question

John Brown avatar image
John Brown asked

Internal resistance of lead acid limiting charge wattage from solar string?

Hi there tech people,

I have a 520 watt solar string, connected in series, so 24 VDC nominal, connected to a Victron MPPT 100/50, monitored with a Victron MPPT Control and a Victron BMV700, charging two parallel 12VDC Victron 200 AH VRLA lead acid batteries (which are quite old).

I am observing a few phenomena, leading me to a few questions:

-----------------------------

OBSERVATION 1: When the battery bank is - for example - at 96% SOC before the sun rises. Early in the morning, after the sun comes up, the BMV suddenly indicates a SOC of 100%, although the MPPT light indicates it is charging in bulk mode and there is hundreds of watts flowing into the battery. Obviously the battery is not at a SOC of 100% at this time.

QUESTION 1: Is it because the solar array is "pushing" voltage into the battery, that the BMV thinks that it is suddenly at 100% SOC shortly after the sun has begun shining on the panels in the early morning?

-----------------------------

OBSERVATION 2: When the battery is almost full and the MPPT is in float mode and there is absolutely zero consumption from inverter (switched off) or any other consumers, I see that the system is still charging with 1-10 watts for a long time.

QUESTION 2: Does this come from the internal resistance of the lead acid battery chemistry, resulting in slower and longer charge stages when compared to a LiFePo4 battery?

-----------------------------

Any ideas?

MPPT ControllersBMV Battery Monitorlead-acid
2 |3000

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

1 Answer
rslifkin avatar image
rslifkin answered ·

Lead acid batteries will often accept a tiny bit of power in float for a very long time once they're effectively full. On a larger bank, this may never drop below a few watts (the 415ah AGM bank in my boat settles around 1w on average after it's been in float for a few days, but never gets to 0).

For the BMV resetting to 100% too early, you may need to adjust the settings for when it synchronizes so you get closer to full charge before it meets the criteria.

2 |3000

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

Related Resources