question

er-minio avatar image

Victron 60Ah AGM acceptable voltage drop under load

General question, I'm learning as I go ahead, and the more I progress the more I realise I don't know anything :-)


I have a small solar setup and my battery is a Victron Deep Cycle AGM 60Ah.


It is connected to a SmartSolar 75|10 and I monitor it via a SmartShunt.

The battery is used to power LED lights and give power to a couple of small electrical loads. Everything is connected to the load output of the charge controller, works perfectly fine. With the lights on I draw around 24W.

Battery charges correctly, everything is fine.

I also have (wired directly to the battery/shunt) a 1000W inverter.
It doesn't get used much, and generally with loads of about 50/150W for 10/15 minutes maximum.

So far so good.


I sometimes want to connect a much bigger load (air compressor). It runs for less than a minute generally, unless it is fully empty, in which case it will run for a couple of minutes, (but this case will happen less than once a month).

The compressor is rated at 700W. When used, the SmartShunt measures about 900W (and a healthy 86A) but the battery voltage drops down to 10.5 under that load.

The SmartShunt indicates about 20 minutes remaining energy (at 50% SoC) under that load.


I used it a couple of times to test it. Battery SoC went down about 10% (as indicated in the app), charged happily in the next day or so.


Questions are:

– how much would be an acceptable voltage drop under an heavy load for that AGM battery (cannot find data online)?

– any pointers on how to properly calculate battery capacity to run something like that?


Thanks in advance.

batteryinverter current draw
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

3 Answers
spirou avatar image
spirou answered ·

I'm surprised it's still going at 86A from a 60Ah non starter battery. C20 current would be OK long term. That would be 12A in your case.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

michelg avatar image
michelg answered ·

Hi, the battery can't simply provide so much Amps during that period of time. 900W = 75A@12V !!

60Ah is the capacity, so you will have 30Ah available to reach a 50%SoC. The battery will probably able to provide the power during 5 minutes, and then you'll reach the cut off voltage to protect it.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

er-minio avatar image
er-minio answered ·

Thanks both. Makes sense.

> probably able to provide the power during 5 minutes

5 minutes is more than double the times it needs, but yes, it is still asking too much.

Adding another (same) battery in parallel bringing to 120Ah would help (the current battery is new) I guess.

2 comments
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Good idea, that would reduce the stress your setup is putting on 1 battery. Just remember that such amount of Amps drawning must not be done for long periods or repeatedly without recharging them.

Michel.

Hi michelg, thanks.

No, the usage for this is extremely low.
I also generally keep the battery monitor app open on my phone when using the inverter, even with lower loads, keeping an eye on draw, etc.


Will order another, same, battery.