still a large jump in SOC when it synchronises then you can increase the charge efficiency factor a few percent. There is a video about setting the Victron shunts at https://youtu.be/mEN15Z_S4kE

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laurapakora91 avatar image laurapakora91 commented ·
Thanks!

I feel like I see this video in my sleep!

Unfortunately while I understand some of it the charge efficiency and tail current still remains a complete mystery to me I will try the tail current and then as you say edit the efficiency factor of this doesn't solve the issue.

Thank you for your assistance!

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kevgermany answered ·

Check your settings for battery as well. Are these really spiral cell? Most AGM will work best with one of the Victron AGM settings.

Your absorption voltage seems too high, unless the battery maker says otherwise, it should be around 14.4V. Setting it too high will shorten battery life. And affect tail current.

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laurapakora91 avatar image laurapakora91 commented ·
Hi,


Thank you for your reply. I have also thought this but when I look at the technical specs of the battery it says charged voltage: 14.6-14.8 and top up charge voltage:13.5-13.8.

Would this make the absorption setting right or still incorrect?

Thanks again!

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kevgermany avatar image kevgermany ♦♦ laurapakora91 commented ·
Assuming these are standard AGM batteries, I'd use the lowest values in this set. I.e. 14.6/13.5. But you're within manufacturer spec already. It's just unusually high.
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