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Richard Budd avatar image
Richard Budd asked

Unexplained voltage reduction during float charge mode

Charger is Blue Smart IP68 24/5. Can anyone explain why after 2 hours into an 8hr hr float charge, the terminal voltage drops from 28v to 27.5 volts, and yet there is no change of status displayed on the charger? I am monitoring terminal voltages for 2 x 12V, 120 AHr brand new series connected Gel batteries using a LASCAR USB, 0-30V data monitor. What causes this specific change and is it part of the dynamic program of the charger - as this seems to occur with two different units, I wonder why this occurs. Thanks BuddyforGWSR

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wkirby answered ·

The terminal Voltage of a lead acid battery will not stay at 28V, it will sag down even if you disconnect the charger. Once the charrger switches to float it will prevent the terminal Voltage going below set float Voltage (27.5V). If you disconnect the charger when it is in float, you'll observe the terminal Voltage sag even lower even if there is no load connected. It will gradually settle at around 12.8V.
That is the nature of lead acid chemistry.

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Richard Budd avatar image Richard Budd commented ·

Thanks WKirby but there was no disconnection - the monitoring was continuous and no intervention or load applied. It seems as if the charger changed the applied voltage at 2 hrs into Float mode and I'd like to know if this is part of the design of the charger or some external characteristic. I've seen this with old batteries as well as using an identical but different charger.

Below is a graph where x-axis is cumulative minutes, y-axis left is total voltage, y-axis right is voltage difference between batteries (to see if I need to use a charge-balancer) and vertical lines taken from a rig I designed to capture charger LED status.


I need to understand the limits and limitations of batteries we use as I look after the electrical maintenance on the carriages of a local heritage railway, as a volunteer. We have over 25 operational carriages and about 15 of the Blue Smart IP68 24/5 chargers. Thanks BuddyforGWSR.


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wkirby answered ·

I have done some asking and thinking.
I did not properly consider your charging environment. You most likely charge outside in the freezing cold, maybe in the sheds if your stock is fortunate enough.

So, the elevated Bulk Voltage that I mentioned will be due to the charger compensating for the low temperature.
The charger does not have remote temperature sensing. When charging commences (bulk), it uses its own internal temperature assuming that the battery is at a close temperature.
The current flowing through the charger during charging will warm it up and the internal temperature will not likely be accurate anymore.
I have been informaed that when the charging current falls below 1A for a period 2 hours it is assumed that the charger will have cooled to a temperature close to that of the battery and it will then actively compensate for the current temperature.

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Richard Budd avatar image Richard Budd commented ·

Thank you - This does explain the clamped voltage for two hours. I shall 'get off your back' now!

It's a shame the manual doesn't explain that detail, but got there in the end.

Thanks once again.

Richard

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tariqc answered ·

I know this is an old question, but it might help another reader. I noticed the same issue with my new 100/50 victron charge controller. I discovered the "Re-bulk Voltage Offset" setting which I think is the reason why you see such a large voltage drop while in float. You must be in expert mode to see this setting. Check the manual for how it operates, but essentially if the voltage drops more than this amount the charge will recycle back to bulk mode. It's defaulted to 0.8v

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